Class 7 Chemistry - BeTOPPERS IIT / NEET Foundation Series - 2022 Edition

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IIT FOUNDATION Class VII

PHYSICS

© USN Edutech Private Limited The moral rights of the author’s have been asserted. This Workbook is for personal and non-commercial use only and must not be sold, lent, hired or given to anyone else.

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Published by

:

USN Eductech Private Limited Hyderabad, India.

PREFACE Our sincere endeavour in preparing this Book is to enable students effectively grasp & understand the Concepts of Physics and help them build a strong foundation in this Subject. From among hundreds of questions being made available in this Book, the Student would be able to extensively practice in each concept exclusively, throughout that Chapter. At the end of each Chapter, two or three Worksheets are provided with questions which shall cover the entire Chapter, helping each Student consolidate his / her learning. This Book help students prepare for their respective Examinations including but not limited to i.e. CBSE, ICSE, various State Boards and Competitive Examinations like IIT, NEET, NTSE, Science Olympiads etc. It is compiled by our inhouse team of experts who have a collective experience of more than 40 years in their respective subject matter / academic backgrounds. This books help students understand concepts and their retention through constant practice. It enables them solve question which are ‘fundamental / foundational’ as well questions which needs ‘higher order thinking’. Students gain the ability to concentrate, to be self-reliant, and hopefully become confident in the subject matter as they traverse through this Book. The important features of this books are: 1.

Lucidly presented Concepts: For ease of understanding, the ‘Concepts’ are briefly presented in simple, easy and comprehensible language.

2.

Learning Outcomes: Each chapter starts with ‘Learning Outcomes’ grid conveying what the student is going to learn / gain from this chapter.

3.

Bold-faced Key Terms: The key words, concepts, definitions, formulae, statements, etc., are presented in ‘bold face’, indicating their importance.

4.

Tables and Charts: Numerous strategically placed tables & charts, list out etc. summarizes the important information, making it readily accessible for effective study.

5.

Box Items: Are ‘highlighted special topics’ that helps students explore / investigate the subject matter thoroughly.

6.

Photographs, Illustrations: A wide array of visually appealing and informative photographs are used to help the students understand various phenomena and inculcate interest, enhance learning in the subject matter.

7.

Flow Diagrams: To help students understand the steps in problem-solving, flow diagrams have been included as needed for various important concepts. These diagrams allow the students visualize the workflow to solve such problems.

8.

Summary Charts: At the end of few important concepts or the chapter, a summary / blueprint is presented which includes a complete overview of that concept / chapter. It shall help students review the learning in a snapshot.

9.

Formative Worksheets: After every concept / few concepts, a ‘Formative Worksheet’ / ‘Classroom Worksheet’ with appropriate questions are provided from such concept/s. The solutions for these problems shall ideally be discussed by the Teacher in the classroom.

10. Conceptive Worksheets: These questions are in addition the above questions and are from that respective concept/s. They are advised to be solved beyond classroom as a ‘Homework’. This rigor, shall help students consolidate their learning as they are exposed to new type of questions related to those concept/s.

11. Summative Worksheets: At the end of each chapter, this worksheet is presented and shall contain questions based on all the concepts of that chapter. Unlike Formative Worksheet and Conceptive Worksheet questions, the questions in this worksheet encourage the students to apply their learnings acquired from that entire chapter and solve the problems analytically. 12. HOTS Worksheets: Most of the times, Summative Worksheet is followed by an HOTS (Higher Order Thinking Skills) worksheet containing advanced type of questions. The concepts can be from the same chapter or as many chapters from the Book. By solving these problems, the students are prepared to face challenging questions that appear in actual competitive entrance examinations. However, strengthening the foundation of students in academics is the main objective of this worksheet. 13. IIT JEE / NEET Worksheets: Finally, every chapters end with a IIT JEE / NEET worksheet. This worksheet contains the questions which have appeared in various competitive examinations like IIT, NEET, AIEEE, EAMCET, KCET, TCET, JIMPER, BHU, AIIMS, CBSE, ICSE, State Boards, CET etc. related to this chapter. This gives realtime experience to students and helps them face various competitive examinations. 14. Different Types of Questions: These type of questions do appear in various competitive examinations. They include:

• Objective Type with Single Answer Correct

• Non-Objective Type

• Objective Type with > one Answer Correct

• True or False Type

• Statement Type - I (Two Statements)

• Statement Type - II (Two Statements)

• MatchingType - I (Two Columns)

• MatchingType - II (Three Columns)

• Assertion and Reasoning Type

• Statement and Explanation Type

• Roadmap Type

• FigurativeType

• Comprehension Type

• And many more...

We would like to thank all members of different departments at BeTOPPERS who played a key role in bringing out this student-friendly Book. We sincerely hope that this Book will prove useful to the students who wish to build a strong Foundation in Physics and aim to achieve success in various boards / competitive examinations. Further, we believe that as there is always scope for improvement, we value constructive criticism of the subject matter, as well as suggestions for improving this Book. All suggestions hopefully, shall be duly incorporated in the next edition. Wish you all the best!!!

Team BeTOPPERS

CONTENTS 1.

Measurement

..........

01 - 20

2.

Motion

..........

21 - 38

3.

Light

..........

39 - 56

4.

Electric Current & Its Effects

..........

57 - 80

5.

Wind, Storm and Cyclones

..........

81 - 94

6.

Sound

..........

95 - 108

7.

Heat

..........

109 - 122

8.

Key and Answers

..........

123 - 184

Chapter -1

Measurement

Learning Outcomes

By the end of this chapter, you will understand • • • • • • •

Physical Quantity Units Types of Physical Quantities System of Units Multiples of Ten Measurement of length Measurement of mass

• • • • •

Measurement of Time Measurement of Area Measurement of Volume Measurement of Density Relative density

1. Introduction In our daily life, we tend to guess the distance of our school from our home, the time taken by us to take our lunch and our body weight. Such guess may not be accurate, but in science measurement needs to be accurate. Therefore, proper measurement techniques need to be learnt. Measurement is the comparision of an unknown quantity with a known standard quantity of same kind. Another important thing is to identify a quantity that can be measured (known as physical quantity). Generally most of the measurements are based on physical quantities like length, mass, time, temperature etc., In measuring a physical quantity one needs to have an idea of its various units, their inter-relationships and also the various devices used for its measurement.

2. Physical Quantity i) ii)

A physical quantity is a quanity that can be measured. Physical quantity is expressed by stating • Magnitude of the physical quantity. • The unit employed. Therefore, physical quantity = magnitude × unit

3. Units Standard Unit The unit plays an important role in the description of a physical quantity. In olden days, the unit used for a particular physical quantity was person dependant like measuring length of cloth through arm-length and its length varied from person to person. To overcome this, a beaurau in paris specified standards units to measure physical quantities more accurately.

Characteristics of Standard Unit 1. It should be definite, without any doubt or ambiguity. 2. It should not change with respect to space and time. 3. It should be reproduced easily . 4. It should be of convenient size to measure small and bulk quantities.

4. Types of Physical Quantities Fundamental Quantities These quantities does not depend on other physical quantities. Example: Length, mass and time are independent fundamental quantities. Derived Quantities These quantities are defined in terms of fundamental quantities. Eg: Area, density, volume are derived quantities.

5. System of Units The fundamental units of length, mass and time taken together form a system of units. For measuring various physical quantities following systems are commonly adopted. Name of the

Unit

System Length

Mass

Time

F.P.S

Foot

Pound

Second

C.G.S

Centimeter

Gram

Second

M.K.S

Metre

Kilogram Second

In S.I system the units for length, mass and time are same as that of M.K.S system.

7th Class Physics

2 Rules for writing symbols of Units Rule 1: A unit named after a scientist must not start with a capital letter when the complete name is written. The work done by a body is 10 joule but not 10 Joule. If only the first letter is used as a symbol, it must be a capital letter. Thus, the work done can be represented a s 10 J. Rule 2: The units other than those named after scientists i.e., all other units are symbolically represented by small letters. If the length of a rod is 2 metres, it is written as 2 m. If the time taken by a ball to hit the ground is 5 seconds, it is written as 5 s. Rule 3: Symbols for units do not take plural form The mass of a body is 10 Kg but not 10 Kgs The force acting on a body is 10 newton and not 10 newtons. Rule 4: No full stop or punctuation mark should be used within or at the end of symbols for units. The mass of a body is 10 Kg but not 10Kg. (full stop () should be omitted) The force acting on a body is 10 N and not 10 N: (colon ( : ) should be omitted)

6. Multiples of Ten The study of nature involves objects of small size like atom, nucleus etc. They belong to the small world (microcosm). If we measure the diameter of hydrogen atom (in microcosm) it will be described by a very small number 0.000,000,000,106m. It also involves celestial objects of the universe like the sun, planets, moon, stars, etc. They belong to the cosmic world (macrocosm). The diameter of the sun will be described by a very large number 1,390,000,000m. The above numbers are exceptionally big and difficult to write or remember. How then can this task be simplified? It can be simplified by expressing in terms of multiples of 10. The following chart shows some multiples of 10 and their corresponding names or prefixes : www.betoppers.com

Positive multiples of 10 10 18 10 15 10 12 109 106 103 102 101

Prefix or Name Exa Peta Terra Giga Mega Kilo Hecto Deca

Symbol

Positive multiples of 10 101 102 103 106 109 1012 15 10 1018

Prefix or Name deci centi milli micro nano pico femto atto

Symbol

E P T G M K H D

d c m  n p f a

Formative Worksheet 1.

2.

3.

4.

5.

6.

7.

Amount of work done is 10 Joule, here 10 stands for (A) Unit (B) Numerical value (C) Both (1) and (2) (D) None of these Which physical quantity unit is same in all systems (A) Length (B) Mass (C) Time (D) Temperature 2 1 cm = ______________ (A) 104 m2 (B) 103m2 2 2 (C) 10 m (D) 102 mm2 10 kg m–3 = _____________ g cm–3 (A) 1000 (B) 1 (C) 10–2 (D) 100 Which of the following are the examples for physical quantity? (A) Length (B) Area (C) Force (D) Energy Convert 100 quintal into nano grams (ng). (A) 105 ng (B) 1016ng 17 (C) 10 ng (D) 1018 ng Convert 1 second into day. (A)

1 Day 86, 400

(B) 24 day

(C)

1 day 24

(D)

1 day 3600

Measurement 8.

9.

10. 11.

12.

13.

14.

15.

1 hectare = _______________. (A) 102 cm2 (B) 104 cm2 (C) 106 cm2 (D) 108 cm2 3 16 kg/m = ___________ g/cm3 (A) 16 × 10–3 (B) 26 × 10–6 –3 (C) 36 × 10 (D) 46 × 10–6 If 1 nano gram = 10x milligram, then x = (A) 6 (B) –6 (C) 9 (D) –9 Match the following Column-I Column-II (A) Giga p) 106 (B) micro q) 1/102 (C) Mega r) 109 (D) Pico s) 10–6 t) 10–12 Choose the correct statement. (A) Number of fundamental quantities are limited. (B) In M.K.S System, there are 7 fundamental quantities (C) Number of fundamental quantities are unlimited. (D) Units of fundamental quantities in M.K.S is same as in SI. Choose the correct options (A) nm is not same as mN (B) 1nm = 10–9m and 1 mN = 10–3N. (C) When we change the unit of measurement of a quantity, its numerical value changes. (D) Smaller the unit of measurement, smaller is its numerical value. Statement - I : Meter is the standard by which we can measure length. Statement - II : Second is the standard by which we can measure time. (A) Both Statements are true, Statement - II is the correct explanation of Statement - I. (B) Both Statements are true, Statement - II is not correct explanation of Statement - I. (C) Statement - I is true, Statement - II is false. (D) Statement - I is false, Statement - II is true. Statement I : Fundamental units are the units for measuring fundamental quantities. Statement II : Metre is the unit of length. (A) Both Statements are true, Statement - II is the correct explanation of Statement - I. (B) Both Statements are true, Statement - II is not correct explanation of Statement - I. (C) Statement - I is true, Statement - II is false. (D) Statement - I is false, Statement - II is true.

3 16. Match the following Column-I (A) Unit of length in F.P.S (B) Unit of mass in C.G.S (C) Unit of time

17. Match the following Column-I (A) Derived unit (B) Fundamental unit (C) Derived quantity (D) Fundamental quantity

Column-II p) m q) foot r) metre s) pound t) second Column-II p) Height q) Thickness r) Area s) Newton t) Kilogram

Conceptive Worksheet 1.

Physical quantity = Numerical value × ________ (A) Standard number (B) Standard Unit (C) Standard value (D) None of these 2. To measure any physical quantity __________ should be required. (A) Time (B) Mass (C) Unit (D) Length 3. The S.I. system has how many basic units? (A) 15 (B) 7 (C) 10 (D) 8 4. 60 kilogram in short form is written as (A) 60 kg (B) 60 kg (C) Both (A) and (B) (D) None 5. Which among the following is the international system of units ? (A) S.I. (B) F.P.S (C) C.G.S (D) M.K.S 6. A physical quantities which are independent of other physical quantities are (A) Fundamental quantities (B) Derived quantities (C) Both (A) and (B) (D) Neither fundamental nor derivered 7. Unit of area is a (A) Derived unit (B) Fundamental unit (C) Both (A) and (B) (D) None of these 8. The unit of speed is a (A) Fundamental unit (B) Derived unit (C) Neither of above (D) Cannot say 9. Which of the following are derived quantities (A) Speed (B) Velocity (C) Perimeter (D) Both (A) and (B) 10. Which of the following quantities are used to derive the quantity density (A) Mass (B) Area (C) Volume (D) Both (A) and (C) 11. Which of the following are fundamental quantities (A) Length (B) Height (C) Radius (D) All of these www.betoppers.com

7th Class Physics

4 12. The symbol for unit of force in M.K.S. (A) n (B) N (C) nN (D) none of these 13. One metric tonne = _________ quintal. (A) 10,000 (B) 1000 (C) 100 (D) 10 14. 1 fermi is equal to how many centimetre (A) 10–15 cm (B) 10–13 cm (C) 10–12 cm (D) 10–10 cm. 15. How many centimetre make 1 nano metre (A) 10–7 cm (B) 10–5 cm (C) 10–15 cm (D) 10–10 cm 2 16. 1 k m = _______. (A) 1 hectare (B) 10 hectares (C) 100 hectares (D) 1000 hectares 17. 1 litre = _______. (A) 1000 ml (B) 1000 cc (C) both (A) and (B) (D) None of these 18. 1 cm2 = _______. (A) 10–10 km2 (B) 10–8 hectares –4 2 (C) 10 m (D) all of these 19. The acceleration of a car is 51840 km/h2. The same in m/s2 is _______. (A) 4 (B) 8 (C) 12 (D) None 20. 0.4 sq.cm is ___________ sq. km (A) 4×10–10 (B) 4×10–11 –11 (C) 0.4×10 (D) 4×10–9 21. 1 g (micro gram) = _______________ g (A) 10–10 (B) 10–8 (C) 10–6 (D) 10–4

I.

Least Count The smallest quantity that can be measured accurately using any instrument is known as least count of that instrument. Least count of regular scale (LC) = 1mm =

1 cm 10

Mesurement of curved lines can be done by using a piece of thread. Thickness of coin by a centimetre scale or millimetre scale is Thickness of one coin =

Thickness  cm or mm  of 'n ' of coins Number of coins  n 

Diametre of a wire Diameter =

Length of the coins Number of turns in the coil

The diameter of spherical object can be measured by placing the object between two wooden blocks as shown in the figure.

7. Measurement of Length Length is the distance between two fixed points. The diameter of spherical object = x2 – x1 SI unit - metre(m)

II.

C.G.S unit - centimetre(cm) The various units used to measure length are mm, c, m, km etc.

m,

The inter-relationship among these units with a standard unit: 1 micro metre = 1

1 millimetre = 1mm = 10 m 1 centimetre 1cm = 10-2m www.betoppers.com

Distance between earth and moon, or earth and sun can’t be done by using conventional units. Such large distances are expressed using following units. III. Astronomical unit(A.U) It is the average distance of the earth from the sun.

m = 10–6 m -3

1kilometre = 103m

Measurement of very large Distances

1 A.U = 1.496 × 1011m

Measurement

Formative Worksheet 18. In decreasing magnitude which of the following is correct ? (A) km, cm, m, mm (B) km, m, cm, mm (C) m, km, cm, mm (D) km, cm, mm, m 19. The number of rounds of a wire around a pencil are 24 and the length of the coil is 4.8cm then what is the diameter of wire ? (A) 24 cm (B) 2.4 cm (C) 20 cm (D) 0.2 cm 20. The distance between Radha’s home and her school is 3250 m. Express this distance into km. (A) 32.5 km (B) 3.25 km (C) 0.325 km (D) 0.0325 km 21. The height of a person is 1.65 m. Express this value in mm. (A) 16.5 mm (B) 165 mm (C) 1650 mm (D) 16500 mm 22. While measuring the diameter of the ball, the inner edges of the wooden block stood at 3.4cm and 4.7cm on a scale. Calculate the diameter of the ball ? (A) 1.3 cm (B) 7.7 cm (C) 0.13 cm (D) 0.77 cm 23. Fill in the blanks (i) 1 cm = mm. (ii) 1 dm = metre. (iii) 1 kilometre = metre. (iv) 1 metre = centimetre. 24. Fill in the blanks with < or > sign. (i) 1 decametre 1 decimetre. (ii) 1 millimetre 1 centimetre. (iii) 1 hectometre 1 kilometre. (iv) 1 decimetre 1 centimetre.

Conceptive Worksheet 22.

23.

24.

25.

26.

What is the SI unit of length? (A) Metre (B) Centimetre (C) Kilometre (D) All of these 4 kilometre are equal to (A) 4, 00, 000 metre (B) 40, 000 metre (C) 4, 000 metre (D) 400 metre 15 cm is equal to (A) 150 mm (B) 15mm (C) 1.5 mm (D) 0.15 mm Which is a correct relationship? (A) 1m= 100 cm (B) 1 cm = 10 mm (C) 1 km = 1000 m (D) all the correct 1 cm = _________ kilometre. (A) 100 (B) 105 (C) 10–5 (D) 10–2

5 27. Arrange the following lengths in their increasing magnitude: 1 metre, 1 centimetre, 1 kilometre, 1 millimetre. (A) 1 centimetre < 1 millimetre < 1 kilometre < 1 metre (B) 1 millimetre < 1 centimetre < 1 metre < 1 kilometre (C) 1 kilometre < 1 metre < 1 centimetre < 1 millimetre (D) none 28. 1 Angstrom = ____. (A) 10-10m (B) 10-8m (C) 10-6mm (D) 10-10cm 29. 1AU = _______. (A) 1.496 × 1011m (B) 1.496 × 109cm (C) 1.496 × 108m (D) 1.444 × 106m 30. The size of bacteria is generally measured in microns. The micrometre  μ m  , is often called the micron. How many microns make up 1 kilometre ? (A) 103 (B) 106 9 (C) 10 (D) 1012 31. Least count of a metre scale is _______. (A) 1cm (B) 0.1mm (C) 0.1cm (D) 0.01mm

8. Measurement of Mass Mass is the amount of matter contained in a body. Mass is measured using devices like common balance, physical balance etc., SI unit of mass is kg. The other units of mass are milligram(mg), gram(g), tonne, quintal. 1mg = 10-3g 1quintal = 100kg 1tonne = 1000kg

9. Measurement of Time The dictionary defines time as the period between two events. Need to Measure Time The need to measure time was felt by human beings a long, long time ago. In every civilization, and every culture, however ancient, there has been evidence of people keeping track of the passage of time. In very ancient times, people used to keep track of the passage of days. As the complexity of their lives increased, people felt the need to split the day into smaller periods. www.betoppers.com

7th Class Physics

6

i)

Today, we measure time for a variety of applications: • We go to school on time and do everything there on time. • The doctor checks how many times our heart beats in one minute. • We need to know when a train or plane will arrive or what is the duration of the journey. • How long a TV programme or a movie will take. • The amount of time a computer will take to perform an operation, and so on. The list is endless . . . Measurement of Time In the beginning, people used calendars to keep track of years, which were divided into months and days (not exactly what we use today, but something similar). Later, people started keeping track of the time of the day. They started building various instruments to divide the day into smaller parts. Measurement of Time An instrument used to measure time is called clock . Ancient Instruments Water Clock Water clock was used in different parts of the world. It worked on the principle of regulated flow of water.

streamed down from the top bulb into the bottom bulb, giving the time.

i)

ii)

ii)

Sundial Sundials use the movement of the shadow of a rod struck upright in the ground, whose shadow changed direction with the movement of the sun across the sky.

iii) Hour Glass It consist of two rounded glass bulbs connected by a narrow neck of glass. The top bulb was filled with sand and a measured amount of sand particles www.betoppers.com

Modern Instruments These instruments were not very accurate. There was a need to have instruments that were more accurate and gave a better precision of time. Pendulum Clock The major breakthrough came in 1656, when Christian Huygens made the first pendulum clock. It consisted of weights and a swinging pendulum. These clocks were much more reliable than the earlier ones.

Atomic Clock Another major advancement in timekeeping was the invention of the atomic clock, which is very accurate.

iii) Stop Watches Today, clocks are used not only to tell the time of the day, but are also used as timers in ovens, in stopwatches used in various athletic events, etc.

Measurement

7

iv) Timer A timer is a special type of clock. It is used to control the sequence of an event. These are used in appliances such as traffic signals, washing machines, and the highly explosive time bombs. 1.

2. 3. Units of Time The SI unit of time is the second. Some other units of time are shown below. 60 seconds 1 minute 60 minutes 1 hour 24 hours 1 day 365 days 1 year 10 years 1 decade 10 decades 1 century 10 centuries 1 millennium Measurement of Time by a Periodic Motion

Any object that is moving is said to be in motion. A motion that repeats itself in equal intervals is called Periodic Motion. Examples: Motion of Earth round the Sun, Motion of Moon round the Earth, Oscillation of Pendulum Simple Pendulum

A small mass that is suspended from a fixed point and allowed to swing freely under the influence of gravity is called a pendulum. An ideal, simple pendulum consists of a small mass (like a stone or a metal ball) called a bob suspended by a string. When the bob moves from one position and comes back to the same position, it is said to complete one Oscillation.

The time taken to complete one oscillation is called the time period of the pendulum. For example, if the bob starts from point A goes to points B and C, and C comes back to point A, it completes one oscillation. The time taken for a pendulum to complete one oscillation does not depend on the extent to which the bob of the pendulum is displaced. The time period does not depend on the mass of the bob used (within reasonable limits). The time period depends on the length of the string or wire used. Greater the length of the string greater is the time period . This means that if the length of the pendulum is fixed, its time period is constant. Scientists began to build new clocks based on this property of the pendulum.

Formative Worksheet 25. One quintal = ______________ton. (A) 10 (B) 100 (C) 0.1 (D) 0.01 26. 1 metric tonne = __________ milligram. (A) 103 (B) 106 (C) 109 (D) 1012 27. 1 microsecond = 10x milliseconds. Find x. (A) +1 (B) + 3 (C) –1 (D) –3 28. 1 kg = –––––––––– tonne. (A) 100 (B) 1000 (C) 10–3 (D) 10–2 29. A truck of weigh 4.4tonnes. Its value in kg? (A) 4400 kg (B) 440 kg (C) 44 kg (D) 4.4 kg 30. How many seconds are equal to 6hours 8min? (A) 86400 sec (B) 43200 sec (C) 1296480 sec (D) 648240 sec 31. Match the following: Column-1 Column-2 (A) 1 minute p) 100 years (B) 1 day q) 365 days (C) 1 year r) 24 hours (D) 1 Century s) 60 s 32. Match the following: Column-1 Column-2 (A) 1 hour p) 1400 min (B) 1 day q) 60 min (C) 1 year r) 36500 days (D) 1 Century s) 8760 hours 33. x is the number of years in a decade. y is the number of decades in a century and z is the number of centuries in a millenium. Find x : y : z. (A) 1 : 1 : 1 (B) 1 : 2 : 1 (C) 1 : 2 : 3 (D) 2 : 3 : 4 www.betoppers.com

7th Class Physics

8

34.

1 minute 60minutes + = x and 60s 1hour

39. In the figure, if the time taken by the bob to move from A to B is 0.5 second, the time period of the pendulum is _________.

1 decade 10decades + = y . Find x + y.. 10 years 1 century (A) 2 (C) 8

(B) 4 (D) 16

35. Imagine this hypothetical situation. The wealth possessed by the richest person of this world is Avogadro number (6.023×10 23 ) of dollars. He thought of donating his entire wealth to the needy and make their living healthy. His life span is assumed to be 90 years and he is now 45. The rate of donation is 5000 dollars per second and from now, every second of his existence is only used for donation. Can he empty all his wealth in this life? Justify your answer. 36. An unusual running race was organised between a rabbit and an ant. By the time rabbit moves 1 m in one second, ant moves 1mm (millimetre) in the same time. How much time will ant takes to cover the distance covered by the rabbit in 1 second? 37. The average lifespan of a lion in the wild is up to 16 years. If average number of days in a year is 365 days. Express this age in hours. (A) 1600

(B) 5840

(C) 140160

(D) 8409600

38. In physics, the Planck time, (tP), is the unit of time in the system of natural units known as Planck units. It is the time required for light to travel, in a vacuum, a distance of 1 Planck length ( 1.6 ×10–35 m) .The unit is named after Max Planck, who was the first to propose it. The value of Planck time is approximately equal to 5.32 ×10–44 s. And Century is one of the larger units of time and is equal to 100 years. Find approximately how many Planck time’s make one century. (Take one year = 365 days) (A) 1 Century = 5.89 × 1052 tP (B) 1 Century = 5.89 × 10 62 tP (C) 1 Century = 5.89 × 1072 tP (D) None of these

www.betoppers.com

(A) 0.5sec (B) 1 sec (C) 1.5sec (D) 2 sec 40. The time period of a simple pendulum is 2 seconds. The time taken by it to complete

1 oscillation,is 2

__________. (A) 1sec (B) 2sec (C) 4sec (D) 8sec 41. The time period of a simple pendulum is 5 seconds. If mass of the bob is increased to 4 times the original, the new time period of it is _______. (A) 20sec (B) 15sec (C) 10sec (D) 5sec 42. There is a pendulum clock showing right time at a given place. The length and mass of it is increased to two times the original. Will there be any change in showing time. If so explain.

Conceptive Worksheet 32. 1quintal = __________kg. (A) 100kg (B) 1000kg (C) 10kg (D) 500kg 33. 1mean solar day = _________. (A) 365days (B) 10years (C) 24hours (D) 60minutes 34. Match the following: Column-1 Column-2 (A) Water Clock p) Sand (B) Sun dial q) Flow of Water (C) Hour glass r) Shadow 35. Which of these is not a unit of time? (A) Second (B) Light year (C) Hour (D) Century 36. Which of these is a type of clock? (A) Hourglass (B) Sundial (C) Pendulum clock (D) All

Measurement

9

37. Which of these clocks would you expect to be the most accurate? (A) Pendulum clock (B) Atomic clock (C) Sundial (D) Water clock 38. The time period of the seconds pendulum is 2 seconds. Now, match the following with respect to seconds pendulum. Column-1 Column-2 A) 1 oscillation p) 8 seconds

39.

40.

41.

42.

B)

1 oscillation 2

q) 2 seconds

C)

1 oscillation 4

r) 1 second

D) 4 oscillations r) 0.5 second If the Charminar superfast express staying 00 hours in Warangal, then the time in 12 hour clock is _________ (A) 12 O’ clock at night (B) 12 O’ clock at noon (C) 18 O’ clock at night (D) 18 O’ clock at noon A passenger goes to Secunderabad railway station. He asked the enquiry counter, “When did Tirumala express come?” The enquiry counter person replied “18 hour 15 minutes.” Then the time in his 12-hour clock was ________. (A) 5 hour – 45 minutes AM (B) 5 hour – 45 minutes PM (C) 6 hour – 15 minutes AM (D) 6 hour – 15 minutes PM Fill in the following blanks: i) _____________(Century/minute) is a very large unit of time. ii) One _____________ is equal to 10 centuries. iii) _____________ made the first pendulum clock. iv) _____________ is the time taken to complete one oscillation. v) _____________ is the SI unit of time. The time period of a simple pendulum depends on (A) The length of the string used (B) The mass of the bob (C) The extent to which the bob is displaced (D) None of these

10. Measurement of Area The amount of surface occupied by an object or a place is called its area. In Standard International (S.I.) system the unit of area is square metre. In short form square metre is written as metre square. The area of a square surface having each side equal to one metre is called square metre (m2). figure shows a square surface ABCD, such that AB = BC = CD = AD = 1 m. The area of the surface enclosed by ABCD (not drawn to scale) is 1m2 Relation between cm2 and m2 : 1m2 = 1 m × 1m  1m2 = 100 cm × 100 cm or 1 m2 = 10000 cm2 or

1cm 2 

1 m 2  104 m2 10000

Multiples of Standard Unit of Area For measuring bigger areas, such as area of field or towns, metre square is a very small unit. Thus, a bigger unit is used, which is called hectare. One hectare is the surface area of a square whose each side is equal to 100 m. Note:100m is called hectometer and 1 square hectometer is called hectare. 1 square hectometer = 1 hectometer × 1 hectometer = 100m × 100m = 10000m2 100m2 is called are  1 hectare = 100 acres = 100 × 100m2 = 104 m2 . 1 hectare = 100 m × 100 m, 1 hectare = 10000 m2 2 or 1m 

1 hectare 104 hectare 10000

For measuring further bigger areas, such as the area of district or a province or a country, even hectare is a very small and inconvenient unit. Thus, a bigger unit is used which is called square kilometre (km2 ). One square kilometre (km2 ) is the surface area of a square whose each side is equal to 1 km 1 km2=1000 m × 1000 m, 1 km2 = 1000, 000 m2 1 km2 = 100 hectares

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7th Class Physics

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Formative Worksheet 43. 1 cm2 = ________. (A) 1/10000 m2 (B) 10–5 m2 2 (C) 1/100000 m (D) 10–4 m2 44. 1 km2 = ____________ hectares. (A) 10000 (B) 1000 (C) 100 (D) 10 45. Statement I : 1 km2 = 1,000,000 m2 Statement II : 1 hectare = 100 Ares (A) Statement - I is true, Statement - II is false. (B) Statement - I is false, Statement - II is true. (C) Both Statements I & II are true. (D) Both Statements I & II are false. 46. The area of a square surface, whose each side is equal to one metre is (A) Square millimetre (B) Square centimetre (C) Square cube metre (D) Square metre 47. A school hall measures 20 m in length and 12 m in breadth. Find the area of the school hall. (A) 1200 m2 (B) 120 m2 2 (C) 240 m (D) 2400 m2 48. The length of the school play ground is 400 m and breadth is 100 m. The area of the play ground is _______ hectares. (A) 3 hectares (B) 2 hectares (C) 4 hectares (D) 5 hectares 49. What is the area of rectangular graph paper having 6cm length and 5cm width ? (A) 11 cm2 (B) 1 cm2 2 (C) 30 cm (D) 60 cm2 2 50. 1 m = ______ x_______hectare. 1 hectare = _____y_____ km2 . 1 mm2 = _____ z_____ km2. (A) x = 10–6, y = 10–4, 10–2 (B) x = 10–4, y = 10–2, z = 10–6 (C) x = 10–2, y = 10–4, z = 10–6 (D) x = 10–4, y = 10–6, z = 10–2

Conceptive Worksheet 43. The amount of surface occupied by an object or a plane is (A) Length (B) Temperature (C) Area (D) Volume 44. Unit of area is a (A) Fundamental unit (B) Derived unit (C) Both (1) and (2) (D) None 45. In C.G.S system the unit of area is (A) s2 (B) kg2 (C) cm2 (D) m2 46. Area of 1sqcm is (A) 100mm2 (B) 1000mm2 2 (C) 10mm (D) 10,000mm2 www.betoppers.com

47. How many small squares are there in 1sq.cm ? (A) 1000 (B) 100 (C) 10 (D) 1 48. Area of irregular shaped bodies are measured using (A) Graph paper (B) Scale (C) Using formula (D) Tape 49. The area of the land is 100m2, here m2 stands for (A) Numerical value of area (B) Unit of area (C) Both (A) and (B) (D) None of these 50. 1km2 = _____________ (A) 1 hectare (B) 10 hectares (C) 100 hectares (D) 1000 hectares 51. 1 hectare = ____________ (A) 1000m2 (B) 10000m2 (C) 100000m2 (D) 100m2

11. Measurement of Volume The space occupied by a substance (solid, liquid or gas) is called volume. The unit of volume in standard international (SI) system is cubic metre. In short form cubic metre is written as m3. One cubic metre (1m3) is the volume occupied by a cube whose each side is equal to 1 m. Submultiples of Unit of Volume Cubic metre is a fairly inconvenient unit for measuring small volumes such as volume of a match box or volume of a glass full of water. A small unit called cubic centimeter (cm3) is used for measuring small volumes. The C.G.S unit of volume is cm3 or cc. Which is used for measuring small volumes. One cubic centimeter (1cm3) is the volume occupied by a cube whose each side is equal to 1 cm. Relation between 1 m3 and 1cm3 1 m3 = 1 m × 1 m × 1 m 1 m3 = 100 cm × 100 cm × 100 m 1 m3 = 1000,000 cm3 or

1cm 3 

1 m3 1000,000

1cm3 = 10–6 m3 Volume of a Rectangular Slab Volume of rectangular slab is the product of its length, breadth and height.

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Volume = Length × Breadth × Height V=l×b×h Measurement of Volume of Liquids The volume of liquids is generally measured in litres (symbol l), the sub-multiple of one litre is millilitre (symbol ml) 1(l) = 1000 ml. One millilitre is also equal to one cubic centimeter (1 cm3 or 1 cc). 1(l) = 1000 ml = 1000 cc or 1000 cm3 The volume of a liquid is measured using a container of known capacity. The space (volume) inside the container is known as its capacity. Different Measuring Jars Measuring jar: It is used to measure the liquids of desired quantity. Measuring Vessels: They are used to measure volume of liquids like milk and oil. Pipette: It is used in laboratories to take specific volume of liquid. Burette: It is also used in laboratories to extract required volume of liquid for the experiment.

Formative Worksheet 51. Statement I : The smaller unit for measuring volume of liquids is milli litre. Statement II : In standard international system volume is measured in cubic metre (A) Statement - I is true, Statement - II is false. (B) Statement - I is false, Statement - II is true. (C) Both Statements I & II are true. (D) Both Statements I & II are false. 52. If length, breadth and height of cuboid are 20 cm, 18 cm and 15 cm, then it volume in M.K.S system is (A) 540 cubic metre (B) 540000 cubic metre (C) 640 cubic metre (D) 640000 cubic metre 53. If length breadth and height of cuboid are 30 m, 28 m and 12 m then it volume in C.G.S system is (A) 1008.00 cubic cm (B) 10080000 cubic cm (C) 10.08000 cubic cm (D) 100.80 cubic cm

54. A rectangular box with lid is made of a metal sheet of thickness 0.5 cm. Its outer dimensions are 40 cm, 36 cm, 21 cm. If the weight of the metal is 2.5 grams per cubic centimetre, then Match the following Column-I Column-II (A) Inner dimensions p) 30240 cubic of the box centimetres (B) Outer volume of q) 2940 cubic the box is centimetres (C) Inner volume of r) 0.39 m, 0.35 m, the box is 0.20 m (D) Volume of the s) 27300 cubic metal sheet centimetres t) 39 cm, 35 cm, 20 cm 55. Find the volume of cuboid of dimensions 5cm, 4cm and 3cm. (A) 60 cm3 (B) 120 cm3 3 (C) 240 cm (D) 480 cm3 56. A measuring cylinder has 75cm3 of water, when a stone is droped water level rises to 125cm3. What is the volume of stone ? (A) 25 cm3 (B) 50 cm3 3 (C) 75 cm (D) 100 cm3 57. A stone of volume 50cm3 lowered into 80cm3 of water in a measuring jar. Then what will be the new reading in measuring jar ? (A) 15 cm3 (B) 30 cm3 3 (C) 45 cm (D) 60 cm3 58. The volume of a rectangular slab is 12 cm3. The length and breadth of the slab are 3 cm and 2 cm respectively. Find its height. (A) 10 cm (B) 8 cm (C) 4 cm (D) 2 cm 59. A boy has purchased a toy, which is in the form of a cuboid. The cuboid has the following dimensions: 0.003 km × 0.03 m × 3 cm. What is volume of this cuboid? (A) 900 cm3 (B) 1800 cm3 (C) 2700 cm3 (D) 3600 cm3 60. When a stone is lowered into a measuring cylinder containing water, the volume becomes 9.3 ml. If the volume of the stone is 5.8 ml, find the initial volume of water in the measuring cylinder. (A) 3.5 ml (B) 5 ml (C) 6.5 ml (D) 8.5 ml 3 61. Express 5cm in terms of cubic millimetres. (A) 500 mm3 (B) 5000 mm3 3 (C) 50 mm (D) 0.5 mm3 www.betoppers.com

7th Class Physics

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Conceptive Worksheet 52. Which of the following measuring vessels are used to measure volume of liquids. (A) Measuring jar (B) Burette (C) Pipette (D) Measuring flask 53. 1 litre = _________________ (A) 1000 millilitre (B) 100 cc (C) Both (A) and (B) (D) None 54. The volume occupied by a cube whose each side is equal to 1 cm is called (A) Cubic centimetre (B) Cubic millimetre (C) Cubic metre (D) None of these 55. Volume of rectangular glass of a regular body is ___________ (A) length + breadth + height (B) length – breadth + height (C) (length × breadth) + height (D) length × breadth × height 56. The space inside a regular or irregular objects is known as its (A) Area (B) Volume (C) Mass (D) Length 57. The unit for the volume of solids in MKS system is (A) cm3 (B) m3 (C) ml3 (D) l3 58. One cubic meter is (A) 106cc (B) 104cc (C) 103cc (D) 109cc 59. In laboratories to take specific volume of liquid we use ? (A) Pipette (B) Burette (C) Measuring jar (D) Measuring vessel 60. 1 litre = ___________. (A) 1000cc (B) 1000cm (C) 100ml (D) 100cm 61. The space occupied by a substance is called _________. (A) Area (B) Length (C) Volume (D) None of these 62. The CGS unit of volume is _________ (A) Cubic centimetre(B) Cubic millimetre (C) Cubic metre (D) Cubic litre 63. One cubic metre is equal to ––––– (A) 106 cc (B) 104 cc (C) 103 cc (D) 109 cc 64. The volume occupied by a cube whose each side is equal to 1cm is called (A) Cubic centimetre (B) Cubic millimetre (C) Cubic metre (D) None of these www.betoppers.com

12. Measurement of Density Consider two substances, say iron and cotton each having the same mass, say 1 kg. They will occupy different volumes. Iron will occupy less volume as compared to cotton having same mass. This is because substances differ from one another in their densities. Density of iron is more than that of cotton. Factors affecting Density of a Substance i) The Space between the Particles : Larger the space between the particles, lesser is the density of the substance. ii) Mass of each Particle : Larger the mass of each particle, more is the density of the substance. The property of matter that describes the relationship between mass and volume is called density. The density of a substance is defined as the mass per unit volume of the substance. Density of a substance (d) =

mass of the subs tan ce(M) volume of the subs tan ce(V) M V Unit of Density The S.I. unit of density = kg/m3 = kg m–3 The C.G.S unit of density = g/cm3 = g cm–3 Relationship between the S.I. and C.G.S units

 d

1kg / m3 



1kg 1kg  3 1m 1m 1m 1m

1000g 1g  100cm 100cm 100cm 1000 cm3

1 g / cm3 or 1000 1g/cm3 = 1000 kg/m3 Density of a substance in S.I. system = 1000 × density in C.G.S system. Example: The density of iron is 7.86 g/cm3 in C.G.S system. The density of iron in S.I. system will be 1000 × 7.86 i.e., 7860 kg/m3. Density of a substance in C.G.S. system  1 kg/m3 

density of the subs tan ce in S.I. system 1000 For example, the density of brass in SI system is 8400 kg/m3. So, density of brass in CGS system = 

Measurement

13

8400  8.4 g/cm3. 1000 Note: Density depends upon the nature of the material of the substance. If the material is same then density is same. Determination of Density of a Solid by using a Measuring Cylinder In order to determine the density of a solid, we must know the mass and volume of the solid. The mass of the solid is determined by a physical balance very accurately. Let it be M grams. In order to find the volume, take a measuring cylinder and pour in it some water. Record the volume of water from the graduations marked on measuring cylinder. Let it be 50 cm3. Now tie the given solid to a fine thread and lower it gently in the measuring cylinder, such that it is completely immersed in water. Record the new level of water. Let it be 75 cm3. Therefore, volume of the solid = (75-50) cm3 = 25 cm3 = V cm3 (Suppose). Knowing the mass and the volume of the solid, the density can be calculated by the formula : Density 

mass M  g / cm3 . volume V

Formative Worksheet 62. Statement I : The density of liquids and gases changes with change in temperature. Statement II : The substances having a density less than 1 g/cm3 will flow in water. (A) Statement - I is true, Statement - II is false. (B) Statement - I is false, Statement - II is true. (C) Both Statements I & II are true. (D) Both Statements I & II are false. 63. A piece of lead weight 232 g and has a volume of 20 cm3, then the density of lead (A) 11.6 g/cm3 (B) 21.6 g/cm3 (C) 31.6 g/cm3 (D) 41.6 g/cm3

64. 5 litre of alcohol has a mass of 4 kg, then the density of alcohol in g/cm3 (A) 0.6 g/cm3 (B) 0.10 g/cm3 (C) 0.8 g/cm3 (D) 0.18 g/cm3 65. Find the mass of 555 cm3 of iron in kg when density of iron is 7.6 g/cm3 (A) 2.218 kg (B) 4.218 kg (C) 6.218 kg (D) 8.218 kg 66. If density of wood is 500 kg/m3, then what will be the value in g/cm3 ? (A) 0.5 g/cm3 (B) 5 g/cm3 3 (C) 0.6 g/cm (D) 4 g/cm3 67. Density in S.I system = _________× density in C.G.S system. 68. If one litre (1000 cm3) of water weighs 1000 g. Hence the density of water is (A) 0.1 g/cm3 (B) 2 g/cm3 (C) 0.2 g/cm3 (D) 1 g/cm3

Conceptive Worksheet 65. The C.G.S unit of density is (A) g cm–3 (B) g cm–2 (C) g/cm (D) g/cm2 66. In order to find the density of a solid we have to find its (A) Mass and area (B) weight and area (C) area and volume(D) mass and volume 67. The density of a body is represented by the expression (A) D = V/M (B) D = M × V (C) D = 1/M×V (D) D = M/V 68. The unit of density in S.I system is (A) kg/m2 (B) kg/m3 –2 (C) kg m (D) kg m–3

13. Relative Density The ratio of the density of a substance to the density of water at 4°C which is called relative density, or specific gravity of the substance. Mathematical expression of Relative Density. Relativedensity =

1)

Density of thesubstance Densityof pure water at 4°C

We notice from the above expression that the relative density is a ratio of similar quantities. So it is a mere number and has no units. Relationship between Density and Relative Density Density of a solid (in S.I. Unit) = R.D. of the solid × Density of water in S.I. Unit = R.D. of the solid × 1000 kg/m3 www.betoppers.com

7th Class Physics

14 2)

Density of a solid(in C.G.S. unit) = R.D. of the solid × Density of water in C.G.S system = R.D. of the solid × 1 g/cm3

Formative Worksheet 69. Choose the correct options. (A) R.D = density of substance ÷ Density of water at 4°C (B) R.D = mass of substance ÷ mass of an equal volume of water at 4°C (C) R.D = Density of substance + Density of water at 4°C (D) R.D = Density of substance + Mass of water at 4°C 70. Statement I : The CGS unit of relative density is gm/cm3 . Statement II : 1 g/cm3 = 1000 kg/m3 (A) Statement - I is true, Statement - II is false. (B) Statement - I is false, Statement - II is true. (C) Both Statements I & II are true. (D) Both Statements I & II are false. 71. Match the following Column-I Column-II a) Density of solid = 1) Relative density bottle b) Relative density of 2) R.D × density of a liquid is determined water by using c) Density of a liquid = 3) 0.80 d) Density of petrol 4) R.D of the solid × density of water (in S.I) 5) R.D of the solid × 1000 kg/m3 72. If relative density of a substance is K then the density of that substance is (A) K kg/m3 (B) 100K kg/m3 3 (C) 1000K kg/m (D) K / 1000 kg/m3 73. What is the mass of air in a room of dimensions 3 m × 4 m × 5 m, when the density of air is 1.30 kg m–3 ? (A) 23.7 kg (B) 39 kg (C) 46.15 kg (D) 78 kg 74. The length of an iron cylinder is 0.8 m and the area of cross-section is 0.45 m2. Find the mass of the cylinder if its density is 7800 kg/m3. (A) 1404 kg (B) 2808 kg (C) 5616 kg (D) 702 kg 3 75. 30 cm of iron weighs 234 g. Find its density in kg m–3 . (A) 3600 kgm–3 (B) 78 kg m–3 (C) 7800 kgm–3 (D) 780 kgm–3 www.betoppers.com

76. If the mass of 2 litres of water is 2 kg, find its density in S.I. system. (A) 1 kg/m3 (B) 10 kg/m3 (C) 100 kg/m3 (D) 1000 kg/m3 77. A sphere of radius 7 cm weighs 490 g. Calculate its density in S.I. system. (Volume of sphere

4   r3 ) 3 (A) 341 kg/m3 (C) 70 kg/m3

(B) 170.5 kg/m3 (D) 35 kg/m3

Conceptive Worksheet 69. The relative density of a liquid is determined by using ____________. (A) Density bottle (B) Density glass (C) Density stopper (D) None of theses 70. The CGS unit of relative density is (A) g/cm3 (B) kg/m3 (C) Both (1) and (2) (D) None of these 71. The relative density of mercury is (density of mercury is 13600kgm–3) (A) 13.6 g/cm3 (B) 0.8 g/cm3 3 (C) 2.5 g/cm (D) None of these 72. The C.G.S unit of relative density is (A) gm/cm3 (B) gm/cm2 (C) gm/cm (D) no unit 73. Relative density is also known as (A) Lighter density (B) Numerical density (C) Specific gravity (D) None of these 74. If M is the mass of an object of volume V, such that ‘D’ is its density, then we can say (A) V = DM (B) MVD = 1 (C) M = VD (D) D = MV 75. C.G.S unit of density is __________ (A) kg/m3 (B) m3/kg (C) g/cm3 (D) cm3/g 76. The density of alcohol is 800 kg/m3. Then the density in g/cm3 is _______ (A) 800000 (B) 0.80 (C) 0.008 (D) 80000 77. 20 cm3 of aluminium has mass 54g. Then 1cm3 of aluminium mass has ____ (A) 74 g (B) 2.7 g (C) 7.2 g (D) 5.4 g 78. A piece of lead weighs 232 g and has a volume of 20cm3. Then the density of lead is ________ (A) 0.0862 g/cm3 (B) 0.0862 cm3/g (C) 11.6 cm3/g (D) 11.6 g/cm3

Measurement

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HOTS Worksheet

Summative Worksheet 1.

A piece of lead weight 232 g and has a volume of 20cm3. Find the density of lead (A) 5.8 g/cc (B) 23.2 g/cc (C) 11.6 g/cc (D) 17.4 g/cc 2. Find the mass of 555 cm3 of iron in kg when density of iron is 7.6 g/cm3 (A) 42.18 kg (B) 4.218 kg (C) 421.8 kg (D) 4218 kg 3. The length of a school compound is 500m and breadth is 120m. Find the area of compound in hectares (A) 3 hectare (B) 6 hectare (C) 12 hectare (D) 24 hectare 4. Find the weight of a body of mass 1 tonne (A) 1 kg.wt (B) 10 kg.wt (C) 100 kg.wt (D) 1000 kg.wt 5. The relative density of mercury is 13.6 what is its density is C.G.S ans SI units (A) 13.6 g/cc (B) 1.36 g/cc (C) 1360 g/cc (D) 13600 g/cc 6. The density of copper is 8900 kgm-3 what would be the value of its relative density (A) 8900 (B) 890 (C) 89 (D) 8.9 7. The time taken to complete 10 oscillations by a seconds pendulum is (A) 5 sec (B) 10 sec (C) 15 sec (D) 20 sec 8. The density of a cuboid of mass 100g with dimenssions 2 cm × 4cm × 5cm is (A) 7.5 g/cc (B) 5.0 g/cc (C) 2.5 g/cc (D) 1.25 g/cc 9. The total mass of two object of mass 2130 kg and 16 tones is (A) 18130 kg (B) 18.13 tone (C) Both (A) and (B) (D) 16213 kg 10. Density of a body is two fifth of the density of water. Find the R.D of that body (A) 0.4 (B) 2.5 (C) 0.2

(D) 0.6

1.

Column-I (A) 1 m (B) 1 pm (C) 1 nm (D) 1 qt

2.

In S.I system (A) All derived units are obtained by multiplying (or) dividing the fundamental units (B) All derived units are obtained by adding the fundamental units (C) All derived units are obtained by subtracting the fundamental units (D) Depends on the physical quantity Statement I : Derived unit is independent unit. Statement II : It is derived from fundamental units. (A) Statement - I is true, Statement - II is false. (B) Statement - I is false, Statement - II is true. (C) Both Statements I & II are true. (D) Both Statements I & II are false. Derived units are the units of (A) Derived physical quantities (B) Fundamental physical quantities (C) Both (A) and (B) (D) None of these Which of the following are fundamental quantities?

3.

4.

5.

Column-II p) 102 kg q) 10–6 m r) 10–4 m s) 10–12 m t) 10–9 m

(A) Mass (B) Length (C) Time (D) All 2 6. 1 cm = ______________ (A) 104 m2 (B) 103m2 2 2 (C) 10 m (D) 102 mm2 7. 10 kg m–3 = _____________ g cm–3 (A) 1000 (B) 1 (C) 10–2 (D) 100 8. How many centimetre make 1 nano metre? (A) 10–7 cm (B) 10–5 cm (C) 10–15 cm (D) 10–10 cm 9. 1 milligram = ________________. (A) 10–6 kg (B) 10–3 g (C) 106 kg (D) 103 g 2 10. 15 mm is equal to (A) 15 × 10–2cm2 (B) 15 × 10–6m2 (C) 15 × 10–12km2 (D) all the above 11. The velocity of a body is 102 mm/nano second, it is also equivalent to (A) 3.6 × 103 km h–1 (B) 3.6 × 105 km h–1 (C) 3.6 × 107 km h–1 (D) 3.6 km h–1

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7th Class Physics

16 12. If 1 g cms–1 = x Newton - second, then the number x is equal to (A) 1 × 10–1 (B) 3.1 × 10–3 –5 (C) 1 × 10 (D) 6 × 10–4 2 13. 1 kg m/s = ______gm-mm/s2 (A) 105 (B) 106 (C) 104 (D) 103 14. Unit of momentum is written as (A) n.s (B) N + s (C) N – s (D) none of these 2 –1 15. If g - cm -sec = x Js, then the number x is equal to (A) 1 × 1047 (B) 1 × 10–7 (C) 1 × 10–5 (D) 1 × 105 16. 1 decade = _________ millennium

1 1 (B) 100 10 (C) 100 (D) 1000 1 m3 = ______________ cm3 . (A) 106 (B) 108 (C) 1010 (D) 1012 One litre container can contain ___________ millilitres of liquid. (A) 500 (B) 1000 (C) 1250 (D) 1500 The volume of a rectangular glass slab of length = 3 cm, breadth = 2 cm and height = 2 cm is ___________ cm3 (A) 10 (B) 12 (C) 14 (D) 14 A book of length 25 cm, breadth 18 cm and height 2 cm. Then the volume of the book is ___________. (A) 900 cm3 (B) 45 cm3 (C) 900 m3 (D) 45m3 Mass of a wooden piece of length 20 cm breadth 10 cm and thickness 0.5 cm is 50g. What is the density of wood ? (A) 0.1 g/cm3 (B) 5.0 g/cm3 3 (C) 0.5 g/cm (D) 5.1 g/cm3 A silk cloth of density ‘D’ units, is cut into 2009 equal parts. The density of each part is

23. From the following sets choose the incorrect representations. i) K.G. for kilogram ii) 0C for degree Celsius iii) mts for metre iv) s for second (A) Odd options (B) First two options (C) Even options (D) Last two options 24. Find the mass of 555 cm3 of iron in kg when density of iron is 7.6 g/cm3 (A) 4218 kg (B) 4.218 kg (C) 42.18 kg (D) 421.8 kg 25. If the length of the wooden cube is 4m and the

1 kg, then the relative density of the wooden 8 cube is ______________ mass is

(A) 17.

18.

19.

20.

21.

22.

(A)

2009 D units 2

(B) D units

(C)

2009 D units 100

(D)

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D units 2009

(A) 19.53 × 10–7

(B)

1 512

1 1 (D) 51200 32 26. A piece of iron has dimensions 3 cm × 1.5cm × 6 cm. If its mass is 205.2g, its relative density is ______________ (A) 27 (B) 7.6 (C) 6.5 (D) 8 27. An iron cylinder of radius 1.4 cm and length 8 cm is found to weigh 369.6 g. The relative density of iron cylinder is _____________ (C)

28.

29. 30. 31.

Take volume of cylinder = r 2  . (A) 7.2 (B) 7.5 (C) 8 (D) 8.2 The length of a cloth measured is 200 cm. Match the following from List - A List - B i) Length a) Numerical value ii) 200 b) Unit iii) cm c) physical quantity (A) i-a, ii-b, iii-c (B) i-c, ii-a, iii-b (C) i-c, ii-b, iii-a (D) i-b, ii-a, iii-c 1 kg = ______________ tonne. (A) 10–6 (B) 10–3 (C) 103 (D) 106 1 micrometre (mm) = ____________ cm (A) 104 (B) 106 (C) 10–6 (D) 10–4 Match the following : List - A List - B i) Gram a) 10–3 kg ii) Microgram b) 10–3 g iii) Milligram c) 10–6 g (A) i-a, ii-b, iii-c (B) i-a, ii-c, iii-b (C) i-b, ii-c, iii-a (D) i-b, ii-a, iii-c

Measurement 32. A syringe has a capacity of 5 ml. Its capacity in cm3 and m3 is respectively are (A) 5 × 10–6 m3, 5 cm3 (B) 5 cm3 , 5 × 10–6 m3 (C) 5 × 10–3 m3, 5 cm3 (D) 5 cm3 , 5 × 10–3 m3 33. A water tank has a capacity of 10, 000 litre. Its value in m3 is (A) 100 m3 (B) 1000 m3 3 (C) 1 m (D) 10 m3 34. The water level of a measuring cylinder is 26 ml. A piece of concrete having a volume of 6 cm3 is immersed in it. The new level of water is (A) 20 ml (B) 26 ml (C) 32 ml (D) 6 ml 35. The mass of an electron is 9.11 × 10–31kg. How many electrons would make 1kg? (A) 1.1 × 1030 (B) 1.1 × 1031 –30 (C) 1.1 × 10 (D) 1.1 × 10–31 36. A vessel of 200gm weight is filled with some kerosene. If weight of the vessel with the kerosene is 270g, then how much kerosene is filled in the vessel? (A) 70ml (B) 80ml (C) 85ml (D) 70ml 37. If 10 copper pieces, each of mass 20g, are placed in the vessel with water, the level of water increases to 300ml in the vessel. Find the initial level of water in the vessel. (A) 100ml (B)1000ml (C) 10ml (D) 0.1ml

1kg x gm 38. If 3 =10 then find the value of x. m cm 3 (A) –2 (B) 2 (C) –3 (D) 3 39. If m1, m2, m3 and m4 are masses of four bodies 0.3 kg, 0.3 mg, 0.3 × 10–6 g and 3000 g respectively. The above, in increasing order of masses, are as follows. (A) m1 > m2 > m3 > m4 (B) m4 > m2 > m3 > m4 (C) m4 > m1 > m2 > m3 (D) m3 > m2 > m4 > m1 40. Mass = Density × Volume; if mass = p, density = q and volume = r, if 10p = 1000q × 100r, if p, q and r in S.I system, then the value of q in C.G.S system is (A) 10–7 g/cm3 (B) 10–5 g/cm3 (C) 10–7 g/cm3 (D) 107 g/cm3

17

IIT JEE Worksheet I.

Single Correct Answer Type

1.

Mass per unit volume of a substance is called ______ (A) Weight (B) Force (C) Density (D) Time Specific gravity bottle is used for determination (A) Volume (B) Mass (C) Density (D) Temperature The density is CGS system = _____ SI system.

2.

3.

(A)

1 1000

1 100 A piece of lead weight 232 gm and has a volume of 20 cm3. Find the density of lead. (A) 11.6 /cm3 (B) 11.2 gm/cm3 3 (C) 12gm/cm (D) 11.6 kg/ cm3 An iron neddle sinkds is water as its density is _______ 1gm/cm3 (A) Greater than (B) Lesser than (C) Equal to (D) None The weight is found by ______ balance. (A) Physical balance (B) Spring balance (C) Sensitive balance (D) All the above The density of body is reperesented by the formula: (C) 100

4.

5.

6.

7.

(B) 1000

(A) d 

(D)

v m

(B) d 

m v

1 mv 8. In order to find the Density of a Solid we have to find its (A) Mass and Area (B) Weight and Area (C) Mass and Volume (D)Weight and Volume 9. The density of wood is 0.65 g/cm3. The density in SI system is (A) 65 kg/m3 (B) 650 kg/m3 3 (C) 6.5 kg/m (D) 0.65 kg/m3 10. Substance which have density more than 1gcm–3 will (A) Sink (B) Float (C) Submerge (D) None (C) d = Mv

(D) d 

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7th Class Physics

18

II. Multiple Correct Answer Type 11. The C.G.S unit of volume is (A) cm3 (B) c.c (C) cm2 (D) m3 12. The density of oxygen (A) 0.000143 g/cm3 (B) 1.43 kg/m3 (C) 0.00143 g/cm3 (D) 1.043 kg/m3 13. Which of the following is the unit of length? (A) Metre (B) Centimetre (C) Foot (D) Second 14. Which of the following is the unit of area? (A) cm2 (B) mm2 (C) m2 (D) m/s2 15. Which of the following is the unit of mass? (A) pound (B) metere (C) gram (D) ton 16. Which of the following is the unit of volume? (A) cubic metre (B) mm2 3 (C) cm (D) (foot)3

III. Paragraph Type Paragraph-I From the below figure, let l, b, h be the lengths of AB, AD and AE. Here l and b are the length and the breadths of the floor and h the height of the room, then H

G F

E h C

D b A

l

B

The length of any surface = Length × Breadth. Using the above information, answer the following questions. 17. If the length, breadth and height of a room are 8 m, 6 m and 5 m respectively, then area of rectangle surface ABFE is (A) 58 sq. m (B) 48 sq. m (C) 68 sq. m (D) 78 sq. m 18. If the length, breadth and height of a room are 4 m, 5 m and 10 m respectively, then Area of rectangular surface BCGF is (A) 50 sq.m (B) 80 sq. m (C) 120 sq. m (D) 210 sq. m 19. If the length, breadth and height of a room are x m, y m and z m respectively, area of rectangular surface CDHG is (A) yz m2 (B) xy m2 2 (C) xz m (D) none www.betoppers.com

Paragraph-II Relative density (R.D)

Density of the subs tan ce = density of water at 4C 20. Calculate the mass of a body whose volume is 2 m3 and R.D is 0.52 (A) 1040 kg (B) 1004 kg (C) 4001 kg (D) 1400 kg 21. An Iron cylinder of radius 3 cm and length 7 cm is found the weight 396 g. The relative density of iron cylinder is (A) 1 (B) 2 (C) 3 (D) 4

IV. Integer Type 22. The ratio between density of substance and density of water at ________ degree celsius is called relative density. (A) 1 (B) 2 (C) 3 (D) 4 23. Radha sees her mother cleaning the room with broom. She takes two broomsticks of different lengths and decides to measure the length of her room. She finds that the length of the room is 18 times the length of the shorter stick. The length of the shorter stick is half the length of the other stick. How many longer sticks will be required to fit along the length of the room? (A) 9 (B) 18 (C) 27 (D) 36 24. Swati has three dolls. She names them as Reena, Teena, and Meena. She then takes a metre scale and measures their heights. She observes that: I. Reena is taller than Teena by 5 cm. II. Meena is shorter than Reena by 4 cm. If the height of Meena is 0.55 m, then Meena is taller than Teena by ______cm. (A) 3 (B) 4 (C) 5 (D) 6 25. Ramesh measures the length of a thread and finds it to be 14hand-spans. He uses it to measure the circumference of a rectangular table. He wounds the thread around the circumference of the table. If the thread is wound around the table two times, then the circumference of the table is ________ hand spans. (A) 14 (B) 28 (C) 42 (D) 56 26. The length of a blackboard is 240 cm. Rakesh tries to measure this length using a small wooden stick of length 40 cm. The number of times that the stick must be revolved over the blackboard to completely cover the length is __________. (A) 6 (B) 12 (C) 18 (D) 24

Measurement

19

V. Matrix Matching D N 30m K A

S

R

H

G

E

F

P Q 40 m

C M L B

27. From the above figure, the length and the breadth of a rectangular flat are 40 metres and 30 metres respectively. Two rectangular paths each 5 metes wide run inside the peat, one parallel to the length of the plot and the other parallel to its breadth, then Column-I Column-II (A) The area of the path KLMN p) 25 square metres (B) The area of the path PQRS q) 200 square metres (C) The area of the path EFGH r) 150 square metres (Which is common to these two path) (D) total area occupied by the paths s) 325 square metres t) 2 × 102 square metres 28. Match the following Column-I Column-II (A) 1 square metre p) The submultiples of standard unit of area (B) 1 square q) The submultiples centimetre of 1 km (C) One hectare r) The submultiples of 100 m (D) One square s) Area of a square surface kilometre having each side is equal to one metre t) Area of a square surface having each side is equal to one cm 29. Match the following Column-I Column-II (A) dimensions of a p) 10 cubic metres cuboid (B) one litre q) The volume of a cube whose side is one centimetre (C) A cubic r) Length, breadth and centimetre height are equal (D) A cubic is a s) length, breadth and cuboid height t) 1000 cubic centimetres 30. Match the following Column-I Column-II (A) Steel p) Density in CGS system is 8.9 g/cm3 (B) Iron q) Density in CGS system is 7.86 g/cm3 (C) Copper r) Density in CGS system is 19.3 g/cm3 (D) Gold s) Density in CGS system is 7.8 g/cm3 t) Density in S.I system is 19300 kg/m3



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20

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7th Class Physics

B y th e e n d o f th i s c h a p te r , yo u w i l l un de r stan d • • •

Rest & Motion Types of motion Vectors and Scalars

• •

Speed Distance and Time Graphs

1. Introduction Mechanics is the branch of physics that focuses on the motion of objects and the forces that cause the motion to change. There are two branches of Mechanics: Kinematics and Dynamics. The word kinematics comes from the Greek word “Kinema” meaning motion. The word dynamics comes from the Greek word “dynamis” meaning power. Kinematics deals with the concepts that are needed to describe motion without any reference to forces or the cause of the motion. The goal of study of kinematics is to develop sophisticated mental models, which serve us in describing the motion of real world objects. Dynamics deals with the relation between the forces and the motion. The motion of objects can be described by words such as distance, displacement, speed, velocity and acceleration. Before we witness the concepts of kinematics, let’s review some basic concepts.

2. Rest & Motion I.

Rest A body is said to be at rest if it does not change its position with respect to the reference point. The objects which remain stationary at a place and do not change their position are said to be at rest. Example :A table and chairs lying in a room are in the state of rest, because they do not change their position with respect to the surroundings of the room.

II.

Motion An object is said to be in motion if it changes its position from one place to another. In other words, the movement of an object is known as the motion of the object. Example: A vehicle has to be driven. Rain drops fall due to gravitational pull.

Chapter -2

Motion

Learning Outcomes

3. Types of Motion All bodies in motion do not move the same way. For example, a car moves straight along the road; a spinning top spins round and round; the strings of a sitar move up and down and the pendulum of a clock moves sideways in either direction. The motion of bodies is classified as under:

I.

Translatory Motion

The motion in which all the particles of a body move through the same distance in the same time is called translatory motion. Examples : i) A car or a train moving along a road or track. ii) A ball rolling on the ground. iii) Firing of a bullet from a gun. iv) A stone hurled from a catapult. vii) An apple falling from a tree. viii) A striker on the carom board going straight to hit the coin, etc. There are two kinds of translatory motion: (a) Rectilinear motion and (b) Curvilinear motion. a) Rectilinear Motion When an object moves along a straight line like a ball dropped from a height, the motion described by the body is called rectilinear motion. Examples: i) A ball rolling on the ground. ii) A car moving on a straight road. iii) A stone falling freely from the roof of a building. iv) A coin moving over a carom board. b) Curvilinear Motion When a body moves along a curved path, the motion described by the body is called curvilinear motion.

7th Class Physics

22 Examples: i) A stone thrown by a boy from a height. ii) A car moving along a curved path. iii) A javelin or shot-put thrown by an athlete. iv) A ball thrown upward at an angle. v) A car or train moving along a curved road or track are all examples of curvilinear motion. II. Rotational Motion A motion in which a body moves about a fixed axis without changing its position is called rotational motion. Examples: i) The motion of the blades of a fan. ii) The wheel of a sewing machine. iii) The motion of a giant wheel. iv) A spinning top v) A spinning wheel (charkha) vi) A rotating potter’s wheel Combination of Translatory and Rotational Motion In some cases, the motion of rotation and the motion of translation occur at the same time. When a drill cuts through a metal piece, it performs the motion of translation as well as the motion of rotation at the same time. Other examples of this kind of motion are the wheels of a moving bicycle or a car, the ball delivered by a spin bowler.

Note:In circular motion, the position of a body itself keeps changing with time, but in rotational motion the position of a body does not change. It is to be noted that circular motion is a special case of rotatory motion. Comparision of Rotational and Circular Motions

Rotational Motion 1. Body stays in same place with moves and changes its position w ith time. 2. The body rotates about an axis may not pass through the body. 3. No linear motion is observed for the body as a whole.

A motion in which the body as a whole moves to-and-fro about its mean position is called oscillatory motion. Examples :

Rotational Motion

III. Circular Motion When a body moves around a circular path, the way an athlete runs around a circular track, such motion is called circular motion. Examples: i) Motion of an electric fan. ii) Motion of a merry-go- round. iii) Motion of a spinning top. iv) Motion of the earth around the sun. www.betoppers.com

2. It moves along a path whose axis passing through it. 3. Body may undergo linear motion.

IV. Oscillatory Motion

Translational Motion

A moving bicycle describes both translational and rotational motion

Circular Motion 1. Body doesn’t stay in same place. It respect to time.

V.

i)

The movement of a swing.

ii)

The oscillation of pendulum of a clock.

iii)

The needle of a sewing machine moving up and down.

iv)

The piston of a motor car, the pressing cylinder of a juice machine, the piston of a spray pump etc.

Vibratory Motion It is another kind of oscillatory motion in which the body does not move as a whole i.e. the entire object does not move but undergoes change in shape or size. Examples: i)

When you pluck a string of a guitar or sitar, it vibrates to and fro about its position of rest and produces music.

Motion

23 ii)

iii)

When you strike the membrane of a drum, it vibrates up and down about its position of rest and produces sound. These vibrations are like the oscillations of a pendulum, except that they are faster. There is another difference. The pendulum swings freely, while the string of a musical instrument is fixed at two ends, and the membrane of a drum is fixed around the circumference of the drum. During breathing, when our chest expands or contracts, the motion is vibratory in nature; all the points on the chest do not move out equally.

Linear Motion of the ball

A spinning ball

ii)

iii)

VI. Periodic Motion

iv)

A motion which repeats itself at regular intervals of time is called periodic motion. Examples: i) The motion of the earth round the sun is periodic. ii) The swinging pendulum of a wall clock. iii) The needle of a sewing machine running at constant speed. iv) The heart beat of a normal human being. v) The motion of a swing. vi) The movement of the lungs during breathing.

v)

vi)

VII. Random Motion A body which has a particular motion that suddenly changes to another kind of motion is said to have random motion. In other words, the motion without any sequence or direction is random motion. Examples : 1) A buzzing bee. 2) A football player on the field. 3) The motion of a hockey ball during a game. 4) The flying motion of a sparrow. 5) A monkey performing antics on a tree.

Formative Worksheet 1.

Which of the following is not making translatory motion ? (A) A ball delivered by a spin bowler (B) A brill that bores a piece of wood (C) Movement of toy car around circular path (D) Moving rear wheel of a bicycle on its stand

2.

VIII. Multiple Motion Sometimes an object manifests two or more types of motion at the same time. Such motion is called multiple motion. Examples: i) When a spin bowler delivers a ball, the ball has a motion of spin (rotatory motion)while it moves towards the batsman (translatory motion).

A person is drawing water from a well. Here, the pulley on which the rope runs has a rotatory motion while the bucket coming up has a translatory motion. A girl riding a bicycle. The wheels of the bicycle rotate (rotatory motion ) and at the same time the bicycle moves forward in a straight or curved path (translatory motion). A drill used by a carpenter for drilling a hole in wood, has both translatory and rotatory motions. This is because while it is being rotated (rotatory motion), it is also getting pushed forward (translatory motion) into the wood. The earth rotates about its axis (rotatory motion) and at the same time it revolves around the sun in a circular path (translatory motion). The mythological Sudarshan Chakra hurled by Lord Krishna to kill his enemies had both translatory as well as rotatory motion.

3.

The motion described by a football ? (A) Curvilinear

(B) Circular

(C) Oscillatory

(D) Non-uniform

Which of the following is a periodic motion ? (A) Freely falling body (B) A swinging pendulum (C) Bullet fired from the gun (D) A flying kite www.betoppers.com

7th Class Physics

24 4.

Which of the following statement is not correct? (A) Needle of sewing machine undergoes circular motion (B) Motion of body thrown upward at an angle is curvilinear (C) Movement of the earth around the sun is circular (D) Ball dropped from a height is rectilinear 5. How are day and nights caused ? (A) Rotation of the earth (B) Gravitational force of earth (C) Mass of the earth (D) None 6. Which of the following is not an example of translatory motion ? (A) A freely falling stone (B) A coin moving over carom board (C) A car or train moving along a curved road or track (D) A ceiling fan 7. From the following pick out the example for rotatory motion ? (A) A spinning wheel (B) Merry-go-round (C) Blades of mixer grinder (D) All the above 8. Which of the are examples of circular motion? (A) Revolution of earth around the sun (B) Motion of the tip of the seconds hand of a wall clock (C) Both (D) None 9. Which of the following posses both rotatory and translatory motion ? (A) Motion of the wheels of a horse driven cart (B) When a drill bores a hole in a wooden piece (C) Ball delivered by a spin bowler (D) All the above 10. The motion of a bob of a simple pendulum is (A) Rectilinear (B) Curvilinear (C) Periodic (D) Rotatory

Conceptive Worksheet 1.

Which of type of motion do the strings of the violin describe? 2. Which type of motion is described by rolling ball ? 3. All oscillatory motions are periodic by nature. Explain . 4. Passengers sitting in a moving train are in _____ with respect to platform, but in _____ with respect to co-passengers in the train ? 5. A body undergoing circular motion may also have linear motion. Explain. www.betoppers.com

6. 7.

A book lying on the table is at___ . A body is said to be in _____ if it changes its position with respect to surroundings with time. 8. Can a body execute both rotatory and translatory motion simultaneously ? 9. Motion of pendulum of a clock is an example of _____ motion. 10. The heartbeat of a normal resting person is ____ .

4. Vectors and Scalars Consider a water melon that weighs 5 kg. Here, the mass is represented by the number 5 along with the unit kg. For specifying the mass of the body, direction is not required, but magnitude. Such quantities are called scalars. The quantities which have only magnitude but no direction are called scalar quantities. Examples: Mass, length, time, volume, density temperature, etc. It is right to express mass of water melon as 5 kg, but it is wrong to express mass of water melon as 5kg towards north. Now assume that you invited your friend for your birthday party and your friend doesn’t know your house. You informed that your house is 200m from a Corporate office. Will your friend find it easy to locate your house ? It becomes difficult to trace the house as the direction is not specified. But if you say it is 200m left or east of the Corporate office, he finds it easy to locate your house and your celebration becomes colourful. That is, for certain quantities, specification of direction is a must. Such quantities that are represented with both magnitude and direction are called vectors. Examples: Displacement, velocity, acceleration, force, weight etc. Representation of a Vector : Any vector can be represented by a straight line 

OA . O

A

The length of the straight line indicates magnitude and the arrow mark (  ) indicates direction.

Motion

25

5. Distance & Displacement The actual length covered by a moving body in between two points, irrespective of the direction in which the body is moving, is called distance. In the figure shown, if the particle performs one complete rotation, then the distance covered by it is 2r .

Formative Worksheet 11. Raju travels the path HS to reach his school from home and SH to reach his home back from school. He leaves to school every day at 9.00 AM and back to home by 3.30 PM. Find the displacement and distance tranversed by him in this time. S

2 km

H

r

12. Find the distance and displacement if the body moves from A to B along the path AQB. The straight line distance between the initial and final positions of a body is called the magnitude of the displacement. The direction of the displacement is given by the direction from the initial position to the final position. The shortest distance covered by a moving body in between two points, in a particular direction, is called the displacement. Consider the following motion depicted in the figure. A man walks 4 metres east, 2 metres south, 4 metres west, finally 2 metres north.

Q

A

B

7m

13. Find the distance and displacement if the body moves from A to B along the path APQB. B

Q

4m 4m

2m

2m

A

P

4m

4m

14. Find the distance and displacement if the body moves from A to B along the path APQRSTUB.

N E

W S

Though the man walked a total distance of 12 metres, his displacement is zero metres. During the course of this motion, he has covered 12 metres of ground (distance = 12m). Yet when he has finished walking, he is not “ out of place” i.e., there is no displacement for his motion (displacement = 0 m). Displacement, being a vector quantity, must give attention and regard to direction. The 4 metres east is cancelled by the 4 metres west; and the 2 metres south is cancelled by the 2 metres north. Both distance and displacement are measured in same units. In CGS and SI systems, the units are metre and centimetre respectively.

A

R 2m S

B

2m

P

4m

4m

4m 6m

T

Q

4m

4m

U

15. Find the distance and displacement if the body moves from A to B along the path APQRB. P 5m

5m A

R

5m

5m

Q

5m 5m

B

16. A boy starts from his house and reach 5 km to reach the market. After purchasing vegetables, he returns back to home travelling the same path. Find the distance and displacement of the boy. www.betoppers.com

7th Class Physics

26 17. A body moving on a circular path covers half of the path in one hour. What is the ratio of the distance covered by the body to its displacement. 18. A body moves from P to S travelling along the path PQRS. 10m S

R 5m

P

Q

10m

Distance + displacement = ? 19. An ant starts from ‘A’ and reaches ‘F’. In doing so, it travels along path that is ‘S’ shaped. If each side of the transversed path is same, then.Find the ratio of distance covered by the ant to its displacement. B

A

C

D

14. 1 MKS unit of distance = ____ CGS unit of distance. (A) 1 (B) 16 (C) 100 (D) 1000 15. The ratio distance to displacement is (A) Zero (B) Always = 1 (C) Greater than (or) equal to 1 (D) Greater than 1 16. If a body completes 1 round of a circular path of radius 7 cm, its displacement is (A) Zero (B) 14 cm (C) 22 cm (D) 44 cm 17. If a body completes 1 round of a circular path of radius 7 cm, its distance is (A) Zero (B) 14 cm (C) 22 cm (D) 44 cm 18. A body starts from A and reaches back A in 1 hour travelling along the path ABCDEFA. Find the distance covered and displacement of the body respectively. A 5m

5m

B

F F

E

5m

20. A rat starts from A and reaches B, running along the path ABCDEB. AB = BC = CD = DE = EB = 5m. C

5m C

E 5m

(A) 60m, 30m (C) 30 m, zero B

5m D

D

(B) 60 m, 15m (D) 30m, 30m

E

5. Speed A

If distance covered by rat is x times its displacement. Then find the value of x.

Conceptive Worksheet 11. The total path covered by a body is called. (A) Distance (B) Displacement (C) Speed (D) Velocity 12. The shortest distance travelled by a body is called its (A) Distance (B) Displacement (C) Speed (D) Velocity 13. The MKS unit of distance is (A) Foot (B) Metre (C) Second (D) Centimetre

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The FASTNESS OR SLOWNESS of a body can be understood from the SPEED of the object. The speed of an object is defined as the distance travelled by it in unit time. Therefore, an object that moves faster covers more distance in a second (unit time) as compared to an object that moves slower. We can calculate the speed of an object if we know the distance it travels in a given time. Distance travelled Speed = Time taken to travel the distance Units of Speed The SI unit of speed is metre per second (m/s) and CGS unit is centimetre per second. Another commonly used unit is kilometre per hour (km/h). When we talk of speed of aeroplanes and cars, it would be more appropriate to use km/h than m/s.

Motion

27

Speed in m/s can be easily converted to km/h by dividing the answer in m/s by 5/18 or multiplying by 18/5. Instrument used to measure Speed The instrument/meter measuring the speed of the vehicle is called a speedometer. The distance travelled by the vehicle is given by an instrument called odometer. Both of them are usually fitted together. These instruments will give the reading of speed and the distance travelled at the same time.

The given information can be represented in both ways as shown below.

Uniform Speed When a body covers equal distances in equal intervals of time (however small the time intervals may be), the car is said to be moving with a uniform speed. Examples: A rotating fan, a rocket moving in space, etc., have uniform speeds. Similarly, an aeroplane flying at 200 km/h or a train running at 90 km/h are moving with uniform speeds.

Non-Uniform Speed When a body covers unequal distances in equal intervals of time, the body is said to be moving with a nonuniform speed. Examples: A train starting from a station, a dog chasing a cat, have variable speeds. Average Speed In practice, very few objects travel at a constant speed for long. For instance, Your school bus may speed up and slow down many times on the way to school. It is difficult to mention the speed of the bus as its speed varies at each moment. In such cases, to represent the speed of the entire journey, we use the concept of Average Speed. Average speed is defined as the total distance travelled divided by the total time taken to travel that distance. Average Speed =

Total Distance travelled Total Time taken to travel the distance

This kind of a chart is called a Line chart or Line Graph. It is created by plotting a series of data points and connecting them to form a line. In this we have taken time on the x-axis and the distance on the y-axis. Some important points to note • When we try to locate an object, we usually relate it to some sort of reference point. This point is usually the origin, the zero point of an axis (x or y). • We provide a numeric value for an object’s location by placing it either in the positive direction (+) or the negative direction (–) in relation to that reference point.

6. Distance and Time Graphs Generally, a vehicle travelling on a road does not travel at a constant speed throughout its journey. If we note the distance travelled by the object at equal intervals of time (like every second or every minute), we will get a clearer picture of its speed at different points of time. The data of the distance travelled and the time taken can be presented either in tabular form or graphical form.

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7th Class Physics

28

1.

2. 3.

We can get the following points by examining the distance – time graph: The steepness of the slope of the graph gives the speed. If the slope is greater (i.e., if the slope is steeper as in line OA of the figure, the speed is greater than the speed shown by line AB,since its slope is flatter). The speed is zero if the graph is flat (i.e., parallel to the ‘time’ axis) as in BC. For a body moving with constant speed, the graph will be a straight line passing through the origin. A motion that can be represented by a straight line as shown in figure is called uniform motion. In other words, a body is said to be in uniform motion if it covers equal distances in equal intervals of time.

Formative Worksheet 21. The odometer of a car reads 36580 km when the clock shows 9:00 AM. fiat is the distance moved by the car if at 10:00 AM the odometer reading has Aanged to 36620 km? Calculate the speed of the car in km/h during this time. 22. Rajiv takes 10 minutes to reach the post office from his house on a I rickshaw. If the speed of rickshaw is 3 m/sec. Calculate the distance between his house and post office. 23. Julia is traveling in a car to school, which is situated at a distance of 40 km from her home. If the car takes 30 minutes to reach to the school, then the speed of the car in km/hour is (A) 40 (B) 80 (C) 100 (D) 120 www.betoppers.com

24. The distance versus time graph of a car is shown in the given figure.

It can be concluded from the given graph that the (A) Car is moving at a uniform speed (B) Speed of the car is increasing uniformly (C) Car travels a distance of 25 m in each second (D) Distance traveled by the car is inversely proportion to time 25. The distance between Paula's house and her school is 12 km. The distance between her school and the public library is 6 km, whereas the distance between the public library and her house is 15 km. Paula reaches her school from home in half an hour and then takes another 15 minutes to reach the public library. She further takes 15 minutes to come back home from the public library. What was Paula's average speed during the entire trip? (A) 11 km/h (B) 22 km/h (C) 33 km/h (D) 44 km/h 26. If the moon revolves around Earth in a circle of radius 385,000 km in 29 days, then its average speed is approximately equal to ______. (A) 11.063 km / h (B) 110.63 km / h (C) 1,106.3 km / h (D) 11,063 km / h 27. An ant is moving around a book, as shown in the given figure. It travels around the book once in every five minutes.

What is the ant's average speed? (A) 0.1 cm/s (B) 0.2 cm/s (C) 0.3 cm/s (D) 0.4 cm/s

Motion

29

(D)

Position

Time

(B)

Position

Time

50

Position

Time

(C)

30. The given graph shows the position versus time graph of a car.

Distance (in km)

(A)

Distance

28. Which position versus time graph represents a stationary body?

40 C

30 A

20

B

10 O 0

30

60

90

D 120 150 180 210

Time (in sec)

(D)

Position

Time

Time

What was the speed of the car during the entire trip? (A) 25 km/h

(B) 50 km/h

(C) 100 km/h

(D) 125 km/h

31. The distance versus time graph of a ball moving with a constant speed is shown in the given graph.

Time

Distance (in metres)

(A)

Distance

29. A boy is cycling at a constant speed. His distance versus time graph will look like 40 30 20 10 0

5

10

15

20

25

(B)

Distance

Time (in seconds)

What is the speed of the ball? (A) 1.2 m/s (C) 12.0 m/s

(B) 1.6 m/s (D) 16.0 m/s

(C)

Distance

Time

Time

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7th Class Physics

30 32. The given figure shows the distance versus time graph of a car.

34. Which of the following position versus time graphs represents a a body at rest? (A)

Position

600 500 400 300 200 100 0

5 10 15 20 25 30 35

Time (in sec) The speed of the car (A) Decreases linearly with time (B) Increases linearly with time (C) Is constant (D) Is zero 33. Ethan and Jacob participate in a bicycle race. One minute from the start of the race, Ethan and Jacob are at the same distance from the starting point. After another minute, Jacob gets ahead of Ethan.

(C)

(D)

Speed of Ethan = Speed of Jacob

During the first minute During the second minute

Speed of Ethan > Speed of Jacob

During the first minute During the second minute

Speed of Ethan = Speed of Jacob

During the first minute During the second minute

Speed of Ethan < Speed of Jacob

Speed of Ethan < Speed of Jacob

Speed of Ethan < Speed of Jacob

Speed of Ethan > Speed of Jacob

Position

Time

(D)

Position

(B)

During the first minute During the second minute

Time

(C)

Which table correctly describes the relationship between the speeds of Ethan and Jacob during first two minutes of the race? (A)

Time

(B)

Position

Distance (in km)

700

Time 35. An ant is moving over a ruler, as shown in the given figure. The digital clocks show the times when the ant is at two different points on the ruler.

Speed of Ethan > Speed of Jacob The speed of the ant is (A) 1 cm/s (B) 3 cm/s (C) 6 cm/s (D) 9 cm/s

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Motion

31

Conceptive Worksheet 19. Which of the following relations is correct? (A) Speed = Distance × Time (B) Speed = (C) Speed =

Distance Time Time Distance

1 Distance× Time The ratio of CGS unit to SI unit of speed is ______. (A) 1:10 (B) 100:1 (C) 1000:1 (D) 1:1000 Speed is a _____. (A) Vector quantity (B) Scalar quantity (C) Fundamental quantity (D) None If a body covers unequal distances in equal intervals of time, then the body is said to move under ____. (A) Uniform speed (B) Non- uniform speed (C) Uniform Velocity (D) Non- uniform velocity A particle moves with a uniform speed (A) The particle must be at rest (B) The particle moves along a curved path (C) The particle moves along a circle (D) The particle moves along a straight line If a particle covers equal distance in equal time intervals, it is said to be in. (A) Uniform Speed (B) Non uniform speed (C) Moving faster (D) Moving slower A bus travels a certain distance with a speed 30 ms-1 and returns with a speed 20ms-1 then find average speed during total journey ? (A) 25 m/sec (B) 20 m/sec (C) 24 m/sec (D) 15 m/sec When body starts at ‘A’ and comes to ‘A’ again as shown by fig ,then the dispalcement is (r is radius) (D) Speed =

20.

21.

22.

23.

24.

25.

26.

27. Speedometer of a car measures ? (A) Average speed (B) Uniform speed (C) Instantaneous speed (D) Distance travelled 28. A body cover 20 m in 1st second , 25 m in 2nd second and 45 m in 3rd second . What is t h e average speed of the body ? 29. A car starts at point A with a speed of 40 kmph and reaches point B. Then the car returns to the starting point A with a speed of 60 kmph. What is average speed of the car? 30. In the clock, the average speed is minimum for the tip of ? (A) Seconds hand (B) Minutes hand (C) Hours hand (D) All the above 31. A crokroach covers a distance of 30 m in 1.5 minutes. What is the speed of cockroach ? 32. A train moves at a speed of 120km/h. How long will it take to cover a distance of 15km? 33. 1kmph = ______ m/s ? 34. A horse runs a distance of 1200m in 3 min and 20s. What is the speed of horse ? 35. The following graph shows the distance travelled by Rajni from home to school and the time taken. Study it and answer the following questions: (i) What distance did Rajni travel in first two minutes? (ii) What distance did Rajni travel in the next two minutes? (iii) What distance did Rajni cover between 8 minutes and 10 minutes from the start? (iv) Are these distances equal? (v) What was the distance travelled between 12 minutes and 14 minutes’? 900 800 700 600 500 400 300 200

?

100

2

(A) 2  r (B) 2r

(C) 0

(D)

r 2

4

6

8

10

12

14

16

18

20

36. Show the shape of the distance-time graph for the motion in the following cases: (i) A car moving with a constant speed. (ii) A car parked on a side road. www.betoppers.com

7th Class Physics

32 (B)

Summative Worksheet Match the following

3.

4.

5.

6.

a)

10 m/s

p)

72 Kmph

b)

20 m/s

q)

1000 cm/s

c)

30 m/s

r)

144 kmph

d)

40 m/s

r)

0.03 km/s

(A) 1 : 3.6

(B) 3.6 : 1

(C) 1 : 1

(D) 2 :1

4



3



2



1



Time in (seconds) 

(C)

The peregrine falcon (Falco peregrinus) is the fastest living creature, reaching speeds as much as 310 kmph when swooping from great heights during territorial displays or while catching pry birds in midair. Express this speed in metre per second. Sudha, Vijay, Shruthi and Rohit participated in a 100 m race. They took 20s, 22s, 25s and 28s respectively to reach the finishing line. Who do you think run the fastest and the slowest in the race. Also express their speed in m/s.



5

0.5 1.0 1.5 2.0 2.5 3.0

Two cars, A and B , are moving on the road with speeds 10 m/s and 36 kmph respectively. If their starting point and ending point is same and if they are moving in the same direction , then find the ratio of the times taken by them to reach the end point once they start at same time from their starting point.

Distance (in mm)

2.

Column-2

Distance (in m)

Column-1



5 4



3



2



1

 0.5 1.0 1.5 2.0 2.5 3.0 Time in (seconds) 

7.

(D) None The graph give below represents the motion of four animals A, B, C and D along a straight line. Identify the one that is moving fastest among the four. A

Cheetah is the fastest land animal and can achieve a peak velocity of 100 km/h up to distances less than 500 m. If it spots its prey at a distance of 100 m, what is the minimum time it will take to get its prey, if the average velocity attained by it is 90 km/ h?

B

Distance (in m)

1.

One of the following graphs represents the motion of snail. Identify it. (A)

5



4



3



2 1

C D

 

0.5 1.0 1.5 2.0 2.5 3.0

Distance (in km)

Time in (seconds)  

100 80



60



40 20

  0.5 1.0 1.5 2.0 2.5 3.0 Time in (hours) 

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8.

(A) A (B) B (C) C (D) D A delivery by Brett Lee from Australia was bowled at 161.8 kmph at Brisbane on November 23, 2000. It was considered as the fastest ball ever bowled by a bowler in One day International Match by then. Now, imagine the following hypothetical situations. A straight road from Brisbane (Australia) and Kingston Jamaica (West Indies) A car is moves on this road with a uniform speed equal to Brett

Motion

9.

33

Lee’s ball mentioned above. If the Also and the distance between them is 14488.92 km, predict how many days approximately would it take by the car to reach Kingston Jamaica. An object covers a distance of 'S' metres in 't' seconds as follows: S (meters)

0

8

20

20

16

10

0

t (sec)

0

2

5

10

12

15

18

16. A car moves with a speed of 40 km/h for 15 minutes and then with a speed of 60 km/h for the next 15 minutes. The total distance covered by the car is: (A) 100 km (B) 25 km (C) 15 km (D) 10 km

HOTS Worksheet 1.

Plot a graph, taking 't' on X-axis and 'S' on Y axis. 10. A car travels a certain distance with a speed of 50 km/h and returns with a speed of 40 km/h. Calculate the average speed for the whole journey. 11. The given figure shows the position of a body at different times. Calculate (i) the speed of the body as it moves for 0 to 5 s (ii) 5 to 7s (iii) 7 to 9 s

Kurt is jogging along the outer edge of a park. The dimensions of the park are shown in the given figure. He takes one minute to complete one revolution around the park. 100 m

50 m

50 m 100 m

2.

Kurt's speed is (A) 1 m/s (B) 5 m/s (C) 50 m/s (D) 100 m/s A toy car is moving away from a wall, as shown in the given figure. As time increases, the distance of the car from the wall also increases, as depicted in the graph.

12. Plot a distance-time graph for the given data and calculate (a) the corresponding physical quantity and, (b) the distance covered by the car at the end of 2.5 s and 6.5 s. S (meters)

0

10

20

30

40

50

60

t (sec)

0

1

2

3

4

5

6

13. The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train. 14. The odometer of a car reads 57321.0 km when the clock shows the time 08:30 AM. What is the distance moved by the car, if at 08:50 AM, the odometer reading has changed to 57336.0 km? Calculate the speed of the car in km/min during this time. Express the speed in km/h also. 15. Salma takes 15 minutes from her house to reach her school on a bicycle. If the bicycle has a speed of 2 m/s, calculate the distance between her house and the school.

The hypothesis that can be derived from the given graph is (A) Distance of the car from the wall = Time (B) Distance of the car from the wall = 2× Time (C) Distance of the car from the wall =

Time 2

(D) Distance of the car from the wall =

Time 3

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7th Class Physics

34

2cm

3cm 6cm

4.

5.

6.

7.

The speed of the ant is the maximum on the edge of length (A) 6 cm (B) 4 cm (C) 3 cm (D) 2 cm The reading of the odometer of a bus, which is about to start its journey, is 42124 km. It reaches its destination after 5 h travelling at a uniform speed of 40 km/h. The odometer of the bus now reads (A) 42194 km (B) 42244 km (C) 42324 km (D) 42384 km The odometer of a scooter reads 14316.0 km. The person is moving with a uniform speed of 30 m/s and reaches his destination after 20 minutes. What is the reading of the odometer when the rider reaches his destination? (A) 14280.0 km (B) 14316.0 km (C) 14332.0 km (D) 14352.0 km In a race, Zaheer and Abbas run with a speed such that the former takes thrice the time more than the latter to cover a distance. What is the ratio of the speeds of Abbas and Zaheer? (A) 1:9 (B) 1:3 (C) 3:1 (D) 9:1 A cyclist is travelling with a uniform speed on a straight road. After traveling 700 m, one of the tyres of the cycle got punctured. He decided to cover the rest of the distance (300 m) by walking with the cycle. He covers the remaining 300 m in 7 minutes. If the cyclist takes a total of 12 minutes to complete his journey, then what is his average speed during the journey? (A) 23.81 m/min (B) 61.28 m/min (C) 83.33 m/min (D) 97.48 m/min

Two cars M and N are standing 1 km away from each other. Car M starts moving towards car N. After 30 s, car N also starts its journey towards car M with the same speed. Both cars meet at a point after 5 min. With what speed were both cars moving towards each other? (A) 1.14 m/s (B) 1.59 m/s (C) 5.91 m/s (D) 6.89 m/s 9. Raj can run twice as fast as Lokesh. Raj runs for 10 min to cover the circumference of a playground. How much time will Lokesh take to cover the circumference of the playground? (A) 5 min (B) 10 min (C) 20 min (D) 40 min 10. A car moves from point M to point N with uniform speed in 3 min. There, the car remains standing for 2 min. Then, it comes back to point M with uniform speed in 2 minutes. (A) Distance

4cm

8.

0

1 2

3

4

5

6

(B)

0

1 2

3

4

5

6

7

Time (in min)

(C)

0

1 2

3

4

5

6

Time (in min)

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7

Time (in min)

Distance

Julian is recording the time take by an ant to travel on the edges of a wooden structure, shown in the given figure. He is amazed to observe that the ant takes exactly one second to travel the length of each edge.

Distance

3.

7

Motion

35

Distance

(D)

(A)

0

1 2

3

4

5

6

7

Time (in min)

11.

The given table shows the distance covered and the time taken by four cyclists M, N, P, and R. Cyclist

Cyclist

Cyclist

M

M

M

N

N

N

P

P

P

R

R

R

(B)

(C)

Which cyclist will take the maximum time to cover any given distance? (A) M

(B) N

(C) P

(D) R

12. A train of length 570 m enters into the Pirpanjal tunnel at 8:25 am. The length of the Pirpanjal tunnel is 11.43 km. The speed of the train is 60 km/h. When the train completely exits from the tunnel, the clock will read (A) 8:37 am

(B) 9:01 am

(C) 9:37 am

(D) 10:12 am

13. Consider the following statements: I.

Car I is parked near a post office

II.

Car II is moving along a straight road with uniform speed

Which of the following distance?time graphs represents the motion of the respective cars?

(D)

14. An airplane starts its journey from Delhi to Chennai at 8:50 am. When it lands at Chennai airport, the clock time is 11:15 am. The shortest distance between Delhi and Chennai is 2095 km. What was the speed of the airplane during its journey? (A) 14.48 m/s

(B) 28.96 m/s

(C) 240.80 m/s

(D) 481.60 m/s

15. A car is moving with a speed of 53 km/h. It can cover a distance of 318 km in (A)

1 1 h (B) h (C) 6 h 6 12

(D) 12 h

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7th Class Physics

36

IIT JEE Worksheet I. 1.

2.

5.

Single Correct Answer Type A race is scheduled. There are four participants only. The given table shows the names of the participants and their respective velocities. Participant

Participant

Ranvir Chauhan

Ranvir Chauhan

Deepak Gupta

Deepak Gupta

Apurva Gupta

Apurva Gupta

Pavan Chauhan

Pavan Chauhan

6.

7.

Who will touch the finishing line just after the winner? (A) Ranvir Chauhan (B) Deepak Gupta (C) Apurva Gupta (D) Pavan Chauhan The position versus time graph of the motion of a boy along a straight path is shown in the given figure.

8.

3.

4.

The total time for which the boy remains in motion is (A) 16 s (B) 10 s (C) 9 s (D) 4s While sitting in his father's car, Zaheer observes that there is no change in the odometer reading of the car for a certain period of time. It can be concluded from Zaheer's observation that the car is (A) at rest (B) gaining speed (C) moving uniformly (D) reducing speed There is some problem in the speedometer of Raju's car. It always indicates a value twice the actual value. One day, he drives the car to a mechanic shop for repair. The shop is 100 km from his house. It takes 5 h to reach the destination with a constant speed. What will be the reading of the speedometer? (A) 20 km/h (B) 40 km/h (C) 250 km/h (D) 500 km/h

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9.

10.

Two athletes, Ajay and Rahul, are participating in a 100 m race. Ajay runs with a speed of 5 m/s. Rahul takes 5 s more than Ajay to complete the race. What is the speed of Rahul? (A) 4.0 m/s (B) 4.5 m/s (C) 5.5 m/s (D) 6.0 m/s A boy ran for 30 minutes and then walked for the same time to cover a total distance of 2.5 km. The boy runs with a speed of 4 km/h. The boy walks with a speed of (A) 1.0 km/h (B) 1.5 km/h (C) 2.5 km/h (D) 3.0 km/h Rohit goes to school by a car. While boarding the car, he saw that its odometer reading was 1020 km. When he reached the school, before getting off the car, he saw that the odometer reading was 1041 km. The speed of the car remained 14 m/s throughout the journey. What is the time taken by the car to reach the school? (A) 400 s (B) 800 s (C) 1400 s (D) 1500 s A few friends were going for a picnic to a nearby hill station in a car. At the start of their journey, they saw that the odometer reading is 1500 km. They reached the picnic spot in 2 h. They were travelling with a uniform speed of 100 km/h. What is the final reading of the odometer? (A) 200 km (B) 700 km (C) 1300 km (D) 1700 km An express train and a passenger train start from the same station at the same time. When the express train reaches place B, the passenger train at the same time reaches place A that is situated midway between the station and Place B. What is the ratio of the speed of the express train to that of the passenger train? (A) 1: 2 (B) 2: 1 (C) 1: 4 (D) 4: 1 Ram can run twice as fast as Shyam. Shyam runs for 5 minutes and Ram runs for 10 minutes. Distances covered by Ram and Shyam ared1 and d2 respectively. What is the ratio d1:d2? (A) 1: 4 (B) 1: 2 (C) 2: 1 (D) 4: 1

Motion

37

11. The distance-time graph for a body with speed is a straight line the time axis. The information in which alternative completes the given statement?

16. Which of the following is an effect of force? (A) Moving a body at rest. (B) Stopping a ball in motion. (C) Changing the direction of motion.

(A)

i

ii

uniform

parallel to

i

ii

non-uniform

parallel to

i

ii

uniform

at an angle with

i

ii

non-uniform

at an angle with

(B)

(C)

(D)

II.

Multiple Correct Answer Type

12. Metre is the SI unit of (A) Length

(B) Distance

(C) Displacement (D) Speed 13. Which of the following is the unit of speed? (A) Metre per second (B) Centimetre per second (C) Kilometre per hour (D) Miles per hour 14. Which of the following are examples for periodic motion? (A) The motion of the earth round the sun is periodic.

(D) Changing the speed of a body in motion.

III. Paragraph Type The actual length covered by a moving body in between two points, irrespective of the direction in which the body is moving, is called distance. Displacement is the shortest distance between two points. The units of both distance and displacement are metre and centimeter. Using the above information answer the following quesitons. 17. If a body completes one complete revolution in a circular path, then (A) Distance = 0 (B) Magnitude of displacement = 0 (C) Both (A) and (B) are correct (D) None of these 18. A man leaves his house at 5.00 a.m. for a morning walk for 2km and returns at 6.00. a.m. What is his displacement traversed by him in this time? (A) 4 km (B) 8 km (C) 2 km (D) Zero 19. In the above question, what is the distance travelled by the man? (A) 4 km (B) 8km (C) 2 km (D) Zero 20. The figure given below shows the path taken by a boy during a walk. Find the total distance covered by the boy. B

(B) The swinging pendulum of a wall clock. (C) The needle of a sewing machine running at constant speed.

4 km

3 km

(D) A car moving on a straight road. 15. Which of the following describe random motion? (A) A buzzing bee. (B) A football player on the field. (C) A car moving on a circular road.

2 km C

D

A

(A) 12 km

(B) 9 km (C) 10 km (D) 5 km

(D) The flying motion of a sparrow. www.betoppers.com

7th Class Physics

38

IV. Integer Type 21.

A body is moving with a speed of 36 kmph. Its speed = ________ m/s.

22.

A body is moving along a circular path and 2012 revolutions round it. What is the total displacement of the body?

23.

A body covers a distance of 3.6km in 1 hour. The speed of the body is ________ m/s.

IV. Matrix Matching (Match the following) 24.

Column – I

Column – II

(A) A car moving

(p) Periodic motion

on a straight road (B) A javelin thrown

(q) Rotatory motion

by an athlete (C) Motion of a

(r)Curvilinear motion

potter’s wheel (D) Motion of heart beats



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(s) Translatory motion

Light

Learning Outcomes      

Light Reflection of light General terms to the reflection Laws of reflection Image Geometrical construction of an image of an extended object in a plane mirror

1. Light

2.

      

Application of reflection Plane Mirror– Its uses Spherical mirrors Terms related to spherical mirrors Uses of Spherical Mirrors Images Formed by Lens Sunlight

Chapter - 53

By the e nd o f t hi s c ha pte r , y o u w i l l u nd e r s ta nd

reflect the light incident on them which on entering into our eyes, make them visible.

If we enter a dark room, objects present there are not visible. However, if a bulb is switched on, everything in the room becomes visible. It shows that for vision the presence of light is essential. Definition of light: It is an invisible energy which causes in us sensation of sight (vision). Since light is obtained from heat energy, i.e., when an object is heated to a temperature beyond [500 0C, we can say that light is a kind of energy]. It must be kept in mind that light energy makes the surrounding objects visible, but by itself it is an invisible energy. For example, if we are seeing a coloured poster, Note: Reflection is possible in case of plane mirror. then we are seeing only the poster and not the A plane mirror is a plane glass plate which is silvered coloured lights reflected fronts it. It is because light at its one surface. The other surface is then is invisible. The various colours reflected from he reflecting surface of the plane mirror. poster excite the retina of the eye, which in turn Types of Reflections sends a message to the brain. It is the brain which finally makes out the colours of the poster. Thus, (a) Regular Reflection we can conclude that light is an invisible energy The phenomenon due to which a parallel beam of which causes in us sensation of vision. light travelling through a certain medium, on striking some smooth polished surface, bounces off from it, as parallel beam, in some other direction is called Reflection of Light Regular Reflection. When a beam of light is incident on a surface, a part Regular reflection takes place from the objects like of it is returned back into the same medium. The looking glass, still water, oil, highly polished metals, part of light which is returned back into the same etc. medium is called the reflected light. Regular reflection is useful in the formation of The remaining part of light is absorbed if the surface images, e.g., we can see our face in a mirror only on on which the incident light strikes is opaque or it is account of regular reflection. However, it causes a partly transmitted and partly absorbed if the surface very strong glare in our eyes. is transparent. Reflection The return of light into the same medium after striking a surface is called reflection. Reflection of light is the process which enables us to see different objects around us. Luminous bodies are directly seen, but non luminous objects are seen only because they

7th Class Physics

40 (b) Irregular Reflection or Diffused Reflection The phenomenon due to which a parallel beam of light, travelling through some medium, gets reflected in various possible directions, on striking some rough surface is called Irregular Reflection or Diffused Reflection. The reflection which takes places from ground; walls; trees; suspended particles in air; and a variety of other objects, which are not very smooth, is irregular reflection. Irregular reflection helps in spreading light energy over a vast region and also decreases its intensity. Thus, it helps in the general illumination of places and helps us to see things around us.

3. General terms to the Reflection (a) Mirror: A smooth polished surface from which regular reflection can take place is called mirror. MM| is the mirror as shown in figure. (b) Incident Ray: A ray of light which travels towards the mirror is called incident ray. AB is an incident ray in the figure. (c) Point of Incidence: The point on the mirror, where an incident ray strikes is called point of incidence. ‘B’ is the point of incidence in the figure. (d) Reflected Ray: A ray of light which bounces off the surface of a mirror, is called reflected ray. BC is reflected ray in the figure. (e) Normal: The perpendicular drawn at the point of incidence, to the surface of mirror is called normal. BN is the normal in the figure. (f) Angle of Incidence: The angle made by the incident ray with the normal is called angle of incidence.  ABN is the angle of incidence in the figure. (g) Angle of Reflection: The angle made by the reflected ray with the normal is called angle of reflection.  CBN is the angle of reflection in the figure. (h) Glance Angle of Incidence: The angle which the incident ray makes with the mirror is called glance angle of incidence.  MBA is the glance angle of incidence in the figure. (i) Glance Angle of Reflection: The angle which the reflected ray makes with the mirror is called glance www.betoppers.com

angle of reflection.  M’BC is the glance angle of reflection in the figure.

4. Laws of Reflection 1.

The incident ray, the reflected ray and the normal lie in the same plane, at the point of incidence. 2. The angle of incidence is always equal to the angle of reflection. Formula for the Angle of Deviation due to Reflection In the figure angle of incidence = i; Angle of deviation = d N A

B

i r M

O

M1

d

c

Consider the straight line AOC, i + r +d = 180° i.e., the sum of angle of incidence, angle of reflection and angle of deviation is 180°.  d = 180 – (i + r) = 180 – (i + i) (i = r)

 d  180 – 2i Therefore, for an angle of incidence i, the angle of deviation is equal to 180 – 2i =   2i Note: The deviation produced by n reflections from two plane mirrors inclined at an angle  is given by D = n(180 –  ) = 360 – 2  , if n = 2 where n is even.

5. Image When the rays of light, diverging from an object point, after reflection or refraction, either actually meet at some other point, or appear to meet at some other point, then that point is called image of that object.

Light

41 Types of images Virtual image

Real image

The rays of light 1. The rays of light after after reflection reflection or refraction or refraction appear to meet at some actually meet at other point. some point. 2. It cannot be caught on It can be caught the screen. on the screen. 3. It is always erect.

It is always real.

Image formed 4. Image of our face in on a cinema plane mirror is a virtual screen is a real image. image.

6. Geometrical Construction of an Image of an Extended Object in a Plane Mirror: (Two-Ray Diagram) Consider an extended object P situated in front of a plane mirror. For geometrical construction, we will consider two points X and Y on this object. First of all we will locate the position of the image, keeping in mind that image formed in a plane mirror is as far behind as the object is in front of it, by taking normal incidence. In order to construct two-ray diagram, from point X| draw two rays straight towards eye along X|E and X|F, cutting the mirror at A and B respectively. Join XA and XB to form incident beam on mirror. Similarly, from point Yj draw two rays towards the eye along Y|E and Y|F cutting at C and D. Join YC and YD to form incident beam on mirror.

Characteristics of an Image formed by a Plane Mirror 1. The image is formed behind the mirror and has the same size as the object 2. The image is inverted laterally. 3. The image is as far behind the mirror as the object is in front of it. 4. The image is virtual. It cannot be received on a screen. 5. The image is erected w.r.t object

7. Application of Reflection (i) Periscope: A periscope is an instrument for observation from a concealed position. In its simplest form it consists of a tube with mirrors at each end set parallel to each other at a 45-degree angle. This form of periscope, with the addition of two simple lenses, served for observation purposes in the trenches during World War I Construction: It consists of a cardboard or wooden tube bent twice at right angles and is provided with two openings. Two plane mirrors (a) are fixed at the bends of the tube at an angle 45°. Both the mirrors face each other. The tube is completely blackened from inside to avoid and reflection from its sides. The parallel rays coming from an object strike the first plane mirror at an angle of 45°. The rays get reflected at an angle of 45°. These reflected rays strike the second mirror and further reflected through an angle of 45°. Thus we can see the image.

Lateral Inversion: The phenomenon clue to which the image of an object turns through an angle of 180" through vertical axis rather than horizontal axis, such that right side of image appears as left or vice versa is called lateral inversion. www.betoppers.com

7th Class Physics

42 Uses: 1. It is used to see above the head of crowds. 2. It is used by soldiers in trench warfare. Disadvantages 1. The final image is not brightly illuminated as light energy is absorbed due to tow successive reflections. 2. Any deposition of moisture or dust on the mirror reduces the reflection almost to nil and hence the periscope cannot be used in places where there is a lot of dust or moisture. Kaleidoscope (Kaleido = beautiful + edos = image + scope = viewing). It is an instrument used to view beautiful images formed by the reflection of the two or more mirrors (usually three) when placed at different angles. Principle: Multiple reflections take place in plane mirrors. This is the principle of ‘Kaleidoscope’.

at one end. Make a lid out of tracing paper to cover the beads or coloured shapes. The lid must be high enough to let the beads or shapes them move about. Look through the clear plastic at the other end of the triangular tube which is now a kaleidoscope. Turn the kaleidoscope while looking through it a see the changing patterns .formed by light bouncing off the aluminium foil. A better kaleidoscope can be made if you can use small mirrors instead of aluminium foil. The kaleidoscope was invented by Sir David Brewster (1781-1868). U ses 1. Kaleidoscope is used as a toy for children. 2. Kaleidoscope is used by costume – designers in cloth mills and fashion designing institutes.

8. Plane Mirror– Its uses 1. 2.

3. Construction:

4. 5. 6.

They are used as looking glass. They are used by opticians to provide false dimensions. These false dimension can be obtained when two mirrors are fixed parallel to each other. They are used in the construction of reflecting periscope. They are used in solar cookers for reflecting the rays of sun into the cooker. They are used for signalling purposes. They are used by barbers to show the customer the back of his head.

Formative Worksheet 1. 2. 3. Cut out a 16 × 16 cm piece of cardboard. Use a pencil to divide the cardboard into 4 strips each 4 cm wide. Use a sharp knife to score along the pencil lines so that the cardboard can be easily folded into 4 strips. Use paste to attach aluminium foil over 2 of the strips at one end. Colour the next strip black. Fold the strips to make a tube with a triangular crosssection. Two of the inside walls of the triangular tube have aluminium foil on them and the inside of the third wall is black. Use adhesive tape to keep the folded cardboard in shape. Attach clear plastic to each end of the triangular tube. Put coloured beads or small coloured shapes on the clear plastic www.betoppers.com

4. 5. 6. 7.

8.

What is the sum of the angle of the incidence, angle of reflection and the angle of deviation equal to? What is the angle of deviation for an angle of incidence i ? For an angle of incidence 30° on a plane mirror, find the following a) angle of reflection b) angle of deviation c) glancing angle of incidence d) Sum of angles of incidence, reflection and glancing A ray is incident on a mirror at a glancing angle ‘g’. Find the angle of deviation. What is the maximum angle of deviation possible for a ray incident on a plane mirror? What is the angle between the incident ray and the reflected ray for a normal incidence? For a certain angle of incidence the angle of deviation and the angle of reflection are equal. Find the angle of incidence. The incident ray and the reflected ray are at right angles. Then find the angle of incidence and the angle of reflection.

Light 9.

10.

11.

12. 13.

14.

15.

16.

17. 18.

19.

20.

43 A ray of light is incident at 50° on the middle of a pair of mirrors arranged at 60° to each other. Calculate the angle of incidence on the second mirror. Two plane mirrors are separated by 120° as the drawing illustrates. If a ray strikes the mirror M1 at 60°, making an angle of incidence, at what angle (q) does it leave the mirror M2? A ray of light is incident on a horizontal plane mirror at an angle of 45°. After reflection from M1, some of the light is incident on another plane mirror. If the ray reflected by the second plane mirror travels horizontally, find the angle between the mirrors. Can a plane mirror form a real image? What about photographing a virtual image? Will you succeed? Can your image in your image in a plane mirror be photographed? An object is situated at a distance 10m in front of the mirror. It is moved to a distance of 4 m towards the mirror. Find the distance moved by the image towards the mirror and the final distance between the image and the object. An object is situated at a distance of ‘x’m in front of a plane mirror. If the object is moved a distance of ‘y’ m towards the mirror, then find the distance moved by the image towards the mirror and the final distance between the image and the object. An object is moving away from the mirror with a speed of 30 m/s. After 2 seconds the distance between the object and its image is found to be 200 m. Find the initial distance between the object and the image. If the mirror is displaced by 10 m away from the object, find the displacement of the image. If both object and mirror move away from each other by 10 m in 1 second, then find the displacement of the image. A clock is kept in front of a plane mirror and the motion of the hands is observed. Do the hands of the clock move in an anticlockwise direction? An object situated at a distance of 10cm infront of a plane mirror. The distance of image from the mirror is __________

Conceptive Worksheet 1.

2.

3.

4.

5.

6.

7.

Statement I : The plane containing the incident ray and the normal is called plane of incidence. Statement II : The plane containing the incident ray and the normal is called plane of reflection. (A) Both Statements are true, Statement - II is the correct explanation of Statement- I (B) Both Statements are true, Statement - II is not correct explanation of Statement - I (C) Statement - I is true, Statement - II is false (D) Statement - I is false, Statement - II is true Match the following: Column-I Column-II (A) Plane of reflection 1) The plane containing the reflected ray and normal (B) Angle of deviation 2) The angle made by incident ray with the mirror (C) Glancing angle 3) Angle through which a ray deviates from its normal path (D) Normal 4) perpendicular drawn to the surface at the point of incidence 5) The angle made by reflected ray with the mirror When the rays of light, diverging from a point, after reflection or refraction, appear to diverge from another point, then the image so formed is called (A) Virtual image (B) Real image (C) Both (A) and (B) (D) Neither (A) nor (B) When the rays of light, diverging from a point, after reflection or refraction actually converge at some other point then that point is _____ image of the object (A) Virtual (B) Real (C) Both (A) and (B) (D) Neither (A) nor (B) The image formed by a plane mirror is always: (A) Real (B) Erect (C) Virtual (D) Both (B) and (C) Reflection is possible in case of (A) Plane mirror (B) Transparent glass (C) Both (A) and (B) (D) Neither (A) nor (B) The part of light which is returned back into the same medium is called (A) Reflected light (B) Refracted light (C) Both (A) and (B) (D) Neither (A) nor (B)

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7th Class Physics

44 8.

9.

10.

11.

12.

13.

14.

Motion and still pictures projected in a cinema hall are ______ images (A) Real (B) Virtual (C) Erect (D) All of these Choose the correct statements: (A) Regular reflection takes place on highly polished smooth surfaces (B) Irregular reflection takes place on rough surface (C) Irregular reflection is also called as diffused reflection. (D) On plane mirror regular reflection takes place. Image of our face in a plane mirror (A) Is a virtual image (B) Is a real image (C) Cannot be taken on the screen (D) Both (B) and (C) If a mirror is rotated by 10°, the reflected ray is rotated by (A) 10° (B) 20° (C) 40° (D) 30° Statement I : All the light rays actually converge at some point Statement II : All the light rays appears to converge at some point (A) Both Statements are true, Statement - II is the correct explanation of Statement- I. (C) Both Statements are true, Statement - II is not correct explanation of Statement - I. (B) Statement - I is true, Statement - II is false. (D) Statement - I is false, Statement - II is true. Statement I : Real images are always inverted Statement II : Virtual images are always erect (A) Both Statements are true, Statement - II is the correct explanation of Statement- I. (B) Both Statements are true, Statement - II is not correct explanation of Statement - I. (C) Statement - I is true, Statement - II is false. (D) Statement - I is false, Statement - II is true. When a mirror is rotated through an angle  the reflected ray from it, turns through an angle of (A) 

(B)

 2

(C) 2

(D) 0

15. The incident ray and the reflected ray from a mirror are mutually perpendicular to each other. The angle of incidence is (A) 90° (B) 45° (C) 22.5° (D) Zero www.betoppers.com

16. A ray of light is incident on a plane mirror at an angle of 60° What is the angle of deviation? (A) 60° (B) 30° (C) 90° (D) 180° 17. An object situated at a distance of 10cm infront of a plane mirror. The distance of image from the mirror is (A) 10cm (B) 20cm (C) 5cm (D) Some other value 18. A man standing in front of a plane mirror is wearing T-shirt on which the word HERO is written. The word HERO will appear to the man in the mirror as (A) (B) (C)

(D)

19. Pravjot is standing in front of a big plane mirror. He can see his image in the mirror. Now, the mirror is moved away from him with a constant speed of 1 m/ s At what speed will Pravjot’s images move away from him? (A) 1 m/s (B) 2 m/s (C) 3 m/s (D) 4 m/s 20. Bunty, Pappu, and Babblee are standing in front of a big plane mirror. The distance between the mirror and Bunty is twice as compared to that between the mirror and Pappu, which is double the distance between the mirror and Babblee. The distance between Babblee and her image is 1 m. What will be the distance between Bunty and his image, if the mirror is moved 1 m back from its original position? (A) 2 m

(B) 4 m

(C) 6 m

(D) 8 m

9. Spherical Mirrors Raj and Laxmi were waiting for their dinner. Raj lifted a stainless steel plate and saw his image in it. Oh! This plate acts as a plane mirror. My image is erect and is of the same size. Laxmi saw her image using the back of a steel spoon. “Raj look here! I can also see my erect image though it is smaller in size. This spoon also acts as a mirror of some kind”, said Laxmi. You can also use a spoon or any curved shining surface to see your image. Now try to do these activities on your own. Activity -1 Take a stainless steel spoon. Bring the outer side of the spoon near your face and look into it.

Light

45 Convex mirror: A spherical mirror whose outer surface is the reflecting surface is called a convex mirror. reflecting surface

opaque inner surface

Do you see your image in it ? Is this image different from what you see in a plane mirror? Is this image erect? Is the size of the image the same, smaller or larger? Now look at your image using the inner side of the spoon.

sphere

Convex mirror

10. Terms related to Spherical Mirrors

Reflecting surface





Silvered surface

 

Silvered surface

Reflecting surface

This time you may find that your image is erect and larger in size. If you increase the distance of the spoon from your face, you may see your image inverted . You can also compare the image of your pen or pencil instead of your face. Understanding The curved shining surface of a spoon acts as a mirror. The most common example of a curved mirror is a spherical mirror. Let’s understand some thing more about these spherical mirrors. A highly polished plane surface is called a plane mirror. A mirror in which the reflecting surface is curved is called a spherical mirror.

In spherical mirrors the polished reflecting surface is a part of a hollow sphere of glass. Depending upon the nature of the reflecting surface of the mirror, spherical mirrors are of two types. Different types of spherical mirrors Concave mirror: A spherical mirror whose inner hollow surface is the reflecting surface is called a concave mirror. opaque inner surface reflecting surface sphere

Aperture: The width (distance) of the spherical mirror from which reflection can take place is called its aperture. It is denoted by MM Pole: The centre of a spherical mirror is called its pole. It is denoted by P. Centre of Curvature: The geometric centre of the hollow sphere of which the spherical mirror is a part is called the centre of curvature of the spherical mirror. It is denoted by C. Radius of Curvature: The radius of the hollow sphere of which the spherical mirror is a part is called the radius of curvature of the spherical mirror. In other words, the M

X

C

M

P

F

M' Concave mirror

X

C

P

F

M' Convex mirror

distance between the pole and centre of curvature of the spherical mirror (PC) is called its radius of curvature. It is denoted by r. Principal axis: The straight line passing through the centre of curvature and the pole of a spherical mirror is called its principal axis (PX). Focus: If a beam of light parallel to the principal axis falls on a concave mirror, all the rays after reflection meet at a point. This point is called the focus (F) of the concave mirror. If a beam of light parallel to the principal axis falls on a convex mirror, all the rays after reflection diverge. If the reflected rays are extended backwards, they appear to come from a point on the principal axis. This point is called the focus of the convex mirror.

Concave mirror

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7th Class Physics

46 M

F

principal axis

M

P

P

focus

focus M'

.

M'

focal length

Focal length

Focal length: The distance between the pole (P) and focus (F) is called the focal length (f). It is denoted by f. f  PF Types of Images formed by Spherical Mirrors A concave mirror forms both real and virtual images. The size and nature of the image formed by a concave mirror depends on the position of the object. A convex mirror forms only virtual, erect and diminished images. The different types of images formed by a concave mirror as follows.

Position of the object

Position of the image

Relative size of the image

Nature of the image

At infinity

At focus (F)

Highly diminished

Real and inverted

Beyond C

Between C and F

Diminished

Real and inverted

At C

At C

Same size

Real and inverted

Between C and F

Beyond C

Enlarged

Real and inverted

At F

At infinity

Highly magnified

Real and inverted

Between F and P

On the other side of the mirror.

Enlarged

Virtual and erect

11. Uses of Spherical Mirrors I.

Uses of Concave Mirror

Concave mirrors are commonly used in torches, search-lights and vehicles head lights to get powerful parallel beams of light. They are used as shaving mirrors to see a lager image of the face. The dentists use concave mirrors to see large images of the teeth of patients. Large concave mirrors are used to concentrate sun light to produce heat in solar furnaces. II.

Uses of Convex Mirrors

Convex mirrors are commonly used as rear-view mirrors in vehicles. These mirrors are fitted on the sides of the vehicle, enabling the driver to see traffic behind him/her to facilitate safe driving. Convex mirrors are preferred because they always give an www.betoppers.com

erect image. Also they have a wider field of view as they are curved outwards.

Formative Worksheet 21. Statement I : A concave mirror has a real focus. Statement II : A convex mirror has a virtual focus. (A) Both Statements are true, Statement - II is the correct explanation of Statement - I. (B) Both Statements are true, Statement - II is not correct explanation of Statement - I. (C) Statement - I is true, Statement - II is false. (D) Statement - I is false, Statement - II is true. 22. What is the nature of image formed when object is placed beyond centre of curvature? (A) Real (B) Virtual (C) B oth A & B (D) No image is formed

Light 23. An object is placed at centre of curvature of a concave mirror whose radius of curvature is 20 cm. The image is formed from pole at a distance is__________ (A) 10 cm (B) 20 cm (C) 30 cm (D) 15 cm 24. When object is placed at centre of curvature for a concave mirror the image formed is (A) Real inverted of same size (B) Virtual erect of same size (C) Real erect of same size (D) virtual inverted of same size. 25. Column-I (A) Focal length of the spherical mirror (B) Radius of curvature is two times of (C) Plane mirror (D) Concave mirror Column-II 1) Virtual image 2) Distance between principal focus and pole 3) Real image 4) Half radius of curvature 5) Line passing through pole and centre of curvature 26. The image formed by a convex mirror of real object is larger than the object. (A) When u  2f (B) When u > 2f (C) For all values of u (D) For no value of u 27. Statement I : The radius of curvature and focal length of convex mirror are 80cm and 40cm Statement II : The relation between radius of curvature and focal length is R = 2f. (A) Both Statements are true, Statement - II is the correct explanation of Statement - I. (B) Both Statements are true, Statement - II is not correct explanation of Statement - I. (C) Statement - I is true, Statement - II is false. (D) Statement - I is false, Statement - II is true. 28. A small boy went to a mirror show and found that there were five mirrors. He went in front of each mirror to see his image. In the first four mirrors, his image appeared smaller in size and in the last mirror, his image appeared magnified. While his image was inverted in the first three mirrors, it was erect in the last two mirrors. How many convex mirrors were there in the mirror show? (A) 1 (B) 2 (C) 3 (D) 4

47 29. A man decides to start his saloon. He went to the market to buy an item that would make the shaving of the beard of his customers comfortable and easier for him.What did the man buy? (A) Concave lens (B) Convex lens (C) Convex mirror (D) Concave mirror 30. A ball is cut into two halves. On the outer surface of one half of the ball, a shiny aluminium foil is pasted. A candle is now placed in front of this shiny surface. The image of the candle formed by the shiny surface is (A) Virtual and smaller in size (B) Real and smaller in size (C) Virtual and bigger in size (D) Real and bigger in size 31. A small child stuck the non-reflecting ends of two mirrors with glue. He saw his inverted image on one side of the mirror and on the other side, he saw his magnified image. The types of mirrors used are (A) Both convex (B) Both concave (C) One concave and the other plane (D) One convex and the other plane 32. A parallelogram is kept in front of a convex mirror. The front view of the parallelogram can be seen in the convex mirror, as shown in the given figure. What is the apparent area of the visible side that can be seen on the convex mirror?

(A) 21 cm2 (B) 32 cm2 (C) 45 cm2 (D) 48 cm2 33. Four students named Anuj, Aruna, Priti, and Denny deliver one statement each about the characteristics of images formed by concave mirrors. The characteristics are listed below. Anuj: “Concave mirrors can form real as well as virtual images.” Aruna: “Concave mirrors can form only small real images.” Priti: “Concave mirrors can form erect as well as inverted images.” www.betoppers.com

7th Class Physics

48 Denny: “Concave mirrors can form large virtual images.” Which student’s statement about concave mirrors is incorrect? (A) Anuj (B) Priti (C) Aruna (D) Denny 34. There are four mirrors labelled as I, II, III, and IV. Mirrors I and IV can form virtual and small images only. Mirror III can form an image that can be obtained on a screen. Mirror II cannot form real images of objects. Which of the given mirrors can form an inverted image of the object? (A) I (B) II (C) III (D) IV 35. Convex mirrors are used as rear view mirrors in vehicles because they (A) Form virtual and larger images (B) Form real and large images (C) Have a small field of view (D) Have a large field of view 36. While reading his science book, Billu decides to prepare a list of devices that involve the use of concave mirrors. The given table shows the list prepared by him. One device is listed incorrectly in the table. Torch Headlight Dentist’s mirror Rearview mirror Solar furnace Which device is included incorrectly in the list? (A ) Torch (B) Solar furnace (C) Dentist’s mirror (D) Rearview mirror 37. John was driving his car. In the rearview mirror, he saw that a vehicle was just behind his car. He was able to read the word on the vehicle. Which of the following figures is inscribed on the vehicle? (A)

(B)

(C)

(D)

38. A

mirror is used as a shaving mirror

because when placed close to the face, it can form

image of the face.

Which alternative gives the correct information to complete the given statement? (i) (ii) www.betoppers.com

(A) Convex Diminished (B) Concave Enlarged (C) Concave Diminished (D) Convex Enlarged 39. A convex mirror tends to diverge a falling parallel beam of light. This property of convex mirrors is used in (A) Solar furnaces (B) Dentists’ mirrors (C) Reflector in torch lights (D) Reflector in street lamps 40. Dentists use concave mirrors for investigations because concave mirrors form (A) Real images (B) Inverted images (C) Magnified images (D) Diminished images 41. Convex mirrors are used as surveillance mirrors in libraries because they form (A) Real, erect, and magnified images (B) Real, erect, and diminished images (C) Virtual, erect, and magnified images (D) Virtual, erect, and diminished images

Conceptive Worksheet 21. The spherical surfaces that have a property of reflection and that are highly polished are called (A) Spherical mirror (B) Plane mirror (C) Plane glass plate (D) Both 1 and 2 22. Concave mirror acts as (A) Converging mirror (B) Diverging mirror (C) Both (A) and (B) (D) Plane mirror 23. Convergence of concave mirror can be decreased by dipping in (A) Water (B) Oil (C) Both Oil& Water (D)Neither Oil nor Water 24. The image formed by convex mirror is always (A) Virtual (B) Erect (C) Diminished (D) Real 25. Which of the following forms a virtual and erect image for all positions of a real object (A) Plane mirror (B) Convex mirror. (C) Concave mirror (D) All the above 26. The diameter of spherical mirror in which reflection takes place is called (A) Radius of curvature (B) Centre of curvature (C) Linear aperture (D) Focal length. 27. For a concave mirror when object is placed at infinity the image is formed at (A) Pole (B) Principal focus (C) Centre of curvature (D) Infinity

Light

49

28. The focal length of concave mirror is -40cm . Its radius of curvature is (A) -20cm (B) +20cm (C) -80cm (D) +80cm 29. Choose the correct statements (A) Convex mirror acts as converging mirror. (B) Image formed by convex mirror is virtual (C) For a concave mirror when object is placed at infinity the image is formed at principal focus. (D) Convex mirror acts as diverging mirror. 30. Convex mirrors can be used as (A) A solar furnace (B) A dentists’ mirror (C) Reflectors of torches (D) Side-view mirror in cars 31. Images formed by a (A) Convex mirror are always diminished (B) Concave mirror are always diminished (C) Concave mirror are always enlarged (D) Convex mirror are always enlarged 32. The dentist’s mirror is a device used by dentists to check the position and size of cavity in teeth. The dentist’s mirror is basically a type of (A) Plane mirror (B) Convex mirror (C) Concave mirror (D) Parabolic mirror 33. Which of the following mirrors are used by cosmetologists for their work? (A) Parabolic (B) Concave (C) Convex (D) Plane 34. The  mirror is used as side view mirror in vehicles. It can form images of objects spread over a  area. The information in which alternative completes the given statements? (i) (ii) (A) Concave Large (B) Convex Small (C) Convex Large (D) Concave Small 35. Concave mirrors cannot be used as (A) Side-view mirror in cars (B) Reflectors of torches (C) Dentists’ mirror (D) Solar furnace

12. Images Formed by Lens What is a Lens? A lens is a piece of glass or any transparent material bound with two surfaces, atleast one of which is a curved surface. The curved surface is spherical

(part of a sphere) in nature. Based on the shape of the curve on the surface of a lens, lenses are grouped into two main types: (i) A convex lens, having a bulge in the centre and with narrow edges. (ii) A concave lens, having a depression in the centre and thick at the edges.

Formation of Spherical Lenses Different Shapes of Spherical Lenses A. Double convex (both the sides convex) B. Plano convex (converging lens) C. Concave convex (convex meniscus) D. Concave concave (concave meniscus) E. Plano concave (diverging lens) F. Double concave (both the sides concave)

Different shapes of Spherical Lenses We call these lenses as glasses or specs (short for a pair of spectacles) which are used by people with poor sight to watch things through them. These are nothing but a pair of lenses made from transparent glass and fixed to a frame which is held on to the eyes. A lens, which is a magnifying glass, is used by watchmakers to see very small parts of the machine as big (large) through them. Kinds of Lens Convex Lens: A convex lens makes the object magnified, when viewed through it. A convex lens is thick in the middle and thin at its edge. When light rays pass through a convex lens, they bend inwards and converge at a common point to form an image of the source of light. Rays from the sun converge to form its image as a bright spot. A convex lens converges light rays. Therefore, it is also called a converging lens. The image formed when the object is placed close to a www.betoppers.com

7th Class Physics

50 convex lens is virtual, erect and magnified. Virtual images cannot be caught on a screen. Images that are caught on a screen are called real images. When the object is placed at a distance from a convex lens, the image formed is real, inverted and diminished.

Convergence of light rays Concave Lens: It is a lens that possesses at least one surface that curves inwards. When light rays are incident on a concave lens, they bend outwards or diverge. The rays diverge away from each other. Thus, a concave lens is also called a diverging lens. A concave lens is thinner at its centre than at its edges, and is used to correct short sightedness. It does not focus at a single point. The image formed by a concave lens is upright, virtual and smaller than the object. For example, the images seen through a peephole are different from normal holes, because these peep holes contain concave lenses.

rainbows arc is always directed away from the sun. It is believed that in the past, Norsemen saw rainbows as bridges for gods to come to the earth from their home in the sky. Norsemen were the inhabitants of Norway. A rainbow lasted for about 9 hours on 14th March, 1994, at a place called Wetherby in Yorkshire, England. Although sunlight appears white, it is composed of seven colours. The colours in a rainbow are the colours of sunlight. A rotating disc has a pencil that serves as a rotator. The disc is covered with violet, indigo, blue, green, yellow, orange and red coloured papers When the disc is rotated, it appears white instead of the individual colours. This is because a mixture of colours of the rainbow in proper proportions produces white colour. The colours of a rainbow can be represented by the acronym: VIBGYOR: V - Violet, I - Indigo, B - Blue, G - Green, Y Yellow, O - Orange and R - Red. Dispersion of Light through a Prism Take a glass prism. Allow a narrow beam of sunlight to pass through a small hole in the window of a dark room to fall on one face of the prism. The light bends when it passes through the prism. Now allow the light coming out of the other face of the prism to fall on a white sheet of paper or a white wall. Different component colours of white light bend differently, and so the constituent colours can be seen separately. Thus, the colours are said to have dispersed after passing through the prism.

Formative Worksheet Applications of Lenses Lenses are used in magnifying glasses, peep holes, cameras, bioscopes, binoculars, telescopes, microscopes and projectors. A refracting telescope uses a concave mirror and a convex lens.

13. Sunlight A band of colours extending from violet to red is a rainbow. A rainbow is formed by the refraction and reflection of the sun's rays through raindrops. Rainbow A band of colours extending from violet to red is a rainbow. A rainbow is formed by the refraction and reflection of the sun’s rays through raindrops. When it is raining in one part of the sky and sunny in another, a rainbow appears. The centre of the www.betoppers.com

42. Farman finds it difficult to read small-sized text from one of his books. He wants to use an optical instrument that helps him to read the book with ease Which of the following optical instruments should Farman choose? (A) Concave lens (B) Convex lens (C) Concave mirror (D) Convex mirror 43. A magnifying glass is simply a (A) Convex lens (B) Concave lens (C) Convex mirror (D) Concave mirror 44. An activity is performed using a candle and an optical instrument as shown in the given figure.

Light

51

Which optical instrument has been used in the 50. Diminished inverted images are produced by activity? (A) Only a concave lens (A) Concave lens (B) Convex lens (B) Only a concave mirror (C) Concave mirror (D) Convex mirror (C) Both convex lens and concave mirrors 45. Rajesh took out the lens from his father’s old (D) Both convex mirrors and concave lens spectacles and kept it in between sun rays and a 51. An image that is real and smaller than the object paper. He varied the distance but he found that the can be produced by paper did not burn. He became upset and looked at (A) A concave mirror only the old spectacles through this lens. (B) A concave lens only Appearance of the spectacles through the lens is (C) Either a concave lens or convex mirror (A) Erect and large (B) Erect and small (D) Either a concave mirror or convex lens (C) Inverted and large (D) Inverted and small 52. James allows a beam of sunlight to pass through a 46. Which optical instrument is used for reading smallglass prism in a completely dark room. He observes size text? a multicolored band or spectrum of light, as shown (A) Concave lens (B) Convex lens in the given figure. (C) Convex mirror (D) Concave mirror 47. The image formed by a concave lens is

,

, and

.

The information in which alternative completes the given statement? (A) i ii iii

It can be concluded from James’ experiment that (A) A prism is made up of different colors real upright enlarged (B) White light is made up of different colors (B) i ii iii (C) A prism can produce light of different colors real inverted enlarged (D) Any triangular object splits white light into (C) different colors i ii iii virt ual upright diminished 53. Ajay throws a narrow beam of white light on a glass prism and observes the phenomenon of dispersion (D) i ii iii of light on the screen. He then placed a slit between virtual inverted diminished the prism and the screen such that the two colours from the top and the three colours from the bottom 48. A convex lens is at the middle than the get obstructed. edges. It is also called a lens. Which of the following pairs of colours still appear on the screen? The information in which alternative completes the (A) Yellow and orange (B) Green and yellow given statement? (C) Blue and green (D) Indigo and blue (A) (B) i ii thinner convergent

(C)

i ii thicker divergent

i (D) i ii thinner thicker convergent

ii divergent 49. The given figure shows a beam of sunlight passing through an optical glass G.

54.

is the third colour in the rainbow when counted from the bottom, while is the third colour when counted from the top. Which colour is the third colour in a rainbow from the bottom and top respectively? i ii (A) (B) i ii Green Blue

Yellow Blue Sunlight

(C)

i ii Red Yellow

(D)

i ii Orange Red

G

The optical glass G is a (A) Concave mirror (B) Convex mirror (C) Concave lens (D) Convex lens www.betoppers.com

7th Class Physics

52 55. Tom is playing with a spinner. The spinner has all the seven colors of the rainbow painted on it, as shown in the figure.

While the spinner is rotating, its color will appear to be (A) Red (B) Black (C) White (D) Yellow 56. White light is composed of seven different colours. These colours can be seen in a rainbow. Which of the following colours is not present in a rainbow? (A) Green (B) Red (C) Black (D) Blue

Conceptive Worksheet 36. Which type of lens is called a converging lens? (A) Concave (B) Plano concave (C) Convex (D) All 37. Which type of lens is called a diverging lens? (A) Concave (B) Plano concave (C) Convex (D) All 38. Which type of image is formed by a convex lens? (A) Real (B) Inverted (C) Virtual (D) All 39. Which type of image is formed by a concave lens? (A) Real (B) Inverted (C) Virtual (D) All 40. Which of the following acts a magnifying lens? (A) Concave (B) Plano concave (C) Convex (D) All 41. Which type of image is formed by a convex lens when an object is placed closer to it? (A) Real (B) Inverted (C) Virtual (D) All 42. The splitting of white light into its constituent colours is called (A) Reflection (B) Refraction (C) Dispersion (D) Diffraction 43. The scientist who suggested that white light is a mixture of seven colours (A) Snell (B) Newton (C) Einstein (D) Rutherford 44. Red, Blue and Green are called (A) Primary colours (B) Secondary colours (C) Tertiary colours (D) Universal colours 45. White light is composed of (A) Three colours (B) Five colours (C) Seven colours (D) Nine colours www.betoppers.com

46. Rainbow is formed due to _______ of light (A) Reflection (B) Refraction (C) Dispersion (D) Diffraction 47. Statement - I: A concave lens is used as a converging lens. Statement - II: A convex lens is used as a diverging lens. (A) I is true, II is false (B) I is false, II is true (C) Both are true (D) Both are false 48. Statement - I: A rainbow is formed by dispersion of light when it passes through tiny droplets of water suspended in air. Statement - II: This band of seven colours is called spectrum or visible spectrum (A) I is true, II is false (B) I is false, II is true (C) Both are true (D) Both are false 49. Statement - I: A real image can be obtained on the screen. Statement - II: A virtual image can be obtained on the screen. (A) I is true, II is false (B) I is false, II is true (C) Both are true (D) Both are false 50. Statement - I: Rainbow is not formed during noon. Statement - II: In the noon earth is in the direction opposite to the sun and hence the rainbow on the earth is not seen. (A) I is true, II is false (B) I is false, II is true (C) Both are true (D) Both are false 51. Which type of lens is used to make fire with it during day time? (A) Concave (B) Plano concave (C) Convex (D) All

HOTS Worksheet 1.

2.

3.

4.

An object is placed at a distance of 20 cm infront of a plane mirror. If the mirror moves towards the object by a distance of 20 cm, then find the displacement of the image. An object is placed at a certain distance infront of a plane mirror. If both object and plane mirror moves towards each other through a distance of 15 cm, then find the distance through which the image moves. If both object and mirror moves away from each other with a speed of 20 m/s, then find (a) the speed of the image (b) speed of the image with respect to object (c) speed of the image with respect to mirror. Two mirrors are inclined at a certain angle  . If a light ray is incident on the first mirror parallel to the second mirror and reflected from the second parallel to the first mirror, then find the value of  .

Light 5.

6.

7.

53 A man 180 cm high stands in front of a plane mirror. His eyes are at a height of 170 cm from the floor. Then the minimum length of plane mirror for him to see his full image is ___. Two plane mirrors are inclined to each other at an angle of 70°. A ray is incident on one mirror at an angle  . The rays reflected from this mirror fall on the second mirror from where it is reflected parallel to the first mirror . What is the value of  ? Two plane mirrors are inclined at an angle of 60° as shown in the figure. A ray of light parallel to Mirror1 strikes the Mirror 2. At what angle will the ray finally emerge?

IIT JEE Worksheet I.

Single Correct Answer Type

1.

Mirrors cast images because they (A) Absorb light (B) Reflect light (C) Refract light (D) Transmit light Regardless of the positioning of an object, which of the following optical devices never produces a magnified or a diminished image? (A) Plane mirror (B) Convex lens (C) Concave lens (D) Convex mirror A mirr or can for m images but a plane glass slab cannot. This is because a mirror (A) Reflects light while a plane glass slab absorbs light (B) Refracts light while a plane glass slab reflects light (C) Reflects light while a plane glass slab refracts light (D) Refracts light while a plane glass slab absorbs light An image of a pencil is formed by a plane mirror at a distance of 40 cm from the pencil. The pencil is moved 5 cm towards the mirror. What will be the new image distance from the mirror? (A) 10 cm (B) 15 cm (C) 30 cm (D) 35 cm A pencil of length 5 cm is kept vertical at a distance of 10 cm from a plane mirror. This pencil is then replaced by another pencil of length twice than that of the earlier one.What is the distance of the image of this new pencil from the mirror? (A) 5 cm (B) 10 cm (C) 15 cm (D) 20 cm A candle and its image are at a distance of 30 cm from each other, as shown in the given figure.

2.

3.

M2

600 M1

8.

9.

10.

11.

12.

13.

Two plane mirrors are placed parallel to each other. The distance between the mirrors is 10 cm. An object is placed between the mirrors at a distance of 4 cm from one of them, say M1. What is the distance between the first image at M 1 and the second image formed at M2 ? A plane mirror is moved away from a stationary object with a speed of 20 cm/s. What is the speed of the image? The incident ray and the reflected ray are at right angles. Then find the angle of incidence and the angle of reflection. (A) 45° each (B) 90° each (C) 30° each (D) 50° each A ray of light is incident on a horizontal plane mirror at an angle of 45°. After reflection, from M1, some of the light is incident on another plane mirror. If the ray reflected by the second plane mirror travels horizontally, find the angle between the mirrors. (A) 50° (B) 22 ½° (C) 67 ½° (D) 90° Two plane mirrors are inclined at an angle ‘  ’. It is found that a ray incident on one mirror at any angle is rendered parallel to itself after reflection from both the mirrors. Find the value of ‘  ’. (A) 90° (B) 45° (C) 35° (D) 75° Two plane mirrors are inclined at 70°. A ray incident on one mirror at an angle  , after reflection, the reflected ray makes an angle of 30° with the second mirror. Find  . (A) 20° (B) 10° (C) 30° (D) 40°

4.

5.

6.

In this case, a i  mirror is used, which is placed ii from the candle. The information in which alternative completes the given statement? (A) (B) i ii i ii plane 15 cm

convex 10 cm

(C) 7.

i ii convex 15 cm

(D)

i ii plane 10 cm

A convex mirror always produces a (A) Real image smaller than the object (B) Real image larger than the object (C) Virtual image larger than the object (D) Virtual image smaller than the object www.betoppers.com

7th Class Physics

54 8.

An activity is performed using a candle and an optical instrument as shown in the given figure.

Which optical instrument has been used in the activity? (A) Concave lens (B) Convex lens (C) Concave mirror (D) Convex mirror 9. The side view mirror of a car forms an (A) Erect and real image (B) Erect and virtual image (C) Inverted and real image (D) Inverted and virtual image 10. Mirrors are placed at different corners of a certain store. A small but complete view of the store is possible through these mirrors. The given figure shows one such mirror.

Which type of mirror is described here? (A) Plane (B) Convex (C) Concave (D) Parabolic 11. A rear-view mirror is a functional type of mirror found on automobiles and other vehicles. It is designed to allow the driver to see the area behind the vehicle. The rear-view mirror of an automobile is a (A) Plane mirror (B) Convex mirror (C) Concave mirror (D) Cylindrical mirror 12. A convex mirror tends to diverge a falling parallel beam of light. This property of convex mirrors is used in (A) Solar furnaces (B) Dentists’ mirrors (C) Reflector in torch lights (D) Reflector in street lamps 13. Concave mirrors cannot be used as (A) Side-view mirror in cars (B) Reflectors of torches (C) Dentists’ mirror (D) Solar furnace

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14. Farman finds it difficult to read small-sized text from one of his books. He wants to use an optical instrument that helps him to read the book with ease. Which of the following optical instruments should Farman choose? (A) Concave lens (B) Convex lens (C) Concave mirror (D) Convex mirror 15. Rajesh took out the lens from his father ’s old spectacles and kept it in between sun rays and a paper. He varied the distance but he found that the paper did not burn. He became upset and looked at the old spectacles through this lens. Appearance of the spectacles through the lens is (A) Erect and large (B) Erect and small (C) Inverted and large (D) Inverted and small

II. Multiple Correct Answer Type 16. Which of the following forms a virtual image? (A) Plane mirror (B) Convex mirror (C) Concave Mirror (D) Convex lens 17. Which of the following forms a real image? (A) Plane mirror (B) Convex mirror (C) Concave Mirror (D) Convex lens 18. Which of the following forms an image of same size? (A) Plane mirror (B) Convex mirror (C) Concave Mirror (D) Convex lens 19. Which of the following forms a magnified image? (A) Plane mirror (B) Convex mirror (C) Concave Mirror (D) Convex lens 20. Which of the following forms a diminished image? (A) Plane mirror (B) Convex mirror (C) Concave Mirror (D) Convex lens 21. Which of the following forms only virtual images? (A) Plane mirror (B) Convex mirror (C) Concave Mirror (D) Convex lens 22. Which of the following forms only real images? (A) Plane mirror (B) Convex mirror (C) Concave Mirror (D) Convex lens

Light

55

III. Paragraph Type Nature of image formed by two plane mirror inclined at an angle  is given by the formula n 

360 . 

If n is an even whole number the number of images = n –1 If n is odd whole number, the number of images = n Using the above information answer the following. 23. Find the total number of images formed by two plane mirrors of same length and their reflecting surfaces are parallel to each other. (A) 5 (B) 6 (C) 10 (D) Infinity 24. Find the total number of images formed by two plane mirrors that are inclined perpendicular to each other. (A) 1 (B) 3 (C) 5 (D) 7 25. Find the total number of images formed by two plane mirrors that are inclined at angle of 45° to each other. (A) 1 (B) 3 (C) 5 (D) 7

IV. Integer Type 26. You are standing in front of a plane mirror, which is approaching you with a speed of 10cm/sec. Then the speed of the image approaching you will be ______________cm/s. 27. A snake is approaching a stationary plane mirror with a speed of 5 m/s. Then it observes that its image is approaching it with a speed of_____________ m/s. 28. Raju is standing at a distance of 15 cm from the plane mirror. The distance between him and his image is ___________cm.

V. Matrix Matching 29. Column A A) A plane mirror B) A convex mirror C) A concave mirror D) A convex lens E) A concave lens

Column B p) Reflector in a torch light q) A magnifying glass r) A looking glass in our homes s) Used in specs to view distant objects clearly t) A rear view mirror

30. Column A A) A plane mirror B) A convex mirror C) A concave mirror D) A convex lens E) A concave lens

Column B p) Only virtual image q) Virtual image magnified r) Virtual image diminished s) Real image magnified t) Real image diminished 

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56

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7th Class Physics

Electric Current and Its Effects

By the e nd o f t hi s c ha pte r , y o u w i l l u nd e r s ta nd  Electric Charge

 Electric Circuit

 Electric Current

 Symbols used in Electrical circuits

 Electric Potential & potential difference

 Connecting Electric Cells in Series

 Sources of Electricity

 Connecting Bulbs in Parallel

 Effects of Electricity

 Conductors and insulators

1. Introduction In 600 B.C. a great scientist ‘Thales’ observed that when amber was rubbed with wool, it acquired a strange property of attracting tiny bits of paper, cork or dry straw towards itself. Later in 16th century ‘Gilbert’ observed the same phenomena in some other substances. Such as ebonite & cat’s skin; glass and silk; sealing wax and wool, etc. He named it as electricity. The source of all electricity and electrical phenomena is Charge.

2. Electric Charge In nature there are two types of charges. They are positive and negative charges. • Electric charges can exist independently either as positive or negative charges. • A positively charged body has more positive charges and negatively charged body has more negative charges. • A neutral body has equal amounts of positive and negative charges. • Positive charge is denoted by +q and negative charge is denoted by -q. • Unit: S.I unit of Charge is Coulomb. •

Static electricity is study of electric charges at rest.

3. Electric Current The rate of flow of charge in a circuit is called electric current. In other words, it is the amount of charge flowing per second. It is denoted by the letter I. If Q is the charge which is flowing through a

Q conductor in time t, then current is given by i  t

Chapter - 64

Learning Outcomes

Unit of Current The S.I unit of current is ampere and it is denoted by the letter ‘A’. The S.I unit of Q is coulomb and that of t is second. Thus, the S.I unit of electric current is

1 coulomb  1A 1 second

Definition of Ampere When a charge of coulomb flows through a conductor in one second, then the current flowing through the conductor is said to be one ampere. Thus, when 1 coulomb of charge flows through a conductor in 1 second, then the current flowing through it is said to be 1 ampere. 1 ampere 

1 coulomb 1 second

Smaller units of Electric Current Sometimes smaller units of current are also used. These are microampere and milliampere. 1 microampere = 1 A  106 A 1 milliampere = 1 mA  103 A Bigger unit of electric current: Sometimes the magnitude of the current flowing in a conductor is very large. This large magnitude of current is expressed in bigger units, such as kilo ampere and mega ampere. 1 kilo ampere (kA) = 1000 A = 103 A 1 mega ampere (MA) = 1,000,000 A = 106 A.

Flow of Current In metals, the moving charges are the electrons constituting the current, while in electrolytes and

7th Class Physics

58 ionized gases, electrons and positively charged ions are the ions moving charges which constitute current. The charge on an electron is negative and is –1.6 × 10–19 coulomb (symbol C). Therefore, IC charge is carried by electrons. Hence if I A current flows through a conductor, it implies that 6.25 × 1018 electrons pass in 1 second across the cross section of the conductor.

3.

4.

The direction of current is conventionally 5. taken opposite to the direction of motion of electrons. If n electrons pass through a cross section of a conductor in time t, then total charge passed Q = n × e and current in conductor Instrument by which current measured: Current is measured by an instrument called ammeter.

4. Electric Potential & Potential Difference a)

We define the electric potential difference between two points in an electric circuit carrying some current as the work done to move a unit charge from one point to the other.

6.

7. 8.

9.

10.

Potential difference (V) between two points 11. Work done  W  = b)

Ch arg e  Q 

V

W Q

The S.I unit of potential difference is volt(V).

 1Volt 

1Joule 1Coulomb

12. 13.

14.

One volt is the potential difference between two points in a current carrying conductor 15. when 1 joule of work is done to move a charge of 1 coulomb from one point to the other. The potential difference is measured by means of an instrument called voltmeter.

Formative Worksheet A flow of 107 electrons per second in a conducting wire . How much current do they constitute? 2. If the electronic charge is 1.6 × 10–19 C, then find the number of electrons passing through a section of a wire per second, when the wire carries a current of 4 ampere. www.betoppers.com

16.

1.

17.

A polythene piece rubbed with wool is found to have a negative charge of 3.2 ×10–7 C. (a) Estimate the number of electrons transferred. (b) Is there transfer of mass from wool to polythene? A comb drawn through person’s hair on a dry day causes 1022 electrons :o leave the person’s hair and stick to the comb. Calculate the charge carried by the comb. A current of 0.5 A is drawn by a filament of an electric bulb : - minutes. Find the amount of electric charge that flows through the circuit An electric bulb draws a current of 0.2 A when the voltage is 220 Calculate the amount of electric charge flowing through it in one hour. Show that one ampere is equivalent to a flow of 6.25 × 1018 electron charges. 1020 electrons, each having a charge of 1.6 –19 × 10 C, pass from a point A towards another point B in 0.1 s. What is the current in ampere? If a current of 10 mA passed through your cell phone battery charger wire for 10 minute , what quantity of electric charge is transferred through the wire to the battery? How much current does your laptop consume if 100 C of charge is transferred through the charger wire in 4 second ? A body has a negative charge of 1 coulomb. It means that __________. A million electrons are added to a pith ball. Its charge is __________. A conductor carries a current of 2A. How long will it take for 1800C of electricity to flow through a given cross-section ? How many electrons flow through a light bulb each second if the current through the bulb is 1.6A ? A current of 1.5A exists in a conductor whose terminals are connected across a potential difference of 100 volt. Find a) the total charge transferred in one minute. b) the work done in transferring the charge. A polythene piece, rubbed with wool, is found to have negative charge of 4  10–7C. The number of electrons transferred from wool to polythene is ______________. When a piece of polythene is rubbed with wool, a charge of –2 × 10–7C is developed on polythene. What mass (if any), is transferred to polythene ?

Electric Current & its Effects 18. An electric cell does 5 J of work in carrying 10 C charge around a closed circuit. The electromotive force of the cell is _______________. 19. A work of 100 joule is performed in carrying a charge of –5 coulomb from infinity to a particular point in an electrostatic field. The potential of this point is ____________. 20. The work done in moving a charge of 20 C from A to B over a distance of 0.2 m is 2 J. The potential difference across A and B is ______________. 21. A charge of 5 C is given a displacement of 0.5m. The work done in the process is 10 J. The potential difference between the two points will be _____________. 22. An electron of charge ‘e’ coulomb passes through a potential difference of V volt. Its energy in joule will be _______________.

Conceptive Worksheet 1.

The unit of charge is (ampere, coulomb, farad, volt) 2. A body can be negatively charged (giving electrons to it, removing some electrons from it, giving some protons to it, removing some neutrons from it.) 3. ___________is the amount of charge flowing through a particular area of cross section of a conductor in unit time (Charge, Electric current, Potential, Energy) 4. The SI unit of electric current is (ohm, volt, ampere, coulomb) 5. One ampere is equal to (1 C/s, 1C × 1s , 1 J × 1 C, 1 J/C) 6. Volt is the SI unit of (potential difference, current, resistance, charge) 7. No current flows between two charged bodies when connected, if they have same (capacity, potential, charge, none) 8. The surface of the earth is taken to be at (infinite potential, negative potential, positive potential, zero potential) 9. Which is bigger: a coulomb of charge or the charge of an electron? (Coulomb of charge, Charge of an electron, Both are same, None) 10. How many electrons are equal to 1 coulomb? (6.25 × 1016, 6.25 × 1017, 6.25 × 1018, 6.25 × 1019)

59 11. ___________ between two points in an electric circuit carrying some current as the work done to move a unit charge from one point to the other. (electric current, electric charge, potential difference, electric power) 12. The SI unit of potential difference (ohm, volt, ampere, coulomb) 13. What is the work done in moving a charge of Q coulomb against a potential difference of V volt? (Q/V , QV, Q+V, Q–V) 14. What is the instrument used to measure potential difference across a circuit. (Ammeter, voltameter, galvanometer, all) 15. The device used for measuring current is (Galvanometer, Ammeter, Voltmeter, Potentiometer)

5. Sources of Electricity Electrical energy available to us from electric power houses, domestic generators, batteries, dry cells, button cells. • Dry cell is the most handy source of electricity which is used in torchlights, watches, clocks. • We make use of button cells in wrist watches, calculators. • A combination of two or more cells is called a battery. • A car battery or batteries used in cars or trucks are combination of ‘six’ or more cells. These batteries once exhausted can be recharged with the help of an electric changer and used again and again. Hence these are called accumulators.

6. Effects of Electricity Electricity is a form of energy, which help us with • Heating Effect • Light Effect • Magnetic Effect • Chemical Effect Heating Effect Here electrical energy is converted into heat energy. •

•

Heating effect of electric current in a wire is used in appliances like electric iron, electric kettle, geyser, immersion rod, room heater etc. Some metals like silver, copper, aluminium are very good conductors of electricity. www.betoppers.com

7th Class Physics

60 •

They offer no obstruction (resistance) to the flow of electricity through them. • In case of an alloy of nickel and chromium (nichrome - an alloy), electric current does not flow easily and it finds obstruction (resistance). This results in heating of the wire. • Similarly filament (very thin wire) made from a metal called ‘tungsten’ offers great obstruction to the flow of the electric current through it. Thus an electric work is done in overcoming this friction. This result in loss of energy. This lost energy appears in the form heat energy and light energy. On passing current through it, it gets heated to a high temperature so that it starts glowing. Fuses: • Fuse is a safety device used for breaking a circuit. • It works based on the principle of heating effect of electricity. • We make use of thin wires from soft metals like lead and tin which melts at a low temperature. • Most common type of fuses are: • Kitkat fuse made from ceramics (porcelain). • Miniature circuit breaker (MCB) there are made like a switch. • Cartridge fuse (Appliance fuse) are generally used in electrical appliances. Resistance: The friction offered to the passage of electric current by a material is called resistance of material. It is the characteristic property of a conductor. Unit: Unit is ‘Ohm’ and it is represented by ‘ Ω ’(omega). Ohm’s Law: The electric current(i) in conductor is directly proportional to the potential difference (V) between its ends at a constant temperature. It is represented by letter R with a symbol V

.

i

V = R i ; where ‘R’ is proportionality constant called as electrical resistance of the conductor. Lighting Effect: Electric current while passed through a filament produces heat and glows. The glow emits light. • Here electrical energy is converted into light energy. www.betoppers.com

•

Lighting effect of electric current is used in bulbs, fluorescent tubes. • Compact fluorescent lights are a modification of fluorescent tubes which emit more light with less of electric energy consumed. Magnetic Effect: Magnetic effect of electricity is of great use in our day to day life, in industries, in the field of medicine etc., • Magnetic effect of electric current was first discovered by ‘Oersted’. • He concluded that electric current produces a magnetic field around the wire. • Here electrical energy is converted into magnetic energy. • When current is passed through a soft iron nail it gets magnetised. Hence this magnet is called as electromagnet. • Electric bell works based on the principle of magnetic effect of electricity. Electromagnet Wrap a wire around a soft iron piece (known as the core). When an electric current is passed through the wire, the iron piece behaves like a magnet.

A magnet made using such an arrangement is called an electromagnet. A solenoid is a device which can be used as an electromagnet.

It is made of a long wire that has been wound many times into a tightly packed coil; it has the shape of a long cylinder. Strength of an Electromagnet The strength of the electromagnet depends on the number of turns of the wire around the core and the amount of current passing through it. The more the number of turns the more will be the magnetic effect.

Electric Current & its Effects

The iron nail attracts two safety pins when wrapped with a coil with more number of turns. What will happen if the current passing through the coil is switched off? The iron piece will lose its magnetic effect, i.e., it will stop behaving like a magnet and, thus, will not attract the safety pin. Uses of Electromagnets Electromagnets are used in cranes to pick up cars in scrap yards and also to separate iron from garbage dump. They are also used in loudspeakers, telephones, electric motors which are used in electric fans, washing machines, refrigerators, etc. Electromagnets are also used in Electric Bell.

61

Step 1: When you push the switch of the bell, an electric current flows to the electromagnet. Step 2: The electromagnet attracts the soft iron strip. The hammer attached to the strip then hits the gong, causing a ring. Step 3: When the soft iron strip gets attracted to the electromagnet, it no longer touches the screw (interrupter) and hence the circuit is broken (much like a switch being turned off). This turns off the electromagnet and it can no longer attract the soft iron strip. The soft iron strip returns to its initial position, touching the screw (interrupter). This results in the circuit being complete, and current flows again. Steps 1 to 3 repeat in quick succession as long as the switch is on. This is how we hear a continuous ring of the bell.

What do you do when you reach your friend’s house to let him/her know that you are at the door? You ring the doorbell. Do you know what makes the bell ring? An electric bell has an electromagnet that pulls a strip of iron which makes the hammer hit the gong to ring the bell. Let’s study how an electric bell works . . . Electric Bell: What do you do when you reach your friend’s house to let him/her know that you are at the door? You ring the doorbell. Do you know what makes the bell ring? An electric bell has an electromagnet that pulls a strip of iron which makes the hammer hit the gong to ring the bell.

Electric Buzzer: Nowadays, we use electric buzzers and music bells, which work on a principle different from that of the electric bell described here. Buzzers are of different types. Chemical Effect: This effect is used in electroplating, electrotyping, purifying metals, electrolysis etc. Here electrical energy can be used to produce chemical reaction in solutions.

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7th Class Physics

62

Formative Worksheet 23. To construct an electromagnet, Morgan uses an iron bar as the core and uses four electrical coils, made of four different materials to construct a solenoid. This is represented in the given figure.

28. An electromagnet is not used in the functioning of which of the following devices? (A) An electric motor (B) A generator (C) A telescope (D) A door bell 29. Adam has an electromagnet in his school’s electrical laboratory.

Which of the following materials can be used to The iron bar will not be converted into an make the rod of an electromagnet? electromagnet if the solenoid is made of a (A) Glass (B) Wood (A) Copper wire (B) Aluminum wire (C) Iron (D) Plastic (C) Silver wire (D) Plastic wire 30. John has to construct an electromagnet for his school project. 24. A light bulb is an incandescent source of light. A light bulb emits light Which of the following figures shows the way in which John should construct the electromagnet? (A) By magnetic induction (A) (B) (B) By the burning of a fuel (C) Because of high temperature (D) Because of the passage of electricity through (C) (D) a gas 25. Joseph wants to pick the iron nails kept on a table.

Which of the following devices should Joseph use to pick the iron nails? (A) A generator (B) An electric motor (C) An electromagnet (D) A magnetic compass 26. Thomas brings four different devices near a current carrying wire.

Which device will show a deflection? (A) The compass (B) The ammeter (C) The voltmeter (D) The barometer 27. Which of the following actions will produce a magnetic field? (A) Rubbing cotton on a glass rod (B) Passing current through a wire (C) Rubbing two iron bars together (D) Charging a conducting sphere www.betoppers.com

31. The hair dryer works on the principle of i  effect of current. The component of the hair dryer that works on this principle is called ii . The information in which alternative completes the given statements? (A) (B) i ii i ii heating

(C)

i

plate

magnetic

ii

magnetic element

(D)

plate

i

ii

heating

element

32. Tempenny constructs a simple electric generator as shown in the figure.

The part of the electric generator which is an electromagnet is the (A) Drill (B) Coil (C) Pencil (D) Bulb

Electric Current & its Effects 33.

63 36. Clarke constructs a device by passing current through a piece of long insulated wire wrapped around an iron core. He uses this device for picking paper clips.

When current flows through the coil, the iron rod is attracted downward because the What is the name of the device constructed by (A) Current through the coil produces a Clarke? gravitational field (A) Electric generator (B) Magnetic compass (B) Current through the coil produces a magnetic (C) Electromagnet (D) Galvanometer field 37. An electric iron works on the principle of i effect (C) Iron rod and the coil carry opposite charges of electric current. The component of an iron that (D) Iron rod and the coil carry like charges works on this principle is its ii. 34. The figure shows an electromagnetic train signal. The information in which alternative completes the When it is switched on, the signal remains in the given statements? upward position, whereas when it is switched off, i ii (A) i ii (B) the signal remains in the downward position as heating element shown in the figure. chemical base (C)

i

ii

heating base

(D)

i

ii

chemical element

38. A doorbell can be constructed by using a coil, a steel gong, and a hammer.

Which part of the electromagnetic train signal is the electromagnet? When current is passed through the coil, the (A) A (B) B (C) C (D) D hammer strikes the gong because it becomes 35. Steve constructs an automatic door latch as shown (A) Heavy (B) Charged in the figure. When the switch is closed, the (C) Magnetized (D) Hot electromagnet attracts the door latch toward itself 39. The presence of a magnetic field created by an and the door can be opened. When the switch is electromagnet can be detected by a opened again, the door latch comes back to its (A) Bulb (B) Compass original position using the spring attached between (C) Coil (D) Battery the core of the electromagnet and the door latch. 40. How many poles does an electromagnet have? (A) One (B) Two (C) Three (D) Four 41. When current is passed through a wire wound around an iron rod, it produces (A) A gravitational field (B) An electric field (C) A magnetic field (D) A nuclear field 42. Four parts of an electric iron are labelled Steve can improve the structure of the automatic as I, II, III, and IV (as shown in the given figure). door latch by (A) Using a wooden or plastic door latch (B) Increasing the number of turns in the electromagnet (C) Using glass rod as the core of the electromagnet The element of the given electric iron is labelled as (D) Increasing the distance between the door knob (A) I (B) II (C) III (D) IV and the electromagnet www.betoppers.com

7th Class Physics

64 43. The element of a heater is (A) Attributed to produce light (B) A coil of low-melting-point wire (C) A coil of high-melting-point wire (D) Attributed to generate electricity 44. An electric fuse of 3 A rating is connected in a branch circuit. Suddenly, a current of amount 4 A flows through the circuit that consists of a bulb. Which of the following events could be a possibility in the given circuit? (A) The bulb will get fused (B) The fuse will melt and break the circuit (C) The fuse will absorb 1 A current and will pass 3 A current only (D) The bulb will continue to work normally getting a 4 A current 45. Electric fuses are (A) Safety devices (B) Heating devices (C) Switching devices (D) Controlling devices 46. Which of the following devices works on the principle of heating effect of current? (A) Switch (B) Battery (C) Electric fuse (D) Socket board indicator 47. While reading about the heating effect of current, Raju decides to list some home electrical devices that work on that principle. He lists some devices (as shown in the given table). His father notices that one device is listed incorrectly in the table. Electrical device: Electric kettle, Motor, Toaster, Geyser, Hair dryer Which device  is  listed incorrectly by  Raju  in  the table? (A) Motor (B) Toaster (C) Hair dryer (D) Electric kettle 48. If a current of 3 A flows through a wire of resistance 8 ohms, calculate the potential difference applied across its ends. 49. What would be the resistance of a conductor if the current flow through it is 0.35 A when the potential differenceacross is 1.4 V? 50. (a) How much current will an electric bulb draw from a 220 V source, if the resistance of the bulb filament is 1200  ? (b) How much current will an electric heater coil draw from a 220 V source, if the resistance of the heater is 100  ? 51. The potential difference between the terminals of an electric heater is 60 V when it draws a current of 4 A from the source. What current will the heater draw if the potential difference is increased to 120 V? www.betoppers.com

Conceptive Worksheet 16. If an iron nail is introduced in a current carrying coil, then it will lead to the formation of (A) A magnetic compass (B) A cell (C) A motor (D) An electromagnet 17. The needle of a magnetic compass gets deflected when placed near a current carrying wire because the current carrying wire produces (A) A gravitational field (B) A magnetic field (C) An electric field (D) A nuclear field 18. A pin is placed near a wire loop, as shown in the given figure. When current flows through the wire loop, the pin gets attracted toward the loop.

Which force attracts the pin toward the loop? (A) Electric (B) Magnetic (C) Frictional (D) Gravitational 19. In an electric bell, i energy is transformed into ii energy. Which row completes the given statement? (A) (B) i ii i ii light heat electrical sound (C)

i

ii

electrical sound

(D)

i

ii

mechanical light

20. Stella has to construct an electric bell for her school project. She has to make an electromagnet in order to complete the construction of the electric bell.

Which of the following materials should Stella use as the core of the electromagnet? (A) Glass (B) Iron (C) Plastic (D) Wood 21. An electric bell uses (A) A permanent magnet (B) A non-magnetic material (C) An electromagnet (D) A liquid magnet

Electric Current & its Effects 22. Electromagnets are used in electrical devices because they (A) Are cheap (B) Are strong (C) Have a long life (D) Can be switched on and off 23. Which of the following characteristics is associated with an ideal fuse element? (A) It should be cheap (B) It should have very low resistance (C) It should have a low melting point (D) It should show the magnetic effect of current 24. Consider the following statements about an electric bell: I. An electric bell works on the principle of magnetic effect of current II. An electric bell consists of an electromagnet III. An electric bell produces sound continuously Among the given statements, (A) Only I is correct (B) Only III is correct (C) Both I and II are correct (D) Both II and III are correct 25. The given figure shows a simple electric circuit that consists of a coil of wire wounded around an iron rod. Some iron filings are placed near one end of the rod. When the switch is closed, some iron filings get attracted towards the rod.

65 The part that moves when the switch is closed is the (A) Gong (B) Hammer (C) Iron bar (D) Wound coil 27. The given figure shows a labelled picture of an electric bell. When the switch is ON, one of the labelled parts of the bell becomes an electromagnet.

Which part of the given electric bell becomes an electromagnet? (A) I (B) II (C) III (D) IV 28. The principle of magnetic effect of current is incorporated in the working of an electric (A) Geyser (B) Toaster (C) Door bell (D) Hair dryer 29. When current is allowed to flow through a coil of wire, the coil (A) Starts vibrating (B) Generates electricity (C) Behaves as a magnet (D) Can repel a piece of iron 30. Cranes used in junk yards for picking scraps have electromagnets attached to their lower ends. Which of the following objects can be picked up using a crane? (A) Broken car (B) Wooden board (C) Old foam sofa (D) Ceramic plate

7. Electric Circuit The iron filings will get attracted towards the iron rod (A) For a very long time (B) As long as the current flows through the coil (C) Until the polarity of the battery is not changed (D) Until the sheet containing the iron filings is not removed 26. The internal construction of an electric bell is shown in the given figure. The switch is in OFF position. When it is switched ON, one part of the bell moves.

An electric cell or dry cell is the source of energy for the bulb to glow and warm up. Let us now learn the way in which this electric energy is made available to the bulb in the torch. Making of a Simple Electric Circuit Step 1: Take out the bulb from bulb from a torch. Examine the bulb carefully. The bulb is a small globe of thin glass enclosing a coiled filament supported on two thick wires. One of these thick wires is connected to the metal casing around the base of the bulb. The other wire is connected to the metal tip at the base. The metal casing and the metal tip at the base are the two terminals of the bulb.

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7th Class Physics

66 Closed and Open Circuit

(a) Inside a torch bulb

Step 2: Take two pieces of insulated wire. Insulated wires have metal wire inside with a plastic covering on the outside. Remove the plastic covering from both the ends of each piece of wire. Fix these wires on the bulb as shown in the figure with the help of Insulating adhesive tape. Or fix the bulb on a bulb holder. The two screws on the bulb holder are the two terminals which are connected to the two terminals on the bulb. The two pieces of wire be connected to the two terminals on the holder, as shown in the picture.

Bulb connected with wires

Step 3: Connect the two free ends of the wires from the bulb or the bulb holder to an electric cell in such a way that one piece of wire is connected to the positive terminals of the cell and the other to the negative terminal of the cell. This may be done with the help of a rubber band or an adhesive tape. When you have finished with connections, the bulb lights up. With your finger trace the path of the electricity from the positive ( + ve) terminal on the cell to the negative (–ve) terminal of the cell. It is a round about path travelled by electricity.

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(b)

The dry cell has two terminals. The central terminal of the dry cell is called positive terminal. The base of the dry cell (which is made of a metal) is called negative terminal. The above figure (a) shows the terminals of dry cell. The figure (b) shows the symbol for dry cell. The long line represents positive terminal of the cell and the small and thick line represents negative terminal of the cell. For this experiment you need a torch cell; a torch bulb marked 1.5 V, cellotape, a plastic coated 1 metre long copper wire and an old used blade. Cut the plastic coated copper wire into two halves A and B. Remove plastic coating from each end of the wire such that 1 cm of plastic is removed. Now fix one bare end of each wire A and B to the terminals of 1.5 V bulb with the cellotape. Fix the other end of wire A to the base of cell with the help of cellotape. Now touch the bare end of wire B to the central terminal of cell as shown in figure. What do you observe? The bulb lights up. This shows that electric current is flowing in wire A and B through the bulb. The path along which electric current flows is called electric circuit. Now remove the wire B from the central terminal as shown in Fig. . What do you observe? The bulb does not glow. It is because electric current does not flow, if the path is broken or path is incomplete.

Closed Circuit or Complete Circuit

Electric Current & its Effects

67

When the path which starts from one terminal of the cell, ends at the other terminal of the cell, without any break, then such a circuit is called complete circuit or closed circuit. When the circuit is closed, then any electric appliance in that circuit starts working. In the present case the bulb starts glowing. Open Circuit or Incomplete Circuit When the path of current, starting from one terminal of the cell to another terminal of the cell is broken or incomplete, then such a circuit is called open

circuit or incomplete circuit. For example, when we remove wire B from central terminal of cell, then the circuit is open circuit or incomplete circuit. Switches are used in the household wiring^ to open or close the electric circuit. When we switch on a particular electric appliance, we close the electric circuit. Conversely, when we switch off an electric appliance, we open the electric circuit.

8. Symbols used in Electrical Circuits COMPO NENTS

SYMBOLS

An electric cell A battery or a combination of cells Plug key or switch (open) Plug key or switch (closed) A wire joint Wires crossing without joining Electric bulb A resistor of resistance R

Variable resistance or rheostat Ammeter Voltmeter A switch, a simple device to ‘close’ or ‘open’ a circuit: An electric circuit passes through a switch. Switch is a simple device which helps us to close or open the circuit. It helps in s a v i n g electricity when not in use. You are always advised to switch ‘off the lights or other gadgets in your home to save electricity. Ammeter is used to measure the current and voltmeter is used to measure the potential difference. More details of ammeter and voltmeter shall be learnt in higher classes.

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7th Class Physics

68

9. Connecting Electric Cells in Series Take a dry cell and a torch bulb. Connect the bulb to the cell using copper wires as shown in Fig. . Observe the intensity- of light. The bulb does not glow brightly.

Now take one more dry-cell and connect two cells as shown in figure. In this method the positive of the first cell is connected to the negative of the second. The negative of the first and the positive of the second are connected to the bulb. The bulb now glows brighter. In the battery torch or battery light two or three dry cells are put into a metal container in series. The positive of one cell is connected to the negative pole of another cell in the series connection, When the, switch is turned on, the circuit is closed and the bulb glows and gives light. Connecting Electric Cells in Parallel Take three dry cells and connect them as shown in fig.. That is all the positive poles of the three cells are connected together, and all the three negative poles are connected together.

These three positives and three negatives are connected to the bulb- You will observe that there is no change in the brightness of the bulb! When cells are connected in parallel, their total electromotive force is the same as that if any one of them. When cells are connected in series, their electromotive force is equal to the sum of the EMF of all the cells used. Connect three torch bulbs in series as shown in figure. Connect this to a dry cell and observe that brightness of each of the three bulbs. Now connect one more dry cell in series with he first cell. Observe the brightness of each of the bulb. Then connect one more dry cell in series with the first two cells. Again observe the bulbs.

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Disconnect one of the three bulbs in the circuit. The circuit becomes open and all the three bulbs stop glowing. In series connection of bulbs,’ if one bulb gets fused, all the other bulbs in the series will stop working. Three bulbs connected in Series

10. Connecting Bulbs in Parallel Connect three bulbs in parallel. That is, one end of each of the three bulbs are connected one wire, the other ends of the three bulbs are con n e c t e d to another wire! These two wires are connected to a dry cell. All the three bulbs glow dimly. Now disconnect one of the bulbs. The other bulbs continue to glow as before.

To study the properties of (i) Series circuit, (ii) Parallel circuit Materials required : a battery of four cells two bulbs of 1 watt each one fused bulb a switch few lengths of connecting wires cellotape.

Method: Connect the bulbs A and B in series by connecting them to connecting wires with the help of cellotape as shown above. Connect the free ends of connecting wires to a battery through a switch. Close the switch. What do you observe? Both the bulbs will glow. However, they will not glow very brightly. Open the switch. What do you observe?

Both the bulbs will stop glowing. Now remove the bulb B and instead fix a fused bulb C as in the above figure. What is your observation? Bulb A does not glow.

Electric Current & its Effects

1.

2. 3.

69

Following are the conclusions from the above rubber. Coating of a conductor with a non-conductor investigation. is called insulation. If we happen to touch a metallic end of a lead In series circuit all the appliances work through which current is passing, it gives an electric simultaneously when switch is closed. Conversely, ‘SHOCK’. The shock may be fatal too or otherwise all appliances stop working when switch is open. it shakes the body and harms the person who has In series circuit, if any, of the appliances goes out suffered the electric shock. Insulation saves a of order, the other appliances stop working. person from electric shock. As the bulbs were not glowing very brightly, it can be concluded that in series the appliances do not work to their full capacity. ormative orksheet Now connect the bulbs A and B in parallel, such 52. If a current of 10 mA passed through your cell phone that they have common positive and common battery charger wire for 10 minute , what quantity negative terminals as illustrated by Fig. , through a of electric charge is transferred through the wire to the battery? switch and a battery. 53. How much current does your laptop consume if Close the switch. What do you observe? Both the 100 C of charge is transferred through the charger bulbs A and B glow very brightly. Now remove the wire in 4 second ? bulb B and instead fix a fused bulb C Fig. . What is 54. How much time is required for 10 Coulombs of your observation? charge to flow past a point if the current is 2 amperes? 55. Which of the following materials is a good conductor of electricity? (A) Plastic (B) Cork (C) Soil (D) Iron 56. The handle of every electrical repairing tool is covered by a certain material to prevent the user from getting an electrical shock. Which of the following materials cannot be used to cover the handle of an electrical repairing tool? The bulb A continues glowing brightly, whereas bulb (A) Tin (B) Glass (C) Wood (D) Rubber C does not glow. Following conclusions can be 57. Mason constructs the circuit shown in the figure. drawn from above investigation. He leaves a gap in the circuit to test the conductivity In parallel circuit all the appliances work of various materials. independently In parallel circuit if one appliance goes out of order, the other continues working. It means that each appliance in parallel circuit can be operated independently by a switch. As the bulbs glow brightly, it means each appliance gets enough electric energy, and hence, works to On inserting which of the following materials in the its full capacity. gap will the bulb not light up? Conductors and Insulators (A) Wood (B) Iron (C) Aluminium (D) Silver The materials which allow the electric current to 58. Which of the following objects is an insulator? pass through them are the conductors of electricity (A) Iron rod (B) Plastic cup and the materials through which electric current (C) Nickel coin (D) Steel spoon does not pass are the non-conductors or the bad 59. Martin is constructing an electrical circuit. He conductors of electricity. Metals are the conductors notices that all electrical metal wires are covered with plastic. of electricity. Non metals like glass, plastic, wood, Electrical metal wires are covered with plastic paper, cloth and rubber are the non-conductors of because plastics are good electricity. (A) Electrical conductors Non-conductors of electricity are also called (B) Electrical insulators insulators. All leads (wires) being used in an electric (C) Heat generators circuit are metallic wires coated with plastic or (D) Heat absorbers www.betoppers.com

F

1. 2.

3.

11.

W

7th Class Physics

70 60. Tommy sets up an electrical circuit as shown in the figure. He connects points P and Q with different materials.

The bulb will light up when points P and Q are connected by a plate made of (A) Iron (B) Glass (C) Wood (D) Plastic 61. Brad sets up an electrical circuit as shown in the given figure. He connects points P and Q of the circuit with four different rods made of four different materials - glass, wood, copper, and asbestos.

67. The given figure shows four wires, a switch, a bulb, and a battery.

The bulb will glow when the given components are connected as (A) (B)

(C)

62.

63.

64. 65.

66.

(D)

The bulb in the circuit will not light up when points P and Q are connected with (A) Glass rod (B) Wood rod (C) Copper rod (D) Asbestos rod Ronald wants to make electrical wires with a wire 68. The given figure shows a simple circuit involving a battery, a switch, and a bulb. drawing machine. He considers using steel, copper, aluminum, and plastic as raw material. The material that Ronald cannot use to make electrical wires is (A) Steel (B) Copper (C) Plastic (D) Aluminum Margaret notices that there are ceramic pulley-like structures attached to the overhead electrical lines If the terminals of the battery are reversed, then near her house. Electrical wires pass over the the bulb will pulleys as shown in the given figure. (A) Glow These pulleys are used as electrical (B) Not glow (A) Insulators (B) Generators (C) Glow for a short time (C) Conductors (D) Transformers (D) Glow after some time Which of the following materials cannot be used as 69. A simple electrical circuit containing two bulbs and an electrical insulator? (A) Wood (B) Rubber (C) Plastic (D) Graphite a battery is shown in the given figure. John is repairing the electrical line of his house. As a precautionary measure, he stands on a wooden plank. John uses the wooden plank because wood is (A) An electrical conductor (B) An electrical insulator (C) A heat conductor If bulb 1 fuses on closing the switch, then bulb 2 (D) A heat absorber will The given figure shows a simple circuit consisting (A) Keep glowing (B) Stop glowing of a bulb and a switch. (C) Glow brightly (D) Glow dimly 70. The symbol for a fuse is (A)

On closing the switch, the bulb will (A) Glow (B) Not glow (C) Glow after some time (D) Glow only for a short time www.betoppers.com

(C)

(B) A

(D)

71. The symbol for a ammeter is (A) (C)

A

(B)

V

(D)

()

Electric Current & its Effects

71 What is the minimum number of switches that must be closed to light at least one bulb? (A) 1 (B) 2 (C) 3 (D) 4 32. The handle of every electrical repairing tool is covered by a certain material to prevent the user from getting an electrical shock. Which of the following materials cannot be used to cover the handle of an electrical repairing tool? (A) Tin (B) Glass (C) Wood (D) Rubber 33. The given figure shows a simple circuit that consists of a bulb, a battery, and two switches.

72. In a parallel circuit of bulbs (A) Same current exists in all the bulbs (B) Same voltage exists in all the bulbs (C) Failure of any bulb leads to a break down in the circuit (D) All of above 73. Column-I Column-II a) Cell 1) V b) Resistance 2) c) Closed switch 3) d) Voltmeter 4) ( ) 5) () 74. When negative terminal of a cell is connected to the positive terminal of the next cell are said to be in (A) Series (B) Parallel (C) Both (A) and (B) (D) Neither (A) nor (B) In the given circuit, when switch I is closed, the bulb 75. When electric cells are connected in series the will electromotive force (A) Glow (B) Flicker (A) Increase (B) Decreases (C) Not glow (D) Flicker after some time (C) Remain same (D) Become zero 34. Which of the following materials is a good 76. Choose the correct option conductor of electricity? (A) The path along which electric current flows is (A) Plastic (B) Cork (C) Soil (D) Iron called electric circuit 35. Robert constructs an electrical circuit as shown in (B) Coating of a conductor with a non-conductor the given figure. Four different points are labeled is called insulator as P, Q,R, and S on the wooden rod. (C) The materials which allow the electric current to pass through them are called conductors (D) The materials which allow the electric current to pass through them are called nonconductors 77. If a voltage V is applied across the bulbs in series ,then (A) The voltage applied is divided among the bulbs In the given circuit, the bulb will glow when the (B) The same current exists in all the bulbs positive terminal of the battery is connected to the (C) The same voltage exists in all the bulbs point (D) The current is divided among the bulbs (A) P (B) Q (C) R (D) S 78. Greater potential difference (or emf ) is obtained in 36. In which of the following circuits can the electrical the circuit when cells are connected in bulb be operated using the switch in the circuit? (A) Series (B) Parallel (A) (B) (C) Both (A) and (B) (D) Neither (A) nor (B)

Conceptive Worksheet 31. The given figure shows a simple circuit containing four bulbs and five switches along with a battery. (C)

(D)

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7th Class Physics

72 37. The circuit shown in the given figure consists of a fan, a switch, and a battery. Andrew wants to connect a bulb in parallel to the fan.

Andrew should connect the bulb in the electrical circuit as shown in figure (A) (B)

40. The given figure shows two bulbs, a switch, a battery, and a fan. Using all the components, David makes a simple electrical circuit putting a bulb on either sides of the fan.

Which of the following circuit diagrams represents the one made by David? (A) (B)

(C)

(C)

(D)

38. Alex constructs the electrical circuit shown in the given figure in his school laboratory. His teacher asks him to add a switch to the circuit such that only bulbs N and P are operated by it.

Alex should add this switch at point (A) I (B) II (C) III (D) IV 39. The given figure shows a simple electrical circuit that consists of two bulbs, and a battery. One switch has to be added to the circuit in order to operate only bulb M.

This switch must be placed at point (A) I (B) II (C) III (D) IV

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(D)

41. The given figure shows two bulbs, a switch, and a battery. Martha makes a simple circuit using all the given components. The circuit is such that a bulb is placed on either sides of the switch.

Which of the following circuit diagrams represents the one made by Martha? (A) (B)

(C)

(D)

42. An electric circuit made by a student is incomplete because one of the wires is too short to connect with the other wire as shown in the given figure. In order to glow the bulb, he has to connect the open ends by inserting an object between them.

Which of the following objects should the student insert between the open ends? (A) Nail (B) Bottle (C) Glass sheet (D) Rubber band

Electric Current & its Effects

73

43. Which of the following objects can conduct electricity? (A) (B)

(C)

(D)

44. The given figure represents four circuit arrangements, I., II, III,  and IV.  Each  circuit consists of an electric cell and a torch bulb.

49. What is the total emf , when three cells of emf’s are 2V,2.5V, 4V are connected in series (A) 4V (B) 2.5V (C) 2V (D) 8.5V 50. What is the total emf, when three cells of emfs are 2V, 2V, 2V are connected in parallel (A) 4V (B) 1V (C) 2V (D) 5V 51. The total emf of three cells of emf’s 1v,1v,1v are connected in series is ____v 52. Which of the following is used as source of electrical energy (A) Electric cell (B) Dry cell (C) Ammeter (D) Voltameter

Summative Worksheet 1.

Electric current flows from ___ terminal to ___ terminal outside the cell through the circuit.

2. 3.

represents. In which case, bulb glows in the circuit. (A)

The bulb  may  glow  in  the  circuit  arrangements labelled as (A) I and II (B) II and III (C) III and IV (D) IV and I 45. An electric cell has i terminal(s). The metal cap of an electric cell represents its ii terminal. The information in which alternative completes the given statement? (A) i (B) i ii ii (C)

two

negative

i

ii

one

negative

(D)

two

positive

i

ii

one

positive

46. The path along which electric current flow is called (A) Electric circuit (B) Electric shock (C) Both (A) and (B) (D) Neither (A) nor (B) 47. Which of the following are the non-conductors of electricity (A) Wood (B) Paper (C) Cloth (D) Rubber 48. Statement I : Switch is used to close are open electric circuit Statement II : When switch is closed , then circuit is closed and when it is opened, then circuit is open circuit (A) Both Statements are true, Statement - II is the correct explanation of Statement- I (B) Both Statements are true, Statement - II is not correct explanation of Statement - I (C) Statement - I is true, Statement - II is false (D) Statement - I is false, Statement - II is true

(B)

(C)

(D)

3. 4. 5. 6.

7. 8. 9.

(A) A (B) B (C) C (D) D Is pure water a conductor of electricity ? Electric current flows in ___ direction only. A cell has two terminals, one terminal is on the metal base, second terminal is on ___. Bulb in this circuit doesn’t glow; why?

A device that is used to break an electric circuit is called ? An electric cell has ___ terminals. Unit of electric current is ___. www.betoppers.com

74 10. Match the following (i) Battery (a) Rechargeable Batteries (ii) Accumulator (b) A combination of two or more cells (iii) Closed Circuit (c) An unbroken path of electricity (d) The path along which electricity travels (A) i - b, ii - c, iii - d (B) i - a, ii - b, iii - c (C) i - b, ii - a, iii - c (D) i - c, ii - d, iii - a 11. (i) Button Cells (a) Torch light, Clocks (ii) Dry Cells (b) Wrist watches, Calculators (iii) Accumulators (c) In Cars, Trucks (d) Space Satellites (A) i - b, ii - a, iii - c (B) i - b, ii - a, iii - d (C) i - c, ii - d, iii - b (D) i - a, ii - b, iii - a 12. Draw in your notebook the symbols to represent the following components of electrical circuits: connecting wires, switch in the ‘OFF’ position, bulb, cell, switch in the ‘ON’ position, and battery. 13. Draw the circuit diagram to represent the circuit shown in figure.

14. Figure shows four cells fixed on a board. Draw lines to indicate how you will connect their terminals with wires to make a battery of four cells.

15. The bulb in the circuit shown in Figure does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.

16. Name any two effects of electric current. 17. When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain. www.betoppers.com

7th Class Physics 18. Will the compass needle show deflection when the switch in the circuit shown by figure is closed?

19. Fill in the blanks: (a) Longer line in the symbol for cell represents its_________terminal. (b) The combination of two or more cells is called a _________. (c) When current is switched ‘on’ in a room heater, it __________. (d) The safety device based on the heating effect of electric current is called a _________. 20. Mark ‘T’ if the statement is true and ‘F’ if it is false: (a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (T/F) (b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F) (c) An electromagnet does not attract a piece of iron. (T/F) (d) An electric bell has an electromagnet. (T/F) 21. Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain. 22. An electrician is carrying out some repairs in your house. He wants to replace a fuse by a piece of wire. Would you agree? Give reasons for your response. 23. Zubeda made an electric circuit using a cell holder shown in Fig. 14.4, a switch and a bulb. When she put the switch in the ‘ON’ position, the bulb did not glow. Help Zubeda in identifying the possible defects in the circuit.

Electric Current & its Effects

75

24. In the circuit shown in figure

. (i)

Would any of the bulbs glow when the switch is in the ‘OFF’ position? (ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?

HOTS Worksheet 1.

It is impossible to construct a torch in the absence of 5. (A) Plastic casing (B) Electric cells (C) Reflector (D) Switch

2.

Among the labelled parts, current does not flow through the

3.

(A) Battery

(B) Switch

(C) Filament

(D) Reflector

The given figure shows a circuit with a cell connected to a bulb and a switch.

When the switch is closed, the bulb lights up because the (A) Switch produces electricity (B) Switch produces electricity (C) Connecting wires produce electricity 4.

(D) Circuit completes on closing the switch Atul has constructed four electric circuits, each consisting of a bell, a bulb, switch and a cell. The bells of the circuits are labelled as 1, 2, 3, and 4, as shown in the given figure.

6.

Which bell will produce a sound when the switch is closed? (A) Bell 1 (B) Bell 2 (C) Bell 3 (D) Bell 4 Suraj has constructed a simple electric circuit. It consists of a bulb, a switch, and a cell, as shown in the given figure. However, the lengths of the wires are not sufficient. Hence, this circuit has two free ends. He has four objects namely a wooden gilli, a match stick, a pen cap, and a nail.

To glow the bulb, Suraj should connect the free ends with the (A) Nail (B) Matchstick (C) Pen cap (D) Wooden gilli Raju has constructed four different circuits with four bulbs, four cells, and four different objects, as shown in the given figure.

Which bulb will glow in the given circuits? (A) Bulb 1 (B) Bulb 2 (C) Bulb 3 (D) Bulb 4

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7th Class Physics

76 7.

8.

Raju bought five similar bulbs and connected them 10. Various parts on the outer surface of an electric with two cells to construct an electric circuit. The cell are labelled as I, II, III, and IV respectively in given figure shows the circuit constructed by Raju. figure (a). In figure (b), an incomplete circuit is shown. M and N are the free ends of this circuit.

In the given circuit, the bulbs that would not glow are (A) Bulb 1 and Bulb 2 (B) Bulb 2 and Bulb 3 (C) Bulb 3 and Bulb 4 (D) Bulb 4 and Bulb 5 The given figure shows two circuits, each consisting of a bulb and a cell. M and N are the terminals of cell I and S and T are the terminals of cell II. The direction of current in the respective circuits is indicated by arrowheads.

To make the bulb glow, free ends M and N of the circuit should be respectively connected to (A) I and II (B) II and III (C) III and IV (D) IV and I 11. Which of the following parts of a torch is not paired with its function? (A) Part of a torch Bulb

(B) Part of a torch Switch

9.

Terminals M and T of the cells are (A) Both positive (B) Both negative (C) Positive and negative respectively (D) Negative and positive respectively A circuit is constructed using two torch bulbs and a cell, as shown in the given figure at the left. The figure at the right shows four arrows whose tips show their respective directions. These arrows have to be placed on each wire segments RS, RV, ST, and TV respectively, according to the directions of current.

The given arrows that can be placed on wire segments RV, VT, ST, and RS are respectively (A) , , , and  (B) , , , and  (C) , , , and  (D) , , , and 

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(C) Part of a torch Reflector

(D) Part of a torch Cell

Funct ion Emits light Funct ion Breaks the circuit Function Completes the circuit Function Provides energy

12. Raj performs an experiment to know the conducting nature of some of the objects using a circuit, as shown in the given figure. He inserts some objects one by one between M and N and observes whether the bulb glows or not. The objects that he inserted are eraser, key, plastic scale, glass bangle, bottle cap, nail, thermocol sheet, and wallet.

Raj will be able to glow the bulb if he inserts (A) Key and nail respectively (B) Key, nail, and wallet respectively (C) Bottle cap and thermocol sheet respectively (D) Eraser, scale, bangle, and bottle cap respectively

Electric Current & its Effects

77

13. The schematic diagram of an open circuit is shown 2. in the given figure.

3.

Which of the following objects is inserted tightly in 4. the gap to make the bulb glow? (A) Key (B) Cork (C) Plastic bottle cap (D) Thermocol sheet 14. The picture of a household torch is shown is the 5. given figure. 6. The electricity required to operate a simple torch is 7. provided by the (A) Bulb (B) Electric cell (C) Switch (D) Curved mirror 8. 15. Different arrangements of an electric bulb with an electric cell are shown in the given figure. The arrangements are labelled as M, N, S, and T 9. respectively.

10.

11.

The bulb will glow in (A) Arrangement N only (B) Arrangement T only (C) Both arrangements M and N (D) Both arrangements S and T

12.

13.

IIT JEE Worksheet I.

Single Correct Answer Type

1.

The metallic wires used for connections in an electric circuit is called. (A) Leads (B) Cells (C) Batteries (D) Insulators

14.

15.

Most handy source of electricity which is used in torch lights are. (A) Dry Cells (B) Button Cells (C) Batteries (D) None In wrist watches ___ are used. (A) Dry Cell (B) Button Cell (C) Battery (D) None Metals are ___ of electricity. (A) Conductors (B) Non Conductors (C) Both (D) None represents. (A) Electric Switch (B) Battery (C) Cell (D) Fuse Electricity name was given by ___. (A) Thales (B) Gilbert (C) Newton (D) Coulomb Combination of two or more cells is called ___. (A) Battery (B) Accumulators (C) Button Cells (D) None ___ is a non-metal but is a Conductors of electricity (A) Graphite (B) Glass (C) Mercury (D) Water Electric current flows from ___ terminal to ___ terminal outside the cell through a circuit. (A) Positive, Negative (B) Negative, Positive (C) Both (D) Cannot say An electric cell converts ___ energy into electrical energy. (A) Chemical (B) Mechanical (C) Wind (D) Light Which of the following energy conversions take place in a torch. (A) Electrical Chemical Light (B) Chemical Electrical Light (C) Electrical Light Chemical (D) Light Chemical Electrical Which of the following is an insulator? (A) Pure Water (B) Impure Water (C) Human Body (D) Earth Car battery and mobile phone battery are rechargeable. Hence these are called ___. (A) Dry Cells (B) Accumulators (C) Button Cells (D) None There are two types of charges, they are ___ and ___.s (A) Positive & Neutral (B) Negative & Neutral (C) Positive & Negative (D) None S.I unit of Charge is (A) Ampere (B) Volt (C) Newton (D) Coulomb www.betoppers.com

7th Class Physics

78 16. The shorter, thicker vertical line in the symbol of Cell represents. (A) The positive terminal (B) The negative terminal (C) The direction of current (D) All of these 17. Over head electric cables passing through poles are NOT insulated because. (A) Insulation will cause energy loss (B) Air is a bad conductor (C) The wires will get heated (D) It is costly 18. Photovoltaic cells produce electricity by using. (A) Wind Energy (B) Solar Energy (C) Geothermal Energy (D) Mechanical Energy of Water 19. What happens to a circuit when the switch is off. (A) The circuit is complete (B) There is a gap in the circuit (C) Electricity flows continuously (D) Electricity flows discontinuously

Using the above answer the following questions. 23. If there is a current of 10 amperes in a circuit for 10 minutes, what quantity of electric charge flows in through the circuit? 24. How much current must there be in a circuit if 100 coulombs flow past a point in the circuit in 4 seconds? 25. How much time is required for 10 coulombs of charge to flow past a point if the rate of flow (current) is 2 amperes?

II. Multiple Correct Answer Type

V. Matrix Matching

30. (A) Solar panels (B) Cells

III. Paragraph Type

(C) Windmills

The rate of flow of charge in a circuit is called electric current. In other words, it is the amount of charge flowing per second. It is denoted by the letter I. If Q is the charge which is flowing through a

Q t

Unit of current: The S.I unit of current is ampere and it is denoted by the letter ‘A’. The S.I unit of Q is coulomb and that of t is second. Thus, the S.I unit of electric current is

1 coulomb  1A 1 second

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26. The current passing through a conductor is 5 ampere .Then the charge that passes through that conductor in 5 minute is _______Coulomb 27. If 60 coulomb of charge passes through a cross section of a conductor in 4 sec, the average current is_________A 28. A light ray is incident on a plane mirror making an angle of 450 with respect to the surface of the mirror. the value of the angle between incident ray and reflected ray is ________° 29. (A) Conductor (B) Insulator (C) Outside the cell through circuit

20. The symbol for switch (A) (B) (C) (D) () A 21. Which of the following are conductors? (A) Silver (B) Copper (C) Aluminium (D) Glass 22. Which of the following are insulators? (A) Glass (B) Plastic (C) Wood (D) Silver

conductor in time t, then current is given by i 

IV. Integer Type

(D) Dams

(p) Wood (q) Graphite (r) Electric current flows from positive to negative (s) Electric current flows from negative to positive (p) Wind energy into electrical energy (q) Heat energy into electrical energy (r) Chemical energy into electrical energy (s) Solar energy into electrical energy (t) Potential energy of water into electrical energy

Electric Current & its Effects

79

31. (A) A path along which electric current flows (p) Switch (B) A device which converts chemical energy into (q) Electric circuit electric energy (C) A device which makes or breaks the circuit (r) Cell (s) Electric current 32. (A) Cell

(p)

(B) Bulb

(q)

(C) Switch

(r) (s)

33. Match the following. Column 1

Column 2

A.

P.

Wire joint

B.

Q.

Plug key

C.

R.

A battery

D.

S.

Electric Cell

34. Match the following. Column 1

Column 2

A.

P.

Resistor

B.

Q.

Rheostat

C.

R.

Wire Cross without joining

D.

S.

Ammeter



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80

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7th Class Physics

Wind, Storm and Cyclones

By the end of this chapter, you will understand    

Air Pressure Winds Thunderstorms and Cyclones Tornado

 Destruction Caused by Cyclones  Meteorological Department  Effective Safety measures

1. Introduction Let’s start the chapter with a tragic article. “Orissa was hit by a cyclone with wind speed of 200 km/h on 18 October 1999. The cyclone smashed 45,000 houses making 7,00,000 people homeless. On 29 October the same year, a second cyclone with wind speed of 260 km/h hit Orissa again. It was accompanied by water waves about 9 m high. Thousands of people lost their lives. Property worthcrores of rupees was destroyed. The cyclone affected agriculture, transport,communication, and electricity supply.” In this article, apart from the tragedy, there is something on which we have to ponder upon... 1. What are cyclones? 2. How are they formed? 3. Why are they so destructive? In this chapter we shall seek answers to some of these questions. Let’s begin by understanding some basic concepts.

Chapter - 35

Learning Outcomes

opposes the motion of a moving object. This is called air resistance. That is why you have to exert yourself and pedal hard when you ride a bicycle. Another example in which air pressure can be observed: Take a glass and fill only 1/3 of it with water. Cover the mouth of the glass with an index card. Now hold the card in place and invert the glass over a sink and remove your hand from the card. The card sticks to the glass. This is due to air pressure. The air outside exerts an upward pressure on the index card. This air pressure is more than the weight of the water in the glass.

3. Winds Winds are caused by variations in air pressure. A wind blows from a region of high pressure to a region of low pressure. The speed of the wind mainly depends on the difference between the pressures of the air in the two regions.

2. Air Pressure The pressure exerted by air on all bodies at all times in all directions is called air pressure.

High speed winds are known to blow away thatched and tiled roofs in rural and semi-urban areas. Strong winds can uproot trees and electric poles, and even snap cables. Anemometer When air moves at high speeds, it creates a low pressure area. The air inside a balloon exerts pressure in all directions, and makes it blow up. Air

The instrument used to measure the speed of wind is called the anemometer. It is usually fixed on the top of a building.

7th Class Physics

82 I.

Effect of Heat on Air

What did you expect? What happens?

Air expands on heating, and the expanded air pushes for more space. Let’s observe few examples to understand the same. Example-1 1.

Take two deep pans - one filled with hot water and the other with cold water. To prove that air expands when heated,



Stretch a balloon across the mouth of a milk bottle and seal it with tape.



Keep this bottle in the hot water pan



The balloon is slowly inflated. This is because the heat from the hot water in the pan heats up the air inside the bottle, which makes the air inside the bottle expand.

Activity-3 1. Hold a strip of paper, 20 cm long and 3 cm wide, between your thumb and forefinger. 2. Now blow over the paper. What do you think will happen to the paper? Will the paper rises up or moves down.

Now place the bottle in the pan with cold water. The balloon deflates and shrinks. This is because the air inside the bottle gets cooled and so contracts. Example-2 Observe a pressure cooker where the steam comes out of the nozzle and escapes upwards. This is because steam escaping from a pressure cooker is lighter than the relative cool air surrounding it. Thus, hot air rises. II. Speed of Wind Vs Air Pressure Let’s understand with the help of following activities. Activity-1: 1. Crumple a small piece of paper into a ball of size smaller than the mouth of an empty bottle. 2. Hold the empty bottle on its side and place the paper ball just inside its mouth. 3. Now try to blow on the ball to force it into the bottle. 4. Try if you can force the paper ball in by blowing into the bottle. You will be surprised to find that it is difficult to force the paper ball. Why is it difficult to force the paper ball into the bottle? Activity-2: 1. Take two balloons of approximately equal size. 2. Put a little water into the balloons. 3. Blow up both the balloons and tie each one to a string. 4. Hang the balloons 8–10 cm apart on a cycle spoke or a stick. 5. Blow in the space between the balloons.

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Understanding the activities 1. In Activity-1, when we blow into the mouth of the bottle, the air near the mouth has higher speed. This decreases the pressure there. The air pressure inside the bottle is higher than near the mouth. The air inside the bottle pushes the ball out. 2. In Activity-2, we observe that when we blew between the balloons, they moved towards each other. How could this happen? This could happen if the pressure of air between the balloons were somehow reduced. The pressure outside the balloons would then push them towards each other. 3. In Activity-3, we observe that when we blew over the paper strip, it goes upwards. Again, this could happen if blowing over the paper reduced the air pressure above the strip. We see that the increased wind speed is, indeed, accompanied by a reduced air pressure. III. Circulation of Wind The equatorial and tropical regions get hotter than the polar regions. The warm air at the equator rises, and the cold air moves in from the polar regions. The air moves due to uneven heating of the earth’s surface between the equator and poles, which results in circulation of wind across the globe.

Wind Storm and Cyclones IV. Direction of Wind Winds are also formed due to uneven heating of land and water in coastal areas. As the earth rotates on its axis from West to East, these wind currents are not exact. There is a difference in the temperatures of the air over sea and over land. Cold winds from the South Pole move north towards the equator, and whereas hot winds from the North Pole move south towards the equator.

83 Reason (R): This is because the heat from the hot water in the pan heats up the air inside the bottle, which makes the air inside the bottle expand.

(A) Both A and R are TRUE, R is the correct explanation of A. (B) Both A and R are TRUE, R is NOT the correct explanation of A. (C) A is TRUE and Statement-R is FALSE (D) A is FALSE and R is TRUE. 5. The density of hot air in relation with cold air. IV. Monsoon Winds (A) Heavier (B) Lighter The word monsoon is derived from the Arabic word (C) Same (D) None “mausam,” which means season. The winds from the ocean carry water in the form of vapours, 6. Statement-I: In a pressure cooker where the steam comes out of the nozzle and escapes upwards. resulting rains over land. These are called monsoon winds. Statement-II: This is because steam escaping from a pressure cooker is lighter than the relative cool ormative orksheet air surrounding it. Thus, hot air rises. 1. Statement-I: Air opposes the motion of a moving (A) Statement I is correct, Statement II is incorrect. object. (B) Statement II is correct, Statement I is incorrect. Statement-II: This is called air resistance. (C) Both statements are correct. (A) Statement I is correct, Statement II is incorrect. (D) Both statements are incorrect. (B) Statement II is correct, Statement I is incorrect. 7. Circulation of wind is due to (C) Both statements are correct. (A) The warm air at the equator rises (D) Both statements are incorrect. (B) Cold air moves in from the polar regions (C) Uneven heating of the earth’s surface between 2. Assertion (A): We have to exert yourself and pedal the equator and poles hard when you ride a bicycle. (D) All the above Reason (R): This is to overcome air resistance. 8. Winds are ____ at South pole. (A) Both A and R are TRUE, R is the correct (A) Cold (B) Hot (C) Very hot (D) None explanation of A. 9. The word monsoon is derived from the Arabic word (B) Both A and R are TRUE, R is NOT the correct (A) Meesam (B) Mensoon explanation of A. (C) Mausam (D) None 10. Take some water in a tin can with a lid. Heat the (C) A is TRUE and Statement-R is FALSE can till the water boils. Immediately put the lid tightly (D) A is FALSE and R is TRUE. on the can. Put the can carefully in a shallow metallic 3. Statement-I: Winds are caused due to same air vessel or a washbasin. Pour fresh water over the pressure. can.What happens to the shape of the can? Statement-II: Wind blows from a region of high (A) Expands (B) Compressed pressure to a region of low pressure. (C) No changes (D) None (A) Statement I is correct, Statement II is incorrect. 11. Statement-I: When you fly a kite, the wind coming (B) Statement II is correct, Statement I is incorrect. from your back help. (C) Both statements are correct. Statement-II: If you are in a boat, it is it easier to (D) Both statements are incorrect. row it if there is wind coming opposite to us. (A) Statement I is correct, Statement II is incorrect. 4. Assertion (A): Stretch a balloon across the mouth (B) Statement II is correct, Statement I is incorrect. of a milk bottle and seal it with tape. Keep this (C) Both statements are correct. bottle in the hot water pan. The balloon is slowly (D) Both statements are incorrect. inflated.

F

W

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7th Class Physics

84

Conceptive Worksheet 1.

2.

3.

4.

5.

6.

7.

8.

9.

When air moves at high speeds, it creates a ____ pressure area. (A) High (B) Low (C) Same (D) None The air inside a balloon exerts pressure in ____ direction. (A) Upward (B) Downward (C) Sideward (D) All The instrument used to measure the speed of wind is (A) Thermometer (B) Barometer (C) Anemometer (D) Manometer What happens to air upon heating? (A) Expands (B) Contracts (C) No change (D) Expands, then contracts What happens to air upon cooling? (A) Expands (B) Contracts (C) No change (D) Expands, then contracts Assertion (A): Stretch a balloon across the mouth of a milk bottle and seal it with tape. Now place the bottle in the pan with cold water. The balloon deflates and shrinks. Reason (R): This is because the air inside the bottle gets cooled and so contracts. (A) Both A and R are TRUE, R is the correct explanation of A. (B) Both A and R are TRUE, R is NOT the correct explanation of A. (C) A is TRUE and Statement-R is FALSE (D) A is FALSE and R is TRUE. Which regions are relatively hotter? (A) Equatorial (B) Tropical (C) Polar (D) None ______are formed due to uneven heating of land and water in coastal areas. (A) Earthquakes (B) Cyclones (C) Winds (D) Tsunamis Earth rotates on its axis from _____ to _____. (A) North, South (B) South, North (C) East, West (D) West, East

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10. Assertion (A): It is it difficult to force the paper ball into the bottle. Reason (R): The air pressure inside the bottle is higher than near the mouth. The air inside the bottle pushes the ball out. (A) Both A and R are TRUE, R is the correct explanation of A. (B) Both A and R are TRUE, R is NOT the correct explanation of A. (C) A is TRUE and Statement-R is FALSE (D) A is FALSE and R is TRUE.

4. Thunderstorms and Cyclones I.

Thunderstorms A storm with lightning and thunder is called a thunderstorm. It occurs due to the convection of air in hot and humid areas.

II. Cyclone A natural calamity caused by difference in air pressure in the atmosphere is called a cyclone. A cyclone is a violently rotating windstorm. Cyclones are dangerous. Cyclones begin as thunderstorms.

In tropical regions like India, thunderstorms are common, but very few thunderstorms convert into cyclones. In India, the eastern coast is more vulnerable to cyclones than the western coast. In different parts of the world, cyclones are known differently. For example, in America, a cyclone is referred to as a hurricane, while the Filipinos and Japanese called it a typhoon.

Wind Storm and Cyclones The largest tropical cyclone recorded was Typhoon Tip that struck Japan in 1979. At its peak strength, the diameter of its eye wall was 2220 kilometres. The wind speeds were recorded at 305 kilometres per hour. III. Formation of Cyclones The formation of a cyclone depends on the speed and direction of the wind, temperature and humidity. A cyclone also arises: 1. Due to the difference in the temperatures of the two regions. 2. A low pressure is created as the air in the high temperature region becomes warm and rises. 3. The gap in the low pressure area is filled by cold air rushing in from the surrounding areas. 4. When the warm air arises, it cools, condenses and forms clouds. 5. When the water vapour in the clouds turns into raindrops, the heat possessed by the water vapour is released into the atmosphere. 6. The process repeats and the release of heat from the water vapour continues. Precautions During a thunderstorm, move away from open garages, metal sheds and water bodies. During a thunderstorm, sit inside a car, a bus or a closed vehicle, or inside a building. During a thunderstorm, do not:  Rest under a tree  Take shelter under an umbrella with a metallic end  Lie down flat, if in an open place  Sit near a window IV. Humidity Humidity is the amount of water vapour in the atmosphere. As the humidity in a region increases, the difference in temperature also increases. This results in the formation of a cyclone. V. Gales Winds moving at high speeds are known as gales. VI. Eye of a Cyclone The calm and clear area at the centre of a cyclone is called the eye of the cyclone. The following flow chart will help you to understand the phenomena that lead to the formation of clouds and falling of rain and creation of storms and cyclones:

85

5. Tornado A rotating column of high speed winds that appears as a dark funnel-shaped cloud reaching from the ground to the sky is called a tornado. A tornado may form within a cyclone. A tornado forms due to the effect of low pressure in the eye of a cyclone. Objects near the base of a tornado, such as cars, dust, debris and even houses, are sucked into its funnel and thrown out at the top. More than a thousand tornadoes occur every year across the world. Most occur in the United States. Tornadoes occur regularly in the regions around the Atlantic Ocean. Weak tornadoes travel with wind speeds of 50 to 60 km/hr, while a violent tornado can travel at a speed of about 400 km/hr. A tornado is a natural disaster, but not as dangerous as a cyclone.

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7th Class Physics

86

6. Destruction Caused by Cyclones 1. 2. 3. 4. 5.

A cyclone causes destruction through: High-rise waves High-speed winds that uproot trees, houses and other property Contamination of drinking water Heavy rainfall that worsens the flood situation Flooding of agricultural land by sea water making it infertile.

7. Meteorological Department

Cyclones cause a lot of damage to land, people and property, and create a lot of havoc in the lives of the people. When a cyclone hits an agricultural field, it makes the land infertile and useless for agriculture. A cyclone spares nothing. It worsens the flood situation. There are indirect impacts of a cyclone as well, such as flooding in coastal areas. Flooding not only damages land, but also pollutes drinking water sources. This can cause epidemics. High waves several kilometres away from the shore indicate an upcoming storm. As a cyclone strikes the coast, the intense winds raise the water into a gigantic wave that is pushed towards the shore. High-speed winds during a cyclone can cause major damage to houses, human beings, animals, trees, power supply, and even communication systems such as telephones. Storm Surge A high wall of water moving towards the shore from the ocean is called a storm surge.

The department that carries out a scientific study of the atmosphere and focuses on weather forecasting and processes. Tropical cyclones are given names. The names are selected from a list decided by the national meteorological organisation of a country, or by a committee of the World Meteorological Organisation. The names of tropical cyclones that cause major death or destruction are not used again, as a tribute to the people who lose their lives in the disaster. Some noteworthy points 1. Cyclone Alert: A warning issued 48 hours before the expected time of a cyclone. 2. Cyclone Warning: A warning issued 24 hours before the expected time of a cyclone.

8. Effective Safety Measures

4.

To keep people safe and secure during a cyclone, the government takes certain measures, such as: Constructing cyclone warning centres Making arrangements to relocate people during cyclones Providing information about a cyclone, and warning people, fishermen, ships, ports, airlines and various government agencies Delivering accurate and effective cyclone forecasts and warnings Safety measures to follow before and during a cyclone: Do not move out unless and until required. Keep a note of all the warnings given by the Meteorological Department. Keep emergency phone numbers of police, ambulance and fire brigade handy. Move people and valuable items to a safe place.

5.

Store hygienic drinking water.

6.

Help neighbours.

7.

Avoid contact with wet electric lines and switches.

1. 2. 3.

4.

1. 2. Storm Tide The combination of a storm surge and a tide is known as a “storm tide”. The most dangerous storms are the ones where the storm surge arrives on the top of a high tide, and then the storm reaches an area that might otherwise have been safe.

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3.

Wind Storm and Cyclones

87

19. Which of the following is formed due to the effect of low pressure in the eye of a cyclone? (A) Typhoon (B) Hurricane A natural calamity caused by difference in air (C) Tornado (D) Gales pressure in the atmosphere is called 20. What is speed of a violent tornado? (A) Thunderstorm (B) Wind (A) 200 km/hr (B) 400 km/hr (C) Cyclone (D) Earthquake (C) 600 km/hr (D) 800 km/hr Statement-I: All thunderstorms are cyclones. 21. Which of the following are damaged due to the Statement-II: All cyclones are thunderstorms. effect of a cyclone? (A) Statement I is correct, Statement II is incorrect. (A) Land (B) People (B) Statement II is correct, Statement I is incorrect. (C) Property (D) All (C) Both statements are correct. 22. The department that carries out a scientific study (D) Both statements are incorrect. of the atmosphere and focuses on weather Cyclone in America is known as forecasting and processes. (A) Typhoon (B) Hurricane (A) Agriculture Department of India (C) Both (D) None (B) Meteorological Department The formation of a cyclone depends on (C) Broad Casting and Communication (A) The speed of wind (D) All the above (B) The direction of wind 23. Which of the following measures keep people safe (C) Temperature (D) All and secure during a cyclone? Which of the following precautions should be taken (A) Cyclone warning centers (B) Arrangements to relocate people during during a thunderstorm? cyclones (A) Sit inside a car (C) Not providing information about a cyclone (B) Move away from open garages (D) Delivering accurate and effective cyclone (C) Move away from water bodies forecasts and warnings (D) All Assertion (A): Humidity results in the formation 24. Statement-I: During a cyclone, not move out unless and until required of a cyclone. Statement-II: Before a cyclone, keep emergency Reason (R): As the humidity in a region increases, phone numbers of police, ambulance and fire brigade the difference in temperature also increases. This handy. results in the formation of a cyclone. (A) Both A and R are TRUE, R is the correct (A) Statement I is correct, Statement II is incorrect. explanation of A. (B) Statement II is correct, Statement I is incorrect. (B) Both A and R are TRUE, R is NOT the correct (C) Both statements are correct. explanation of A. (D) Both statements are incorrect. (C) A is TRUE and Statement-R is FALSE onceptive orksheet (D) A is FALSE and R is TRUE. Statement-I: Winds moving at high speeds are 11. The convection of air in hot and humid areas known as gales. results in Statement-II: The calm and clear area at the centre (A) Thunderstorm (B) Wind of a cyclone is called the head of the cyclone. (C) Rain (D) none (A) Statement I is correct, Statement II is incorrect. 12. A violently rotating windstorm is a (B) Statement II is correct, Statement I is incorrect. (A) Thunderstorm (B) Wind (C) Both statements are correct. (C) Cyclone (D) Earthquake (D) Both statements are incorrect. 13. Which of the following area is more vulnerable to cyclones in India? (A) Eastern coast (B) Western coast (C) Northern coast (D) Southern coast

Formative Worksheet 12.

13.

14.

15.

16.

17.

C

18.

W

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7th Class Physics

88 14. The largest tropical cyclone recorded in (A) India (B) America (C) Japan (D) Australia 15. Which of the following should not be done during a thunderstorm? (A) Rest under a tree (B) Take shelter under an umbrella with a metallic end (C) Lie down flat, if in an open place (D) All the above 16. The amount of water vapour in the atmosphere is called (A) Water cycle (B) Humidity (C) Vapour pressure (D) None 17. Winds moving at high speeds are known (A) Typhoon (B) Hurricane (C) Gales (D) All 18. A rotating column of high speed winds that appears as a dark funnel-shaped cloud reaching from the ground to the sky is called a (A) Typhoon (B) Hurricane (C) Tornado (D) Gales 19. In which of the following countries, tornadoes occur frequently ? (A) India (B) America (C) Japan (D) Australia 20. Which of the following is more dangerous? (A) Wind` (B) Rain (C) Cyclone (D) Tornado 21. A high wall of water moving towards the shore from the ocean is called a (A) Storm tide (B) Storm surge (C) Water tide (D) Air surge 22. A cyclone causes destruction through: (A) High-rise waves (B) High-speed winds that uproot trees, houses and other property (C) Contamination of drinking water (D) All

Summative Worksheet I.

Fill in the blanks

1. 2.

Wind is ____________ air. Winds are generated due to ____________ heating on the earth. Near the earth’s surface ____________ air rises up whereas ____________ air comes down.

3.

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4.

Air moves from a region of ____________ pressure to a region of ____________ pressure. 5. Air around us exerts ______ in all directions. 6. Winds carrying _______ bring rain. 7. Air ______ on heating while ________ on cooling. 8. A cylcone is called a ______ in the American continents and ______ in Japan and Phillippines. 9. The word monsoon is derived from the Arabic word _____, which means ‘season’. 10. ___________ is used to measure speed of wind.

II. Say True or False 11. Increased wind speed is, indeed, accompanied by a reduced air pressure. 12. Air moves from the region where the air pressure is low to the region where the pressure is high. 13. The warm air is lighter than cold air 14. If a storm is accompanied by lightning, we must take shelter under an isolated tree. III. VERY SHORT ANSWER QUESTIONS 15. What is wind? 16. You see that leaves of trees, banners or flags flutter when the wind is blowing. Why is it so? 17. What is the main cause of wind? 18. What is the main cause of wind movements on the earth? 19. What happens when temperature is increased at a place? 20. Which factor is increased the speed of wind? 21. From which word monsoon is derived? 22. What is a monsoon? 23. In which areas does the thunderstorms develop very frequently? 24. What is thunderstorm? 25. What is a cyclone? 26. What are the other names of cyclones? 27. What are the factors which contribute to the development of cyclones? 28. What is called the eye of the storm? IV. SHORT ANSWER QUESTIONS 29. What is wind? What is the cause of wind? 30. What is cyclone and how is it formed? 31.   What are the different reasons for wind current? 32. What are monsoon winds? 33. What are the advantages and disadvantages of rain?

Wind Storm and Cyclones

89

34. 35. 36. 37. 38.. 

What is Thunderstorm? 2. What are the precautions in thunderstorms? What are tornadoes? What is eye of a storm? What are the stepstaken by government for cyclone warning. 39.    Describe the action on the parts of the people. 40. What are the other precautions if you are staying in a cyclone hit are? 41. Suggest two methods to find out wind direction at a given place. 3. 42. State two experiences that made you think that air exerts pressure. 43.   You want to buy a house. Would you like to buy a house having windows but no ventilators? Explain your answer. 44. Explain why holes are made in hanging banners and hoarders. 45. How will you help your neighbours in case cyclone approaches your village/town? 46. What planning is required in advance to deal with the situation created by a cyclone? 47. What happens when we blow air between the two balloons of nearly equal size hanging on a stick? 48. Why hot air goes up? 49. Explain that how uneven heating between equator and the poles generates wind current. 50. What is an anemometer? Explain with the help of a diagram?

HOTS Worksheet 1.

4.

Which of the following statements regarding the safety measures to be followed during cyclones is incorrect? (A)  One should not go out unless advised (B)  One should not drink contaminated water (C)  One should not cooperate with neighbours (D)  One should not touch wet switches and power lines During a thunderstorm four students Shashank, Somak, Sanjeev, and Sanjay take shelter in different places, as shown in the following table.

Name

Place of shelter

Sanjeev

under an umbrella

Shashank

under a tree

Somak

on a scooter

Sanjay

inside a car

Who is safe from the effects of the thunderstorm? (A) Sanjeev

(B) Shashank

(C) Somak

(D) Sanjay

When a tornado occurs, people should (A) Climb to the roof (B) move out to the open field

Which of the following graphs correctly represents the relationship between wind speed and the distance to which wind-dispersed seeds travel?

(C) Take shelter under a  tree (D) Bend down on their knees 5.

When a cyclone strikes one should (A) Go fishing (B) Move to a shelter

(A)

(B)

(C) Move out into an open place (D) Climb onto the roof of the building 6.

Which of the following measures is a safety measure to be followed during a thunderstorm? (A) Taking shelter under a tree (B) Standing on an open ground

(C)

(D)

(C) Taking shelter inside a bus (D) Standing on a roof-top www.betopperps.com

7th Class Physics

90 7.

Mutum has recently got to know about a thunderstorm warning in his town. He decides to take shelter in the underground basement of his house. On his way to the basement he picks up four objects along with him. These are a radio, a water bottle, a music system, and a torch. Which of the following objects will not be useful to Mutum in the basement during a thunderstorm?

8.

(A) Radio

(B) Torch

(C) Water  bottle

(D) Music system

In order to be safe from the effect of tornadoes a place should have (A) Many  doors

(B) Many  windows

(C) A wooden foundation (D) An underground basement 9.

Which of the following statements regarding the safety measures to be followed during cyclones is incorrect?

(A) Low-pressure area and region Q is the region where wind speed is high (B)  High-pressure area and region Q is  the  area where wind speed is low (C)  Low-pressure area and region Q is  the  area where wind speed is low (D)  High-pressure area and region Q is  the  area where wind speed is high 12. A low pressure condition is formed during cyclones over a region because of the (A) Release of heat by air (B) Release  of  heat  by water  vapour (C) Absorption of heat by land surface (D) Absorption of heat by water vapour 13. Which figure correctly depicts  the  conditions required for the formation of thunderstorms? (A) (B)

(A) One should not go out unless advised (B) One should not drink contaminated water (C) One should not cooperate with neighbours

(C)

(D) One should not touch wet switches and power lines 10. Thunderstorms occur because of the collision between (A) Two hot air columns

(D)

(B) Two cold air columns (C) Descending cold and ascending hot air (D) Ascending cold and descending hot air 11. The formation of a cyclone is depicted in the given figure.

14. Cyclones are formed when (A) Warm air does not rise up (B) Winds do not move in high speed (C) There is a drop in the air pressure (D) There is a drop in the humidity levels In the given figure, region P is the www.betoppers.com

Wind Storm and Cyclones

91

15. The centre of a cyclone is called Its diameter varies from

.

18. A hot air balloon moves in the air as a result of the heating of the air inside the balloon.

.

The information in which alternative completes the given statements?

(A) i

ii

focus

10 to 20 cm

i

ii

focus

10 to 30 km

(B) This heating of air inside the balloon (A) Makes the air lighter

(C)

(B) Makes the air heavier

i

ii (C) Causes contraction of air

eye

(D)

10 to 20 cm

i

ii

eye

10 to 30 km

16. Consider the following statements regarding cyclones. I The Western coastline of India is more prone to cyclones than the Eastern coastline. II Technological advancements make it possible to predict the occurrence of cyclones. III A cyclone develops as a result of the combination of high speed winds and pressure differences. Among the given statements, (A) Only I is  incorrect (B) Only III is  incorrect (C) Both I and II are incorrect (D) Both II and III are  incorrect 17. A loud sound is heard, when a tyre of a bus or a truck bursts.This loud sound is the result of (A) High gravitational force on the tyres (B) Low gravitational force on the tyres (C) High air pressure in the tyres (D) Low air pressure in the tyres

(D) Causes condensation of air 19. After washing clothes, Vikram dries them under the Sun, as shown in the given figure. He observed that when a high speed wind blows, the clothes lift up in the air.

The clothes lift up because high speed wind is accompanied by (A) Increase in the pressure of air (B) Decrease in the pressure of air (C) Increase in the weight of clothes (D) Decrease in the weight of clothes

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7th Class Physics

92 20. Most parts of India receive rainfall during which is caused by the of a

,

23. Land breeze is the name given to the wind that moves from the land to the sea. It blows because

. It is an example

(A) Of the intense cooling of the seas during winters

breeze.

(B) Of the intense heating of the land surface during summers

The given statements are completed by the information in alternative

i (A)

summers

ii loo

(C) The temperature of air above the sea is lower than that above the land

iii

(D) The temperature of air above the sea is higher than that above the land

land 24.

(B)

winters

monsoons

land

(C)

summers

monsoons

sea

(D)

winters

loo

sea

 breeze is the name given to the wind that moves from sea to land. It blows when the temperature of air above the sea is

than

that above the land. The information in which alternative completes the given statements?

21. Which of the following statements is incorrect?

i

ii

(A)

S ea

higher

(B)

S ea

lower

(C)

Land

higher

(D)

Land

lower

(A) Monsoon winds are an example of land breeze (B) Land breeze blows mainly during winters (C) Sea breeze blows mainly during summers (D) The direction of winds change seasonally 22. Wind currents are generated because of the difference in temperatures of different regions. One of the reasons for these differences in temperatures is the uneven heating of the Earth’s surface, as a result of its shape. The direction of winds in different regions is shown in the given figure.

The winds move in a curved trajectory in each climatic zone of the Earth because of its

25. If three balloons A, B and C are inflated with air at different temperatures T A, TB and TC, respectively, they will rise to different heights as shown in the figure. 

What will be the correct relation among their temperatures?

(A) Shape

(B) Rotation

(A) TA >TB > TC

(B) TA  TB

(D) TA = TB = TC

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Wind Storm and Cyclones

IIT JEE Worksheet I. 1.

2.

3.

4.

5.

6.

7.

8.

9.

93

II.

Multiple Correct Answer Type

11. The air inside a balloon exerts pressure in ____ direction. Single Correct Answer Type (A) Upward (B) Downward A cyclone warning can be issued: (C) Left side (D) Right side (A) 6 hours in advance 12. Circulation of wind is due to (B) 12 hours in advance (A) The warm air at the equator rises (C) 24 hours in advance (B) Cold air moves in from the polar regions (C) Uneven heating of the earth’s surface between (D) 48 hours in advance the equator and poles Pick the odd one out of the following: (D) Even heating of the earth’s surface between (A) Monsoon (B) Typhoon the equator and poles (C) Hurricane (D) Cyclone 13. Which regions are relatively hotter? In which did Orissa was hit by a cyclones in the (A) Equatorial (B) Tropical recent past: (C) Polar (D) None (A) 1998 (B) 1999 14. The formation of a cyclone depends on (C) 2000 (D) 2001 (A) The speed of wind (B) the direction of wind Air moves from: (C) Temperature (D) Soil condition (A) The region of low pressure to high pressure 15. Which of the following are damaged due to the (B) The region of high pressure to low pressure effect of a cyclone? (A) Land (B) People (C) Air does not depend upon the pressure (C) Property (D) Jobs (D) Air does not move The winds from oceans carrry water and bring rain 16. Which of the following precautions should be taken if you are staying in a cyclone hit area are called: (A) Do not drink water that could be contaminated. (A) Cyclone (B) Typhoon Always store drinking water for emergencies. (C) Monsoon (D) None of these (B) Do not touch wet switches and fallen power What happens if air is heated? lines. (A) Contracts (B) Expands (C) Do not go out just for the sake of fun. (C) No change (D) First expands then (D) Do not pressurise the rescue force by making contracts undue demands. The moving air is called (A) Tsunami (B) Cyclone III. (C) Typhoon (D) Wind High-speed winds and air pressure difference can cause (A) Tsunamis (B) Cyclones 17. (C) Typhoons (D) Earth quakes

Paragraph Type We see that the increased wind speed is, indeed, accompanied by a reduced air pressure. When we blow into the mouth of the bottle, the air near the mouth has _____speed.

Which of the following made it easier to monitor (A) Higher (B) Lesser cyclones? (C) Same (D) none (A) Satellites (B) RADARs (C) Both (D) None 18. Blow up two balloons with some water inside them. 10. Choose the correct statement. Tie each one to a string and hang them apart on a cycle spoke or a stick. What happens if we blow in (A) Increased wind speed is, indeed, accompanied the space between the balloons? by a reduced air pressure. (B) The warm air is lighter than cold air (A) They move away (B) They come closer (C) Both (B) They do not move (D) None (D) None www.betopperps.com

7th Class Physics

94

19. Hold a strip of paper, 20 cm long and 3 cm wide, between your thumb and forefinger.Now blow over the paper. The strip (A) Goes downwards

(B) Goes upwards

(C) Remains horizontal

IV. Integer Type 20. Cyclone Alert is a warning issued ____ days before the expected time of a cyclone. 21. Cyclone Warning: A warning issued ___ days before the expected time of a cyclone.

V. Matrix Matching 22.

(i) (ii) (iii) (iv) (v)

Column A Wind carrying water Moving air Measures the speed of wind Dark funnel shaped clouds Develops in India very frequently



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(i) (ii) (iii) (iv) (v)

Column B Thunderstorms Tornado Monsoon Wind Anemometer

(D) None

6 Chapter -3

Sound

Learning Outcomes By the end of this chapter, you will understand • • •

Sound Sound Produced By Humans Propagation Of Sound

• • •

1. Sound If somebody calls you from behind, you will quickly turn around. What makes you do so? We turn back in response to a call because of the sound heard by us. We are able to talk to each other because of the sound produced by us. We are able to predict the distance of a train only by listening to the sound it produces. Similarly, we can distinguish between different musical instruments because of the sounds they produce. Sound is a form of energy that causes the sensation of hearing. How do you realize that an alarm bell is ringing? Vibrating bodies produce sound

2. Sound produced by Vibrating Objects Do you know how a sound is produced?  To find out, let us perform the following activities.

The Human Ear Characteristics of Sound Noise Pollution

Carefully, observe the surface of water in the utensil. Do you see concentric circles moving on the water surface? These are vibrations in water, produced by vibrations of the utensil body, on beating. Therefore, it can be concluded that a vibrating body produces sound. The back and forth movement of an object produces sound. An object moving back and forth is said to be in vibration. Hence, sound is produced by vibrating objects.

3. Sound produced by Musical Instruments Musical instruments such as flute, guitar, tanpura etc.  produce  sounds  by vibrations of its vibrating parts. A guitar produces sound because of the vibrations in its plucked strings. When we hit the skin of a tabla, it starts vibrating. This produces sound. The skin of a tabla is known as stretched membrane.

Musical instrument

Vibrating part that produces sound Stretched membrane

i)

ii)

Take a rubber band and stretch it between two poles. Now, pluck the rubber band in the middle. Can  you  hear  any  sound?  Does  the rubber band vibrate when it produces a sound? On plucking a stretched rubber band or a stretched string, it vibrates rapidly and produces a sound. Take a cooking utensil and pour some water in it. Now, beat the utensil with a rod. You will hear a sound.

Stretched string

Stretched string

Air-column

7th Class Physics

96 Musical instrument

Vibrating part that produces sound Air-column

Let us understand better by performing a small activity. Take a piece of paper and cut two small rectangles out of it. Now, put these two pieces of paper one above the other such that there is a small gap between them.

Stretched membrane A rectangular slit

When you beat the skin of a drum, it vibrates and hence, produces sound. The sound produced by a drum is not only that of the skin, but the drum is itself forced to vibrate. Hence, the sound produced by a drum is the sound of vibration of the whole instrument.

4. Sound Produced By Humans While watching a group of children playing hide and seek, Himesh observed how a blind-folded boy was able to catch the other children by hearing the sounds produced by them. This made him wonder how sounds are produced by humans. While singing a song, put your hand on your throat. You will find a part of your throat moving up and down. This part of your throat is known as the voice box or larynx. The larynx is responsible for producing sounds in humans. It moves when we swallow something.

Now, blow air through this small gap. Can you hear a sound? Your  voice  box  functions  in  the  same way. We can hold these pieces of paper tightly as well as loosely. In the same way, our vocal chords can be held tightly or loosely with the help of the muscles attached to them. Different people have different vocal chords. Due to this reason, we all have a different voice quality.

5. Propagation of Sound We are able to hear the bursting of crackers even when we are standing at a distance. How is it possible? How does the sound produced by a cracker reach us? The sound of a bursting cracker reaches us through air. It shows that sound can travel through air. Experiment to demonstrate that sound requires a medium for propagation. Requirements: Electric bell, Glass bell jar, Vacuum pump, Battery Method: Place the electric bell inside the glass bell jar and connect it to a battery. When the circuit is closed, you can hear the bell ring. The jar contains air and sound travels through this air.

We know that a vibrating object produces sound. Then, which part of our body vibrates to produce sounds? The voice box consists of two vocal chords. These chords are arranged in such a manner that there is a small gap between them. This small gap allows air to pass through. When we speak, air is forced into this small gap by the lungs. This prompts our vocal chords to vibrate and hence, produce sounds. www.betoppers.com

Now, remove the air from the jar with the help of the vacuum pump connected to the bell jar. As the air is taken out, the loudness of the sound slowly decreases until the sound becomes too faint. Finally you cannot hear the bell even though the hammer of the bell is seen striking the gong as before. Allow air to enter the jar gradually. You will hear the sound slowly increasing. Result: Sound cannot propagate in the absence of a material medium like solid, liquid or gas.

Sound

97 Stethoscope is an example of sound travelling through solids. Doctors use stethoscopes to listen to your heartbeat.

A material medium is necessary for the propagation of sound. Vacuum is devoid of any material.  Hence,  sound  cannot  travel through vacuum. Outer space is devoid of any material medium. Hence, no sound can be heard in outer spaces. To communicate in such areas, astronauts use walkietalkies. A walkie-talkie is an instrument, which uses radio waves for the transportation of messages. Can sound travel through liquids? To find out whether sound can travel through liquids, let us perform the following activity. Take a metal plate and spoon. Place them inside a bucket filled with water. Hit the plate with the spoon in such a way that it does not touch the body of the bucket. Now, carefully place your ear near the surface of water.  Are you able to hear the sound produced? You can hear the produced sound. Sound reaches your ear after travelling through water. Hence, we can say that sound travels through liquids. Interesting point to note Dolphins communicate with each other by sending high pitched squalls. It shows that sound can travel through water.

Can sound travel through solids? Place your ear on one end of a long table. Ask your friend to tap the table from the other end. Do you hear any sound? You can hear the produced sound. Sound reaches your ear after travelling through the table. This indicates that sound can travel through solids.

Sound can travel in solids, liquids, and gases. However, sound cannot travel in vacuum.

6. The Human Ear The ear is one of the sensory organs in our body. We hear sounds with the help of our ear. It is the eardrum present in the ear, which vibrates when vibrating air hits its surface. This vibration gives us the sense of hearing. To understand the functioning of an eardrum, let us perform an activity. Take an empty plastic bottle with a rectangular base and put some dry rice in it. Now, speak loudly through the mouth of the bottle and tell your friend to carefully observe the rice grains. As you speak, the rice grains jump up from the surface of the bottle because your sound causes the base of the bottle to vibrate. The eardrum performs the same function. It vibrates and moves to and fro when it receives a sound. Working of the human ear Before understanding the working of a human ear, let us first glance at the different parts of the human ear.

Eardrum The eardrum is a thin membrane, which is stretched tightly in the middle ear. It is also called the tympanic membrane. If damaged, then it may result in severe pain, loss of hearing, discharge of ear fluids, ringing in the ear, etc.

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7th Class Physics

98 Once you lose your hearing, it is gone forever. It cannot be repaired or regained. If you are standing on a platform, you can easily tell if a train is coming towards you or going away from you, even though you are not able to see it. How is this possible? As a train draws near you, the sound produced by it keeps on increasing. This vibrates our eardrum. As soon as the train leaves, the sound produced by it keeps on decreasing. Hence, vibrations produced by it also decrease. Hence, you can easily tell if the train is coming towards you or going away from you. Interesting facts The frog is the only animal whose eardrum is present outside the body i.e., behind the eye. Hearing in snakes Snakes have no external ears such as humans. They cannot hear a snake charmer’s sound. However, they can sense sound with the help of their sense of touch and respond to the movement of the trumpet.

4.

5.

i ii (A) speed drying or wetting

Formative Worksheet 1.

2.

3.

A police officer, who is investigating a robbery in a locality, presses the door bell of house number 211.To alert people, a door bell emits (A) Heat energy (B) Light energy (C) Sound energy (D) Potential energy Kevin touches four objects, I, II, III, and IV. He makes the following conclusions: Object I is hot. Object II is vibrating. Object III is  cold. Object IV is sticky. Among the given objects, the object producing sound is (A) I (B) II (C) III (D) IV Which of the following musical instruments is not matched with its vibrating part? (A)

Musical instrument Vibrating part Guitar Stretched string

(B)

Musical instrument Vibrating part Jaltarang Water column

(C)

Musical instrument Dhol

Vibrating part Stretched membrane

(D) Musical instrument Vibrating part Ghatam Air column www.betoppers.com

The plucking of a stretched metal wire produces sound because (A) Of change in atmospheric pressure around the wire (B) It is made up of a good conductor of electricity (C) of generation of heat (D) It starts vibrating The human vocal cord controls the i of sound by  ii  itself. The information in which alternative completes the given statement?

6.

7.

8.

9.

(B)

i ii speed loosening or tightening

(C)

i ii quality drying or wetting

(D)

i ii quality loosening or tightening

Which part of the human body vibrates to produce sounds? (A) Windpipe (B) Eardrum (C) Vocal cord (D) Nasal cavity vs = Speed of sound in solid medium vl = Speed of sound in liquid medium vg = Speed of sound in gaseous medium When arranged in the descending order of their magnitudes, the speed of sound in the three different media is (A) vs > vl > vg (B) vs > vg >vl (C) vg > vs > vl (D) vl > vg > vs The sound coming from an alarm clock cannot be heard if it is kept inside a box that is (A) Filled with liquid only (B) Filled with both solid and liquid (C) Made up of iron with air inside it (D) Devoid of any material medium inside What should  a  hearing-impaired  person  wear  in order to enhance his hearing power by amplifying sound? (A) Ear-horn (B) Stethoscope (C) Mobile headset (D)Jackhammer’s headphone

Sound

99

10. The given table lists the main parts of the human ear that take part in hearing. Parts of the human ear I Ear canal II Three tiny bones III Eardrum IV Cochlea The order in which sound vibration moves through the human ear is (A) I   II    III    IV (B) I   III    II    IV (C) III   I    IV    II (D) III   IV    I    II 11. Vibration can be defined as the (A) Stretching of a membrane (B) Roughness of a membrane (C) Circular motion of a membrane (D) To and fro motion of a membrane

Conceptive Worksheet 1.

2.

3.

4.

5.

Sound is produced by (A) Burning a candle (B) Beating a drum (C) Photosynthesis (D) A battery When two people talk with each other, energy is transferred through (A) Electric currents (B) The movement of mouth (C) Sound waves (D) Wind force One end of a ruler is fixed on a table and the other end is struck The vibration of the ruler produces (A) Heat (B) Light (C) Sound (D) Electrical Tom spreads some sand particles on a drum-head. He strikes the drum with a hammer. The striking of the drum will cause the sand particles to (A) Cut through the drum-head (B) Get stuck to the drum-head (C) Move to and fro on the drum-head (D) Move up and down on the drum-head When a stretched membrane is beaten, it starts a 

. This phenomenon is used to make

(A)

i moving forward

ii veena

(B)

i ii moving forward drum

(C)

i vibrating

ii veena

(D)

i vibrating

ii drum

6.

Which of the following actions is not related to the production of sound by humans? (A) The forcing out of air by the lungs through the windpipe (B) The expansion of the lungs in order to fill air (C) The loosening of the vocal cord (D) The vibration of the vocal cord 7. Justin and his friend Daniel stand at the two ends of a large empty hall. Justin makes a sound with a tuning fork by hitting the wall at his end. His friend Daniel puts his ear against the wall at the other end. He hears two sounds at an interval. Daniel hears two sounds because (A) Justin makes two sounds (B) A tuning fork has two vibrating arms (C) Sound br eaks into two perpendicular components (D) Sound travels at different speeds through air and wall 8. Two fishing boats are at mid sea. One of the boats sends two sound signals (to the other boat) at the same time; one through air and the other through water. The second boat will receive (A) The sound signal sent through water first (B) The sound signal sent through air first (C) Only the sound signal sent through air (D) Both the sounds signals at same time 9. Sound waves cannot travel through (A) Wood (B) Gass (C) Gasoline (D) Vacuum 10. With reference to the human ear, exposure to excessive sound may damage its (A) Pinna (B) Eardrum (C) Ear canal (D) Eustachian tube

.

The information in which alternative completes the given statements?

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7th Class Physics

100

7. Characteristics of Sound Consider a pendulum moving continuously from left to right such that it passes through point C each time. This type of to and fro motion of an object is known as vibration   or  oscillation.

same phase is called wave length. It is denoted by the Greek letter  (lambda).

Ex: 

Unit : The S.I unit of wave length is meter (m). Wave velocity It is the distance travelled by a wave in one second. Expression for wave velocity We know that, velocity of the wave =

distance travelled by the wave time taken Suppose a wave travels a distance  in time T,, Frequency The number of oscillations of a vibrating body per second is known as the frequency of oscillation. It is measured in hertz (Hz). If in one second, the pendulum passes point C 10  times  then,  its frequency of oscillation is 10 Hz. Time period The time required to complete one oscillation is known as the time period of an oscillation. It is given by the inverse of the frequency of oscillation. It is expressed in seconds (s). Hence, Time period 

1 frequency

Example: An  object  oscillates  at  the  rate  of  2 oscillations per second. What is its time period?

1 1   0.5s frequency 2 Hence, the time period of the given object is 0.5 seconds. Amplitude Amplitude of an oscillation is the distance covered on either side of the mean position. In the above example of a pendulum, distance covered by the bob on either side of point C gives its amplitude. Time period 

Extra Milestone Wavelength Sound propagates in the form of waves. Wavelength of a wave is the length of one wave. The distance travelled by the wave in one time period is called the wavelength or The distance between two nearest particles which are in the www.betoppers.com

 T [i.e. the relation between wave velocity, wavelength and time period] Hence T is the time taken by one wave. We know then v 

1 becomes the number of waves per second T and this is known as frequency (f) of the wave. So the above equation can also be written as, v = f×  Where, v = velocity of the wave; f = frequency;  = wavelength In other words velocity of a wave = frequency × wavelength Thus, the velocity (or speed) of a wave in a medium is equal to the product of its frequency and wavelength. The formula v = f ×  is called wave equation. that

Loudness You can easily distinguish between the difference in sounds produced by a tabla and guitar. Similarly, there are many sounds that you can identify without looking at the objects producing them. This shows that there are some characteristics that make these sounds different. Let us perform an activity to understand better. Take a plastic ball and throw it on the ground. When it hits the floor, it produces a sound. Again, throw the ball with greater force. In this case, the sound produced will be louder and the height to which it bounces will also be higher. The height to which

Sound the ball bounces gives the measure of its amplitude of vibrations. Hence, we can say that loudness increases with an increase in amplitude. Loudness of sound is proportional to the square of the amplitude of the vibration produced. Loudness of sound is expressed in decibels (dB). As the amplitude of vibrations increase, the loudness of a sound also increases. Loudspeakers increase the amplitude of the produced sound. Due to this, the sound produced becomes very loud. When we burst crackers, a loud sound is produced. Hence, they have large amplitude. Whispering has small amplitude, whereas shouting has large amplitude. Hence, shouting is louder than whispering. Pitch We can easily distinguish between the sound of a drum and the whistle of a train. Why is it so? The sound produced by two different objects can be equally loud. Even then we can distinguish between them. How? The frequency of an oscillation determines the shrillness or pitch of a sound. If the frequency of vibration is higher and the sound produced is shrill, it will have a high pitch. Similarly, if the frequency of oscillations is small then, the sound produced will have a low pitch. Sound produced by a drum is an example of low pitched sound. However, it has large amplitude. Sound produced by the buzzing of a bee is shriller than the roar of a lion. This suggests that buzzing of a bee has higher frequency of oscillations than the roar of a lion. Likewise, the voice of a child is always shriller than the voice of a man. Male voice is less shrill than female voice. This is because of the pitch of sound. Female voice has a higher pitch in comparison to the male voice. When your teacher writes on the blackboard with a piece of chalk, sometimes the chalk makes a displeasing sound.

Audible and Inaudible Sounds We know that we need a vibrating body for the production of sound. Can we hear the sound of all vibrating bodies? The fact is that sounds of frequencies less than about 20 vibrations per second (20 Hz) cannot be detected by the human ear. Such sounds are called inaudible. On the higher side, sounds of frequencies higher than about 20,000 vibrations per second (20 kHz) are also not audible

101 to the human ear. Thus, for human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz. Note: Some animals can hear sounds of frequencies higher than 20,000 Hz. Dogs have this ability. The police use high frequency whistles which dogs can hear but humans cannot. The ultrasound equipment, familiar to us for investigating and tracking many medical problems, works at frequencies higher than 20,000 Hz.

8. Noise Pollution Pollution occurs when there is an excess of some unwanted entity. Noise pollution occurs when there is an excess of unwanted sounds in the environment. It is one of the biggest problems of modern era. Noise pollution like air pollution is largely created by humans. It is mainly caused by factories, vehicles, construction instruments such as jackhammer, bulldozer, leaf blower, air conditioner, desert cooler etc. Loudspeakers and crackers produce noise pollution. Televisions and transistors running on high volumes can also contribute to noise pollution. However, the worst offenders of noise pollution are transportation vehicles. Effects of noise pollution: Noise pollution can lead to many health related problems like:  Insomnia  Loss of hearing  Hypertension  Severe headache  Stress related diseases  Aggressiveness in behavior How can noise pollution be controlled? To control noise pollution, we must control its source. Hence, silencers must be installed in vehicles such as motorcycles, cars, trucks, buses, and other noise producing machines. We should watch television and listen to music at a low volume. Also, use of loudspeakers as well as horns of buses and trucks should be minimized. Regular maintenance of automobiles should be done so that noise produced by them can be kept under check. All industrial work should be done away from residential areas. More trees should be planted in residential areas as they help in reducing noise.

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7th Class Physics

102

Formative Worksheet 12. Ivan shakes one end of a rope attached to a wall. This produces a wave motion in the rope. He observes that the first peak of the wave reaches the wall in 5 seconds, and a total of 15 peaks are produced in the vibrating rope in 5 seconds. The length of the rope is 3 meters. What is the wavelength of the wave produced in the rope? (A) 10 cm (B)15 cm (C) 20 cm (D) 25 cm 13. Alex hits a tuning fork on a wall. The fork begins to vibrate. He connects a digital counter with it and records the number of complete vibrations. His recordings are listed in the given table. Time (s) Number of vibrations 1 256 5 1,280 15 3,840 14. What is the time period of a pendulum if its bob oscillates 100 times in two seconds? (A) 0.01 s (B) 0.02 s (C) 0.1 s (D) 0.2 s 15. Eric throws a stone in a river. The stone travels a distance that is 100 m away from the river bank. He observes that the first wave comes to the bank 2 seconds after the stone touches the water surface. He counts that a total of 20 waves are created in 2 seconds. What is the distance between two consecutive waves? (A) 2 m (B) 5 m (C) 20 m (D) 50 m 16. The maximum displacement  of  a  sound  wave  on either side of the mean position of its vibration is the measure of its (A) Amplitude (B) Frequency (C) Period (D) Speed 17. The intensity of sound waves is measured in (A) Hertz (B) Joules (C) Meters (D) Decibel 18. The famous band of Beetles has used an ultrasonic whistle in one of their songs. The sound produced by the whistle cannot be heard by humans, but it can be heard by dogs. From the given information, it can be inferred that the sound produced by the whistle may have a frequency, which (A) Is less than 5 Hz (B) Is less than 80 Hz (C) Lies between 20 kHz to 25 kHz (D) Lies between 20 Hz to 20 kHz www.betoppers.com

19. Humans cannot hear all the sounds produced in their surroundings. The sounds whose frequencies fall in the audible range can only be heard by humans. The lower limit of the audible range for human ears is 20 Hz and the upper limit is a × 10b Hz. What are the respective values of a and b? (A) 1 and 2 (B) 2 and 4 (C) 3 and 2 (D) 1 and 3 20. I. The amplitude determines the pitch of the sound. II. For human ears, the audible range is 20 Hz to 20,000 Hz. III. Sound can travel through water. Among the given statements, only (A) I is correct (B) III is  correct (C) I and II are  correct (D) II and III are  correct 21. Objects that produce sound: I Fire  crackers II Siren III Mouth organ IV Fire alarm V Factory  machines Which of the given objects does not produce sound that is harmful to the human ear? (A) I (B) II (C) III (D) V 22. Nowadays, government and local bodies are taking initiatives in roadside plantation. This helps in (A) Reducing noise pollution (B) Reducing traffic congestion (C) Increasing the durability of roads (D) Increasing the visibility of drivers

Conceptive Worksheet 11. Which of the following sound waves produces the loudest sound?

(A)

(B)

Sound

(C)

(D)

12. With reference to sound, the number of to and fro motions of a membrane in per unit time is expressed in the unit of (A) Angstrom (B) Second (C) Meter (D) Hertz 13. Julie hears the ringing of a bell at a nearby church. She hears the bell ring 24 times in one minute. The ringing frequency of the bell is (A) 0.2 Hz (B) 0.3 Hz (C) 0.4 Hz (D) 0.5 Hz 14. A membrane is vibrating at a rate of 20 oscillations in two  seconds.  The  sound  produced  by  the membrane is (A) Loud (B) Shrill (C) Of good quality (D) Out of the audible range 15. The loudness of a sound is determined by (A) The amplitude of the vibrations of the sound (B) How fast the source is moving to and fro (C) The size of the source of the sound (D) How it is reflected from a surface 16. Cruz is standing in a pond. He throws a stone in the pond at some distance away from him. A wave is produced on the water surface. He observes that 30 peaks reach him in 5 seconds. What is the frequency of the produced wave? (A) 5 HZ (B) 6 HZ (C) 30 HZ (D) 150 HZ 17. The bob of a simple pendulum is oscillating with a frequency of 25 Hz. How much time will the pendulum take to make 100 oscillations? (A) 1 s (B) 2 s (C) 4 s (D) 6 s 18. Which of the following frequencies of sound is not heard by humans? (A) 18 Hz (B) 180 Hz (C) 1800 Hz (D) 18000 Hz

103 19. High noise levels can injure the ear drum and cause deafness. Which of the following devices can lead to the production of high noise? (A) Fan (B) Alarm clock (C) Washing machine (D) Leaf-blowing machine 20. Which of the following effects is unlikely to occur because of noise pollution? (A) Loss of hearing (B) Breathing problem (C) Sleeping disorder (D) High blood pressure

Summative Worksheet 1.

2.

3.

4.

5.

Choose the correct answer. Sound can travel through (A) Gases only (B) Solids only (C) Liquids only (D) Solids, liquids and gases Which of the following voices is likely to have minimum frequency? (A) Baby girl (B) Baby boy (C) A man (D) A woman In the following statements, tick ‘T’ against those which are true, and ‘F’ against those which are false. (a) Sound cannot travel in vacuum. (b) The number of oscillations per second of a vibrating object is called its time period. (c) If the amplitude of vibration is large, sound is feeble. (d) For human ears, the audible range is 20 Hz to 20,000 Hz. (e) The lower the frequency of vibration, the higher is the pitch. (f) Unwanted or unpleasant sound is termed as music. (g) Noise pollution may cause partial hearing impairment. Fill in the blanks with suitable words. (a) Time taken by an object to complete one oscillation is called _________. (b) Loudness is determined by the ________ of vibration. (c) The unit of frequency is _________. (d) Unwanted sound is called _________. (e) Shrillness of a sound is determined by the _________ of  vibration. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

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7th Class Physics

104 6.

7.

8. 9. 10. 11.

12. 13.

The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration? Identify the part which vibrates to produce sound in the following instruments. (a) Dholak (b) Sitar (c) Flute What is the difference between noise and music? Can music become noise sometimes? List sources of noise pollution in your surroundings. Explain in what way noise pollution is harmful to humans. Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer. Sketch larynx and explain its function in your own words. Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain?

HOTS Worksheet 1.

3.

4.

5.

6.

The given figure shows a human ear with parts labelled as I, II, III, and IV.

7.

2.

Which part of the ear vibrates with sound and sends these vibrations to the inner ear? (A) I (B) II (C) III (D) IV John places an alarm clock inside a box as shown in the given figure. He then creates a vacuum inside the box. When the alarm goes on, John does not hear the alarm sound. John does not hear the alarm sound because (A) The walls of the box block the sound (B) Sound cannot travel through vacuum (C) There is no medium between John and the box (D) The inside wall of the box reflects the sound back

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8.

Which of the following statements about sound produced by human beings is incorrect? (A) Larynx produces sound. (B) Larynx is also called voice box. (C) Vocal cords are cords stretched across larynx. (D) Windpipe modifies the quality of voice produced by larynx. Which of the following statements is incorrect about human ear? (A) The inner ear sends sound signals to the brain. (B) The outer ear collects sound from the surroundings. (C) Vibrations present in sound waves help in vibration of eardrum. (D) The eardrum vibrates to reproduce sound that goes to the brain. Which part of a tabla vibrates to produce sound? (A) Wooden shell (B) Stretched membrane (C) Stretched side strings (D) Ring holding the strings Which of the following acts cannot produce sound? (A) Striking a stretched metal cord (B) Beating a stretched membrane (C) Blowing into a narrow pipe (D) Heating a stretched string Hearing impiarment is caused by either excessive noise or injury, disease, age, or by a combination of any of them. Many devices are used by the hearing impaired persons. Ear-horn is one of them. Ear-horn is a device that is used to (A) Cure any infection in ear (B) Cancel the noise pollution (C) Protect ear from external agents (D) Amplify and enhance the hearing power Jackhammer is an electric device that is used to drill on hard surface. It produces unwanted sound that is very loud and irritating. The person who works with a Jackhammer generally wears a headphone known as Jackhammer’s Headphone. What does the Jackhammer’s Headphones do? (A) Prevents the worker from brain hemorrhage (B) Connects the worker with his manager (C) Absorbs and cancels the noise (D) Entertains the person

Sound 9.

10.

11.

12.

13.

In medical sciences, doctors use ultrasound devices to investigate and track the affected internal parts of the body that are not visible directly. The technique is called sonography. These devices use sound to observe internal human parts and work at a frequency greater than 20 KHz. The sound emitted by ultrasound equipments are (A) Inaudible (B) Audible (C) Louder (D) Feeble Jackhammer is an electric device that is mainly used for drilling on a hard surface. In this process, a large sound is produced. The person who works with jackhammer generally wears a ‘protection with jackhammers’ to avoid this. With reference to the given information, what does the ‘protection with jackhammers’ device do? (A) Protects ear from noise (B) Protects eye from dust (C) Protects from electric shock (D) Protects from wind and rain Telephone operators answer the calls on telephones. A sound is easily transmitted and clearly heard over a telephone conversation if it is of high frequency. Mostly women are recruited as telephone operators because they can (A) Do hard work (B) Produce louder sound (C) Produce high pitched sound (D) Work continuously for many hours Humans are able to hear because of the vibration of a stretched thin membrane. This membrane is called eardrum. If elasticity of the eardrum is loosened, then (A) It will vibrate rapidly (B) It will not vibrate properly (C) Inaudible sound can be heard (D) Sound cannot be heard at all Suraj and Pankaj are playing with a specially designed electric rod. The rod is inserted through a glass vacuum sphere. On switching the current, the rod can break and separate at point O. Suraj is tapping the rod at one end and Pankaj is able to hear a sound at the other end of the rod. The situation is shown in the given figure. After closing the switch, the rod breaks at O. Pankaj will  i to hear the tapping because sound ii travel iii .

105

Which alternative gives the correct information to complete the given statement? (A)

i continue

i (B) continue

ii can

iii through vacuum

ii cannot

iii between beaked points

(C)

i unable

ii iii cannot through vacuum

(D)

i unable

ii can

iii between beaked points

14. There is a hollow glass partition in a room. It divides the room in two compartments. Two brothers X and Y are sitting in each compartment. An alarm clock, which is placed inside the glass partition, is producing sound, as shown in the given figure. Both brothers are able to hear the sound. All the air from within the glass partition is removed with the help of a vacuum pump.

After evacuating the partition completely, (A) The alarm will produce a louder sound (B) Both will be unable to listen to the sound (C) Only X will be able to listen to the sound (D) The alarm will produce a high pitch sound

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7th Class Physics

106 15. An electric drum is enclosed inside a specially designed closed glass chamber, as shown in the given figure. On switching the current, the hammer repeatedly strikes the membrane of the drum. Hence, the membrane vibrates and produces sound that can be heard at a distance. Now, the chamber is evacuated by closing the switch of the vacuum pump.

As the chamber is evacuated, the (A) Frequency of hammering decreases (B) Membrane stops vibrating (C) Hammer stops striking (D) Sound cannot be heard 16. Dolphins and whales communicate under water using sound. They can see each other with their eyes and also by producing sound. Dolphins and whales are able to communicate in water because sound (A) Can travel in water (B) Travels fastest in water (C) Does not scatter in water (D) Does not lose energy in water 17. Microphone is an electrical device that is used to amplify a human voice to be heard at a distance. It consists of a thin membrane called diaphragm that 

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in response to

.

The information in which alternative completes the given statement? (A)

i rotates

ii electricity

(B)

i rotates

ii sound

(C)

i ii vibrates electricity

(D)

i vibrates

ii sound

18. Astronauts in deep space use radio devices to communicate with each other. The radio device converts the sound produced by the first astronaut into radio signals that reach to the radio device of the second astronaut. There, this radio signal converts back into sound so that the second astronaut can hear the voice of the first. In outer space, astronauts use radio devices instead of direct sound to communicate because (A) Sound cannot travel in vacuum (B) It is very difficult to speak in vacuum (C) Radio devices are very cheap and easily available (D) Vocal cords do not vibrate to produce sound in outer space. 19. Underwater detection of objects is possible in navigation because (A) Underwater objects can also produce sound (B) The speed of sound is the highest in water (C) The energy of sound increases in water (D) Sound can travel through water 20. Astronauts use walkie-talkie to communicate to each other in deep space. Walkie-talkie radiates radio waves to connect with the respective pair. Astronauts use walkie-talkie to communicate in deep space because (A) Sound cannot travel through vacuum (B) Range of the walkie-talkie is very large (C) Walkie-talkie amplifies the sound produced by astronauts (D) Radio waves cannot be transmitted without walkie-talkie 21. When a stretched thread is plucked, it starts vibrating. The given information is used to make a (A) Flute (B) Veena (C) Manjira (D) Jaltarang

Sound 22. The presence of excessive and unwanted noise in the surroundings may cause (A) Vision defects (B) Mental problems (C) Hearing impairment (D) Breathing difficulty 22. Which of the following preventive measures cannot be attributed to roadside noise pollution? (A) Plantation on the roadside (B) Installing silencers in vehicles (C) Regular servicing of engines (D) Smoothening of roads

IIT JEE Worksheet I.

Single Correct Answer Type

1.

Every source of sound is (A) Vibrating Body (B) Stationary Body (C) Moving Body (D) None Sound travels fastest in (A) Gases (B) Liquids (C) Solids (D) None The frequency of ultrasonic vibrations is (A) < 20 Hz (B) 20Hz - 20,000 Hz (C) > 20,000 Hz (D) None Sound produced by irregular vibrations of a material is (A) Music (B) Noise (C) Loudness (D) None The conditions for hearing sound are (A) There must be a vibrating body to produce sound (B) There must be a material medium to propagate (C) There must be a receiver to receive the sound vibrations (D) All Why the thunder is heard after the flash of lightning ? (A) Light travels very fast compared to sound (B) Sound travels very fast compared to light (C) Light and sound travel with same velocity (D) None The Frequency of electric tuning fork is 8000 Hz. Will this sound be audible ? (A) The sound will be audible (B) The sound will not be audible (C) May or may not be audible (D) None

2.

3.

4.

5.

6.

7.

107 8.

We differentiate between voices of two different persons without seeing them by ___ of sound produced ? (A) Pitch (B) Loudness (C) Quality (D) All 9. When a drum is hit softly, a soft sound is produced. Because, the vibration of ___. (A) Amplitude is large (B) Amplitude is less (C) Frequency is less (D) Frequency is more 10. Mosquitos produce high pitch sound, because_____. (A) Their frequency of vibration is high (B) Their frequency of vibration is low (C) Amplitude is low (D) Amplitude is high

II. Multiple Correct Answer Type 11. Which of the following produces sound? (A) Beating of drums (B) Heating an iron rod (C) Vibrating strings (D) Movement of charges 12. Through which of the following media, sound can propogate (A) Air (B) Water (C) Iron rod (D) Vaccum 13. Which of the following are the parts of a human ear? (A) Retina (B) Cochlea (C) Cornea (D) Pinna 14. Which of the following statements are correct? (A) The unit of frequency is Hertz (B) The unit of time period is second. (C) The unit of wave velocity is metere per second (D) Loudness of sound is expressed in decibels. 15. Which of the following sounds cause noise pollution? (A) The sound coming from factories. (B) The sound created by heavy vehicles. (C) Cry of a child. (D) Cutting of vegetables.

III. Paragraph Type Velocity of sound is given by the expression, v=f×  Where, v = velocity of the wave ; f = frequency ;  = wavelength Using the above information answer the following questions

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7th Class Physics

108 16. The velocity of sound in air is 344 m/sec. What is the wavelength of a sound wave of frequency 32Hz? (A) 9.75m (B) 10.75m (C) 7.25m (D) 8.25m 17. A wave travels at a speed of 330 m/s. If the wavelength is 2.2cm, what will be the frequency of the wave? (A) 15,000Hz (B) 14,500Hz (C) 15,500Hz (D) 14,000Hz 18. Calculate the wavelength of radio waves of frequency 109Hz . The speed of radio waves is 3×108 m/s. (A) 20 cm (B) 35 cm (C) 30 cm (D) 25 cm

19. The wavelength of the wave generated by a tuning fork of frequency 256 Hz. velocity of sound in air at ordinary conditions is 340 m/s is (A) 1.33m (B) 2.3m (C) 0.1m (D) 0.3m 20. A tuning fork produces 256 waves in 2 seconds. The frequency of the tuning fork is (A) 1.120 Hz (B) 125 Hz (C) 128Hz (D) 130Hz

IV. Integer Type 21. The time period of oscillation is 1 second. Its frequency = ______Hz. 22. The time period of oscillation of a body is 0.25 sec. Its time period is ________ seconds. 23. The time period of seconds pendulum is _______ seconds.

V. Matrix Matching 24. (A) Audio vibrations of lower frequency (B) Audio vibrations of higher frequency (C) Frequencies up to 40,000 Hz can heard by

26. (A) Galton Whistle (B) Bat (C) Ultrasonic Vibrations are used (D) Reflection of Sound 27. (A) Frequency < 20 Hz (B)Frequency 20 Hz - 20,000 Hz (C) Frequency > 20, 000 Hz

28. (A) Frequency (B) Amplitude (C) Time period (D) Sound waves 29. (A) Sounds which have pleasing effect on ears (B) Sounds which do not have pleasing effect on ear (C) In Musical sounds (D) Noise is produced due to 30. (A) (B) (C) (D) 31. (A) (B) (C)

Veena, Guitar Mrudangam, Table Flute, shahnai Harmonium, Piano Pitch Loudness Two notes of same pitch and loudness are distinguished by

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

(p) Audio Vibration (q) Bass Effect (r) Shrill Effect (s) Dogs (t) Dolphins (p) Dolphins (q) Dogs (r) Ultra sonic Vibration (s) For homogenising milk (t) In Megaphone (p) Ultrasonic (q) infrasonic (r) Audible range (s) Reflection of sound (t) Absorption of sound (p) Mechanical form (q) Reflected sound (r) Maximum displacement of vibrating body (s) Time taken to complete one vibration (t) Hertz (p) Amplitude & frequency of vibration don’t change suddenly (q) Music (r) Irregular vibrations in a material (s) Noise (t) Sound from construction site (p) Membrane instruments (q) Wind instruments (r) Keyboard instrument (s) Stringed Instruments (p) Amplitude (q) Frequency (r) Quality

By the e nd o f t hi s c ha pte r , y o u w i l l u nd e r s ta nd

 Temperature

 Measuring of heat

 Measurement of Temperature

 Conduction of Heat

 Thermometers

 Convection of Heat

 Heat

 Radiation of Heat

1. Temperature Mr. Raj was on his way home after office. The sun was beating down and he was sweaty and uncomfortable. As soon as he reached home, he sat down for a while. Then he went straight to the refrigerator for a bottle of cool water.

Why did the bottle of water feel cool while it was so hot outside? The answer lies in the concept of temperature. Temperature indicates the degree of ‘hotness’ or ‘coldness’ of a body or a place. An Experiment – Hot & Cold are relative Aim: To show that the terms ‘hot’ and ‘cold’ as we feel them are only relative terms. Materials needed:Three glasses, hot water (as hot as you can bear to dip your finger in), ice-cold water, and water at room temperature.

Hot wat er

Water at room t emp

Ice cold water

Method (i) Pour hot water, water at room temperature, and the ice-cold water into three separate glasses. (ii) Keep the three glasses side by side on a table with the glass containing water at room temperature in the middle.

7 Chapter - 04

Heat

Learning Outcomes

(iii) Dip the index finger of one hand in the icecold water and that of theother hand in the hot water. Hold the fingers in the water for some time(for about a minute). (iv) Then dip both the index fingers into the water at room temperature. Observation: You will find that to one finger, the water feels hot and to the other it feels cold. Conclusion: This is because the finger that was in ice-cold water has become cold, and when it was placed in the water at room temperature, it felt that the water was warm, relative to the earlier experience of ice-cold water. Similarly, the finger that was first in hot water will feel the water at room temperature to be relatively cold. So you see the terms ‘hot’ and ‘cold’ are only relative terms. There always has to be a reference body with respect to which we can say whether a given body is hot or cold. Therefore, it is important that we use an instrument, rather than our own judgement to measure temperature.

2. Measurement of Temperature We measure temperature with an instrument called the thermometer. All thermometers measure temperature by making use of some property of a substance that varies with temperature. One such property is the change in volume (expansion and contraction) with temperature. The substance which is generally used in a conventional thermometer is either mercury or alcohol.

Temperature Scales Just like we use different units to measure length (like inch, centimetre, etc.), we use different units

7th Class Physics

110 to measure temperature. These different units are represented by different temperature scales. Here is a simplified description of how a temperature scale is defined. Two reference temperatures are chosen and the difference between these two temperatures are divided into a certain number of divisions. Each division is called one degree. The most commonly used reference temperatures are the melting point of ice and the boiling point of water. The two commonly used scales are the Celsius and the Fahrenheit scales. There is a third scale, the Kelvin scale which you will learn in higher classes. I. Celsius Scale This scale is indicated by °C [read as degree Celsius (in honor of Anders Celsius) or degree 1 centigrade]. On this scale the melting point of ice is taken as 0 °C and the boiling point of water as 100 °C. The difference between these two points is divided into 100 degrees. II. Fahrenheit Scale This scale is indicated by °F (read as degree Fahrenheit). On this scale, 32 °F is taken as the melting point of ice and 212 °F as the boiling point of water. The difference between these two points is divided into 180 degrees

Conversion Formula Two simple equations given below will help us to scale convert one temperature scale to another.

9  F =  C×  4  32 5  5  F  32  where F is for Fahrenheit and C is 9 for Celsius. C=

3. Thermometers I.

Mercury Thermometer In 1714, Gabriel Fahrenheit invented the first mercury thermometer. Mercury is used in thermometers because of the following reasons: • Mercury remains in the liquid state for a wide range of temperature. • It melts at – 38.87 °C and boils only at 356.58°C. • It is relatively easy to see because of its silvery grey colour. • It does not stick to glass. • It has a fairly uniform rate of expansion for a wide range of temperatures.

II. Alcohol Thermometer Thermometers in which alcohol is used have some advantage over mercury thermometers. Alcohol is cheaper and less harmful than mercury. Alcohol thermometers can measure much lower temperatures (up to -115 °C) than mercury. However, the main disadvantage of using an alcohol thermometer is that these thermometers can measure temperature only up to 78 °C while mercury thermometers can measure much higher temperatures.

.

Reading a Conventional Thermometer A laboratory thermometer is shown in. It consists of a thin glass tube which is sealed at one end and www.betoppers.com

Heat

111 has a bulb at the other end. The bulb is generally filled with mercury or alcohol. Mercury appears as a silvery grey line and alcohol looks like a red line in 1. the thermometer. To read the temperature on this thermometer, you just read the number on the scale at the tip of the red or silvery grey line. Remember to mention the unit used (Celsius or Fahrenheit). The range of temperature which a laboratory thermometer can measure is from -10°C to 110°C.

Formative Worksheet A laboratory thermometer generally reads temperature from i while a clinical thermometer reads temperature from ii . The information in which alternative completes the given statement? (A) i ii - 10°C to 35°C 42°C to 110°C

(B)

i

ii

- 10°C to 110°C 35°C to 42°C

(C)

i

ii

- 10°C to 42°C 35°C to 110°C

(D)

i

ii

35°C to 42°C - 10°C to 110°C

Temperature

Temperature

A metallic ball at 40°C is dropped in a vessel containing water at 20°C. Which of the following statements corresponding to the above statement is correct? (A) There will be an increase in temperature of both the ball and water. (B) There will be a decrease in temperature of both the ball and water. (C) Heat will flow from the ball to the water. (D) Heat will flow from the water to the ball. A solid substance having a temperature of 90°C is kept in a beaker. Some water having temperature of 60°C is then poured into the beaker. After sometime, the temperature of the water will be (A) 60°C (B) 90°C (C) Below 60°C (D) Between 60°C and 90°C Which of the following graphs represents rate of cooling of hot water? (A) (B)

Time

(C)

Time

(D) Temperature

Temperature

The thermometer used to measure our body temperature is called a clinical thermometer. It is generally a mercury thermometer, though nowadays digital thermometers are becoming very popular. 2. Characteristic feature of Mercury Thermometer • There is a little arrow (at 98.4 or 98.6 °F) showing the normal body temperature. • There is a constriction or ‘kink’ in the tube near the bulb. This kink has been made to ensure that the mercury in the thermometer does not contract (and flow back into the bulb) before the temperature has been read. Precaution in handling Thermometer 3. • Wash the thermometer well and dip it in an antiseptic liquid before and after using it. • Be careful while using a thermometer. It can break easily. Remember, mercury is a toxic substance. Digital Thermometer 4. These days digital thermometers are used to measure temperature. These thermometers do not use mercury. The temperature is displayed as a number.

Time

Time

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7th Class Physics

112 5.

The given table lists some materials as conductors or insulators. Conductor

Insulator

Iron

Copper

Aluminium

Plastic

Which material is listed incorrectly in the table? (A) Iron (B) Plastic (C) Copper (D) Aluminium 6.

Column A

Column B

i

Black shirt

a Summers

ii

White shirt

b Laboratory thermometer

iii - 10°C to 110°C c Clinical thermometer iv

35°C to 42°C

Conceptive Worksheet

2.

3.

What is the temperature range of a clinical thermometer? (A) 32°C to 38°C (B) 35°C to 42°C (C) 38°C to 45°C (D) 40°C to 45°C Which of the following units cannot be used to measure temperature? (A) Fahrenheit (B) Celsius (C) Kelvin (D) Pascal In the given thermometer, the bigger marks read one degree. Also, there are ten small divisions between them.

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5.

d Winters

The alternatives in the given table can be correctly matched as (A) i  d, ii  a, iii  c, iv  b (B) i  a, ii  d, iii  b, iv  c (C) i  d, ii  a, iii  b, iv  c (D) i  a, ii  d, iii  c, iv  b 7. The body temperature of an animal is 86 °F. Express the same temperature in degree Celsius. 8. The day temperature in Delhi on a hot day was 45 °C. Express this temperature in degree Fahrenheit. 9. Express the following temperatures on the Fahrenheit Scale. (a) 35 °C (b) 15 °C 10. Express the following temperatures on the Celsius Scale. (a) 95 °F (b) 41 °F

1.

4.

6.

How can one small division be read in the given thermometer? (A) 0.1°C (B) 0.5°C (C) 0.05°C (D) 0.01°C What is the temperature range for a clinical thermometer on the Fahrenheit scale? (A) 94°F – 110°F (B) 90°F – 108°F (C) 94°F – 108°F (D) 93°F – 109°F Which of the following temperatures represents the average human body temperature? (A) 25°C (B) 30°C (C) 37°C (D) 48°C A i thermometer has a temperature range of ii The information in which alternative completes the given statement? (A) i ii laboratory 37°C - 57°C

(B)

i

ii

clinical 35°C - 42°C

(C)

i

ii

clinical 25°C - 45°C

(D)

i

ii

laboratory 40°C - 140°C

7.

8.

Before a clinical thermometer is used, it should be ensured that the mercury level is (A) At 42ºC (B) At 37ºC (C) Below 35ºC (D) Between 35ºC and 42ºC For an experiment, Mahima requires water at 80ºC. Which type of thermometer can she use to measure the temperature of water? (A) Only clinical thermometer (B) Only laboratory thermometer (C) Both clinical and maximum-minimum thermometer (D) Both laboratory and maximum-minimum thermometer

4. Heat Have you stirred hot tea or milk with a stainless steel spoon? Next time you do this, leave the spoon in the hot liquid for a little while. Now, touch the handle of the spoon. What do you notice? The handle of the spoon would have got a little warm.

Heat

113 Why did the handle of the spoon become warm?

When there is a difference in temperature between two bodies, a certain type of energy flows between them. This energy is called heat energy. When this heat energy flows into a body, it warms the body while if it flows out of a body, it cools the body. In other words, the hot body loses heat energy while the cold body gains heat energy. Flow of Heat energy Heat energy flows from a region of higher temperature to a region of lower temperature and never in the other direction. Heat flow continues till the temperatures of both the hot and the cold body become the same (whose value will be somewhere in between the temperatures of the hot body and the cold body). Flow of Heat Energy Hot body

Cold Body

Activity – Direction of Flow of Heat Aim: To show that heat energy flows from a hot body to a cold body. Materials needed:A coin, tongs, boiling water, cold water, and two glasses. Method: 1. Take two glasses and fill them to three-fourths of the capacity with cold water. Boil water in a vessel and drop a coin in the boiling water. Leave it for some time so that the coin gets really hot. 2. Ask an adult to use the pair of tongs to pick up the hot coin and drop it in one of the glasses filled with water. 3. After about 2 minutes, dip your finger in the two glasses, first in the glass without the coin and then in the glass with the coin. Observation: You will observe that the water in the glass in which you dropped the hot coin will be warmer. Conclusion: Heat energy has been transferred from the hot body (coin) to the cold body (water).

5. Measuring Heat Just as we measure length in centimetres and metres, heat energy (and any other form of energy for that matter) is measured in calories or joules. In the SI system (which you know is the international standard for the system of units), heat energy is measured in joules. The symbol used to represent ‘joule’ is ‘J’. Note Temperature is not energy. However, if we know the temperature of a substance, we can calculate the amount of heat energy it contains.

6. Conduction of Heat We now know that heat is a form of energy and that it flows from a region of higher temperature to a region of lower temperature. But how does this happen? The methods by which heat is transferred from one region to another are called the modes of heat transfer. There are three modes of heat transfer, namely, conduction, convection and radiation. We know that the metal objects heat up when they come in contact with hot solids or liquids. Just like the stainless steel spoon becoming hot when left in a hot liquid. How do you think this happens? The heat from the hot liquid moves to the stainless steel spoon by a process called conduction. How does Conduction Occur? We know that matter is made up of tiny particles called atoms and molecules. We know that in solids, the particles are very tightly packed and they cannot move around freely. However, they can vibrate about a fixed point. Let us take the example of a metal rod being heated at one end by a flame. Heat increases the vibrations of the particles at that end of the rod which is in contact with the flame. These particles collide (bump) and transfer their energy to their more slowly moving neighbors further away from the flame, and make them vibrate faster. They in turn pass the vibration on to their neighbors even further away from the flame, and so on. In this manner, heat energy is transferred from one particle to the next, although each individual particle remains in its original location. Conduction is the primary mode of heat transfer through a solid. Conduction of heat energy can occur within a body or between two bodies when they are in contact with each other. However, the rate of conduction of heat is different in different materials. Also some www.betoppers.com

7th Class Physics

114 materials allow heat energy to flow through them easily while others do not. Conductors & Insulators The substances that conduct heat easily are called conductors. For example, metals are good conductors of heat. Substances such as wood, straw, clay, rubber, glass, and Bakelite (a kind of plastic) do not conduct heat very well and are called insulators. Air and water are also poor conductors of heat. Some materials such as wool, fur, and bird feathers are good insulators because they trap air between their fibres. Activity – Rate of Conduction Aim: To demonstrate that the rate of conduction of heat is different for different materials. Materials needed:Rods of equal diameters and equal lengths made of aluminium, copper, and iron (you will find these in your school lab), two wooden slabs, small metal balls, wax, and flame Method: 1. Take two rods at a time, say, first the copper and aluminium rods. 2. Stick the metal balls to the rods at equal distances using wax. 3. Light the burner and make your observations. 4. Repeat the experiment with iron and aluminium. Observation: You will see that the balls drop fastest for copper, then aluminium and slowest for iron. Conclusion: Of the three metals, the rate of conduction is highest for copper and the lowest for iron. Practical Applications of Conduction 1. Both good and bad conductors are useful to us, depending on our needs. We use good conductors (metals) for making cooking utensils, and heat-resistant plastic for making the handles for these vessels. 2. We wear woolen clothes in winter because wool is a bad conductor of heat. Thus, it helps in retaining body warmth. The wool fibre has a series of curls and these are called ‘crimps’. These crimps create small air pockets. The greater the number of crimps, the greater is the number of air pockets which can hold and trap air. Thus, a thin layer of air is created which insulates us from the cold weather and also prevents body heat from escaping into the surroundings. www.betoppers.com

7. Convection of Heat This mode of heat transfer is seen in liquids and gases in which the molecules are less densely packed and are free to move. Consider the following example . When a vessel containing water is heated, after sometime, the water at the top of the vessel also becomes hot. How does this happen? The water at the bottom of the vessel gets hot, and since hot water is lighter than cold water, it rises up, carrying the heat energy with it. The colder and denser water on the top falls to the bottom of the vessel and gets heated in its turn. This process continues and results in a circulating stream of hot and cold water. As a result, the “whole water in the vessel gets heated. This method by which heat is transferred by the mass movement of the liquid or gas itself is called convection. Activity – Convection Transfers Heat Aim: To show that heat transfer in water is due to convection. Materials needed:A glass beaker made of heatresistant glass/glass, test tube, candle, match box, potassium permanganate crystals/artificial food colouring, and a spoon. Method 1. Fill the beaker to the half way mark with clear water. 2. Take a pinch of the potassium permanganate crystals/food colouring and drop it gently to the bottom of the beaker. 3. Let it settle for a few minutes. Observation: You will see that the water at the bottom of the beaker is coloured, but the water at the top is quite clear. Light the candle and pick up the beaker very gently and hold it on top of the candle flame. You will see columns of coloured liquid rise up in the beaker. Conclusion: This is due to convection. Water molecules at the bottom get heated up and rise, carrying the heat energy with them. Practical Applications of Convection 1. The principle of convection of heat can be used in many applications. 2. When you want to warm a fluid, you should warm it from below. If a room has to be warmed with a room heater, the heater should be placed at a lower level, so that the room gets warmed evenly. 3. Similarly, an air conditioner used for cooling a room should be placed at a higher level. 4. In olden times, buildings were designed where windows and ventilators were cleverly placed so that the buildings were kept cool in summers.

Heat

115 Sea Breeze and Land Breeze Sea Breeze: Convection plays a major role in maintaining a moderate temperature in places near the sea. Land masses (beach, coastal town, or city) heat up much faster than water bodies (sea, ocean) during daytime, and cool down much faster during the night. This difference in temperature sets up a wind pattern. During the day, the air above land rises since it is warm and cooler air from over the sea flows in to take its place. This gives rise to a sea breeze (sometime in the afternoon) which cools the land. Land Breeze: In the night, since land cools down 1. much faster than the sea, the cooler air over land flows out to take the place of warmer air over the sea which rises, setting up a land breeze.

8. Radiation of Heat

2. Radiation is the form of heat transfer that can occur in the absence of a medium. Thus, it is the mode of heat transfer through vacuum. While heat radiations can travel through vacuum, they can also travel 3. through media such as air, water, etc. All bodies give out energy that travels in the form of radiation (much like light) through space. Energy from the sun reaches us through radiation. Bodies that absorb this radiation become hot. Radiation of Heat & Colour The amount of heat energy that is absorbed by a body depends on its colour. Bodies that are black absorb more of radiated heat than white bodies. We feel more comfortable wearing white and lightcoloured clothes in summer. This is because white clothes absorb comparatively less amount of heat than dark clothes, and therefore keep us relatively cooler. The reverse applies in winter; we are more comfortable in dark clothes. This is because we need to absorb as much heat as possible from our surroundings to keep ourselves warm, and dark clothes absorb more heat than light-coloured ones. The converse is also true. Black and dark bodies also radiate more heat than light-coloured bodies. Activity – Radiation of Heat Aim: To show radiation of heat. Materials needed: A source of heat, like a room heater or an electric bulb, and adult supervision. Method: 1. Switch on the room heater or the electric bulb. 2. Wait for a few minutes, and put your hand close to it without touching.

Observation: You will observe that you can feel the heat of the heater/bulb even when you do not touch it. Conclusion: Heat energy is reaching your hand from the heater/bulb in the form of radiation. In the above activity, you can confirm that the heat energy reaching your hand is being carried by radiation and not convection. Hold your hand a little below the bulb. The heat which you feel below the bulb is mostly due to radiation because hot air always rises up. Practical Applications of Radiation Heat radiations travel in vacuum and in air just like light rays. That is why electric room heaters have mirrors behind the heating coil. This helps in focussing the heat from the heating coil in front of the heater. Solar panels used in households for heating water, are designed to maximize the absorption of heat from the sun. A black metal sheet is used to increase heat absorption. A thermometer called the black bulb thermometer uses the principle of heating by radiation to measure the amount of radiation received by sources such as the sun. Preventing Loss of Heat A thermos flask is a very good example of how heat loss by all three modes of heat transfer, conduction, convection, and radiation, is minimized. Let us see how this is done. Heat loss due to conduction is minimized by using insulating materials (like plastics) for the outer casing and the cap of the thermos flask. The inner jar is a double-walled bottle made of glass or stainless steel. The space between the two walls is a vacuum, so that heat loss due to conduction is minimized. This also reduces heat loss due to convection since there are no air molecules to carry-away the heat. Heat loss due to radiation is minimized by making the surface of the jar highly reflective, so that heat radiations are reflected back into the jar..

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116

Formative Worksheet

16.

11. In solids, heat flows from a i temperature to a ii temperature by the process of iii . The information in which alternative completes the given statement? (A) i ii iii

i breeze blows during the ii from the land to the sea. The information in which alternative completes the given statement? (A) (B) i ii i ii Land day

(C)

higher lower convection

(B)

i

ii

(C)

i

ii

iii

higher lower conduction

(D)

i

ii

iii

17.

The wax on which rod will melt first? (A) Glass (B) Plastic (C) Wooden (D) Aluminium 13. Four pins (I, II, III, and IV) are fixed to a metallic rod with the help of wax. The rod is heated at one end.

ii

Sea night

Air aluminium

(C)

i

Aluminium air

(D)

ii

Air iron

i

ii

Iron aluminium

18. During the daytime, the air over the land becomes __i_ and rises up. The cold air from the __ii__ starts moving towards the __iii__to fill the vacuum created. This movement of air is known as __iv__. The information in which alternative completes the given statements? (A) i ii iii iv hotter sea land sea breeze

(B)

i

ii

iii

iv

hotter land sea land breeze

(C)

i

ii

iii

cooler sea land

(D)

i

ii

iii

cooler sea land

Which of the following pins would drop first? (A) I (B) II (C) III (D) IV 14. Kitchen utensils are provided with a copper base because (A) Copper base increases their durability (B) Copper base makes them attractive (C) Copper is a good conductor of heat (D) Copper is a bad conductor of heat 15. A litre of water at 50°C is mixed with another litre of water at 70°C. What will be the temperature of the mixture? (A) 20°C (B) 120°C (C) Between 50 °C and 70 °C (D) More than 70 °C, but less than 120 °C www.betoppers.com

i

i is a good conductor of heat, but ii is a bad conductor of heat. The information in which alternative completes the given statement? (A) i (B) ii i ii

lower higher convection

12. Four rods made up of glass, plastic, aluminium, and wood are covered with wax at one end. The other ends of these rods are inserted in a container containing hot water.

(D)

ii

Sea day

iii

lower higher conduction

i

Land night

iv sea breeze iv land breeze

19. Four rods made of iron, wood, paper, and plastic are connected by a straight metallic rod, as shown in the given figure. The rods are heated by a hot filament placed near them. After sometime, it is observed that the temperature of the iron rod is more than that of the other three rods.

Heat transfer within the rods takes place by (A) Conduction (B) Convection (C) Radiation (D) Induction

Heat

117

20. When a layer of air receives enough heat from 10. When an ice cube is kept on the palm, it starts melting because Earth’s surface, it expands and becomes less dense. (A) Of the presence of sweat on the palm This layer is then pushed upward by buoyancy. The (B) Of the atmospheric and body pressure on the cool and heavy air sinks under this layer. This cycle ice cube repeats with the re-heating of the cool air. (C) The heat energy of the palm gets transferred The phenomenon described in the preamble is to the ice cube known as (D) The heat energy of the ice cube gets transferred (A) Radiation (B) Induction to the palm (C) Conduction (D) Convection 11. Which of the following substances is an insulator? 21. Heat transfer by the process of i requires (A) Iron (B) Steel medium while no medium is required for the heat (C) Plastic (D) Aluminium transfer by the process of ii . The information in which alternative completes the 12. Which of the following statements is incorrect regarding the process of conduction? given statement? (A) (A) There should be no medium. i ii (B) The objects should be solid in state. radiation convection (C) Two objects should be in contact with each (B) other. i ii (D) The temperature of the objects should be convection conduction different. (C) i ii 13. Which of the following examples is not an application of the process of convection? conduct ion convection (A) Ventilation in houses (D) i ii (B) Land breeze and sea breeze convection radiation (C) Central heating of buildings (D) Wooden handles of cooking utensils 22. Convection is a process of heat transfer by the actual 14. Which colour would you prefer to wear in movement of molecules. Convection can take place summers? only in I and II . Heat transfer by III can (A) White (B) Purple (C) Black (D) Red take place without any medium. 15. Which of the following methods of heat transfer The given statements are correctly completed by does not involve the movement or vibration of alternative particles? (A) I II III (A) Radiation (B) Circulation solids liquids radiation (C) Convection (D) Conduction (B) 16. Ravi kept some water for boiling for making tea. I II III To mix sugar, he stirred it with a spoon. After some solids gases conduction time, he observed that the temperature of the other (C) I II III end of the spoon had changed. How did the temperature of the spoon change? liquid gases radiation (A) It became hot by the process of conduction. (D) I II III (B) It became cold by the process of conduction. (C) It became hot by the process of convection. liquids solids conduct ion (D) It became cold by the process of convection 17. On a sunny afternoon, Ram has to go out. His onceptive orksheet mother gives him umbrella to make him feel 9. Which of the following statements is incorrect? comfortable. Ram feels comfortable under the (A) With heating, the temperature increases umbrella because (B) With cooling, the temperature decreases (A) Material of umbrella reflects heat (C) Temperature is the measure of degree of (B) Atmosphere under the umbrella absorbs heat hotness of a body (C) Umbrella prevents the heat from reaching Ram (D) Heat always flows from a low temperature to (D) Atmosphere under the umbrella slowly transmits high temperature body heat to the body

C

W

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7th Class Physics

118

Summative Worksheet 1.

2.

3.

Ramesh fills a black and a white-coloured container with an equal amount of water and leaves them in mid-day sun for about an hour. When he checks the temperature of water in both the containers using a thermometer, he finds that the water in the blackcoloured container is hotter. Which of the following statements cannot be concluded from the given 6. activity? (A) Dark-coloured clothes are preferred during summers. (B) Light-coloured clothes should not be worn in winters (C) Dark-coloured clothes are bad reflectors. (D) Light-coloured clothes are bad absorbers Which of the following statements is not true? (A) Loss and gain of heat by human body takes 7. place by radiation. (B) Heat is transferred from the sun to the Earth by radiation. (C) Heat transfer by radiation requires a medium. 8. (D) All hot bodies radiate heat. Which table correctly matches the mode of transfer of heat in air and an iron nail? (A) Object Mode of transfer of heat

(B)

(C)

(D)

4.

5.

Water

Convection

Iron nail

Conduction

Object

Mode of transfer of heat

Water

Convection

Iron nail

Convection

Object

Mode of transfer of heat

Water

Conduction

Iron nail

Conduction

Object

Mode of transfer of heat

Water

Conduction

Iron nail

Convection

Which of the following statements regarding land and sea breezes is correct? (A) Land as well as sea breeze is caused by convection. (B) Land as well as sea breeze is formed by conduction. (C) Sea breeze is formed by radiation, while land breeze is formed by convection. (D) Land breeze is formed by radiation, while sea breeze is formed by conduction.

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9.

Some examples of transfer of heat are I. Heat felt from a bonfire II. Shaking hands with someone III. Heating a pot containing water IV. An ice cube cooling down the hand of its holder Which of the given examples are examples of heat transfer by conduction? (A) I and III (B) I and II (C) II and IV (D) I, III, and IV Which of the following instances involves heat transfer by conduction? (A) Formation of land and sea breezes (B) Heating a room using a heat convector (C) Container with white surface getting cold in shade (D) Measuring the temperature of a sick person with a thermometer The process of conduction (A) Generally occurs in gases (B) Requires a medium for heat transfer (C) Is a measure of the hotness of solids (D) Occurs when water is heated in a beaker A teacher asked four of his students to measure the temperature of tap water in a beaker. His students Prateek, Qaif, Romila, and Suraj measured the temperature, as shown in the given figure.

Who placed the thermometer correctly to obtain the result? (A) Prateek (B) Qaif (C) Romila (D) Suraj Four people, P, Q, R, and S, had gone for a checkup to a medical camp. The doctors present there measured their body temperatures using four different thermometers. The readings on the thermometers are shown in the given figure.

Whose temperature matches with the normal body temperature of a healthy human being? (A) S (B) P (C) Q (D) R

Heat

119

10. The given figure represents the thermometer with 14. When an ice cube is kept on the palm, it starts melting because which a doctor has just measured the temperature (A) Of the presence of sweat on the palm of a patient. (B) Of the atmospheric and body pressure on the ice cube (C) The heat energy of the palm gets transferred to the ice cube (D) The heat energy of the ice cube gets transferred to the palm 15. For an experiment, Mahima requires water at 80ºC. Which type of thermometer can she use to measure the temperature of water? (A) Only clinical thermometer What is the temperature of the body of the patient? (B) Only laboratory thermometer (A) 39°C (B) 39°F (C) Both clinical and maximum-minimum (C) 39.6°F (D) 39.6°C thermometer 11. The device used for measuring the degree of hotness (D) Both laboratory and maximum-minimum of an object is called a (A) Telescope (B) Barometer thermometer (C) Thermometer (D) Stethoscope 12. The temperatures of liquids I, II, III, and IV were OTS orksheet measured using a thermometer. The various readings 1. Fill in the blanks with the correct words. that the thermometer displayed are shown in the a. The degree of hotness or coldness of a body given figure. is called ___________.

H

2.

The decreasing order of the given liquids according 3. to their hotness is (A) (I)> (II) > (III) > (IV) (B) (II) > (I)> (III) > (IV) (C) (IV) > (III) > (I)> (II) (D) (III)> (IV) > (II) > (I) 13. Ravi kept some water for boiling for making tea. 4. To mix sugar, he stirred it with a spoon. After some time, he observed that the temperature of the other end of the spoon had changed. How did the temperature of the spoon change? (A) It became hot by the process of conduction. (B) It became cold by the process of conduction. (C) It became hot by the process of convection. (D) It became cold by the process of convection.

W

b. Heat is a form of ____________. c. Heat energy is measured in ___________. Write true or false. (A) Temperature is a form of energy. (B) Heat energy flows from a hot body to a cold body. (C) Temperature is measured in joules. (D) When heat energy flows into a body it warms the body. Which of these thermometers would you use to measure body temperature? (A) Alcohol thermometer (B) Laboratory thermometer (C) Clinical thermometer (D) Any mercury thermometer Heat energy will flow from our body to the surroundings if (A) The surroundings are at a higher temperature than our body. (B) The surroundings are at the same temperature as our body. (C) The surroundings are at a lower temperature than our body. (D) There is no relationship between heat flow and temperature.

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7th Class Physics

120 5.

6.

7.

Give one point of difference between the following a) A Clinical Thermometer and a Mercury Thermometer b) Celsius scale and Fahrenheit scale The given figures show the change in temperature of ice when it is heated.

ice at –2°C water at 10°C What is the change in temperature as ice converts to water? (A) 2°C (B) 5°C (C) 10°C (D) 12°C Charles heats some water in a beaker and then measures the temperature of water using a thermometer. After this, he keeps the water away from the burner for two hours and then again measures the temperature of water.

The information in which alternative completes the given statement? (A) i ii (B) i ii (C) i ii (D) i ii X 50°C

9.

Y 40°C

The information in which alternative completes the given statement? (A) i (B) i ii ii

(C) i

ii

Y 37.8°C

8.

After heating

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After removing the heat source

Y 30°C

The body temperatures of four people W, X, Y, and Z were taken using four different thermometers I, II, III,and IV respectively. The reading of each thermometer is shown in the given figure. Among the four people, the body temperature of i is the highest and is equal to ii .

W 39.4°C

80°C 30°C What is the total decrease in the temperature of water? (A) 30ºC (B) 50ºC (C) 60ºC (D) 80ºC Sujit took two beakers I and II containing equal amounts of two different liquids X and Y respectively. He heated both liquids for 30 seconds. He then kept both the beakers away from the heat source for 5 minutes. He then measured the temperature of X and Y as shown in the given figure. The temperature of liquid i is lower after the removal of the heat source, and the decrease in its temperature is ii .

X 60°C

X 37.0°C

(D) i

ii

Z 39°C

10. Define the following: (a) Conduction (b) Convection (c) Radiation 11. Conduction, convection, and radiation are (A) Different modes of heat energy (B) Different modes of heat transfer (C) Different measures of temperature (D) All of these 12. Convection does not occur in (A) Vacuum (B) Liquids (C) Gases (D) Air 13. In fluids, heat transfer takes place primarily due to (A) Conduction (B) Convection (C) Radiation (D) None 14. Give one point of difference between the following. a) Conduction of heat and Convection b) Conductors and Insulators 15. Which of these is a good conductor of heat? (A) Wood (B) Straw (C) Water (D) Copper 16. Which of these is a bad conductor of heat? (A) Wool (B) Gold (C) Copper (D) Stainless steel

Heat

121

17. Name the mode of heat transfer in each of the following cases: (a) In a vessel of boiling water. (b) Heat energy reaching us from the sun. (c) A vessel kept on a hot stove becomes hot. (d) Warm breeze from the land flows towards the sea. (e) A spoon kept in a hot tea cup becomes warm. 18. a) Name the mode of heat transfer from one body to another where it is essential for the two bodies to be in contact. b) Name the mode of heat transfer that can take place even in vacuum. 19. Give reasons. a) Woolen clothes are worn in winter. b) Light cotton clothes are worn in summer. 20. a) Why is mercury used in thermometers? Give three reasons. b) List the special features of a mercury clinical thermometer. 21. Match the following. Column A Column B (A) Insulator p. Unit of heat energy (B) Energy q. Good conductors (C) Fahrenheit r. Rubber (D) Joule s. Thermometer (E) Metals t. Heat

IIT JEE Worksheet

A steel cup at 400oC is dropped in a container of water at 40oC, then (A) Heat will flow from cup to water (B) Heat will not flow from cup to water or from water to cup (C) Heat flow from water to cup (D) The temperature of cup increases 6. Normal temperature of human body is (A) 37oC (B) 98.4oF (C) Both (D) None 7. Which of the following is NOT correct in ‘heat transmission through conduction’ ? (A) Heat flows only when one end of an object is hot and other end is cold. (B) When heat is flowing through two objects they must remain in contact with one another. (C) Once the two ends of the object through which heat is flowing attain the same temperature conduction of heat stops. (D) In conduction, heat is transferred through the moving particles of medium from one place to another. 8. Heat transmission through convection does not take place in (A) Liquids (B) Gases (C) Solids (D) None 9. While sitting near a heater or a bonfire, you feel warm. The heat from the heater reaches you by _____ process (A) Conduction (B) Radiation (C) Convection (D) All 10. Handles of electrical appliances and cooking utensils are made from ? (A) Bakelite (B) Plastics (C) Wood (D) All 5.

I.

Single Correct Answer Type

1.

Which body feels hot ? (A) A body losing heat (B) A body gaining heat (C) Both (D) None II. Under what condition the flow of heat is rapid. (A) One of the two bodies is too hot 11. (B) One of the two bodies is too cold (C) There is a big difference in the temperature of two bodies (D) All __ is the physical quantity which measures the 12. amount of heat in a body. (A) Temperature (B) Energy (C) Mass (D) None One litre of water at 30 o C is mixed with one litre of water at 60o C, the temperature of the mixture will be (A) 90oC (B) > 60oC but < 90oC o (C) 30 C (D) Between 30oC & 60oC

2.

3.

4.

Multiple Correct Answer Type Choose the correct statements: (A) S.I. unit of temperature is kelvin (B) Temperature is a scalar quantity. (C) S.I. unit of heat is joule(J). (D) 1 cal = 4.2 J Mercury is the commonly used thermometric liquid because (A) It can be easily obtained in pure state (B) It does not stick to glass tube thermometer (C) It has a very high density (D) It has very low freezing point and a very high boiling point.

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7th Class Physics

122 13. Choose the correct option : (A) Temperature is a scalar quantity (B) Heat energy is also called thermal energy (C) The device for measuring the temperature of a substance is called a thermometer (D) Temperature of a body decides the direction of heat flow from the body 14. Choose the correct option : (A) Two bodies of same substance having different masses may have same temperature but different amount of heat (B) Two bodies of same substance having different masses may have same amount of heat but different temperature (C) Heat contents of a body do not decide the direction of heat flow from the body (D) The thermometer which has mercury as the thermometric liquid is called mercury thermometer

III. Paragraph Type

15

16.

17.

18.

19.

Paragraph-I Temperature can be measured in °C and °F The boiling point of water is (A) 180°C (B) 12°C (C) 40°C (D) 100°C The melting point of ice is (A) 0°C (B) 273°C (C) 40°C (D) 100°C Normal temperature of human body is (A) 98.4ºF (B) 120ºF (C) 80ºF (D) 37ºF Paragraph-II For measuring temperature Celsius scale, Fahrenheit scale , Kelvin scale and Reaumur scale may be used. The scale on which ice point is taken as 0°C and steam point is taken as 100°C is (A) Celsius scale (B) Fahrenheit scale (C) Kelvin scale (D) Reaumur scale The scale on which ice point is taken as 32°F and steam point is taken as 212°F. (A) Celsius scale (B) Fahrenheit scale (C) Kelvin scale (D) Reaumur scale

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20. The scale on which ice point is taken as 273 K and steam point is taken as 373K. (A) Celsius scale (B) Fahrenheit scale (C) Kelvin scale (D) Reaumur scale

IV. Integer Type 21. 1 kilo calorie = _________ calories 22. Ice point in Fahrenheit is ______°F

V. Matrix Matching 23. A) Temperature B) Heat C) Centigrade Scale

(p) Energy (q) 0oC - 100oC (r) Force (s) Measurement of Energy

24. A) Silver, Copper, Lead, Iron (p) Solids B) Wood, Wool, Bakelite, (q) Fluids Plastics C) Convection (r) Conductors (s) Insulators 25. A) Process due to which a solid directly

B) Alcohol thermometer C) Liquid metal 26. A) Land breeze blows during B) Sea breeze blows during C) Dark colour clothes are preferred during

(p) To measure very low change into gaseous state temperature (q) Sublimation (r) To measure high temperature (p) Summer (q) Winter (r) day (s) Night



IIT FOUNDATION Class VII

PHYSICS SOLUTIONS

© USN Edutech Private Limited The moral rights of the author’s have been asserted. This Workbook is for personal and non-commercial use only and must not be sold, lent, hired or given to anyone else.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior written permission of USN Edutech Private Limited. Any breach will entail legal action and prosecution without further notice.

Utmost care and attention to the details is taken while editing and printing this book. However, USN Edutech Private Limited and the Publisher do not take any legal responsibility for any errors or misrepresentations that might have crept in.

Published by

:

USN Eductech Private Limited Hyderabad, India.

CONTENTS 1.

Measurement

..........

123 - 132

2.

Motion

..........

133 - 144

3.

Light

..........

145 - 154

4.

Electric Current & Its Effects

..........

155 - 164

5.

Wind, Storm and Cyclones

..........

165 - 170

6.

Sound

..........

171 - 178

7.

Heat

..........

179 - 184

1. MEASUREMENT SOLUTIONS

FORMATIVE WORKSHEET KEY 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

C

C

A

C

A,B, C,D

B

A

B

A

B

*

A

B,C

B

B

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

*

*

B

D

B

C

A

*

*

C

C

D

C

A

A

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

*

*

A

B

*

*

C

A

D

A

D

*

D

C

C

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

D

C

C

C

B

C

B

C

*

A

B

B

D

C

A

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

B

C

A

C

B

A

*

D

A

B

*

C

D

B

C

76

77

D

A

HINTS/ SOLUTIONSTO THE SELECTED QUESTIONS 4.

6.

10kg 10  1000 g  3 m3 102 cm 

= 10 × 10–3 g cm–3 = 10–2 g cm–3 100 quintal = ____ ng 1 quitanl = 100kg  100 quintal = 100 × 100 kg = 107 g –9 1 ng = 10 g 

7.

1quintal 107 g  9 1ng 10 g

 1 quintal = 1016 ng. 1 day = 24 hours = 24 × 60 min = 24 × 60 × 60s

1second = 

1 days 24  60  60 1 days 86400

1 hectare = 104 cm2 1 gcm–3 = 1000 kgm–3 1kgm–3 = 10–3 gcm–3  16 kgm–3 = 16 × 10–3 gcm–3 10. 1 nanogram = 10–9g 1 milligram = 10–3 g 8. 9.



1nanogram 109 g  3  106 1milligram 10 g

 1 nanogram = 10–6 milligram (or) x = –6 11. 1 giga = 109 1 micro = 10–6 1 mega = 106 1 pico = 10–12  A  r; B  s; C  p; D  t 16. A  q; B  p; c  t 17. (A) Derived unit  Newton (B) Fundamental unit  Kilogram (C) Derived quantity  Area (D) Fundamental quantity  Height and thickness

7th Class Physics

124 18. 1 km = 1000 m 1 m = 100 cm 1 cm = 10 mm  1km > 1m > 1cm > 1mm 19. Number of turns (n) = 24 Length of the coil (L) = 4.8cm

L 4.8  cm n 24 = 0.2cm (or) 2mm 20. Distance = 3250 m 1000 m = 1km Diameter 

3250 km 1000 = 3.25 km h = 1.65 m 1m = 100 cm = 1000 mm ( 1cm = 10 mm)  1.65 m = 1.65 × 1000 mm = 1650 mm Diameter of the ball = x2 – x1 = 4.7 – 3.4 = 1.3 cm (i) 1 cm = 10 mm (ii) 1 dm = 10–1 m (iii) 1 kilometre = 1000 m (iv) 1 m = 100 cm (i) 1 deca metre = _________ decimetre 1 decametre = 10m 1 decimetre = 10–1m 3250 m 

21.

22.

23.

24.

10 decimetre 101 = 100 decimetre (ii) 1 millimetre = ________ centimetre 1mm = 10–3 m 1 cm = 102 m  1 decametre 

1mm 103  1cm 102  1mm = 10–3 × 10–2 cm = 10–5 cm (iii) 1 hectometre = ________ kilometre 1 hectometre = 100m 1 kilometre = 1000m

(iv) 1 decimetre = _________ centimetre 1 decimetre = 10–1 m 1 centimetre = 10–2 m 1 decimetre 101  1 centimetre 102  1 decimetre = 10 centimetre 25. 1 quintal = 100 kg 1 ton = 100 kg

1Quintal 100  1ton 1000  1 quintal = 10–1 ton = 0.1 ton 26. 1 metric tonne = 1000 kg 1 mg = 10–3 g 



1 metric tonne = 106 × 103 g = 109 g 27. 1 microsecond = 10–6 s 1 millisecond = 10–3 s

1 micro second 106  1 milli second 103  1 micro second = 10–3 milli second.  x = –3 28. 1 tonne = 1000 kg 

1 hectometre 100  1 kilometre 1000  1 hectometre = 10–1km = 0.1 km

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1 tonne 1000 = 10–3 tonne 1 tonne = 1000 kg  4.4 tonnes = 4.4 × 103 kg 6 hours = 6 × 60 min = 6 × 60 × 60 s = 21600 s 8 min = 8 × 60 s = 480 s 6 hours + 8min = (21600 + 480)s  = 22080 sec 1 min = 60 s 1 day = 24 hours 1 year = 365 days 1 century = 100 years  The right match is ABCD / srqp. 1 hour = 60 min 1 day = 24 hours = 1440 min 1 year = 365 days = 8760 hours 1 century = 100 years = 3650 days 1kg 

29. 30.





1 metric tonne 1000  1000 g  1 mg 103 g

31.

32.

Measurement Solutions 33. 1 decade = 10 years  x = 10 1 century = 100 years = 10 decades  y = 10 1 millenium = 10 centuries  z = 10 x:y:z=1:1:1 34.

125 38. Plank’s time (tp) = 5.32 × 10–44 s 1 century = 100 years But 1 year = 365 × 86400 s 1C 100  365  86400  1t p 5.32  10 44

1 minute 60 s  1 60 s 60 s 60 minutes 60 minutes  1 1 hour 60 minutes x  11  2

1 decade 10 years  1 10 years 10 years

x + y = 2 + 2 =4 35. Total amount = 6.023 × 1023 No. of years left = 90 – 45 = 45 years Rate of donation = 5000 dollars / sec To find out the total amount he can donate in 45 years, we need to find the number of seconds in 45 years = 45 × 365 × 24 × 60 × 60 = 1419120000 s  1.419 × 109 s As he can donate 5000 dollars per second, the total amount he can donate in 45 years is: 5000 × 1.419 × 109 = 7 × 1012 Hence, he cannot empty his health 45 years. 36. Speed of rabbit = 1 m/s Speed of ant = 1mm/s = 10–3 m/s Distance covered by ant = 1 m time taken = ?

t

distance 1m  3  103 s speed 10 m/s

37. 1 day = 24 hours 1 year = 365 days  16 years = 16 × 365 days = 5840 days But 1 day = 24 hours  16 years = 5840 × 24 hours = 140160 hours

1C 31.53  108  t p 5.32  10 44



1C  5.9  1052 tp

 1C  5.9  1052 tp 39. Time period = tAB + tBA + tAC + tCA But tAB = tBA = tAC = tCA = 0.5 sec  Time period = 4 × 0.5 sec = 25sec 40. Time period is the time taken to complete 1 oscillation

10 decades 10 decades  1 1 century 10 decades y 1 1  2



 Time taken to complete

1 oscillation 2

Time period 2   1s 2 2 41. Time period of a simple pendulum is independent of the mass of the bob of the pendulum.  Time period = 5 seconds 42. Time period of a pendulum depends on the length of the pendulum. Greater the length, Greater is the time period. As the length is increased, time period also increases. Increase in mass of the bob has no effect on the time period of the pendulum. 43. 1cm2 = ________ m2 . 

1 cm 

1 m 100

 1  1cm   m  100 

2

2

1 2 m 104 = 10–4 m2 2 1 km = 100 hectares 1 km2 = (1000m)2 = 106 m2 1 hectare = 100 Acres = 104 m2 So, both the given statements are correct. 1m × 1m = 1m2 = 1 sq.m l = 20 m b = 12 m Area = lb = 20 × 12 = 240 m2 

44. 45.

46. 47.

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7th Class Physics

126 48. l = 400 m b = 100 m Area = _______ hectares Area of the play ground = 400 × 100 = 4 × 104 m2 4 2 But 10 m = 1 hectare  4 × 104 m2 = 4 hectares 49. l = 6 cm b = 5cm Area = lb = 6 × 5 = 30 cm2 50. (i) 1 hectare = 104 m2  1m2 = 10–4 hectare  x = 10–4 (ii) 1 km2 = 100 hectare  1 hectare

1  km 2 100 = 10–2 km2

 y = 10–2 km2 (iii) 1km = 1000 m = 1000 × 100 cm = 1000 × 100 × 10 mm  1km = 106 mm (or) 1 km2 = (106 mm)2 = 1012 mm2 (or)

 1  1mm2   12  km 2  10  = 10–12 km2  z = 10–12 km2 51. Both the given statements are correct 52. l = 20 cm b = 18 cm h = 5cm Since, we need volume in MKS system, we have to express all the above units in MKS system.  l = 20 × 10–2 m = 2 × 10–1 m b = 18 × 10–2 m h = 5 × 10–2 m [ Km = 10–2 m] V = lbh = (2 × 10–1) (18 × 10–2) (5 × 10–2) = (2) (18) (5) × 10–5 = 180 × 10–5 = 1.8 × 10–3 cubic metre. 53. l = 30 m = 30 × 102 cm b = 28 m = 28 × 102 cm h = 12 m = 12 × 102 cm V = lbh = (30 × 102) (28 × 102) (12 × 102) = 30 × 28 × 12 × 106 = 10080 × 106 cm3 www.betoppers.com

54.

55. 56. 57. 58.

Outer dimensions of rectangular box are 40 cm, 36 cm, 21 cm.  thickness of the box = 0.5 cm  The inner dimensions of the box are (40 – 1)cm (36 – 1) cm (21 – 1)cm (subtract twice the thickness from the outer dimensions) Hence the outer volume of the box = 40 × 36 × 21 = 30240 cubic cm The inner volume of the box = 39 × 35 × 20 = 27300 cubic cm Volume of the metal sheet = 30240 – 27300 = 2940 cubic cm v = 5 × 4 × 3 = 60 cm3 Volume of the stone = Final volume = Initial volume = 125 – 75 = 50cm2 New reading = 50 + 80 = 130 cm3 12 = 3 × 2 × h

12  2cm 6 V = 0.003 km × 0.03 m × 3 cm = 300 cm × 3 cm × 3cm = 2700 cm3 Initial volume = Final volume – Volume of stone = 9.3 – 5.8 = 3.5 ml 5cm3 = __________ mm3 5cm3 = 5 × (10 mm)3 = 5 × 1000 mm3 Both the statements are correct. Mass = 232 g Volume = 20 cm3 h 

59.

60.

61.

62. 63.

Density



mass volume

232  11.6g/cc 20 64. Mass = 4kg = 4000 g Volume = 5 litre = 5000 ml = 5000 cc 

4000  0.8g/cc 5000 65. Volume = 555 cm3 Density = 7.6 g/cm3 d

mass volume  Mass = volume × density = 555 × 7.6 = 4218 g = 4.218 kg

Density 

Measurement Solutions

127

66. 500 kg/m3 = ________ g/cm3 1kg/m3



1000 g

10 cm  2

3

77. r = 7cm m = 490 g d=?

d 100 g 106 cm3 = 10–3 g/cm–3  500 kg/m3 = 500 × 10–3 gcm3 = 0.5 g/cm3 3 –3 67. 1kg / m = 10 g / cm3 

68. Density 

1000  1g/cc 1000

Density of the substance 69. RD  Density of water at 4 °C 70. Relative density has no units. 1 kg/m3 = 10–3 g/cm3 (or) 1 g/cm3 = 1000 kg/m3 Hence statement I is false and statement II is true. 71. a-4, 5; b-1; c-2; d-3 72. Density of a substance in SI system = D relative density × density of water in SI system = k × 103 kg/m3 = 1000 k kg/m3 73. Volume of air = 3 × 4 × 5 m3 = 60 m3 Density of air = 1.3 kg/m3 Mass = Volume × density = (60 × 1.3) kg = 78 kg 74. l = 0.8m A = 0.45m2 d = 7800 kg/m3 m=? M=v×d =A× l × d = (0.45 × 0.8 × 7800)kg = 2808 kg

m v

4 v  πr 3 3 4 22   7 7 7 3 7  d

4  22  49 3 cm 3 490 4  22  49 3

490  3 4  22  49 = 0.340 g/cm3 = 0.340 × 1000 kg/m3 = 340 kg/m3 

234 g/cm3 30 = 78 g/cm3 (or) 78 × 103 kg/m3 76. m = 2kg v = 2 litre = 2000 ml = 2000 cm3 = 2000 × (10–2m)3 = 2000 × 10–6 m3 = 2 × 10–3 m3 75.

d

d

2 kg/m3 = 103 kg/m3 3 2  10 www.betoppers.com

7th Class Physics

128

CONCEPTIVE WORKSHEET KEY 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

B

C

B

C

A

A

A

B

D

D

D

D

D

B

A

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

C

C

D

A

D

C

A

C

A

D

B

B

A

A

C

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

C

A

A

*

D

D

B

*

A

D

*

A

C

B

C

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

A

D

A

B

C

B

A

A

A

D

B

B

A

A

A

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

C

A

A

A

A

D

D

BD

A

D

D

A

C

C

C

76

77

78

B

B

D

HINTS/ SOLUTIONSTO THE SELECTED QUESTIONS

15. 1 nano metre = 10–9 m = 10–9 × 102 cm = 10–7 cm 2 –2 2 18. 1cm = (10 m) = 10–4 m2 = 10–4 (10–3 km)2 = 10–4 × 10–6 km2 = 10–10 km2 1km2 = 100 hectares –10  10 km2 = 10–10 × 100 hectares = 10–8 hectares 16. a = 51840 km/h2 = ___________ m/s2

51840

km 51840  103 m  2 h2  3600s  

51840  103 m m/s 2 3600  3600

20. 0.4 m2 = ___________ km2 0.4m2 = 0.4 × (10–2 m)2 = 0.4 × 10–4 m2 = 4 × 10–3 m2 = 4 × 10–3 × (10–3 km)2 = 4 × 10–3 × 10–6 km2 = 4 × 10–9 km2 21. 1μg  106 g 23. 4 km = 4 × 103 m 24. 1 cm = 10mm  15 cm = 15 × 10mm = 150 mm 26. 1km = 1000 m = 1000 × 102 cm = 105 cm 27. 1mm < 1cm < 1m < 1km 30. 1 km = 1000 m

1μm  106 m



5184 36  36





5184  4m/s 2 1296

 1km  103  106 μm

1km 103 m  1μm 106 m

 109 μm www.betoppers.com

Measurement Solutions

129

34. (A) Water clock = Flow of water (q) (B) Sun dial = Sand (p) (C) Hour glass = shadow (r) 35. Light year is the distance travelled by light in one year. 38. 1 oscillation = Time period = 2s

1 oscillation = 1 second 2

40.

41.

46.

76.

77.

HINTS/ SOLUTIONSTO THE SELECTED QUESTIONS 1.

d

2.

M

=v×d = 555 × 7.6

1 oscillation = 0.5 second 4 4 oscillation = 2 × 4 seconds = 8 seconds Time = 18 hour. 15 min *(24 hr. clock) Time in 12 hour clock = (18 – 12) . 15 min = 6.15 min PM (i) century (ii) millenium (iii) Christian Huygens (iv) second 1cm2 = _______mm 1cm2 = 1(10mm)2 = 1 × 102 mm2 = 102 mm2 We know 1g/cm3 = 1000 kg/m3  1kg/m3 = 10–3 g/cm3  800 kg/m3 = 800 × 10–3 g/cm3 = 0.8 g/cm3 3 20 cm = 54 g

= 4218 g = 4.218 kg 3.

= 60000 m2 = 6 hectare 4.

Weight of 1kg body = 1 kg wt 1 tonne = 1000 kg  Weight of 1 tonne body = 1000 kg.wt

5.

dcgs = 13.6g/cc dSI = 13.6 × 103 kg/m3

d

RD 

7.

Seconds pendulum 

= 10 × 2 = 20 seconds

9.

SUMM ATIVE WORKSHEET KEY 1

2

3

4

5

C

B

B

D

A

6

7

8

9

10

D

D

C

C

A

T = 2 seconds

Time taken to complete 10 oscillation = 10T

d

=

m 232 = =11.6g/cm3 v 20

density in SI system 8900   8.9 1000 1000

6.

8.

54 g 20 = 2.7g

= 500 × 120 m2

A

1 cm3 

78.

mass 232   11.6g/cc volume 20

mass 100  volume 2cm  4cm  5cm 100 g/cm3 = 2.5 g/cm3 40

m1 = 2130 kg m2 = 16 tonnes To add, we need to express the quantities in same system of units.  m1 + m2 = 2130 kg + 16000 kg

= 18130 kg (or)

= 2.13 tonne+ 16tonne = 18.13 tonnes

2 density of water 2 10. RD = 5 = = 0.4 density of water 5

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7th Class Physics

130 = 60 × 60 sec

HOTS WORKSHEET

 1second  1

2

3

4

5

6

7

8

*

D

B

A

D

D

C

A

9

10 11 12

13 14 15 16

A

D

D

C

C

C

B

C

17 18 19 20

21 22 23 24

A

C

B

B

A

B

A

B

25 26 27 28

29 30 31 32

A

B

B

B

B

C

B

B

33 34 35 36

37 38 39 40

D

A

C

C

A

C

C

A

HINTS/ SOLUTIONSTO THE SELECTED QUESTIONS 1.

6

1μm  10 m 1pm = 10–12 m 1nm = 10–9 m 1q = 100 kg 6. 1cm2 = (10 mm)2 = 102 mm2 7. 1 kg/m3 = 10–3 g/cm3  10kg/m3 = 10 × 10–3 g/cm3 = 10–2 g/cm3 8. 1 nano metre = 10–9 m = 10–9 × 102 cm = 10–7 cm –3 9. 1 mg = 10 g = 10–3 × 10–3 kg = 10–6 kg 10. 15 mm2 = 15 × (10–1 cm)2 = 15 × 10–2 cm2 = 15 × 10–2 × (10–2m)2 = 15 × 10–2 × 10–4 m2 = 15 × 10–6 m2 = 15 × 10–6 (10–3km)2 = 15 × 10–6 × 10–6 = 15 × 10–12 km2 11. Velocity = 102 mm/nano second 1km = 1000 m = 1000 × 100 cm = 1000 × 100 × 10 mm = 106 mm  1mm = 10–6 km 1 nano second = 10–9 second 1 hour = 60 min www.betoppers.com

1 hours 3600

1 nano second 

109 hours 3600

102 mm 102  106 km  9 nano second 10 / 3600hours = 102 × 103 × 3600 km/hours = 3.6 × 108 km/hr –1 12. 1g cms = x kgms–1 

1gcm 1  103 kg  102 m  s s –5 = 10 kgms–1 13. 1kgm/s2 = gm.mm/s2 1kgm 1 103 gm  106 mm  s2 s2 103 gm  mm s2 14. Momentum = Force × time  Unit of momentum = Unit of force × Unit of time = Newton × second = N-s –2 –1 15. 1g cm sec = _____ kgm2 sec–1 

3 2 1g cm 2 1  10 kg  10 m   sec sec

kgm 2 sec –7 2 = 10 kgm sec–1 1 decade = 10 years 1 millenium = 10 centuries = 10 × 100 years = 100 decades ( 1 decade = 10 years) 1m3 = (102 cm)3 = 106 m3 1 litre = 1000 ml V=3×2×2 = 12 cm3 V = 25 cm × 18 cm × 2 cm = 900 cm3 M = 50g V = 20cm × 10cm × 0.5cm = 100cm3  103  104

16.

17. 18. 19. 20. 21.

2

d

50  0.5g / cc 100

Measurement Solutions 22. Density of given substance is constant at a given temperature. 23. Kilogram is represented as ‘kg’. 24. M = v × d = 555 × 7.6 = 4218g = 4.128 kg 25. l = 4cm  V = (4 × 4 × 4)m3 = 64 m3

1 m  kg 8 d 

m 1/ 8  v 64 1 1  kg / m3 64  8 512

RD 

Density 2n SI 1000

1 1  512 1000 = 1.953 × 10–3 × 10–3 = 19.53 × 10–7 

26.

d

m 205.2  g / cc v 3  1.5  6

205.2   7.61g / cc 27 Density in cgs system 1 = 7.61 27. r = 1.4 cm l = 8 cm m = 369.6 g RD = ? RD 

V  r 2l 

22  1.4  1.4  8cm3 7

m 369.6  7 d   7.51 v 22  1.4  1.4  8 29. 1 tonne = 1000 kg  1kg = 10–3 tonne

131 32. 5ml = 5cc = 5 × (10–2 m)3 = 5 × 10–6 m3 33. 1 litre = 10–3 m3  10,000 L = 104 × 10–3 m3 = 10 m3 34. v1 = 26 ml v2 = 6cm3 = 6ml Vf = v1 + v2 = 26 + 6 = 32 35. Me = 9.11 × 10–31 kg No. of electrons (n) = ? Total mass = 1kg Total mass = No. of electrons × mass of each electron  1 = n × 9.1 × 10–31 1031  1.1 1030 9.1 36. Mass of kerosene = 270g – 200g = 70g  1g occupies = 1cm3 volume of kerosene Volume of kerosene = 70cm3 = 70ml. 37. Total mass of all pieces = 20 × 10 = 200g i.e. 200g mass displaces 200ml water in the vessel Now, the present level of water in vessel = 300ml  initial level of water in the vessel = 300 – 200 = 100ml. n 

103 g g 10 x  10 x  10 3  x = –3 6 3 10 cm cm 3 39. m1 = 0.3 kg = 300 g m2 = 0.3 mg = 0.3 × 10–3g = 3 × 10–4 g = 0.0003g m3 = 0.3 × 10–6 g = 3 × 10–7 g = 0.0000003g m4 = 3000 g  m4 > m1 > m2 > m3 40. r = p/104q = 10–4 p/ q r in S.I. system = 10–4 × kg/m3

38.

r in C.G.S. system

1000g 10 6 cm3 = 10–7 g/cm3 = 10–4 ×

30. 1m  106 m = 10–6 × 102 cm = 10–4 cm 31. 1gram = 10–3 kg

1g  106 g 1mg = 10–3 g www.betoppers.com

7th Class Physics

132

 T = R – 5 = 59 – 5 = 54 cm.  55 = 54 + x x = 1 cm. 

IIT JEE WORKSHEET 1

2

3

4

5

6

7

8

D

C

B

A

A

B

B

C

9

10

11

12

13

14

15

16

B

A

A,B, C

A,B, C

A,B B,C

A, C, A, C, D D

18. 19. 20.

21.

Length of thread = 14 hs n=2 Circumference

17 18

19

20

21

22

23

24

B

A

C

A

B

4

9

1

25 26

27

28

29

30

LR = 240 cm. LW = 40 cm. LR = nLW

7

MT

MT

MT

MT

n

6

HINTS/ SOLUTIONSTO THE SELECTED QUESTIONS 17.

25.

Area of rectangle surface ABFE = 8 × 6 = 48 sq.cm Area of rectangle surface BCGF = 5 × 10 = 50 sq.cm Area of rectangle surface CDFG = x × z = xz sq.cm V= 2 m3 R.D = 0.52 Density of the body = R.D × Density of water = 0.52 × 1000 kg/m3 = 520 kgm3 Mass of the body = Density × Volume = 520 × 2 = 1040 kg Volume of iron cylinder =  r 2 l

22  3  3  7  198 cc = 7 m 396   2 g / cc v 198 RD = Density of the substance in cgs RD = 2 LS = x LL = 2x Give LR = 18 x = 9 × 2x LR = 9(LL) =9 R = T + 5 cm _______ (1) M = R – cm _______ (2) M = 0.55 _______ (3) M = 55 cm. M = T + x = ? _______ (4) Substituting (3) in (2), we get R=m+4 = 55 + 4 = 59 cm.

26.

27.

28. 29. 30.

14  7 hs 2

L R 240  6 LW 40

The area of the path KLMN = 40 × 5 = 200 square metres The area of the path PQRS = 30 × 5 = 150 square metres The area of the path EFGH = 5 × 5 = 25 square metres total area occupied by the Paths = (the area of the rectangle KLMN) + (The area of the rectangle of PQRS) – (The area of the square EFGH) because the square path EFGH is counted twice. So, the required area = 200 + 150 – 25 = 325 square metres. The right match is: A – q, t; B – r; C – p; D – s A – s; B – p,t; C – r; D – q A – s; B – p,t; C – q; D – r A – r; B – q; C – p; D – r, t

Density =  23.

24.

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 

2. MOTION SOLUTIONS

FORMATIVE WORKSHEET KEY 1 C, D

2 D

23 B 33 A

24 A 34 C

3 B

4 5 6 7 A A A, B, C D 11 to 23 are non objective questions. 25 26 27 28 29 C C C D A 35 A

8 C

9 D

10 C

30 B

31 B

32 C

HINTS/ SOLUTIONS TO THE SELECTED QUESTIONS 11.

14. S

2 km

H

R 2m S

P

Distance

= length of the path AQB

A

2r D 22 1   7 2 2 7 2

Q

16. 4m

A

(i)

(ii)

4m

Distance

5m

U

5m

P

= Length of the path APQB = AP + PQ + QB = 4m + 4m + 4m = 12m Displacement = AB = PQ = 4m

5m

Q

5m

B

Distance

= 11cm (ii) Displacement = AB = 7cm B

4m

B



13.

T

Q

4m

R 5m

5m

(i)

6m

P

15.

7m

4m

4m

(i) Distance covered = (2 + 4 + 6 + 4 + 2 + 4 + 4 + 4)m = 30m (ii) Displacement = AB = AX + XR + RS + SB = 2m + PQ + 2m + TU = 2m + 6m + 2m + 4m = 14m

Q

A

B

2m 4m

(i) Distance covered = path covered by Raju = HS + SH = 2km + 2km = 4km (ii) Displacement = Distance between initial and final position As both the initial final positions are same, the displacement is zero. 12.

A

17.

= (5 + 5 + 5 + 5)m = 20m Displacement = AB = AQ + QB = 5m + 5m = 10m Distance = 5km + 5km = 10km Displacement = zero ( he returns back home) Let the body starts from A and reach B covering half the path of the circle.

2r D  2 2 Displacement = AB = D Distance covered 

7th Class Physics

134



Distance D / 2    Displacement D 2

Ratio =  : 2 18.

10m S

R 5m

P

Q

10m

Distance covered = (10 + 5 + 10)m = 25m Displacement = PS + RQ = 5m Distance + Displacement = 25m + 5m = 30m  19.

Distance covered = AB + BC + CD + DE + BE = 5m + 5m + 5m + 5m + 5m = 25m Displacement = AB = 5m Given Distance = x × displacement  25= 5x

x

B

A

C

D

Initial reading of the odometer = 36580 km Final reading of the odometer = 36620 km Initial time = 9:00 A.M. Final time = 10:00 A.M. Total distance travelled = 36620 - 36580 km = 40 km Total time taken = 10:00 A.M. - 9:00 A.M. = 1.00 hr

F

E

Speed =

Let the length of each side be ‘x’ units.  AB = BC = CD = DE = EF = x Distance covered = AB + BC + CD + DE + EF =x+x+x+x+x = 5x Displacement = AF According to pythagorous theorem. AF 2 = AE2 + EF 2 (or) AE



 AD  DE 



x  x

2

2

21.

25 5 5

40 km = 1 hr = 40 km/hr 22.

 EF2

 x2

 4x 2  x 2

23.

Therefore, the speed of the car will be 40 km/hr. Speed of rickshaw = 3 m/sec. Total time taken = 10 min = 600 sec. We know that, The distance covered = Speed x Time = 3 m/sec. x 600 sec. = 1800 m. Therefore, the distance between his house and post office will be 1800 m. or 1.8 km. This simple problem can be solved by applying the following formula to calculate speed,

 5x 2

Speed 

 5x 

Distance 5x 5   Displacement 1 5x

 Ratio  5 :1

20.

C

Total distance travelled Total time taken

Speed of the car 

Distance between the house and the school Total time taken by the car to reach the school

D

 B

Total distance covered Total time taken

E

40km 40km   80km / hour 30minutes  1   hour  2 

The correct answer is B. A

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Motion Solutions

135 28.

24.

29.

25.

It can be observed from the given distance versus time graph that the car travels a distance of 25 m (50 – 25) every 2 seconds. Since the distance traveled by the car per unit time is constant, it can be concluded that it is moving at a uniform speed. The correct answer is A. The speed of a body is given as the distance traveled by it per unit time. Total distance traveled by Paula = 12 km + 6 km + 15 km = 33 km Time taken by Paula to cover this distance = 30 minutes + 15 minutes + 15 minutes = 60 minutes =1h Hence, Paula’s speed can be determined as

Distance travelled 33km   33km / h Time taken 1h The correct answer is C. The speed of a body is given as the distance traveled by it per unit time. Distance traveled by the moon = 2 × r Where, r = 385,000 km Hence, the distance traveled by the moon = (2π × 385,000 km) = 770,000π km Time taken by the moon to complete one revolution around Earth = 29 days = (29 × 24 hours) = 696 hours Hence, the moon’s speed can be determined as

30.

If a body is stationary, then its position will not change with time i.e., its position versus time curve will run parallel to the time axis, as shown by the graph in alternative D. The correct answer is D. The speed of an object is given as the distance traveled by it per unit time. Since the boy is cycling at a constant speed, he covers equal distances in equal intervals of time i.e., his distance versus time graph will be a straight line having a definite slope, as shown by the graph in alternative A. The correct answer is A. The speed of an object is given as the distance traveled by it per unit time. It can be observed from the given graph that the distance traveled by the car in going from point O to point C is 75 km and the distance traveled by it in going from point C to point D is 75 km. The total time taken by it to travel from point O to

180 minutes  3hours 60 Hence, the speed of the car can be determined as point D is 180 minutes.

Speed 

26.

Speed 

27.

Distance travelled 90cm   0.3cm / s Time taken 300s The correct answer is C.

Distance travelled 75km  75km  Time taken 3h

150km  50km / h 3 The correct answer is B. The speed of an object is given as the distance traveled by it per unit time. It can be observed from the given graph that the ball covers a distance of 40 meters in 25 seconds. Hence, its speed can be determined as 

31.

Distance travelled 40m   1.6m / s Time taken 25s The correct answer is B. The speed of the car is the distance traveled by the car per unit time. It can be observed from the given graph that the car travels equal distances in equal intervals of time. Hence, the speed of the car is constant. During the first minute of the race, Jacob and Ethan cover equal distances. Hence, their speeds are equal during the first minute. After two minutes from the start of the race, Jacob gets ahead of Ethan, i.e., Jacob covers more distance in the second minute than Ethan. Therefore, during the second minute, Jacob’s speed is more than that of Ethan. Speed 

Distance travelled 770,000 km   Time taken 696h

1,106.3 km / h The correct answer is C. The speed of a body is given as the distance traveled by it per unit time. The distance traveled by the ant is equal to the book’s perimeter = 20 cm + 15 cm + 20 cm + 15 cm = 90 cm Time taken by the ant to travel 90 cm = 5 minutes = (5 × 60) = 300 seconds Hence, the ant’s speed can be determined as

Speed 

Speed 

32.

33.

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7th Class Physics

136 34.

35.

In order to answer this question, the student must be able to identify the position versus time graphs representing  various  types  of  motion.  In the wave motion, the position of the particle varies. This is shown in the position versus time graph given in alternative C. The wave motion comprises of consecutive crests (peaks) and troughs (valleys). The correct answer is C. The ant travels a distance of (6 cm – 0 cm =) 6 cm on the ruler. The time taken by the ant to travel a distance of 6 cm is given by the difference between the times on the two clocks, i.e., 10:22:41 “ 10:22:35 = 6 seconds. Hence, the ant travels 6 cm in 6 seconds, i.e., the

7. 8. 9. 10. 17.

Motion Yes. Motion of wheels of a moving car. Periodic Periodic Motion Distance = Circumference of the circle  2 r

22 7 7 = 44cm Distance covered = 5m + 5m + 5m + 5m + 5m + 5m = 30m Displacement = 0 Distance = Speed × time (or)  2

18.

19.

6cm  1cm / s . 6s The correct answer is A. speed of the ant is

Speed 

CONCEPTIVE WORKSHEET

20.

distance time

CGS unit of speed cm / s  SI unit of speed m/s

KEY 11 A 21 B

12 B 22 B

13 14 15 16 17 B C C A D 23 24 25 26 27 D A C C C

18 C 30 C

19 20 B A

HINTS/ SOLUTIONS TO THE SELECTED QUESTIONS 1.

Vibratory Motion. Because strings does not move as a whole.

2.

25.

cm s  s 100cm



1 100

Total distance Total time Let ‘x’ be distance covered with 30m/s  The distance covered with 20m/s is also x.  Total distance = 2x Let t1 and t2 be time taken to cover the respective distances with speeds 30m/s and 20m/s. Distance = speed × time (or) time Average speed 



Linear and rotational. As ball rolling down it covers some distance along with the rotation. 3. In oscillatory motion body moves to-and-fro or back-and-forth at regular intervals of time. Hence all oscillatory motions are periodic in nature. 4. Motion, Rest 5. Movement of a toy car around a circular road. It covers a distance of circumference of circle during its circular motion. 6. Rest www.betoppers.com



distance speed

t1 

x 30

t2 

x 20

 t1  t 2  

x x  30 20 2x  3x 5x  60 60

Motion Solutions

137

Average speed 

Total distance Total time

2x  5x / 60

31.

Total distance covered = (20 + 25 + 45 )m = 90m Total time taken = 3s Therefore, Average Speed

32.

Time  t  

33.

Total distance covered 90   30m / s Total time taken 3 When the body covers distance in different in different times by different velocity Average speed is given by the velocity 

29.

2v1 v 2 Averager Speed  v  v 1 2



2v1 v 2 v1  v 2

2  40  60 40  60 = 48 kmph Let’s consider one complete revolution Distance covered by each hand same. But the time taken to complete one revolution is different for each ts = 60s tm = 60 min = 3600 s th = 12 hrs = 12 × 60 × 60s 

30.

1000 10 5 m/s   m/s 60  60 36 18

1 kmph  34.

S 15km  Speed 120km / h

1 1  h   60  7.5min 8 8 It means distance of 1 km travelled in 1 hour i.e., its velocity = 1 km h–1 

v1 = 40 kmph = v2 = 60 kmph Average speed

S 30m   0.33ms1 t 90s Distance covered ‘S’ = 15 km Speed = 120 km/h Therefore Speed 

2x  60  5x = 24 m/s 28.

Distance covered (S) = 30 m Time (t) = 1.5 min = 90 s

5 m/s 18

Distance travelled Time taken Distance = 1200 m Time taken = 3 min and 20 sec = 3 min + 20 sec = 3 x 60 sec + 20sec ( 1 min = 60 sec) = 180 + 20 = 200 sec Speed 

1200  6sec 200 (i) 100 m (iii) 650 m are not equal (v) 60 m. Speed 

35.

(ii) 80 m (iv) No, these distances

37.

Total distance total time As the distance is same for all, Average speed 

1 time More is the time taken, less is its average speed. As hour’s hand taken more time, its average speed B minimum. Conversely, the average speed is more for the end of a second’s hand. Average speed 

(i) distance-time graph of a car moving with a constant speed

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7th Class Physics

138 Time = ?

Distance Time  speed 14488.92 161.8 = 89.54 hrs 

SUM MATIVE WORKSHEET

9. 30

HINTS/ SOLUTIONS TO THE SELECTED QUESTIONS 1.

2.

3.

(a) (b)

24 20

10m/s = 100 cm/s 20m/s = 72 kmph

16

30 km / s = 0.03 km/s (c) 30m/s  1000 (d) 40m/s = 2 × 72 kmph = 144 kmph Note: 10m/s = 36 kmph 10m/s = 36 kmph So, their speeds are same. As the distance covered by them is same, they take same time to reach the ending points Speed = 310 kmph = _________ m/s We know

12 8 4 0

10.

5 m/s 18 = 86.1 m/s Fastest one taken least time to complete the race.  Sudha is the fastest and Rohit is the slowest.

6. 7. 8.

5 m / s = 25 m/s 18

Distance 100   4sec Time  time 25 C C Distance = 14488.82 km Speed = 161.8 kmph

11.

(i) (ii) (iii) 4m

10

12

14

16

18

2  50  40 50  40

3 Speed  m / s 5 From 5 to 7 seconds it is at rest = 0m/s Distance covered from of to 9sec = 7 – 3 = Time = 2sec. Speed 

4  2m / s 2

12. 70 60 50 40 30 20 10 0

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8

2  50  40 90 = 44.45 kmph

Speed of Rohit 

Speed = 90 kmph  90 

6



100  5m / s 20

100  3.57m / s 28 Distance = 100 m

4



5 kmph  1m / s 18

Speed of Sudha 

5.

2

2v1 v 2 Average speed  v  v 1 2

 310kmph  310 

4.

89.54  3.73days 24

No. of days 

(ii) distance -time graph of a car parked on a side road

1

2

3

4

5

6

7

8

9

Motion Solutions 13.

139

Here, it is given that, The distance between two stations = 240 km Time taken to cover this distance = 4 hr. We know that,

Speed =

HOTS WORKSHEET KEY

Distacne Time

1 B 11 C

240 km = 60 km/hr..Therefore, speed of the train 4 hr 14.

will be 60 km/hr. In this question, it is given that, Initial reading of odometer = 57321.0 km Final reading of odometer = 57336.0 km Initial time = 08:30 A.M. Final time = 08:50 A.M. Now, Total distance covered = Final reading of odometer - Initial reading of odometer. = 57336.0 km – 57321.0 km = 15.0 km Total time taken = Final time – Initial time = 08:50 AM - 08:30 AM = 20 min. We know that, speed

=

Distance covered Time taken

=

15km = 0.75 km/ 20min

1.

2.

3.

min)

15.

16.

9 10 C D

The total distance covered by Kurt is (50 m + 100 m + 50 m + 100 m =) 300 m. He takes one minute to complete one revolution around the park, i.e., a distance of 300 m. Therefore, the distance covered by him in one

300m  5m , i.e., his speed is 5 m/s. 60 This question requires the student to formulate a hypothesis based on cause and effect relationship. In the given experiment, the lapse of time is the cause and the increase in the distance of the car from the wall is the effect. It can be observed from the graph that the distance of the car from the wall is equal to the time lapsed. Hence, the hypothesis can be formulated as Distance of the car from the wall = Time Speed is defined as distance traveled per unit time. The ant covered each edge of the given structure in one second. In one ledge, the ant travels 6 cm in one second. It is the maximum distance traveled by the ant in one second. Hence, its speed was the maximum when it traveled the edge of length 6 cm. Time of travel, t = 5 h Speed of the bus, v = 40 km/h Speed is given by the relation, second is

= 0.75 × 60 (1 hr = 60

= 45 km/hr. Therefore, the distance moved by the car = 15km. According to the question, Speed of the bicycle = 2 m/s Total time taken = 15 min = 900 sec. We know that, Distance covered = Speed x Time = 2 m/s × 900 sec. = 1800 m Therefore, the distance between Salma house and the school is 1800 m or 1.8 km. (ii) 25 km

3 4 5 6 7 8 A C D C C B 13 14 15 C C C

HINTS/ SOLUTIONS TO THE SELECTED QUESTIONS

min Speed in km/h

2 A 12 A

4.

Speed  v  

Distance  d  Time  t 

 Distance covered, d = vt = 40 × 5 = 200 km Distance covered = Final reading of the odometer “ Initial reading of the odometer Initial reading of the odometer = 42124 km Final reading of the odometer = Initial reading of the odometer + Distance covered = 42124 + 200 = 42324 km Therefore, the final reading of the odometer is 42324 km. www.betoppers.com

7th Class Physics

140 5.

6.

The odometer reads the distance travelled by a vehicle. Speed of the scooter = 30 m/s Time taken = 20 min = 20 × 60 = 1200 s Distance travelled by the scooter = Speed × Time = 30 × 1200 = 36000 m = 36 km Initial reading of the odometer = 14316.0 km Final reading of odometer = 14316.0 + 36 = 14352.0 km Hence, the odometer shows a reading of 14352.0 km when the rider reaches his destination. In a racing event, both participants have to run the same distance. Let this distance be d. Time taken by Abbas = ta Time taken by Zaheer = tz = 3ta Speed is given by the relation:

Speed 

distance of

It takes 5 min i.e., 5 × 60 = 300 s to meet. The distance travelled by car N in 300 s is 300v m. Therefore,  300v 



Distance Time

1000v  30v 2

1000  30v  300v 2

 1000 – 30v = 600v  1000 = 630 v v 

d Speed of Zaheer, v z  3t a

Hence, both cars were moving with a speed of 1.59 m/s.

v a d 3t a 3    vz t a d 1

9.

1000  1.59m / s 630

Speed is given by the relation

Distance Time

Hence, va : vz = 3 : 1 Average speed is given by the relation

Speed 

Total distance travelled Total time taken Distance covered on the bicycle = 700 m Distance covered by walking = 300 m Hence, total distance = 700 + 300 = 1000 m Total time taken in the journey = 12 min (given)

Let v be the speed of Raj and  Lokesh i.e.,

100  83.33m / min 12 Let the speed of both cars be v m/s. In 30 s, car M covers (v × 30) = 30v m. Now, the distance between both cars is reduced to (1000 – 30v) m. Since both cars were moving with the same speed, they will meet at mid point of distance (1000 – 30v) m.

Lokesh has to cover a distance of 10v with speed. Hence, the time required by Lokesh to cover the distance is given as

Average speed 

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be the speed of

Raj runs for 10 minutes. Hence, the distance covered by Raj is given as Distance = Speed × Time = v × 10 = 10v

 Average speed 

8.

1 1000  30v  m 2

d Speed of Abbas, v a  t a

 ratio,

7.

Hence, car N will meet car M after travelling a

10.

The distance – time graph represents the nature of motion of a moving object. A distance”time graph that is a straight line making an angle with the time or distance-axis represents the motion of a body in uniform speed. It is given that initially, the car covered a distance (MN) in 3 min. This motion can be represented graphically as shown in the given graph.

Motion Solutions

141 The car moves uniformly to reach the initial point M in 2 minutes. Therefore, the car travelled a distance MN in 2 min, i.e., between time 5 min and 7 min (as shown in the given graph).

The distance – time graph is a straight line parallel to the time-axis represents an object which is not moving. It is given that at point N, the car remains standing for 2 minutes, i.e., between time 3 min and 5 min. This motion can be represented as shown in the given graph.

11.

Hence, the motion of the given car can be represented as shown in alternative D. The speed of a moving body is given by the ratio of the distance travelled to the time taken, i.e., Distance Speed  Time

The following table shows the speed of the four given cyclists.

Cyclist Distance (m)

12.

Time (min)

M

166

7

N

205

9

P

281

13

R

333

15

Speed =

Distance (m/min) Time

166  23.71 7 205 9

 22.78

281  21.62 13 333 15

 22.20

From the above table, it is clear that the speed of cyclist P is the minimum among the given cyclists. Hence, cyclist P will take maximum time to cover any given distance. Speed, distance, and time are related as Distance Speed  Time Distance covered by the train = Length of the tunnel + Length of the train = 11.43 + 0.57 = 12 km Speed of the train = 60 km/h 12 1 60  h  12 min Time taken by the train to cover 12 km will be 60 5 5 Hence, the clock will read (8:25 + 0:12) = 8:37 am when the train exits from the tunnel. www.betoppers.com

7th Class Physics

142 13.

A distance – time graph represents how fast a body is moving. If the distance – time graph of a body is a straight line parallel to the time-axis, then the speed of the body is zero, i.e., the body is not moving at all. Statement I says that  car I is parked or standing near a post office, i.e., car I is not moving at all. Hence, its distance”time graph will be a straight line parallel to the time-axis (as shown in the graph below).

Given that, Speed of the car = 53 km/h Distance covered by the car = 318 km Let the time taken to cover 318 km be t.

53 

318 t

318  6h 53 Hence, the car will cover the given distance in t

6 h.

IITJEE WORKSHEET KEY

The distance – time graph of a body moving with uniform speed is a straight line inclined at an angle with the time-axis. Car II is moving on a straight road with a constant speed. Hence, its distance – time graph will be a straight line inclined at an angle with the time-axis (as shown in the graph below).

1 C 7 D 13 ABCD

2 B 8 D 14 ABC

3 4 5 6 A B A A 9 10 11 12 B D C ABC 15 16 ABD ABCD

HINTS/ SOLUTIONS TO THE SELECTED QUESTIONS 1.

The speed of a person indicates how fast a person can run. The greater the speed, the faster the person can run and vice-versa. It is clear from the given table that the speed of the four participants is related as vR > vA > vP > vD..

14.

15.

Distance . Time Total distance covered by the plane = 2095 km Total time of flight of the plane = 11:15 “ 8:50 = 2 h and 25 min = 120 + 25 = 145 min. 2095 Speed  km / min 145 2095  1000   240.80m / s 145  60 Hence, the speed of the given airplane = 240.80 m/s Speed, distance, and time are related by the formula: Distance Speed = Time Speed is given by the ratio

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2.

Where, vR = speed of Ranvir Chauhan = 6 m/s vA = speed of Apurva Gupta = 4 m/s vP = speed of Pavan Chauhan = 3 m/s vD = speed of Deepak Gupta = 2 m/s In this situation, Ranvir is the fastest. Hence, he will be the winner. Also, it is clear from the above data that Apurva is the second fastest among the four participants. Hence, Apurva will be a runner up in the race. Hence, Apurva will touch the finishing line just after the winner. The boy is in motion for the first four seconds; then he is at rest for the next six seconds, and is in motion for the last six seconds. Therefore, he is in motion for a total time of (4 s + 6 s =) 10 s. The correct answer is B.

Motion Solutions 3.

The odometer is a device that measures and

7.

143 Distance covered by the car = Final reading of the odometer – Initial reading of the odometer Final reading of the odometer = 1041 km Initial reading of the odometer = 1020 km Distance covered by the car = 1041 – 1020 = 21 km = 21000 m Speed of the car = 14 m/s Distance = Speed × Time

shows the distance covered by a vehicle. Distance covered = Final reading of the odometer – Initial reading of the odometer Since there is no change in the odometer reading for a certain period of time, this indicates Final reading of the odometer – initial reading of

Time 

the odometer = zero  Distance covered = 0

8.

Time taken by the car to reach school = 1500 s The correct answer is D. Speed of the car, s = 100 km/h

9.

Time taken by the car to reach the picnic spot, t = 2h Total distance covered by the car = Speed × Time = 100 × 2 = 200 km Total distance covered by the car = Final reading of the odometer – Initial reading of the odometer Initial reading of the odometer = 1500 km Final reading of the odometer = Total distance covered by the car + Initial reading of the odometer = 1500 + 200 = 1700 km  Final reading of the odometer = 1700 km Let the distance between station and place B

 The car is at rest.

The correct answer is A. 4.

Speedometer is a device that measures and reads the average speed of moving objects. The speed of the car is given by the formula,

Speed 

Distance covered by the car Total time take

Where, Distance covered by the car = 100 km Total time taken = 5 h

100  20km / h 5 Hence, actual speed of the car = 20 km/h However, the speedometer reads twice the actual speed. Thus, the speedometer will give a reading of 40 km/h. For Ajay, Speed 

5.

Speed = 5 m/s Distance = 100 m Distance = Speed × Time

Time 

Distance 100   20s Speed s

For Rahul, The time taken by Rahul is 5 s more than the time taken by Ajay. Time taken by Rahul = 20 + 5 = 25 s Distance covered by Rahul = 100 m Distance = Speed × Time

Speed 

Distance 100   4m / s Speed 25

 The speed of Rahul is 4 m/s.

Distance 21000   1500s Speed 14

be d. It is given that place A is midway between the station and place B. Therefore, distance between place A and the

d 2 Distance covered by the express train = d Distance covered by the passenger train = The time taken by both the trains is the same i.e., t. We know, Distance Speed  Time Distance d  Speed of the express train, se =   Time t ____________ (i) Distance d  Speed of the passenger train, sp   Time 2t ____________ (ii) d  2t se : sp   2 :1 t d The ratio of the speed of the express train to that of the passenger train is 2: 1. station =

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7th Class Physics

144 10.

It is given that Ram runs twice as fast as Shyam. It means that the speed of Ram is twice that of Shyam. Let speed of Shyam = x  Speed of Ram = 2x Time for which Ram runs, t1 = 10 minutes = 10 × 60 = 600 s Time for which Shyam runs, t2 = 5 minutes = 5 × 60 = 300 s Distance covered by Ram, d1= Speed × Time = 2x × 600 … (i) Distance covered by Shyam, d2 = Speed × Time = x × 300 … (ii) Ratio, d1: d2

d1 2x  600   4 :1 d2 x  300 17.

Therefore d1: d2 is 4: 1. As the body comes to its initial position of start, its displacement is zero and distance is equal to circumference of the circle.

18.

As the body comes to its initial position of start, its displacement is zero.

19.

The distance travelled is equal to the total path covered by the body and is equal to 4 km.

20.

Distance = AB + BC + CD = 4 + 3 + 2 = 9 km.

21.

36 

22.

Zero

23.

Distance = 3.6 km = 3600 m

5 m / s  10m / s 18

Time = 1 hr = 3600 s

Speed  24.

3600  1m / s 3600

A– t; B – r ; C – q ; D – p

 

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3. LIGHT SOLUTIONS

FORM ATIVE WORKSHEET 21 22

23 24 25 26 27

28

29 30

B

B

A

A

D

31 32

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B

C

B

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4.

C

HINTS / SOLUTIONS TO THE SELECTED QUESTIONS 1. We know that, d = 180 - 2i or d = 180 – ( i + r ) ( i = r ) or d + i + r = 180° Therefore, sum of angle of incidence , angle of reflection and angle of deviation equals to 180° 2. Given, Angle of incidence = i Angle of deviation d = ? Angle of reflection = r Consider the straight line IOB We know,

5.

6.

7.

3.

Angle made in a straight line is 180°  i + r + d = 180° ( i = r)  i + i + d = 180° d = 180 ? 2i  Given, Angle of incidence (i) = 30° Angle of reflection (r) = ? Angle of deviation (d) = ? Glancing angle of incidence (g) = ? i+r+g=?

a) We know, i = r  r = 30° b) We know, d = 180 - 2i  d = 180 – 60  d = 120° c) We know, g = 90 – i  g = 90 – 30  g = 60° d) i + r + g = 30 + 30 + 60 = 120° Given, Angle of deviation d = ? We know, d = 180 – 2 i  d = 180 – 2(90 – g)  d = 180 – 180 + 2g  d = 2g How to get i? We know, g = 90 – i  i = (90 – g) dmax = ? We know, d = 180 – 2i dmax  (180 – 2i)max  Subtracting quantity should be minimum ? I=0  dmax = 180 – 0 = 180° Let us understand what a normal incidence is. The ray is passing along the normal. Such an incident ray passing through the normal is known as normal incidence. Note : For normal incidence, i = 0 Angle of reflection, r is 0. Hence the angle between incident ray and reflected ray is zero. Let the angle of incidence be ‘x’  i = x ________ (1) Angle of deviation (d) = angle of reflection (r)  d = r _________ (2) From (1) and (2), i=r=d=x ( i = r) From the figure, i + r + d = 180° ( AOC is a straight line and in a straight line the angle = 180°) x + x + x = 180°  3x = 180  x = 60°

7th Class Physics

146 8.

9.

Given, i + r = 90° i=? We know, Angle of incidence i = angle of reflection r i + r = 90° i + i = 90° 2i = 90° i = 45° As i and r are same,  i = r = 45° Angle between the mirrors  ABC = 60° Let the ray be incident at D, middle of AB. It reflects and strikes BC at E. Angle of incidence on AB = 50° We know, Angle of reflection = angle of incidence  Angle of reflection at AB = 50° Glancing angle of incidence g = 90 – i = 90 – 50 = 40° Glancing angle of reflection = 90 – r = 90 – 50 = 40°

But, how to find ‘x’ ? From the figure, it is clear that the reflected ray makes 30° angle with the mirror and M1OM2 forms a triangle. So, 30 + 120 + x = 180  x = 30° From the figure, It is clear that  = 90 – 30 = 60° 11. Suppose OM1 is the horizontal plane mirror and OM2 the second mirror and OM2 the second mirror, then the angle between the mirrors is q.

From the figure, ABM,  OBC  45 Since CD and OM1 are parallel, DCM 2   According to the laws of reflection, DCM 2  BCO  

Now, in  COB, OCB  CBO  COB  180

   45    180    67

1 2

So, the angle between the plane mirrors  67 In triangle DBE. B  E  D  180  60  E  40  180

1 2

12. Yes, this is possible.

 E  80 On drawing a normal to BC at E. Glancing angle of incidence at E = 80°  Angle of incidence on BC = 90 – 80 = 10° 10.

Incident angle for mirror M1, i1 = 60° We know, according to the laws of reflection, i=r Angle of reflection, r1 = 60° www.betoppers.com

When a convergent beam is incident on the plane mirror, the resulting beam is a convergent beam, which meets at a point ‘P’. A real image is formed at ‘P’.

Light Solutions

147

13. A virtual image cannot be caught on a screen as 17. Let initial object distance = x the reflected rays appear to meet. Photographing  Image distance = x virtual image is possible and your image can be Initial distance between object and image = 2x photographed. Displacement of mirror = 10 m away from the 14. Let an object be placed at ‘A’ at a distance of object 10 m from the plane mirror placed at M. New object distance = (10 + x) m New image distance = (10 + x) m Displacement of image = (Final distance between object and image ) Displacement of image = (Final distance between object and image ) – ( Initial distance between object and image) Displacement of image = 2x + 20 – 2x = 20 m Its image is formed at the same distance, 10 m by 18. We know that a virtual image cannot be caught on the plane mirror at B. a screen. But our eye lens forms a real image (here, Distance moved by the object (AA’) = 4 m virtual image acting as virtual object) on the retina Distance moved by the image (BB’) = 4 m of our eyes. Final distance between the image and the object Let O, M and I be the initial positions of the object, (A’B’) the mirror and the image respectively. And the = A’O + OB’ initial distance between object and mirror be ‘x’ m. = 6 + 6 = 12 m 15. Initial object distance (OA) = Initial image distance (OA’) = ‘x’ m Object is moved through a distance of ‘y’m

 images moved by ‘y’ m  New object distance (OB) = New image distance (OB’) = (x – y) m  Final distance between the image and object (BB’) = (x – y) + (x – y) = 2 (x – y) m 16. Let initial object distance = x  Image distance = x Initial distance between object and image = 2x =? Distance travelled by object in 2 seconds = 30× 2 = 60 m away from the mirror Final object distance = (x + 60) m  Final image distance = (x + 60) m Final distance between object and image = (2x + 120)m 2x = 200 – 120 2x = 80 Initial distance between object and image = 2x = 80 m

The object and the mirror move away from each other by 10m such that their new positions are O’ and M’ respectively and I’ is the final position of the image. Displacement of object (OO’) = 10 m Displacement of mirror (MM’) = 10 m  New object distance OM’ = O’ O + OM + MM’ = 10 + x + 10 = (x + 20)m  New image distance M’ I’ = (x + 20)m Displacement of image = II’ = ? Displacement of the image = II’ = MI’ – (MM’ + M’ I’) – MI ________ (1) Substituting the values of MM’, M’ I’ and MI from the data in (1), we get Displacement of the image = II’ = (10 + x + 20) – x = 30 m Therefore, the displacement of the image is equal to 30 m. Conclusion: If both mirror and object move away from each other by ‘x’ m, then the displacement of the image is equal to ‘3x’ m. 19. Yes 20. Image distance = Object distance = 10 cm 21. Both Statements are true, Statement - II is not correct explanation of Statement - I. www.betoppers.com

148 22. When the object is placed beyond C, the image is formed between C and F in front of the mirror. The image is Real, inverted and diminished. 23. For an object placed at C for a concave mirror, the image is formed at C itself. Therefore, the image distance is equal to the object distance i.e., 20 cm. 24. For an object placed at C for a concave mirror, the image is formed at C itself. The nature of image is Real, inverted and of same size. 25. (A) Focal length of the spherical mirror – distance between principal focus and pole (2) – half the radius of curvature (4) (B) Radius of curvature is two times of – distance between principal focus and pole (2) (C) Plane mirror – Virtual image (1) (D) Concave mirror – Real image (3) The right match is : A – 2, 4; B – 2; C – 1; D – 3 26. Whatever may be the position of object, a convex mirror always forms a virtual, erect and diminished image. 27. R = 2f Both Statements are true, Statement - II is the correct explanation of Statement - I. 28. The image formed by a convex mirror is always diminished and erect. Since the image formed by the last mirror is magnified, it is a concave mirror. Again, since the images formed by the first three mirrors are inverted, they are all concave mirrors. Since the image formed by the fourth mirror is diminished and erect, it is the only convex mirror placed in the mirror show. 29. Concave mirror produces a magnified image of the object, when it is placed very close to this mirror. This property of concave mirror is used to make shaving mirror, which allows the barber to have an enlarged view of the beard of his costumers, so that he can shave it comfortably. Therefore, the item bought by the man will be a concave mirror. 30. As the outer surface is covered and is used for reflection, it act as a convex mirror. So, it forms a virtual and diminished image. 31. The image formed by a concave mirror may be inverted or erect, and magnified or diminished. The nature of the image formed by a concave mirror depends on the distance between the object and the mirror. In the given case, an inverted and a magnified image of the boy is formed by the respective sides of the two mirrors that are stuck together. Hence, both the mirrors are concave mirrors. www.betoppers.com

7th Class Physics 32. The image formed by the convex mirror is erect and smaller in size than that of the object. The actual area of the front view of the parallelogram is shown in the following figure.

33.

34.

35.

36.

37.

Actual area = 8 × 4 = 32 cm2 Since the image formed in a convex mirror is smaller than the object, the apparent area of the visible side seen on the convex mirror must be less than 32 cm2. Hence, the area of the image can be 21 cm2. A concave mirror can form real images of any size with respect to the object size depending on the object distance from the mirror. Thus, a concave mirror can form both small and large real images. Hence, the statement delivered by Aruna is incorrect. The image formed by mirror III can be obtained on a screen. Hence, mirror III can form a real image. This implies that mirror III is a concave mirror. Since only concave mirrors can form real and inverted images of objects, mirror III can form inverted images. Convex mirrors form only virtual images that are smaller than the object. Because of its shape, a convex mirror can cover a large field of view in front of it. Therefore, a small convex mirror can form small and upright images of larger vehicles such as cars, trucks, etc. Because of this large field of view and its ability to form small upright images, convex mirrors are used as rear view mirrors in vehicles. Rearview mirrors of vehicles form small, erect, and virtual images. This is a characteristic of images formed by convex mirrors. Since Billu has prepared a list of devices that involve the use of concave mirrors, it is the rearview mirror that is included wrongly in the given table. The rear view mirror of a vehicle is simply a convex mirror. A convex mirror forms virtual images smaller than the object. It is the property of a mirror that it always forms laterally inverted images. Therefore, all asymmetric letters in the English alphabet get laterally inverted when viewed in a mirror. When the figure  is viewed in a convex mirror, it appears as the word   (as  shown  in  the  given figure).

Light Solutions

38.

39.

40.

41.

42.

43.

149 44. In the given activity, the candle is behind the instrument and its image is also formed behind. The image is erect, virtual in nature, and diminished in size. Only a concave lens can form this type of image of an object. Therefore, the optical instrument used in the activity is a concave lens. 45. The fact that the lens was unable to burn the paper indicates that the rays of the sun coming towards the paper were not being converged by the lens i.e., the rays were diverged by the given lens. Therefore, the lens used by Rajesh is a concave lens. The image formed by a concave lens is always erect and smaller in size than the object. Therefore, the appearance of the spectacles through the lens is erect and small. 46. It is the convex lens that is used as magnifying glass for reading small text. The reason behind this is that it forms an enlarged, erect, and virtual image of the text. 47. The image formed by a concave lens cannot be obtained on a screen. Hence, it is a virtual image. Virtual images are erect or upright. Concave lenses form only diminished images. Hence, whenever an object is seen through a concave lens, the image of the object is virtual, upright, and small in size. 48. Convex lens is thicker in the middle than the edges, as shown in the given figure.

Hence, the figure actually inscribed on the vehicle is larger than what John saw in the car’s rear view mirror. The correct figure is shown in alternative B only. A shaving mirror consists of a concave mirror because it forms an enlarged and erect image of the object, when the object is placed close to it. Therefore, an enlarged and erect image of the face can be seen which helps in having a closer look of the face for a better shave. Street lamps are used to light a large open area. Hence, the light emitted from these lamps must spread over a large area. This job is successfully done by convex mirrors because they diverge the parallel light rays coming from the light source. Hence, convex mirrors are used as reflectors in street lamps. Dentists are required to see the enlarged and upright images of teeth and throats to view their minute details. Since a concave mirror can form magnified and erect images, it is used by the dentists to examine a patient. The correct answer is C. Surveillance mirrors in libraries and supermarkets or safety mirrors in shops and stores must have a large field of view so that they can form images of a larger area. Hence, the image formed by such a All the incoming parallel rays bend inwards and meet mirror must be erect and diminished. Also, the erect at a point after passing through a convex lens, as images are always virtual in nature. Since a convex shown in the following figure. mirror always forms virtual, erect, and diminished images, they are used as surveillance mirrors in libraries. It is the convex lens that is used as magnifying glass for reading small text. The reason behind this is Hence, convex lens is also called the convergent that it forms an erect and virtual image of the text. lens. A convex lens acts as a magnifying glass. This is 49. In the given figure, a parallel beam of sunlight because a convex lens forms virtual magnified converges to a point after passing through the image of an object on the same side of the object glass G. Hence, G is a converging lens or a convex as represented in the given figure. lens. It is the property of a convex lens to focus all parallel incident rays to a point on its focus. 50. An image smaller than the object can be formed by concave lens, convex lens, convex mirrors, or concave mirrors. But inverted and real images are formed only by convex lens and concave mirrors. Hence, convex lens and concave mirrors can form A magnifying glass is used to see the magnified diminished and inverted images. image of an object. www.betoppers.com

7th Class Physics

150 51. An image that is smaller than the object is produced by either a convex or concave lens or mirror. But a real image can only be produced by either a concave mirror or convex lens. Hence, either a concave mirror or convex lens can produce an image that is real and smaller than the image of an object. 52. The light coming from the sun is white light. When it is passes through a glass prism, it splits into seven colors as a result of the refraction of white light on the prism’s surface. Hence, it can be concluded that white light is made up of seven colors. 53. When white light falls on a prism, it splits into seven colours. The colours are red, orange, yellow, green, blue, indigo, and violet (from top to bottom). The sequence of the colours as they appear on the screen is shown in the given figure:

CONCEPTIVE WORKSHEET 1

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B

B

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A

A

ABC

A

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27 28

29

30

A

A

D

ABC AB

C

B

ABC

D

31 32

33

34

35

36

37 38

39

40

A

C

B

C

A

C

A

D

C

C

41 42

43

44

45

46

47 48

49

50

C

B

A

C

C

D

A

C

C

C

C

51 C

It can be observed that the top two colours on the screen are red and orange, whereas the three bottom 2. colours are blue, indigo, and violet. 3. When these five colours are removed from the screen, only yellow and green are visible. This is also shown in the given figure.

54. There are seven colours present in a rainbow. From top to bottom, the order of the colours is red, orange, yellow, green, blue, indigo, and violet. Therefore, the third colour from top is yellow and the third colour from bottom is blue. 4. 55. The spinner has all the constituent colors of white light painted on it. While the spinner is rotating, all the colors combine and give a white appearance. It is for this reason that the spinner appears white. 56. The seven constituent colours of white light are red, orange, yellow, green, blue, indigo, and violet. These colours can be seen in a rainbow in that order from top to bottom. Black is not present in a rainbow. A rainbow is shown in the given figure.

HINTS / SOLUTIONS TO THE SELECTED QUESTIONS A – 1; B – 3; C – 2; D – 4 When the rays of light, diverging from a point, after reflection or refraction, appear to diverge from another point, then the image so formed is called virtual image.

When the rays of light, diverging from a point, after reflection or refraction actually converge at some other point then that point is real image of the object.

18. The image formed by a plane mirror is of the same size and  upright.  However,  the  right  appears  left

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Light Solutions and the left appears right in the mirror. Hence, the word printed on the T-shirt will appear in the mirror as given in alternative C. 19. In a plane mirror, image distance is always equal to object distance. The mirror is moved away from Pravjot at a rate of 1 m/s. This means that the distance between Pravjot and the mirror is increased by 1 m in each second. Thus, the image distance is also increased by 1 m in each second. Hence, the distance between Pravjot and his image increases by 2 m in each second i.e., the image recedes from Pravjot at a rate equal to 2 m/s. 20. We know that image distance from a plane mirror is equal to object distance. The distance of Babblee from the mirror is half the distance between Babblee

151 traffic. Since concave mirrors cannot provide large field of view, these cannot be used as side-view mirror in cars.

HOTS WORKSHEET 1. 3. 4. 7.

40 cm 2. 45 cm (a) 60 m/s (b) 80 m/s (c) 40 m/s 600 5. 90 cm 6. 500 The angle that the ray makes after each reflection is clearly labelled in the adjacent figure . It is clear that the final ray makes an angle of 600 with the horizontal, or in other words it emerges parallel to Mirror 1.

M2

600 30

0

and her image, i.e.,

0

30

30.

31.

32.

33.

34.

35.

Hence, the distance of Pappu from the mirror is 2 × 50 = 100 cm, and the distance between Bunty and the mirror is 2 × 100 = 200 cm = 2 m. If the mirror is moved 1 m back from its original position, then the distance of Bunty from the mirror will be 2 + 1 = 3 m. Hence, the distance between Bunty and his image will be two times, i.e., 2 × 3 = 6 m. The side-view mirror of a car must have a large field of view so that the driver can view large traffic behind him. Since convex mirrors provide a large field of view, these are used as side-view mirror in cars. The size of the images formed by a convex mirror is smaller than that of the object size. The images can never be larger than the object. Hence, a convex mirror always forms diminished images. The dentist’s mirror is basically a concave mirror. A concave mirror produces a large image of an object. This mirror helps in determining the position and size of cavity present in teeth. Hence, it is used in the dentist’s mirror. The nature of work of cosmetologists is such that they have to view the large image of a small face area by holding a mirror close to the face. A concave mirror produces a large image of an object. Hence, cosmetologists use concave mirrors for their work. Convex mirrors are used as side view mirrors in cars and scooters because they can form images of objects spread over a large area. Therefore, these help the drivers to see the traffic behind them. The side-view mirror of a car must have a large field of view so that the driver can view the large

600 600

8.

9.

600

M

1 The first image at M1 is formed at a distance of 4 cm behind it. The first image at M2 is 6 cm behind it. The second image at M 2 is at a distance of 4+10=14 cm behind. M2. Therefore, the distance between the first image at M 1 and second image at M 2 is 4+14+10=28 cm. The speed of image =

Distance travelled by image Time Let us consider that the mirror has moved for 1 second.  The distance travelled by mirror = 20 cm/s Let the initial distance between object and mirror be x. From the figure, it is clear that OM = x  MI = x Let M’ be the final position of the mirror.  New object distance = OM’ =( x + 20) cm  New image distance = M’I’ = ( x + 20) cm The distance moved by the image II’ = MI’ – MI = 20 + x 20 – x = 40 cm

 Speed of image =

40  40 cm / s 1

10. 45° each 11. Suppose OM1 is the horizontal plane mirror and www.betoppers.com

7th Class Physics

152 OM2 the second mirror. Let the angle between the mirrors be 

Light ray travels along the path PQRS, such that RS is the reflected ray of second mirror.

According to the laws of reflection, The angle of incidence on first mirror be  .

ABM1  OBC1  45 Again, since CD and OM 1 ar e parallel, DCM 2   According to

the

laws

of

BQR  90   ( It is the glancing angle)

reflection,

Given, CRS  30

DCM 2  BCO   Now, in COB .

 BRQ  30 (g i  g r )

OCB  CBO  COB  180 or  + 45° +  = 180°

1 2 12. Let PO and OQ be the two plane mirrors inclined at an angle  A light ray AB is incident on PO at B. BC is its reflected ray incident on OQ at C, such that CD is the final reflection. Given, AB||CD,  =?

In QBR,(90           10

IIT JEE WORKSHEET

  = 67

1

2

3

4

5

6

7

8

9

10

B

A

C

B

B

B

D

C

B

B

11

12

13 14 15

16

17

18

19

20

B

D

A

AB AC BC CD CD CD D D

21

22

23 24 25

AB CD

D

B

B

B

D

HINTS / SOLUTIONS TO THE SELECTED QUESTIONS

Let x be the angle between BC and the mirror OQ. ACQ  x (Glancing Angles are equal) As AB and CD are parallel to each other, ABC  CDB  180 CDB 180  2

1.

Here, the student is required to investigate the phenomena of absorption, reflection, refraction, and transmission of light. A mirror reflects light that is incident on it. As a result, a virtual image is formed on the other side of the mirror. Hence, mirrors cast images because they reflect light.

2.

The image formed by a plane mirror is such that its size is always equal to the object size. Plane mirrors never produce magnified or diminished images.

3.

A ray of light incident on the surface of a mirror gets reflected. This leads to the formation of an image, as shown in the following figure.

From the figure, x + x + (180 –2  ) = 180°. x In BOC,(90  )      180    90 Therefore, the angle between two mirrors is 90°. 13. AB and BC are two mirrors and the angle between them is 70°.

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Light Solutions

153 5.

The distance of the image from the plane mirror is same as the distance between the plane mirror and the object. This distance is independent of the height of the object, that is, even if we increase the height of the objects, the distance between the mirror and the object will remain the same. In the given case, the initial distance between the pencil and the plane mirror is 10 cm. When this pencil is replaced by another pencil of length twice the previous one, the distance between its image and the mirror remain unaffected. Therefore, the distance between the image of the new pencil and the mirror is 10 cm.

A plane glass slab is a transparent material and refracts light. A light ray incident on the surface of a plane glass slab gets displaced, as shown in the following figure. Therefore, it cannot form an image. 6.

The image formed by a plane mirror is erect, laterally inverted, and of the same size as that of the object. On the other hand, the image formed by a convex mirror is erect and diminished. In the given case, the image is of the same size as the object. Therefore, the mirror used in the given case is a plane mirror.

Hence, a mirror can form images because it reflects light while a plane glass slab cannot do so because it refracts light. 4.

In case of reflection in a plane mirror, the image distance from the mirror surface always remains the same as the object distance from the mirror surface. It is given that the distance between the 7. pencil and its image is 40 cm. Hence, the pencil is placed at a distance of

40  20 cm from the mirror.. 2

The image formed by a plane mirror is such that its distance from the mirror is same as the distance between the plane mirror and the object. Hence, the mirror is placed in the middle of the candle and its image. Therefore, the plane mirror is placed 15 cm away from the given candle. A convex mirror always produces virtual images smaller than the size of the object.

8.

In the given activity, the candle is placed in front of the instrument and its image is formed on the instrument. The image is erect, virtual in nature, and enlarged in size. Only a concave mirror can form this type of image of an object when the object is placed very near to it. Therefore, the optical instrument used in the activity is a concave mirror.

9.

The side view mirror of a car is a convex mirror. It forms erect and virtual images of distant objects.

Now, the pencil is moved 5 cm towards the mirror. Hence, the new object distance will be 20 – 5 = 15 cm. The new image distance from the mirror will be equal to the object distance that is 15 cm (as shown in the given figure).

10. The images formed by the described mirrors are small and cover a large area. The field of view of the described mirrors is large. Only convex mirrors have a large field of view that produces small images. 11. The rear-view mirror of an automobile is a convex mirror. The convex mirror enables the driver to see a wider area behind the vehicle.

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7th Class Physics

154 12. Street lamps are used to light a large open area. Hence, the light emitted from these lamps must spread over a large area. This job is successfully done by convex mirrors because they diverge the parallel light rays coming from the light source. Hence, convex mirrors are used as reflectors in street lamps. 13. The side-view mirror of a car must have a large field of view so that the driver can view the large traffic. Since concave mirrors cannot provide large field of view, these cannot be used as side-view mirror in cars. 14. It is the convex lens that is used as magnifying glass for reading small text. The reason behind this is that it forms an erect and virtual image of the text. 15. The fact that the lens was unable to burn the paper indicates that the rays of the sun coming towards the paper were not being converged by the lens i.e., the rays were diverged by the given lens. Therefore, the lens used by Rajesh is a concave lens. The image formed by a concave lens is always erect and smaller in size than the object. Therefore, the appearance of the spectacles through the lens is erect and small. 26. 20 27. 10 28. 7 29. A-r; B-t; C-p; D-q; E-s 30. A-p; B-pr; C-qst; D-qst; E-pr 

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4. ELECTRIC CURRENT & ITS EFFECTS SOLUTIONS

FORMATIVE WORKSHEET 21 22

23

24

25 26 27

28 29 30

D

C

C

C

31 32

33

34

35 36 37

38 39 40

D

B

B

A

B

C

41 42

43

44

45 46 47

70 71 72

C

C

C

B

A

D

73 74

75

76

77 78

*

A

AC

B

A

A

C

C

B

B

A

C

B

C

A

A

107 × 1.6 × 10-19  1.6  1012 A 1 Charge on one electron (e) = 1.6 × 10–19 C Let the number of electrons be n. Current, I = 4 A We know that, Q ne I= = t t

It 4 1   2.5  1019 e 1.6  1019 (a) Here, Q = 3.2 × 10–7 C, e = 1.6 × 10–19 C Number of electron transferred,

Q 3.2  107   2  1012 e 1.6  1019 (b) Yes, there is a transfer of mass from wool to polythene. Here, n = 1022, e = 1.6 × 10–19 C  Q = ne = 1022 × 1.6 × 10–19 = 1.6 × 103 C As the comb has excess of electrons, so Charge on comb = -1.6 x 103 C. n

4.

7.

B

n

3.

6.

C

HINTS / SOLUTIONS TO THE SELECTED QUESTIONS 1. Number of electrons, n = 107 Current, I = ? We know that, Q _________ I= (1) t 17 Charge of 10 electrons = 107 × Charge of 1 electron = 107 × 1.6 × 10–19 C Substituting the above value in (1), we get

2.

5.

Current, I = 0.5 A Time, t = 10 min = 600 s Charge, Q = It = 0.5 A × 600 s      = 300 C. Here, I = 0.2 A, t = 1 h = 3600 s Charge, q = I × t = 0.2 × 3600 = 720 C. Here, I = 1 A, t = 1 s Total charge, q = I × t = 1 × 1 = 1 C Charge on one electron, e = 1.6 × 10–19 C Number of electrons



8.

Total charge 1C  Charge on one electron 1.6  1019 C

= 6.25 × 1018 Here, n = 1020, e = 1.6 × 10–19 C, t = 0.1 s Total charge, Q = ne = 1020 × 1.6 × 10–19 = 16 C

Q 16   160A t 0.1 9. i = 10 mA = 10 ×10–3A t = 10 min = 10 × 60 sec. Q = it = 10 × 10–3 × 10 × 60 =6C 10. Q = 100 C t = 4 sec Q 4 i  = 0.04 A t 100 11. Number of electrons acquired, Current, I 

q 1   6.25  1018 , i.e., it has e 1.6  1019 acquired 6.25 ×1018 additional electrons. 12. q = n e = 106 ( –1.6 × 10–19) C = –1.6 × 10–13 C. 13. Formula : Q = It  1800 = 2 × t or,, n

1800 = 900 sec = 15 min 2 14. Q = I × t = 1.6 × 1 = 1.6 Col. Each electron carries a charge of q = 1.6 × 10–19 Col. Hence the number of electrons flowing through the t=

1.6  1019 19 1.6  10 15. a) Q = I × t = 1.5 × 60 = 90 coulomb b) W = VQ = 100 × 90 = 9000 joule.

bulb is

16.

n

n

q 4  107   2.5  1012 19 e 1.6  10

7th Class Physics

156

28. An electric motor converts electrical energy into mechanical energy. A generator converts mechanical energy into electrical energy. q   transferred = me × n = me ×   A telescope is used for viewing objects situated at e large distances. A door bell converts electrical energy into sound work 5   0.5V e.m.f. of cell = energy. ch arg e 10 29. A metallic material must be used as a core in an w 100 electromagnet. Hence, iron can be used to make Potential    20 volt. the rod of an electromagnet. q 5 30. Electromagnet can be formed by putting a magnetic WAB = q (VB – VA) material inside an electric coil. As long as the current WAB 2 flows through the coil the core magnetic material 1   1  10 V .  (VA – VB) = – behaves as an electromagnet. q 20 31. The working of the hair dryer is based on heating w 10 effect of electric current. When electric current is V    2V passed through the hair dryer that has a coil of wire q 5 called the element, the element becomes red hot Energy = work done = charge × potential = 1 eV and gives out heat. Hence, the element of the hair joule. dryer works on the principle of heating effect of In order to answer this question correctly, the student electric current. needs to classify materials as electrical insulators 32. A simple electromagnet can be constructed by and conductors. The iron bar will be converted to winding a current carrying coil over a core. Hence, an electromagnet if the solenoid is made up of an the coil in the electric generator constructed by electrical conductor. Plastic is an electrical insulator. Tempenny is an electromagnet. Therefore, electricity will not pass through the 33. The current flowing through a coil produces a solenoid made of plastic. Hence, if a plastic wire is magnetic field which attracts the iron rod downward. used to make the solenoid, the iron bar will not be 34. A simple electromagnet is constructed by winding converted into an electromagnet. a current carrying coil of wire over an iron rod. This question is successfully answered by the student Therefore, in the electromagnetic train signal shown who knows that a light bulb emits light as a result of in the figure, part A represents an electromagnet. the heating up of the filament. This happens because 35. The strength of an electromagnet is increased by of the conversion of electrical energy into heat increasing the number of turns in it. This implies energy and light energy. that the latch will work more effectively with the Hence, a light bulb emits light because of high increase in the strength of the electromagnet as it temperature. will pull the latch with greater force. A generator is used for converting mechanical 36. An electromagnet is a type of temporary magnet in energy into electrical energy. which magnetic field is induced by a flow of electric An electric motor is used for converting electrical current. This magnetic field helps the electromagnet energy into mechanical energy. in attracting magnetic substances such as paper An electromagnet is used for generating a magnetic clips. field. 37. When an electric current passes through a A magnetic compass is used for detecting magnetic conducting wire, it becomes hot. The phenomenon fields. of heating of the wire in response to electric current A current carrying wire produces a magnetic field is called the heating effect of electric current. An around it. Hence, only a compass, which can be electric iron works on this principle. It is the element used to detect magnetic fields, when brought near of an electric iron that becomes hot when electricity the wire, will show a deflection. is supplied to it. A current carrying wire produces a magnetic field around it.

17. No. of electrons transferred, n 

18. 19. 20.

21. 22. 23.

24.

25.

26.

27.

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q e

Mass

Electric Current & Its Effects Solutions

157

38. When current is passed through the coil, a magnetic 44. An electric fuse works on the principle of heating effect of current. When a current exceeds the rating field is produced by the electric current that of the fuse element, it melts and breaks the circuit, magnetizes the iron hammer. The magnetized stopping the flow of current. When 4 A current flows hammer is attracted toward the gong, which is through the given fuse and circuit, the fuse will melt usually made up of steel. Hence, the magnetized and break because the amount of current is more hammer strikes the gong to produce sound energy than the fuse rating. in the doorbell. 39. A magnetic compass contains a magnetized needle 45. An electric fuse consists of a wire of low melting that is free to align itself with Earth’s magnetic field. point (called the filament). It works on the heating A magnetic compass will show a deflection when effect of current. When excess current flows brought near an electromagnet. through the filament, a large amount of heat is 40. Every magnet, whether temporary or permanent, produced. As a result, the filament melts and breaks has two poles called North Pole and South Pole. the circuit, stopping the flow of current in the circuit. 41. There are two ways of producing a magnetic field. This protects electrical appliances from damage A magnetic field is produced by magnetic dipoles caused by an excessive amount of current in the that are present in permanent magnets. It is also circuit. Hence, electric fuses are safety devices. produced by moving charges or flowing electric current. Hence, when current is passed through a 46. The heating effect of current states that a wire becomes hot when current is allowed to pass wire wound around an iron rod, it magnetizes the through it. When the amount of current flowing iron rod by aligning the magnetic poles inside the through a wire is enough to produce a large quantity iron rod, producing a magnetic field around it. of heat, the wire can even melt and break. This 42. An electric iron is a heating device. It produces principle is involved in the working of an electric heat when electricity is allowed to pass through its fuse. It consists of a wire of low melting point. The element. The element of an electric iron is a coil of wire, called filament, melts and breaks in response wire. The coil becomes hot when current flows to a large amount of current that produces heat in through it. The element of the given iron is labelled the wire. as III (as shown in the given figure).

The given table explains the labelled parts with their respective names. Label

Part

I II

Terminals Insulating body

III IV

Element Handle

47. Motor is an electric device, which works on electricity. It converts electrical energy into mechanical energy. It is not a heating device. Therefore, it does not work on the principle of heating effect of current. Hence, Raju should remove motor from the list of electric devices that work on the principle of heating effect of current. 48. Here, I = 3 A, R = 80 By Ohm’s law, V = IR = 8 × 3 = 24 49. Here, V = 1.4 V, I = 0.35

R

V 1.4  4 I 0.35

43. A heater is a heating device that works on the principle of heating effect of current. It consists of 50. (a) For electric bulb: a heating filament and a coil of wire. The melting V = 220, R = 1200  point of the wire is very high. When current is allowed to pass through this coil, it produces a large From Ohm’s law, amount of heat. The high melting point of the heater V 220V filament ensures that the filament does not melt on I= = = 0.18A excessive heating produced by the heating effect R 1200Ω of current.

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7th Class Physics

158 (b) For electric heater coil. V = 220 V, R  100 From Ohm’s law,

I=

V 220V = = 2.2 A R 100Ω

Clearly, the heater coil draw much more current than the electric bulb. 51. When potential difference, V = 60V, Current (I) = 4 A By Ohm’s law, Resistance, R =

V 60V = = 15Ω I 4A

When the potential difference is increased to 120 V, the current drawn by the heater will be

V 120V   8A R 15 i = 10 mA = 10 ×10–3A t = 10 min = 10 × 60 sec. Q = it = 10 × 10–3 × 10 × 60 =6C Q = 100 C t = 4 sec Q 4 i  = 0.04 A t 100 t=? Q = 10 C i=2A Q = it = 10 × 2 = 20 C Electric current can pass through iron. Hence, Iron is a good conductor of electricity. The handle of an electrical repairing tool should be made of an electrical insulator. Tin is a good conductor of electricity, i.e., electric current can pass through it. Hence, tin cannot be used to cover the handles of electrical repairing tools. Wood is a bad conductor of electricity. The circuit will not become complete on inserting wood in the gap. Hence, the bulb will not light up. Insulators are those materials that do not conduct electricity. Plastics cannot conduct electricity. 68. By reversing the terminals of the battery in the Hence, plastic cup is an insulator. given circuit, the direction of the current flowing In order to answer this question correctly, the through the circuit will reverse. The functioning student must be able to classify materials as of the bulb does not depend on the direction of electrical insulators and conductors. Electric wires current flowing through it. Hence, there will be are coated with plastic because plastic is a good no effect on the operation of the bulb. The bulb electrical insulating material. This minimizes the will remain glowing as before. unavoidable contact of the wire with other parts of the circuit or living tissues. I

52.

53.

54.

55. 56.

57.

58.

59.

60. In order to answer this question, the student must be able to identify whether a material is a good or bad conductor of electricity. To light up the bulb, points P and Q must be connected by an electrically conducting material. Iron is a good electrical conductor. Hence, the bulb will light up when points P and Q are connected with a plate made of iron. 61. In order to answer this question, the student must be able to classify materials as electrical conductors and insulators. Copper is a good conductor of electricity. Therefore, when points P and Q are connected with a copper rod, a short circuit will occur in the battery and current will not pass through the bulb. Hence, the bulb will not light up. 62. Plastic is an electrical insulator. Hence, plastic cannot be used to make electrical wires. 63. Overhead electric lines transfer a high voltage of electricity and ceramics is an insulator of electricity. Therefore, ceramic pulleys are used to prevent electric wires from touching the poles. 64. Graphite is an allotrope of carbon and is a good conductor of electricity. Hence, it cannot be used an electrical insulator. 65. Wood is a very good electrical insulator. Therefore, the possibility of receiving an electric shock while working on an electric line is minimized by standing on a wooden plank. 66. This simple problem can be solved by analyzing the given circuit. The complete circuit should consist of a bulb, a switch, and an energy source, such as a battery. The given circuit does not contain any battery or energy source. Hence, the bulb will not glow on closing the switch. 67. The bulb will glow, when the constructed circuit is closed. Using the given four wires, bulb, switch, and battery, a closed circuit can be constructed as,

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Electric Current & Its Effects Solutions

159

69. If bulb 1 gets fused, then the equivalent circuit can be represented asi.e., the circuit breaks. Since the circuit of bulb 2 becomes incomplete, it will stop glowing.

19. In an electric bell, electricity is used to produce sound energy. Therefore, in an electric bell, electrical energy is converted into sound energy.

73. a  2; b – 3; c  5; d – 1

CONCEPTIVE WORKSHEET 16

17

18 19 20

21

22 23 24 25

D

B

B

B

C

D

26

27

28 29 30

31

32 33 34 35

B

D

C

A

B

A

36

37

38 39 40

41

42 43 44 45

A

A

C

B

C

A

46

47

48 49 50

51

52

A

ABD

B

3V

A

B

C

A

D

C

18. Moving electric charges produce a magnetic field around themselves. The flow of current in the wire loop is caused by moving electrons. This means that when current flows through the wire loop, a magnetic field is produced around the wire. Hence, the pin gets attracted toward the loop because of magnetic force.

C

C

D

C

D

D

B

A

B

20. The core of an electromagnet should be made up of a ferromagnetic substance. Hence, Stella should use iron as the core of the electromagnet. 21. An electric bell uses a magnet that can be turned on and off. 22. Electromagnets are temporary magnets. When electric current flows through an electromagnet, it gets magnetized. The magnetic field of an electromagnet can be switched on and off according to convenience. 23. An ideal fuse element should melt when a large amount of current flows through it. Although its working is based on the phenomenon of the heating effect of current, for the fuse element to work effectively, it must have a low melting point so that it melts when the amount of current crosses the prescribed upper limit.

HINTS / SOLUTIONS TO THE SELECTED QUESTIONS 1. Coulomb 2. Giving electrons to it 24. An electric bell consists of a conducting coil inside 3. Electric current which an iron rod is placed. When current flows 4. Ampere through the coil, the iron rod converts into an 5. 1 C/s electromagnet. Hence, statement II is correct. 6. Potential difference The phenomenon of converting a magnetic material 7. Potential into an electromagnet in response to the current is 8. Zero potential called the magnetic effect of current. Thus, an 9. Charge of an electron 19 electric bell involves the magnetic effect of current. 10. 6.25 × 10 Hence, statement I is also correct. 11. Potential difference 12. Volt 25. The given phenomenon is based on the magnetic 13. QV effect of current. If the switch of the circuit is 14. Voltameter closed, then the current starts flowing through the 15. Ammeter wounded wire. Hence, the iron rod inside the coil 16. The iron nail will act as a core and since iron is a becomes an electromagnet. This magnet attracts magnetic material, it will be transformed into an the nearby iron filings. An electromagnet remains a electromagnet. magnet as long as the current flows through it. If 17. This question requires the student to know that a the current in the coil is ceased, then the iron bar magnetic field is produced around moving charges. looses its magnetism and becomes a simple iron The needle of a magnetic compass is deflected only bar. At this point, the iron bar releases the attached by the presence of a magnetic field. Hence, a current iron filings. carrying wire produces a magnetic field around itself.

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160

7th Class Physics

26. The working of an electric bell is based on the 30. An electromagnet is a temporary magnet that uses magnetic effect of current. When the switch is electric current to generate a magnetic field around closed, current flows through the wound coil. An it. The electromagnet behaves like a magnet when iron bar (placed inside the coil) becomes an electric current flows through it. It is by virtue of electromagnet. This electromagnet pulls a nearby this property that electromagnets attract magnetic iron hammer. As a result, the hammer moves and particles such as iron, cobalt, and nickel toward strikes the gong that produces a sound. Hence, it is themselves. Hence, broken cars, which are made the hammer that moves when the switch of the bell up of magnetic materials, can be picked up using is closed. cranes that have electromagnets attached to their 27. An electric bell works on the principle of magnetic lower ends. effect of current. When current is allowed to flow 33. On closing switch I, the given electrical circuit will through the coil of wire, an iron bar placed inside not be complete. Hence, on closing switch I,the the coil becomes an electromagnet. The given table bulb will not glow. lists the names of the labelled parts of a bell. 34. Electric current can pass through iron. Hence, Iron is a good conductor of electricity. Label Part 35. The electrical circuit constructed by Robert will I Gong be complete when the positive terminal of the II Hammer battery is connected to the point P. Hence, the bulb will glow when the positive terminal of the III Metal strip battery is connected to the point P. IV Coil of wire 36. The circuit shown in alternative A will be complete Since the coil of wire is labelled IV, it is the part when the switch is closed. As a result, the bulb which gets converted into an electromagnet. will glow. However, the circuit will not be complete 28. A door bell works on the principle of the magnetic when the switch is open. As a result, the bulb will effect of current. A door bell is a device that not glow. Therefore, the bulb can be operated by produces sound. It consists of an iron piece placed the switch in the circuit. inside a current-carrying coil of wire. When a current 37. Andrew should connect the bulb and the fan such is passed through this coil, the iron piece becomes that the voltage across the fan and the bulb is the an electromagnet. The electromagnet attracts an same. Hence, Andrew should connect the bulb as iron strip. As a result, a hammer (attached to the shown in alternative A. iron strip) is pulled towards the gong. This strike 38. Alex should add the switch in such a way that produces sound. The working principle of a door only bulbs N and P are controlled by it, i.e., the bell is illustrated in the given figure. circuits of bulbs N and P should  be  incomplete when the switch is open and complete when the switch is closed. Hence, Alex should add the switch at point III. 39. If a switch is placed at point I, then it will operate only bulb M. Bulb N will not be affected by the switch. 40. The simple circuit containing all the components and the fan between the two bulbs is shown in the circuit diagram given in alternative B. 29. A coil of wire behaves as a bar magnet when a 41. The simple circuit containing all the components current flows through it. The coil behaves as a bar and the switch between the two bulbs is shown in magnet. The coil is called solenoid, which acts as the circuit diagram given in alternative C. an electromagnet. The coil behaves as a magnet as 42. The student should insert an object that can allow long as the current flows through it. When the electricity to pass through it. A nail is made of current stops flowing, the coil looses its magnetic iron. Since iron allows electricity to pass through property and becomes a simple coil. This effect is it, nail is a good conductor of electricity. In order called the magnetic effect of current. to complete the circuit, the student should connect the open ends of the wires with a nail of sufficient length. www.betoppers.com

Electric Current & Its Effects Solutions 43.

44.

45.

46. 47. 48. 49. 50. 51. 52.

Although the lead of a pencil is encased within a 10. i - b, ii - a, iii - c wooden frame, it allows electric current to pass 11. i - b, ii - a, iii - c through. Since the given pencil is sharpened from 12. both ends, it can conduct electricity. Component of The bulb can glow only when two terminals of an electrical circuit electric cell are connected to two terminals of a Connecting wires bulb respectively. In such arrangements, a circuit Switch in the ‘OFF’ is termed as closed circuit. An electric device such position as a bulb works in a closed circuit only. It can be observed that the terminals of the cell and the bulb Bulb are correctly connected in arrangements Iand IVonly. Hence, cir cuit Cell arrangements Iand IV are  correct  and  the  bulb will glow. Switch in the ‘ON’ The pictorial representation of the electric cell position is represented in the given figure. Battery It can be observed that an electric cell has two terminals called positive terminal (+) and negative terminal (–). The metal cap attached to one end of the cell is its positive terminal. The path along which electric current flow is called an electric circuit. Non metals like glass, plastic, wood, paper, cloth and rubber are the non-conductors of electricity. Both the statements are true. In series, total emf = sum of the individual emfs = 2 + 2.5 + 4 = 8.5 V In parallel, emf always remains constant. In series, total emf = sum of the individual emfs = 1 + 1 + 1 = 3v Electric cell

SUMMATIVE WORKSHEET 1.

2. 3. 4. 5. 6.

7. 8. 9.

161

Electric current from cell flows from positive terminal to negative terminal outside the cell through circuit. Electric bulb. No. pure water is an insulator, but salty water is a conductor of electricity. Electric current flows in one direction only. Metal cap. Cells are not placed properly. Positive terminal of one cell is to be connected to negative terminal of another cell. A device that is used to break an electric circuit is called Switch. An electric cell has two terminals. Unit of electric current is Ampere.

Symbol

13. In the given circuit, one terminal of the bulb is connected with one terminal of the cell while their other terminals are connected to a safety pin. However, the safety pin is not connected with one of the drawing pins. Thus, the circuit is not complete. Hence, safety pin represents a switch in ‘OFF’ position. The circuit diagram of the given figure is shown in the following figure.

14. To make a battery, the negative terminal of one cell must be connected to the positive terminal of the next cell. The wire indicates the manner in which the given cells should be connected with each other, as shown in the following figure.

15. The bulb in the circuit is not glowing because the two cells are not connected properly. To make the bulb glow, the negative terminal of one cell should be connected to the positive terminal of the other cell, as shown in the given figure.

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162 16. The two effects of electric current are (i) Heating effect of electric current (ii) Magnetic effect of electric current 17. When a current is switched on through a wire, the wire starts behaving as a magnet. Hence, when a compass needle is placed near the given current carrying wire, it gets influenced by the magnetic effect of electric current and gets deflected from its North-South position. 18. No. The given circuit does not have any current source. In the absence of current, the wire does not behave as a magnet and hence, the compass needle will not show any deflection. 19. (a) Longer line in the symbol for cell representsits__positive__terminal. (b) The combination of two or more cells is called a __battery__. To make a battery, the negative terminal of one cell is connected to the positive terminal of the next cell and so on. (c) When current is switched ‘on’ in a room heater, it __produces heat__. Working of a room heater is based on the phenomenon of heating effect of electric current. (d) The safety device based on the heating effect of electric current is called a __fuse__. Fuse is a safety device, which limits current in circuit and breaks the circuit if the current exceeds its maximum limit. 20. (a) False To make a battery of two cells, the negative terminal of one cell must be connected to the positive terminal of the other cell. (b) True The wire of the fuse is made of special material which has a low melting point. When large current passes through a fuse, it melts quickly and breaks the circuit. (c) False Working of an electromagnet is based on the magnetic effect of electric current. An electromagnet behaves as a magnet when current passes through it. Hence, it will attract a piece of iron. (d) True An electric bell has a piece of iron wound by a coil. www.betoppers.com

7th Class Physics When current passes through this coil, it behaves as an electromagnet.

21. No. Electromagnets can only attract magnetic materials. Plastic bag is a non-magnetic material and will not be attracted by an electromagnet. Hence, an electromagnet cannot be used for separating plastic bags from a garbage heap. 22. No. The electrician cannot be allowed to replace the fuse in the house by a piece of wire. This is because every wire cannot be used as a fuse filament. A fuse filament must have a low melting point such that it could melt and break in response of large amount of current. Most of the wires have high melting points. 23. One of the reasons may be that the rubber band used in the cell holder may not be tight enough to keep the two cells in contact with each other. If the cells are not in proper contact with each other, then the circuit will not be complete and current will not flow through the circuit. Hence, the bulb will not glow. The other reason may be that the two cells are not connected properly. The negative terminal of one cell must be connected to the positive terminal of the other cell. 24. (i) No. When the switch is in ‘OFF’ position, the current does not flow through the circuit. Hence, none of the bulbs will glow. (ii) Bulbs will glow simultaneously. When the switch is moved to the ‘ON’ position, then all the bulbs will glow at once. This is because they all are connected to the same battery and switch.

Electric Current & Its Effects Solutions

HOTS WORKSHEET

163 6.

KEY 1 2 B D 11 12 C A

3 4 5 6 7 D B A B A 13 14 15 A B C

8 9 10 C C D

7.

HINTS/ SOLUTIONS TO THE SELECTED QUESTIONS 1.

2.

3.

4.

In an electric torch, electric cells are the source of current. Without electric cells, it is impossible to construct a torch. In an electric torch, current is produced by the 8. battery. Current passes into the bulb filament through the switch. Hence, current flows through switch, battery, and the bulb filament forming a circuit. Current does not pass through the reflector because it is not a part of the circuit. The bulb glows when current flows through it. On closing the switch, the circuit gets completed. Hence, current flows from the cell through the bulb. Current flows through the circuit only when the circuit is complete. A circuit that has no break is called a closed or a complete circuit. In the circuit that consists of bell 2, the two terminals of the electric cell are connected to one terminal of the bulb and one terminal of bell 2 respectively. Also, the bulb and the bell are connected to each other and the switch is closed. Hence, the bulb will glow and bell 2 will ring and produce a sound. 9.

5.

A nail is made of iron, which is a good conductor of electricity. It allows the current to flow through it. If the free ends of the circuit are connected with the iron nail, then the circuit becomes complete. A current starts to flow in the bulb, and hence, it starts glowing. Hence, Suraj should use the nail to connect the free ends.

The bulb of the circuit that is closed will glow. One terminal of the cell of the circuit, which consists of bulb 2, is connected to one terminal of the bulb and the other terminal of the cell is connected to a safety pin. The circuit is complete because safety pin is made up of iron which is a good conductor of electricity. Therefore, bulb 2 will glow. The bulb through which current will flow will glow. Current flows only through a complete circuit. In the circuit constructed by Raju, bulb 1 and bulb 2 are connected in open circuit because the wire is cut in this sub-circuit. Hence, no current will flow through bulb 1 and bulb 2. The current will flow only through bulbs 3, 4, and 5. The direction of current that externally flows through a closed circuit is from the positive to the negative terminal of the cell. Since the current is flowing from terminal M to terminal N of cell I, terminal M must be a positive terminal. Again, it is clear from the second circuit that the current is flowing externally from terminal S to terminal T of cell II. Hence, terminal T must be a negative terminal. The following table shows the nature of the terminals of the given cells. Terminal

Nature

M

Positive

N

Negative

S

Positive

T

Negative

Hence, terminals M and T are positive and negative respectively . Current flows in a closed circuit externally from the positive to the negative terminal of the cell. Hence, current will flow in the given circuit, along the direction as shown in the following figure.

It is clear from the circuit diagram that the directions of current in the wire segments RV, VT, ST, and RS are   respectively..

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7th Class Physics

164 10.

A bulb glows only if current flows through its filament. It is possible only if its two terminals are connected respectively with two terminals of the cell. An electric cell produces electricity through its two terminals only. Hence, if the free ends of the circuit are respectively connected to the terminals IV and I of the cell, then the bulb would glow. This is shown in the following figure.

This arrangement is shown in the given diagram labelled as T only. Hence, electricity would flow through the filament of the bulb in arrangement T only and, hence, the bulb would glow.

IITJEE WORKSHEET 1 2 3 4 5 B A A A A 11 12 13 14 15 B A B C D 21 22 AB AB C C

11.

12.

13.

14.

15.

6 A 16 A

7 8 9 10 A A A A 17 18 19 20 B B B BC

Reflector (curved metallic surface that surrounds the bulb) is a device that is used in a simple torch HINTS/ SOLUTIONS TO THE SELECTED to focus the emitted light in a particular direction. QUESTIONS It is not a part of its circuit and, hence, it does not 23. i = 10 A, t = 10 × 60 sec. complete the electric circuit of the torch. Q=? Bulb of the given circuit will glow if a conducting Q = it = 10 × 10× 60 object is inserted between ends M and N. Key = 600 C and nail are made of iron which is a conducting 24. Q = 100 C, t = 4 sec material. Hence, the bulb will glow if Raj would Q 100 insert key and nail respectively. i   25 A . t 4 Bulb will glow only when the circuit is closed. This 25. Q = 10 C, i = 2A is possible only when a conducting object is t=? inserted in between the gaps of the given circuit. Q = it Since a key is made of iron that allows electricity Q 10 to flow through it, it is a conducting object. Hence, (or) t    5sec key can be inserted tightly in the gap to make the i 2 bulb glow. 26. i = 5A, t = 5 min = 5 × 60 =300 sec Q = it Torch is a simple electric device that gives off = 5× 300 light. It consists of a bulb that emits light by drawing = 1500 C electric energy from the electric cells that are enclosed in a cylindrical metallic case. Hence, the 27. Q = 60C, t = 4 sec,i = ? electricity required for the operation of the torch is provided by the electric cells.

i

A bulb glows only if the current flows through its filament. It is possible only if its two terminals are respectively connected to the two terminals of the 28. cell. Electricity is provided to an electric bulb 29. through both its terminals only as shown in the 30. following figure. 31. 32.

60  15A 4 Not relevant A  Q; B  P; C  R A  s; B  r; C  p; D  t A  q; B  r; C  p A  r ; B p ; C q

33. 34.

Q t



A– S, B – R, C– Q, D – P A– R, B – P, C– Q, D – S 

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5. WIND, STORM AND CYCLONES SOLUTIONS

FORMATIVE WORKSHEET

10. Anemometer 11. True 12. False

KEY

13. True

1

2

3

4

5

6

7

8

14. False

C

A

B

A

B

C

D

A

15. The moving air is called wind.

9

10

11

12

13 14

15

16

C

B

A

C

B

B

D

D

16. Leaves, banners and flags flutter due to the pressure exerted bv the air.

17 18

19

20

21 22

23

24

A

C

B

D

ABD

C

A

B

17. The air moves from the region where the air pressure is high to those regions where the air pressure is low. 18. The main cause of wind movements on the earth is the difference in temperature.

CONCEPTIVE WORKSHEET KEY

1

2

3

4

5

6

7

8

B

D

C

A

B

A

AB

C

9

10 11

12

13 14

15

16

D

A

A

C

A

D

B

17 18 19

20

21 22

C

D

B

C

B

C

D

19. On heating the air expands and becomes lighter. 20. Reducing of air pressure. 21. The word monsoon is derived from the Arabic word mausam which means season. 22. The winds from the oceans that carry water and bring rain is called monsoon. 23. Thunderstorms develop in hot, humid, tropical areas like India very frequently. 24. The swift movement of falling water droplets along with the rising air create lightning and sound. Such situation is called thunderstorm. 25. It is a very low pressure system with very high speed wind revolving around it.

SUMM ATIVE WORKSHEET HINTS/ ANSWERS TO THE SELECTED QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9.

moving uneven warm, cooler high, low pressure water from oceans/rivers expands, contracts hurricane, typhoon mausam

26. Cyclones are also called: (i) Hurricane (ii) Typhoon 27. Wind speed, wind direction, temperature and humidity contribute to the development cyclones. 28. The centre of a cyclone in a calm area is called the eye of the storm. 29. Moving air is called wind. Air moves from high pressure region to the low pressure region. It is caused due to the difference in air pressure between the two places. Due to the heat of the sun air becomes warm, producing a low pressure. So, the warm air rises up and the cool air from the surrounding moves towards the area.

166 30. Winds blowing at a speed of 120 km per hour or above are known as cyclone. A cyclone is known by different names in different parts of the world. It is called ‘hurricane’ in the American continent. It is called typhoon in Japan, China and Philippines. In Australia, it is called Willy-Willies. Formation of cyclone: The formation of cyclone is a very complex process. Due to heat of the atmosphere water gets evaporated and changes to water vapour. When water vapour changes back to water due to condensation, the heat is released to atmosphere and warms the air around. Then the hot air rises up causing a drop in pressure. More air from surrounding regions not only rushes in but also starts spiraling. This cycle is repeated. This chain of events ends with the formation of a very low-pressure system with very high speed winds evolving around it. This weather condition called a cyclone. Factors like wind speed, wind direction temperature and humidity contribute to the development of cyclone. 31 The different reasons for wind currents are : i)     Uneven heating between the equator and the poles due to shape of earth and its inclination. ii)   Uneven heating of land and water. 32. The seasonal winds bring rain between June and September in India and South Asia are known as Monsoon winds. There are two types of monsoon winds. They are 1. Summer Monsoon winds ( Sea breeze) 2. Winter Monsoon wind ( Land breeze) Summer Monsoon Winds: In summer near the equator, the land become warm and the temperature of the land increases. T h e air above the land becomes warm and rises up. This causes the air from the ocean blow towards the land. These are known as Summer Monsoon Winds. Winter Monsoon Winds: In winter, the direction of the wind flow gets reversed. It blows from the land to the oceans. These are known as Winter Monsoon Winds. 33. Advantages 1.  It provides water for agriculture. 2. It recharges the underground water sources. 3.  It helps in recharging the rivers, ponds etc. Disadvantages 1. Excessive rain causes water from the river flow out of their bank causing flood.

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7th Class Physics 34. Thunderstorms develop in hot, humid tropical areas very frequently. The rising temperature produces strong upward rising winds. These winds carry water droplets upward. At a certain height these tiny water droplets freeze and fall down again. Because of their swift movement, the water and ice particles rub against each other and build up a negative electric charge in the cloud. The electric charge is then released by the clouds by the stroke of  lightning and thunder is produced. 35. The precautions in thunderstorms are as below. 1.  It  is well  advised not  to take  shelter under  an isolated tree, because an isolated tree is prone to lightning. While in a forest, it is advised to take shelter under a small tree 2.  Do not lie on ground. 3.  Do  not  take  shelter  under  an  umbrella  with  a metallic rod, as pointed metallic materials are prone to lightning. 4. Do not sit near a window. Open garages, storage sheds, metal sheds are not safe places to take shelter. 5. If one is in water, he should get out and go inside a building. 6. A bus or car is a safe place to take shelter. 36. A tornado is a dark funnel shaped cloud that reaches from the sky to the ground. The neck of the funnel sucks up anything which comes in its way. The tornadoes may form within cyclone. The wind in a violent tornado can circle around at a speed of 300 km per hour. 37. The centre of a cyclone is a calm area. It is called the eye of the storm. The diameter of the eye varies from 10 to 30 km. It is a region free of clouds and has light winds. 38. The steps taken by government for cyclone warning are as under. 1. With the help of Satellite Weather Observation Radars, the formation of cyclone, its direction and intensity is detected by the meteorological department. 2. The news of the formation of cyclone , its direction and intensity etc is immediately telecasted and broadcasted at least 48 hours before the arrival of the cyclone. 3. Cyclone warning is telecasted and broadcasted every hour or half hour.

Wind Storm and Cyclones Solutions 39. 1.     We should not ignore the warning issued by the meteorological department through television, radio or news papers. 2. We should make necessary arrangement to shift the essential household goods, domestic animals and vehicles etc to safer places. 3. Avoid driving on roads through standing water. 4. Keep ready the phone numbers of all emergency services like police, fie brigade and medical centers. 40. The precautions in a cyclone hit area are as under. 1. Do not drink water that could be contaminated. Always store drinking water for emergency. 2.       Do not touch wet switches and fallen power lines. 3.       Do not go out just for the sake of fun. 4.       Do not pressurize the rescue force by making undue demands. 5.       Cooperate  and  help  your  neighbours  and friends. 41. Method 1 : We can find the wind direction using a Wind Vane or Weather Vane or W e a t h e r Cock.  It consists of a a flat metallic sheet generally cut into shape of a cock or ar row which is mounted on disk and free to move around a axial shaft and point in the direction of wind . They are typically used as an architectural ornament to the highest point of a building. Method 2 : We can find the direction of light wind using smoke from aggarbatties. The direction of smoke will indicate the wind direction. 42. Two experiences that made us think that air exerts pressure are: 1.  Whiling cycling against the direction of the wind, it is difficult to ride. It shows that the air exerts pressure. 2.  While rowing a sailboat in the direction of the wind, it is always easier to row and while rowing a sailboat against the direction of the wind, it is very difficult to row. It also shows that the air exerts pressure. 43. No, I would not like to buy a house having windows but no ventilators. Warm air rises up and cool air comes downward. So, to make a stream of cool and fresh air to continuously flow to the house through the windows, there must be some ventilators in the upper part of the wall.

167 44. Holes are made in hanging banners and hoarders to allow air to pass through them as a result of which there will be less pressure of air on the banners or hoarders. Unless it is done, these may be uprooted or distorted due to the pressure of the air. 45. 1. I will help my neighbours in case cyclone approaches my village/town by the following ways. 2. I will help my neighbours in shifting their household articles, domestic animals etc to a safe place. 3. I will provide phone numbers of all emergency services like police, fie brigade and medical centers. 4. I will give them safety advice like avoiding driving on roads through standing water, not touching wet switches etc. 46.    A  good  planning  in  advance  will  minimize  the damages to be caused by a cyclone. Hence, the following planning is required. 1. The cyclone warning issued by the meteorological department through television, radio or news papers must not be ignored and followed carefully. 2. The household articles, domestic animals, vehicle etc should be shifted to safe places. 3. The telephone numbers of essential services like police, fire brigade and medical centers etc should be kept ready to meet the emergency   situation. 47. We know that increased wind speed is accompanied by a reduced air pressure. When we blow air between the two balloons the pressure between them is reduced. The pressure outside the balloon would then push them towards each other. 48. When we heat the air then it expands and occupies large space. We know that when same thing occupies more space, it becomes lighter. The warm air is therefore lighter than the air. It is because hot air goes up. 49. The regions close to equator get maximum heat from the sun. This makes the air warm rises up and the air from the regions in the 0-30 degrees latitude belt on either side of the equator moves in. These winds blow from the north and the south towards the equators. At the poles the air is colder than at latitudes about 60 degree. The warm air at these latitudes rises up and the cold wind from the poles rushes in. In this way, wind circulation is set up from poles to the warmer latitudes. www.betoppers.com

7th Class Physics

168 50. The device which is used to measure wind speed is called anemometer. Since wind speed plays an important role in the formation of storms an anemometer becomes useful to predict the cyclones or storms in cyclone prone areas.

HOTS WORKSHEET KEY 1

2

3

4

5

6

7

8

A

C

D

D

B

C

D

D

9

10

11

12

13

14

15

16

C

C

A

B

D

C

D

A

17

18

19

20

21

22

23

24

C

A

B

C

A

B

D

B

25 C

HINTS/ ANSWERS TO THE SELECTED QUESTIONS 1.

Seeds are dispersed by wind, water, and animals to different locations. Seeds dispersed by wind depend on the speed of the wind to travel long distances. Hence, the relationship between wind speed and distance travelled by seeds is direct. With the increase in the wind speed, the distance covered by seeds also increases. 2. During a cyclone, people should cooperate and help neighbours and friends get out to safer places. The correct answer is C. 3. During thunderstorms people should take shelter inside closed places such as a house or a closed vehicle. Sanjay is safe from the thunderstorm because he is sitting inside a car that is safe from the lightning that frequently occurs during thunderstorms. The correct answer is D. 4. When a tornado strikes, people should bend down on their knees and protect their head with their hands. The correct answer is D. www.betoppers.com

5.

Cyclones are accompanied with strong winds and heavy rain. Thus, one should move to a shelter during a cyclone for safety. The correct answer is B. 6. During thunderstorms people should take shelter inside closed places such as a bus. The correct answer is C. 7. A music system is an object that will not be useful to Mutum while taking shelter from a thunderstorm. The correct answer is D. 8. A tornado is accompanied by very high speed winds. Thus, an underground basement with no windows is a safe place during a tornado. It remains unaffected by the high speed winds that accompany a tornado. The correct answer is D. 9. During a cyclone, people should cooperate and help neighbours and friends get out to safer places. The correct answer is C. 10. A thunderstorm is a weather phenomenon that is generated when the air mass becomes unstable . Hot and humid air moves upward. This upward rising wind contains water droplets. When the air reaches higher up in the atmosphere, these water droplets start condensing and form clouds. This transformation is accompanied by the release of heat that the air carries with it. When these clouds becomes so heavy that they are unable to hold the water, this water begins to fall down as water droplets along with the cold air.This continuous movement of falling water (cold air) and rising hot air creates a thunderstorm as shown in the following figure.

The correct answer is C.

Wind Storm and Cyclones Solutions 11. The centre of a cyclone is known as the eye of the cyclone. It is a low-pressure area and is relatively calm. Therefore, region P in the  given figure  is a low-pressure area. High-speed winds flow in the outer section of a cyclone. Therefore, region Q in the given figure is the region that experiences high-speed winds. The correct answer is A. 12. Cyclones are formed in regions in which air pressure is  dropping.  Pressure  drops  because  of the release of heat as a result of the condensation of water vapour. The correct answer is B. 13. Thunderstorms occur as a result of the interaction of ascending hot air and descending cold air. This is correctly depicted by the figure in alternative D. The correct answer is D 14. Cyclones are formed in hot and humid regions in which the air pressure is dropping. A drop in air pressure occurs because of the intense heating of the air above that region owing to the high temperature in the region.This hot and humid air rises up and creates a low pressure zone. Cold air starts moving toward this low pressure zone from all the directions. As a result, this low pressure zone turns into the centre of a storm. The air revolves around this centre at very high speeds, thereby forming a cyclone. The correct answer is C. 15. The centre of a cyclone is a calm area. Its pressure is low in comparison to the outside pressure of the cyclone. It is called the eye of a cyclone. Its diameter may vary from 10 km to 30 km depending upon the size of the cyclone. The correct answer is D. 16. The Western coastline of India is less prone to cyclones than the Eastern coastline both in terms of frequency and intensity. The correct answer is A. 17. Air in the tyres of all vehicles is filled at a pressure higher than the normal atmospheric pressure. When tyres of buses and trucks burst, the high pressure air inside the tyre suddenly comes into contact with the relatively low pressure region in the atmosphere outside. Hence, the air escapes at a very high speed from the high pressure region toward the low pressure region. This sudden interaction of the two different pressure regions causes the loud sound that is heard when a tyre bursts. The correct answer is C.

169 18. When air is heated up, it spreads and occupies a larger space. Hence, hot air becomes lighter than the cold air. When the air inside the balloon is heated up, the air becomes lighter due to expansion. This lighter air moves up, thereby lifting the balloon. The correct answer is A. 19. High speed wind is always accompanied with low pressure. When a high speed wind blows over the clothes, it creates a low pressure area over the clothes. As a result of this, air pressure below the clothes become relatively higher than the air pressure above it. Hence, air from below the clothes rushes upward with high speed. This rush of air forces the clothes to move toward the low pressure area, thereby, causing the clothes to lift in the air. The correct answer is B. 20. In summers, the land warms up faster and remains at a relatively higher temperature than the oceans. This high temperature causes the air over the land to rise up. In order to fill the vacant space, cold air from the ocean moves toward the land. This results in the blowing of sea breeze during the summers. Such winds are known as monsoon winds in India. Since these winds contain high amounts of water vapour, most of the rainfall received by most parts of India is during the summers. The correct answer is C. 21. In summers, the land heats up faster and maintains a relatively higher temperature than the oceans. This high temperature causes the air over the land to rise up. In order to fill the vacant space, cold air from the oceans moves toward the land. This results in the blowing of a sea breeze during the summers. These are known as the monsoon winds. Since these winds contain large quantities of water vapour that cause rainfall, the rainfall received by most parts of India is during the summer months, brought about as a result of these winds. The correct answer is A. 22. When the winds move from a high pressure region to a low pressure region in the Northern hemisphere, owing to the rotation of the Earth, the direction of these winds is rightward. Similarly when the winds blow from a high pressure region to a low pressure region in the Southern hemisphere, its direction is deflected toward the left. Thus, rotation of the Earth is responsible for the relatively curved trajectory of the winds. The correct answer is B.

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7th Class Physics

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23. During winters, land cools down much faster than the sea because land loses the heat much quickly than water. Thus, the hot air above the surface of the sea rises up. In order to fill this vacant space, cold air from the land moves toward the sea. This results in the blowing of land breeze during winters. The correct answer is D. 24. During day time when the temperature above the land is higher than that above the sea, the air above the land rises because hot air is relatively lighter than cold air. As a result, cold air from the sea blows toward the land to occupy the empty space. This blowing of wind from sea to land is called sea breeze. Sea breeze blows only during daytime. The correct answer is B. 25. The density of air is inversely proportional to the temperature. In the given figure, the air of balloon A has the lowest density, so it has the highest temperature, which means density of air in balloon A < density of air in balloon C < density of air in balloon B. So, the correct relation among their temperatures are TA >TC > TB. The correct option is C.

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Column A Wind carrying water Moving air Measures the speed of wind Dark funnel shaped clouds

(v)

Develops in India very frequently

(iii) (iv) (v) (ii) (i)

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Column B Monsoon Wind Anemometer Tornado Thunderstorms

6. SOUND SOLUTIONS

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HINTS / SOLUTIONS TO THE SELECTED QUESTIONS 1. In order to answer this question correctly, the student needs to identify different forms of energy. A door bell is a device that produces sound. Hence, it emits sound energy to alert people. 2. Sound is caused by vibration. Since object II is vibrating, it can be concluded that object II is producing sound. 3. A jaltarang is  a  musical  instrument  made  of  a combination of water-filled bowls or glasses. In each bowl, water is filled up to different levels. When the bowls are gently struck in succession by a stick, the air column of each bowl vibrates to produce different frequency sounds. The combined effect is a pleasant sound. Hence, the air column is the vibrating part of the jaltarang. 4. Vibr ating objects pr oduce sound. When a stretched metal wire is plucked, it starts vibrating. This, in turn, produces a sound. 5. The vocal cord vibrates when the lungs force air out through it. This results in the vibration of the vocal cord, thereby causing sound to be produced. The vocal cord can loosen or tighten itself in order to control the quality of the voice produced. Hence, the quality of a sound depends on the thickness or thinness and tightness or looseness of the vocal cord. 6. When the human lungs force air out through the vocal cord, it starts vibrating. Hence, a sound is produced. 7. The speed of a wave is directly proportional to the density of a particular medium. The speed at which sound travels in liquids is lower than the speed at which it travels in solids. However, the speed at which sound travels in liquids is higher than the speed at which it travels in gases. Hence, when

arranged in the descending order of their magnitudes, the speed of sound in the three different media is vs > vl > vg. 8. Sound travels through material media only. Solids, liquids, and gases are all material media. Therefore, sound cannot pass through a box devoid of any material medium inside. Hence, the sound produced by an alarm clock cannot be heard if it is kept in a box that is devoid of any material medium inside. 9. An ear-horn is used to enhance the hearing ability of a person with weak hearing power. This device amplifies the incoming sound in order to increase its loudness, thereby enabling the person to hear the sound. 10. The vibration coming from a sound-producing object passes through the ear canal (I) to the eardrum (III). In response to the vibration, the eardrum starts vibrating. The eardrum transfers its vibration to the three tiny bones (II). These bones vibrate in accordance with the vibration of the eardrum. These vibrations are received by a coil-shaped tube known as the cochlea (IV). The cochlea contains sound-sensitive nerve cells that pass the sense of vibration to the brain. 11. The to and fro or back and forth motion of a membrane is called its vibration. 12. The first peak of the wave travels a distance of 3 meters in 5 seconds. Hence, the speed of the wave

3m 3  m/s . 5s 5 A total of 15 peaks are produced in 5 seconds. Ther efore, the frequency of the wave is

15 peaks  3Hz . 5 seconds The speed, wavelength, and frequency of a wave are related as, is 

7th Class Physics

172 Wavelength 

Speed Frequency

Ther efore,

wavelength

3 3 1 5   0.20m  20cm 3 5 3 5 13. The tuning fork completes a total of 256 vibrations in one second. Therefore, the frequency of vibrations of the fork is 256 Hz. It completes one

1  = 0.004 seconds. 256 14. The time period of a pendulum is given by the time taken by it to make one complete oscillation. It is given by the inverse of the frequency of the pendulum. complete vibration in

1 Time period  Frequency Frequency is given as: Frequency 

Number oscillations Time taken

 Time period 

Time taken Number of oscillations

It is given that: Time taken = 2 s Number of oscillations = 100

18. Humans can hear sounds of frequency lying between 20 Hz to 20 kHz. This is called the ‘audible range’ for humans. The sound produced by an ultrasonic whistle lies outside the audible range. Humans cannot hear ultrasonic sounds. Since the sound produced by an ultrasonic whistle is greater t h a n 20 kHz, the sound produced by the given whistle may have a frequency range of 20 kHz to 25 kHz. 19. The audible range for human ears is 20 Hz – 20000 Hz. Therefore, the upper limit of the human audible range is 20000 Hz or 2 × 10 4  Hz.  Hence,  the respective values of a and b are 2 and 4. 20. The pitch of the sound is determined by the frequency, not by the amplitude. Hence, statement I is  not  correct. The normal human audibility range is 20 Hz to 20,000 Hz. Therefore statement II is correct. Sound requires a material medium to travel. Hence, sound can travel through water. Therefore, statement III is  also  correct. 21. Mouth organ is a musical instrument that is used to produce melodious sound. The produced sound is not harmful to the human ear. 22. Trees and plants, planted along the roadside and in and around residential colonies, cut down the traffic noise reaching the residents. This helps in reducing noise pollution.

CONCEPTIVE WORKSHEET

2 1  Time period    0.02s 100 50 15. The first wave travels a distance of 100 m in 2 seconds. Hence, the speed of the wave is

KEY

100  50m / s . The number of waves created per 2 second is the frequency of the wave. Therefore, 20  10Hz . 2 Hence, the distance between two consecutive waves, i.e., wavelength

Speed of the wave 50   5m Frequency of the wave 10

16. The amplitude of a sound wave is given by the maximum displacement of its vibration from the mean position. 17. The intensity of sound wave is defined as the amount of energy transported through a medium per unit time. It is measured in decibels. www.betoppers.com

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HINTS / SOLUTIONS TO THE SELECTED QUESTIONS

the frequency of the wave is

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Sound is generated by an object that produces sound. Sound is produced on beating a drum. The correct answer is B. When one person speaks he/she releases sound waves. These waves are detected by the ear of the second person. Hence, when two people talk with each other, energy is transferred through sound waves. The correct answer is C.

Sound Solutions 3. 4.

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The vibrating ruler produces a sound. The correct answer is C. When the drum-head is struck, it starts vibrating up and down so that it produces sound. The sand particles on the head of the drum also start vibrating because of the vibration of the drum-head. Hence, the striking of the drum will cause the sand particles to move up and down on the drum-head. The correct answer is D. When a stretched membrane is beaten, it starts moving to and fro. The membrane is said to be in vibration. Since vibration produces sound, a stretched membrane, when beaten, produces sound. This principle is used to make musical devices which consist of stretched membranes, for example, a drum. When the head of a drum is beaten, it starts vibrating, thereby producing sound. The correct answer is D. Only vibrations can produce sound. No parts vibrate when the lungs expand to fill air inside itself. Hence, the expansion of lungs is not related to the production of sound. The correct answer is B. To answer this question correctly, the student needs to know that sound waves travel at different speeds through different mediums. Sound waves can travel through any medium except vacuum. Therefore, Daniel hears two sounds of the tuning fork (one that travels through the wall and the other that travels through air). Sound waves travel faster through solids, than through air Hence, Daniel hears the two sounds of the tuning fork at an interval. The correct answer is D. This simple problem can be solved by the student who knows that sound travels faster through water, than through air. Hence, the second boat will receive the sound signal sent through water earlier, than the sound signal sent through air. The correct answer is A. Sound waves require a particle medium to travel through it. Vacuum is a medium that does not contain any particles. Hence, sound waves cannot travel through vacuum. The correct answer is D. Why alternatives A, B and C are wrong: Wood, glass, and gasoline are particle media. Hence, sound waves can travel through these materials.

173 10. The eardrum is a thin membrane present in the ear which vibrates in response to a sound. The amount of vibration of the eardrum depends on the loudness of the sound. If the loudness of a sound increases, then the amplitude of the vibration of the eardrum also increases. Excessive sound forces the eardrum to vibrate with large amplitude. As a result, it may get damaged. The correct answer is B. 11. The loudness of a sound depends on its amplitude. Larger the amplitude, louder will be the sound and vice-versa. Amplitude of a wave is the height of the crest and trough of the wave. It can be observed that the height of the crest and trough is the maximum of a wave represented in alternative C. Hence, the loudest sound is represented by wave given in alternative C. The correct answer is C. 12. Hertz (Hz) is the unit of frequency. Frequency is defined as the number of oscillations or vibrations in per unit time. Hence, the number of to and fro motions of a membrane in per unit time is measured in hertz. The correct answer is D. 13. The student is asked to calculate the ringing frequency of the bell, which is given as

Numbers of rings Time interval Julie heard the bell ring 24 times in one minute (60 s) Frequency 

 The ringing frequency of the bell 

24  0.4Hz 60

The correct answer is C. 14. The frequency of vibrations of a membrane is given by the number of oscillations in one second. It is given as:

Numbers of oscillations Time taken It is given that: Number of oscillations = 20 Time taken = 2 s Frequency 

20  10Hz 2 The frequency of a sound determines whether the sound is audible or not. The audible frequency range for human ears is 20 Hz “ 20000 Hz. A sound having a frequency of 10 Hz cannot be heard by humans because it is out of the audible range. The correct answer is D.  Frequency 

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7th Class Physics

174 15. Sound is produced because of the vibrations of a body, called sound source. The amplitude of the sound is determined by the amplitude of the vibrations. The larger the amplitude of the vibrations, the louder is the sound. Hence, the loudness of a sound is governed by the amplitude of the vibrations. The correct answer is A. 16. The frequency of a wave is measured by the number of peaks or troughs produced in a second. Therefore, the frequency of the produced wave is

Number of peaks 30 peaks  = 6 peaks/second Time 5 seconds = 6 Hz The correct answer is B. 17. Frequency is given by the number of oscillations of a vibrating pendulum in one second. It is given as: Total number of oscillations Total time taken It is given that the frequency of the pendulum is 25 Hz and the number of oscillations is 100. Total time Frequency 

Number of oscillations 100   4s Frequency 25 Hence, the given pendulum will take four seconds to make 100 oscillations. The correct answer is C. 18. Humans can hear sounds whose frequency falls in the audible range. The range of audible frequencies for human ears is 20 Hz to 20000 Hz. Sounds of frequency less than 20 Hz or greater than 20000 Hz is not heard by humans. A sound of frequency 18 Hz does not lie in the audible range. Hence, this sound is not heard by humans. The correct answer is A. 19.d A leaf-blowing machine is a device that is used to clean pavements covered with leaves. The sound produced by it may be harmful to the human ear. Hence, a leaf-blowing machine can produce high noise. The correct answer is D. 20. Sound pollution affects the ears directly and can result in loss of hearing. Indirectly, it can also affect the function of the brain. This can lead to high blood pressure (hypertension), anxiety, and sleeping disorder. However, it is unlikely for breathing problem to occur as a result of noise pollution. The correct answer is B. required  

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SUMM ATIVE WORKSHEET 1.

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Sound can travel through solids, liquids, and gases. Sound requires a medium to travel through. Solid, liquid and gas provide the medium for sound. Hence, sound can travel through solids, liquids and gases. The voice of an adult man is of lower pitch in comparison to the voices of a baby boy, a baby girl and a woman. Since frequency of a sound is directly proportional to its pitch, man’s voice is of minimum frequency in comparison to a boy, a girl, or a woman’s voice. (a) True Sound requires a medium to travel through. Since vacuum is devoid of any medium, sound cannot travel through it. (b) False The number of oscillations per second of a vibrating object is known as its frequency. Time period is the time required to complete one oscillation. (c) False Loudness of a sound is proportional to the square of the amplitude of its vibration. When the amplitude of vibration of a sound is large, the sound is very loud. The sound is feeble for small amplitude. (d) True Humans cannot hear sounds of all frequencies. Humans can hear a sound whose frequency falls in the range of 20 Hz”20,000 Hz. The sound having frequency out of this range is inaudible to humans. (e) False The pitch of a sound is proportional to its frequency. As the frequency of vibration increases, the pitch of the sound also increases and vice-versa. A sound is said to be high pitched if its frequency of vibration is high, and is low pitched for a small frequency of vibration. (f) False Unwanted or unpleasant sounds are known as noise. Sounds that are melodious and pleasing to ear are known as music. (g) True Unwanted or unpleasant sounds are known as noise. If one is subjected to loud unpleasant sound continuously for a long time, then it may cause temporary hearing impairment.

Sound Solutions

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(a) Time taken by an object to complete one oscillation is called __time period__. (b) Loudness is determined by the __amplitude__ of  vibration. (c) The unit of frequency is __hertz (Hz) __. (d) Unwanted sound is called __noise__. (e) Shrillness of a sound is determined by the __frequency__ of  vibration. (Shrillness is also called pitch of the sound. Pitch is directly proportional to the frequency of vibration. Hence, shrillness is determined by the frequency of vibration.)

5.

Frequency of oscillations is defined as the number of oscillations of a vibrating body per second. It is given by F r e q u e n c y 

Number of oscillations 40   10Hz Total time 4

The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency. T i m e 1

1

period   Frequency of oscillation  10  0.1s 6.

The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency.

1 Time period  Frequency of oscillation Frequency of oscillations = 500 Hz

1  0.002s 500 (a) Dholak is a musical instrument. It consists of a stretched membrane called its head. When the head is beaten gently, the stretched membrane sets into vibration. Since sound is produced when an object vibrates, the dholak produces  a  sound. (b) Sitar is  a  musical  instrument.  It  consists  of stretched strings. When a string is plucked, it sets into vibration. Since sound is produced when an object vibrates, the sitar produces a sound. (c) Fluteis a hollow pipe. When air is blown over its mouth, the air inside the pipe is set into vibration. As a result, a pleasant sound is produced. Time period  

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The sound that is pleasing to the ear is called music. For example, the sound produced by violins, pianos, flutes, pungs, etc. The sound that is unpleasing to the ear is called noise. Some examples of noise are as follows: (i) Sound produced by horns of buses and trucks (ii) Sound of electrical generators (iii) Sound of a gun shot (iv) Sound produced by jackhammers Yes. Music can become noise when played at high volumes. 9. Some sources of noise pollution are as follows: (i) Televisions and transistors running at high volumes (ii) Loudspeakers and crackers (iii) Horns of buses, cars and trucks (iv) Home appliances such as mixer, desert cooler, etc. 10. Noise pollution can lead to a number of healthrelated problems. Some of them are as follows: (i) Hearing loss (ii) Insomnia; inability to sleep (iii) Hypertension (iv) Severe headache (v) Stress 11. There will be more noise in the house which is along the roadside. This is because noise produced by transportation vehicles may cause trouble to the residents. The intensity of noise decreases with the distance between the source and the listener. Hence, it is better to take the house that is three lanes away from the roadside. 12. Larynx is a part of the throat. It is responsible for production of sound. A sketch of a human larynx is shown in the following figure.

Larynx moves when we swallow something. Inside the larynx, there are two vocal cords. There is a small gap between them. This small gap allows air to pass through. When we speak, air is forced into this small gap by the lungs. This prompts vocal cords www.betoppers.com

7th Class Physics

176 to vibrate. Since vibrating objects produce sound, sound is produced due to the vibration of vocal cords. 13. The speed of sound is less than the speed of light. Hence, light reaches us before the sound during a lightning, which is accompanied by thundering.

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HINTS / SOLUTIONS TO THE SELECTED QUESTIONS 1. The eardrum, labelled as III in  the  given  figure, consists of a stretched membrane. When sound falls on it, it vibrates and passes the signals to the inner ear. 2.

Sound is produced and carried by the vibration of a medium or matter molecules. In vacuum, no vibration of medium is possible. Therefore, sound cannot travel through vacuum. Vacuum is created inside the box. Therefore, the vibration of the clock cannot be carried by any medium to the walls of the box. Hence, no sound is heard outside the box.

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Wind pipe blows wind into the slit present in the larynx. This action produces sound. Wind pipe does not modify the quality of sound produced by larynx.

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The eardrum does not reproduce sound. It vibrates to the tune of the incoming sound and sends these signals to the inner ear from where the signals go to the brain. Hence, the statement in alternative D is incorrect.

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It is the stretched membrane of the tabla that produces sound. Other parts such as the wooden shell and the stretched strings of a tablaenhance quality of the sound produced. The ring encircling the membrane acts as the frame to hold the stretched membrane. Therefore, the sound is produced by the stretched membrane of a tabla.

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Sound is produced by to-and-fro motion. Heat cannot bring about to-and-fro motion in a string. Hence, sound cannot be produced by heating. Ear-horn is used by hearing impaired persons who have weak hearing power. This device amplifies the incoming sound to enhance the amplitude and hence, enhance the loudness of the sound. Therefore, a hearing impaired person can detect and hear the normal sound by using this aid. Jackhammer’s headphone is  a  device  that  a person who works with jackhammer wears because it is a noise-canceling device. Noise produced by jackhammer is very loud and if exposed to a relatively long time, then it may affect the listening ability of the working person. The Jackhammer ’s Headphones absorb  and cancel or minimize this noise. Human ear can detect sounds whose frequencies lie in between 20 Hz to 20 kHz. This is called the audible range of frequency. The frequency of sound that is outside this range is termed as inaudible sound. Inaudible sounds cannot be heard by humans. Since ultrasonic equipment emits sound of frequency greater than 20 kHz, which lies outside the audible range, the emitted sound is inaudible. A jackhammer produces excessive noise that could damage the hearing perception of the person who continuously works with it. To prevent any damage to the hearing, the person wears a hearing protection that is designed to protect ears from the excessive noise of jackhammers. It is evident from the given information that a jackhammer produces noise. Hence, ‘protection with jackhammers’ protects ear from noise. The frequency of sound produced by women or girls is very high, that is, women can produce high pitched sounds. High frequency sound can be easily transmitted and clearly heard over a distance through telephone cable. Hence, women are recruited as telephone operators because they can produce high pitched sound. The rate of vibration of eardrum depends on the air vibration and on its elasticity. A thin membrane vibrates properly when it is stretched tightly. It will not vibrate properly if its elasticity is loosened. Sound requires a material medium to travel through. Since vacuum is devoid of any material medium, sound cannot travel in it. On closing the switch, the rod breaks and separates from point O within the vacuum sphere. At this point, as Suraj will tap the rod, sound will reach to one of the broken ends.

Sound Solutions

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It cannot reach to the other broken end because sound cannot travel through vacuum. Therefore, tapping sound will not reach the other end of the rod. Hence, Pankaj will unable to hear the tapping after closing the switch. Sound requires a material medium to travel through. Since vacuum is devoid of any material medium, sound cannot travel in it. In the given case, the hollow glass partition behaves like a vacuum after air is evacuated from it. Therefore, the alarm sound will not reach the respective boundaries of the partition. Hence, both X and Y will be unable to hear any sound. Sound requires a material medium to travel through. Since vacuum is devoid of any material medium, sound cannot travel in it. Hence, after evacuating the chamber, no sound can be heard from the drum. Sound is the result of vibration of a membrane. It can propagate in a medium only. Dolphins and whales are the species that reside underwater. They make sound that propagates in water by vibrating its molecules. The vibration of water molecules is received by the other dolphin. Hence, communication underwater is possible because sound can travel in water. Microphone is a device that converts sound into electrical signal like the human ear converts sound into electrical impulse. In a microphone, diaphragm plays the role of an eardrum. It vibrates in responseto sound and converts the incoming sound into electrical signal. The space that is devoid of any medium is called as vacuum. In outer space, there is no medium particle. Since sound cannot travel along vacuum, it is not possible for the astronauts to communicate using direct sound. Sound requires a medium to travel. Therefore, sound can travel through water. This property of sound is used in the detection of objects that are situated below the water surface. The sound produced by SONAR can penetrate and pass through water. Sound requires a medium to travel. In deep space, there is no medium present in which sound could travel through. Since radio waves can travel in vacuum, astronauts use walkie-talkie, which itself uses radio waves, to communicate with each other.The correct answer is A. Veena is a musical instrument that consists of a number of strings that are stretched along its length, as shown in the following figure.

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When its strings are plucked, it produces a pleasant sound. The qualities of sound vary with tension and length of the strings. Veena uses the principle of production of sound when a stretched string is plucked. The correct answer is B. 22. Excessive and unwanted noise in the surrounding air attributes to noise pollution. Some of the problems caused by noise pollution are listed below. i. Hearing impairment ii. Anxiety iii. High blood pressure iv. Sleep disorder It is evident from the list that noise may cause hearing impairment. 23. Roadside noise pollution is primarily created by vehicles’ engines and horns. Making road smooth may result in the reduction of air pollution. This preventive measure cannot be attributed to the reduction of roadside noise pollution. Some of the preventive measures to reduce the roadside noise pollution are listed below. i. Plantation of trees along the roads and around residential buildings helps in reducing noise. ii. Installing silencers in the vehicles also helps in reducing noise. iii. Regular servicing of vehicle engines is a must in order to reduce road side noise pollution.

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7th Class Physics

178 HINTS / SOLUTIONS TO THE SELECTED QUESTIONS 16. v = 344 m/s, f = 32Hz,  = ? We know, v = f 

 λ =

v f

344 = 10.75 m 32 17. v = 330 m/s  = 2.2 cm = 0.022m f=? We know, v = f  =

 f=

v 330 = Hz = 15,000 Hz λ 0.022

18. = ? f = 109Hz v = 3 × 108 m/s We know, v = f 

v 3×108 = m = 3 × 10–1m = 30 cm. f 109 19. f = 256 Hz v = 340 m/s =? Speed, v = f   λ=

v 340 = = 1.33m f 256 20. In 2 seconds, the number of waves produced = 256  In 1 second the number of waves produced  =

265 = 128 2 Therefore Frequency of the tuning fork = 128 Hz. A-q; B-r; C-s A-q; B-r; C-s; D-t A-q; B-r; C-p A-t; B-r; C-s; D-p A-q; B-s; C-p; D-r A-s; B-p; C-q; D-r A-q; B-p; C-r =

24. 25. 26. 27. 28. 29. 30.

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7. HEAT SOLUTIONS Heat flows from hotter object to colder object. Therefore, in this case, heat will flow from the solid substance to water. As a result, the temperature of Q.no 1 2 3 4 5 water will increase. However, the temperature of Key B C D D C water cannot become more than 90°C. Therefore, Q.no 6 11 12 13 14 the temperature of water will be in the range of 60°C to 90°C.The correct answer is D. Key C C D A C 4. When the hot water starts cooling, it loses heat to 15 16 17 18 19 Q.no the atmosphere as heat flows from the hotter body Key C B B A A to the colder body. Thus, with increase in the time, Q.no 20 21 22 the temperature of water decreases. The correct answer is D. Key D D C 5. Substances that allow heat to pass through them HINTS/SOLUTIONS TO THE SELECTED easily are called conductors. Metals such as copper and aluminium are good conductors of heat. They QUESTIONS allow heat to pass through them easily. The correct answer is C. 6. Dark-coloured clothes such as a black shirt absorb 1. There are two types of thermometer. most of the heat that is incident on them. Thus, darkcoloured clothes should be worn in winters. On the 1. Clinical thermometer other hand, light-coloured clothes reflect most of 2. Laboratory thermometer the heat that falls on them. Thus, light-coloured clothes should be worn in summers. The thermometer that is used to check the The range of laboratory thermometers is generally temperature of a human is called clinical from –10°C to 110°C. They are used to measure thermometer. The temperature range that can be the temperature of any object other than the human measured using a clinical thermometer is 35°C to body. On the other hand, the range of a clinical 42°C. This is because the temperature of a human thermometer is from 35°C to 42°C. It is used to rarely goes below 35°C or above 42°C. measure the temperature of human body. The correct answer is C. 7. We know that C = 5/9 (F – 32) = 5/9 (86 – 32) = 5/9 × 54 = 30 Therefore, 86 °F = 30 °C 8. We know that F = (C × 9/5) + 32 We are given that C = 45, A laboratory thermometer is used to measure Fahrenheit = (45 × 9/5) + 32 °F = (81+ 32) °F temperature of different objects. The range of = 113 °F temperature that a laboratory thermometer can 9. We know that F = (C × 9/5) + 32 measure is generally from –10°C to 110°C. a) C = 35 Fahrenheit = (35 × 9/5) + 32 °F = (63+ 32) °F = 95 °F b) C = 15 Fahrenheit = (15 × 9/5) + 32 °F = (27+ 32) °F = 59 °F 10. We know that C = 5/9 (F – 32) a) F = 95 °F The correct answer is B. C = 5/9 (95 – 32) = 5/9 × 63 = 35 °C 2. Heat flows from a body at higher temperature to a body b) F = 41°F at lower temperature. Hence, heat will flow from the ball C = 5/9 (41 – 32) = 5/9 × 9 = 5°C to the water. The correct answer is C.

FORMATIVE WORKSHEET

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7th Class Physics

180 11. Conduction is a process in which heat is transferred from the hotter end to the colder end. In solids, heat is generally transferred by the process of conduction. The correct answer is C. 12. Aluminium, being a metal, is a good conductor of heat. Heat is transferred from a hotter end to a colder end of an object by the process of conduction. Therefore, wax starts melting first as a result of conduction. The correct answer is D. 13. On heating, the wax melts. Heat is transferred from the hotter end to the colder end of an object by the process of conduction. Thus, heat is transferred from the end nearest to the flame to the other end. Thus, pin I which is nearest to the flame will drop first. The correct answer is A. 14. Copper is a good conductor of heat. Hence, kitchen utensils are provided with a copper base. The correct answer is C. 15. Heat flows from a body at a higher temperature to a body at a lower temperature. Therefore, heat flows from water at 70 °C to water at 50 °C. Hence, the temperature of the mixture will be in between 50 °C and 70 °C. The correct answer is C. 16. In coastal areas, people experience an interesting phenomenon because of the process of convection. During night time, water cools down more slowly than land. Hence, cool air from the land moves towards the sea. This phenomenon is called land breeze. Thus, land breeze is the cool air that blows from the land to the sea during night time. The correct answer is B. 17. Metals such as aluminium allow heat to pass through them. Hence, aluminium is a good conductor of heat. On the other hand, air is a poor conductor of heat. The correct answer is B. 18. During the daytime, the land gets heated more quickly than the water. The air present over the land becomes hot and rises up. The air over the sea being cooler takes its place. Thus, the warm air from the land moves towards the sea to complete the cycle. This cycle is known as the sea breeze. The correct answer is A. 19. Here, the student needs to classify various ways of heat transfer. Conduction is a process of transfer of heat within a material itself or between materials in contact with each other. Hence, heat transfer within the rods takes place by conduction. www.betoppers.com

20. During convection, the hot air near the surface of Earth moves up, while the cool and denser air moves toward the surface of Earth. The correct answer is D. 21. A medium is required for the heat transfer by the process of convection. The process of transferring heat through a fluid (liquid or gas) is called convection. On the other hand, no medium is required for heat transfer by the process of radiation. For example, the earth receives heat energy from the sun by the process of radiation. The correct answer is D. 22. Convection is the transfer of heat energy by the actual movement of molecules. Since molecules are tightly packed in solids and cannot move from their positions, heat transfer by convection cannot take place in solids. Thus, heat transfer by convection can take place only in liquids and gases. Heat transfer by radiation involves energy transfer by electromagnetic radiation. Electromagnetic radiations do not require any medium to travel. Therefore, heat transfer by radiation can take place without any medium. Hence, the given statements are correctly completed by row C. The correct answer is C.

CONCEPTIVE WORKSHEET Q.no

1

2

3

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Key Q.no

B 6

D 7

A 8

C 9

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B 11

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HINTS/SOLUTIONS TO THE SELECTED QUESTIONS 1. Clinical thermometers are used to measure the temperature of the human body. The temperature of the human body rarely goes below 35°C or above 42°C. Hence, clinical thermometers have a temperature range of 35°C to 42°C. The correct answer is B. 2. The units used to measure temperature are Fahrenheit, Celsius, and Kelvin. Pascal is a unit of pressure. The correct answer is D.

Heat Solutions 3.

181

If the bigger marks read one degree and there are 12. The process of heat transfer from the hotter end of ten small divisions, then one small division will read an object to a colder end is called conduction. In solids, heat is generally transferred by the process 1  0.1C . of conduction. Therefore, it requires a medium. 10 The correct answer is A. The correct answer is A. 4. Fahrenheit scale (°F) measurement of temperature 13. Wooden handles of cooking utensils is an application was used earlier. The temperature range of a clinical of the principle of conduction of heat. Wood is a thermometer is 35°C – 42°C, which is equivalent poor conductor of heat. Hence, cooking utensils are to 94°F – 108°F range on the Fahrenheit scale. provided with wooden handles. The correct answer is C. The correct answer is D. 5. The human body temperature may vary depending on various factors. However, the average human 14. Light colours absorb less radiation than dark colours. body temperature is 37°C. Thus, it makes us feel more comfortable during The correct answer is C. summers. 6. A clinical thermometer is used to measure the The correct answer is A. temperature of the human body. Since the average body temperature is 37°C, the temperature of the 15. Radiation is the phenomena by which electromagnetic rays travel without any medium. human body does not go above 42°C or below 35°C. These rays heat the surface without actually heating Hence, a clinical thermometer has a temperature the medium. range of 35 – 42°C. The correct answer is B. The correct answer is A. 7. Before a thermometer is used, it should be ensured that the mercury level is below 35ºC. A clinical 16. On dipping the spoon in the hot tea, one end of spoon is exposed to hot tea while the other end is in contact thermometer is designed to measure the temperature with comparatively less hot surroundings. Also, of a human body. The average body temperature is transfer of heat takes place from high temperature around 37ºC. Thus, clinical thermometer has a range to low temperature. Metals are good conductors of of 35ºC to 42ºC. heat. In solids, transfer of heat takes place through The correct answer is C. conduction. Thus, the temperature of spoon 8. Laboratory thermometers are used for accurately increases by the process of conduction. measuring the temperature in a variety of mediums. The range of laboratory thermometers is –10ºC to The correct answer is A. 110ºC. Clinical thermometer is used for measuring the 17. The light of the sun contains heat. During summers, this heat is very intense and a person who is exposed temperature of human body only. A maximumto direct sunlight feels very hot. An umbrella minimum thermometer is used to measure the prevents this heat from reaching to the person maximum and minimum temperatures of a day. directly. Hence, the person under the umbrella feels The correct answer is B. comfortable. 9. Heat flows from a body at higher temperature to a body at lower temperature. The correct answer is C. The correct answer is D. 10. Heat energy always flows from a body at higher SUMM ATIVE WORKSHEET temperature to a body at lower temperature. Hence, the ice cube starts melting because the heat energy Q.no 1 2 3 4 5 of the palm gets transferred to the ice cube. The Key A C A A C transfer of this heat causes the ice cube to melt on Q.no 6 7 8 9 10 the palm. Key C D B C D The correct answer is C. 11 12 13 14 15 Q.no 11. The materials which do not allow heat to pass Key C B A C B through them are called insulators or poor conductors of heat. Plastic does not allow heat to pass through it. Therefore, it is an insulator of heat. www.betoppers.com

7th Class Physics

182 HINTS/SOLUTIONS TO THE SELECTED QUESTIONS 1.

2.

3.

4.

5.

be transferred to the ice cube and this would result in the melting of the ice and the cooling of the hand of the holder. Hence, examples II and IV represent It can be observed from the given activity that dark heat transfer by conduction. colours absorb more heat than light colours. This The correct answer is C. suggests that dark-coloured clothes should be 6. A thermometer has a bulb containing mercury. avoided in summers when the temperature is very When this bulb is brought in contact with a high. substance, transfer of heat take place from the The correct answer is A. substance to the bulb and then to the mercury. Heat transfer by radiation does not require any Mercury expands as it absorbs heat. Hence, the medium. For example, no medium is present level of mercury in the thermometer rises. This is between the Earth and the sun, but heat is an example of heat transfer by conduction. transferred from the sun to the Earth. This transfer The correct answer is D. of heat takes place by radiation. 7. The process of conduction, which occurs in solids, The correct answer is C. involves heat transfer from the hotter end to the Heat transfer in an iron nail takes place by cooler end. The transfer of heat by this process conduction. If an iron nail is heated at one end, then always requires a medium. Thus, conduction is not heat is transferred from the hotter end to cooler possible in vacuum. end of the iron nail by the process of conduction. The correct answer is B. Water is a bad conductor of heat. Heat transfer in 8. While measuring the temperature of an object, the water takes place by the process of convection. thermometer should not be tilted. It should be upright When water near the heat source becomes hot, hot and surrounded from all the sides by the substance water rises and cold water from the side moves whose temperature is to be measured. The bulb of down towards the source of heat. This process the thermometer should not touch the surface of continues till all the water gets heated and this the container in which the substance is present. Thus, process is called convection. among the given students, only Suraj has noted the The correct answer is A. temperature in the correct manner. In coastal regions, land breeze is formed during The correct answer is D. nighttime when the land becomes cooler than the 9. The normal body temperature of a human being is sea. The hot air present above the sea rises up and 37°C. The figure of the thermometer that was used the cooler air above the land moves to take its place. for measuring the body temperature of Q shows Hence, a heat transfer takes place and the cooler the mercury thread at 37°C. Thus, the temperature land becomes warm at night. of Q is equal to that of the temperature of a normal human being. The correct answer is C. 10. Human temperature is measured by clinical thermometers on the Celsius scale, which is indicated by °C. As shown in the figure, the mercury thread is three divisions above the bigger mark During the day, the land gets heated faster than showing 39°C. Since the larger divisions represent water. As a result, the air above the land become one degree and there are five smaller divisions hot and rises. The relatively cooler air from sea between two larger divisions, one small division moves toward the land to takes its place. The warm air also moves toward the sea. Hence, the heat is 1 reads  0.2C . Hence, three small divisions will transferred due to the movement of air form land 5 to sea and vice-versa. read 0.6°C. Therefore, the temperature of the The correct answer is A. patient is 39.6°C. During conduction, heat is transferred by the direct The correct answer is D. contact between the two substances. While shaking 11. The degree of hotness of an object is measured by hands, heat from the warmer hand is passed on to its temperature. The device used for measuring the the cooler one. Similarly, the hand of the ice cube temperature of an object is called a thermometer. holder will be relatively warmer. Hence, heat will 12. Temperature is the measure of the hotness of a

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Heat Solutions

183 Thermometer The thermometer used to measure our body temperature is called a clinical thermometer. Mercury thermometers are used to measure any temperature. b) Celsius scale and Fahrenheit scale Celsius scale is indicated by °C. On this scale the melting point of ice is taken as 0 °C and the boiling point of water as 100 °C. Fahrenheit scale is indicated by °F. On this scale, 32 °F is taken as the melting point of ice and 212 °F as the boiling point of water. The temperature of ice before heating = –2°C The temperature of water = 10°C Therefore, the net change in temperature = 10°C – (–2°C) = 12°C The correct answer is D. From the given figures: Temperature of water after heating = 80ºC After two hours, the temperature of water = 30ºC Hence, the total decrease in the temperature of water is 80ºC – 30ºC = 50ºC. The correct answer is B. Temperature is the measure of hotness of substances. The higher the temperature, the hotter will be the substance.The temperatures of X and Y after removing the heat source are 40°C and 60°C respectively. Hence, liquid X is at a lower temperature after the removal of the heat source. The decrease in the temperature of liquid X = 90°C – 40°C = 50°C. The correct answer is A. In the given thermometers, one bigger division represents 1°C and one smaller division represent 0.2°C. Mercury thread in thermometer I is two divisions above the 39°C mark. Thus, the temperature is 39.4°C. The correct answer is A. 10. a) Conduction Conduction of heat energy can occur within a body or between two bodies when they are in contact with each other. b) Convection This method by which heat is transferred by the mass movement of the liquid or gas itself is called convection. c) Radiation Radiation is the form of heat transfer that can occur in the absence of a medium. 11. Conduction, convection, and radiation are different modes of heat transfer. The correct answer is (B)

substance. Higher the temperature of a substance, hotter the substance is. On this basis, the decreasing order of the given liquids according to their hotness is (II) > (I)> (III) > (IV) The correct answer is B. 13. On dipping the spoon in the hot tea, one end of spoon is exposed to hot tea while the other end is in contact with comparatively less hot surroundings. Also, transfer of heat takes place from high temperature to low temperature. Metals are good conductors of heat. In solids, transfer of heat takes place through conduction. Thus, the temperature of spoon 6. increases by the process of conduction. The correct answer is A. 14. Heat energy always flows from a body at higher temperature to a body at lower temperature. Hence, the ice cube starts melting because the heat energy 7. of the palm gets transferred to the ice cube. The transfer of this heat causes the ice cube to melt on the palm. The correct answer is C. 15. Laboratory thermometers are used for accurately measuring the temperature in a variety of mediums. 8. The range of laboratory thermometers is –10ºC to 110ºC. Clinical thermometer is used for measuring the temperature of human body only. A maximumminimum thermometer is used to measure the maximum and minimum temperatures of a day. The correct answer is B. 9.

HOTS WORKSHEET 1.

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3. 4.

5.

a. b. c. (A)

Temperature Energy Joule False. It s the measurement of hotness or coldness of a body. (B) True. (C) False. Temperature is measured in Celsius or Fahrenheit Scale. (D) True. Clinical Thermometer. The correct answer is (C) Heat always flows from a body at higher temperature to lower temperature. Heat energy will flow from our body to the surroundings if the surroundings are at a lower temperature than our body. The correct answer is (C) a) A Clinical Thermometer and a Mercury

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7th Class Physics

184 12. Convection does not occur in Vacuum. The correct answer is (A) 13. In fluids, heat transfer takes place primarily due to convection. The correct answer is (B) 14. a) Conduction of heat and Convection Conduction of heat energy can occur within a body or between two bodies when they are in contact with each other. The method by which heat is transferred by the mass movement of the liquid or gas itself is called convection. b) Conductors and Insulators The substances that conduct heat easily are called conductors. For example, metals are good conductors of heat. Substances such as wood, straw, clay, rubber, glass, and Bakelite (a kind of plastic) do not conduct heat very well and are called insulators. 15. As Copper is a metal, it is a good conductor of heat. The correct answer is (D) 16. Wool is the bad conductor of heat. The correct answer is (A) 17. a) Convection b) Radiation c) Conduction d) Convection e) Conduction 18. a) Conduction b) Radiation 19. a) We wear woolen clothes in winter because wool is a bad conductor of heat. Thus, it helps in retaining body warmth. b) This is because white clothes absorb comparatively less amount of heat than dark clothes, and therefore keep us relatively cooler. 20. a) Reasons for use of Mercury in thermometers. i) Mercury remains in the liquid state for a wide range of temperature. ii) It melts at – 38.87 °C and boils only at 356.58 °C. iii) It is relatively easy to see because of its silvery grey colour. iv) It does not stick to glass. v) It has a fairly uniform rate of expansion for a wide range of temperatures. b) Special features of Mercury Clinical Thermometer www.betoppers.com

i)

There is a little arrow (at 98.4 or 98.6 °F) showing the nor mal body temperature. ii) There is a constriction or ‘kink’ in the tube near the bulb. This kink has been made to ensure that the mercury in the thermometer does not contract (and flow back into the bulb) before the temperature has been read. 21. A  r ; B  t; C  s ; D  p ; E  q

IIT JEE WORKSHEET 1. 2. 3.

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6. 7. 8. 9. 10. 11. 12. 13. 14. 15 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

A body gaining heat feels hot. D Temperature is the physical quantity which measures the amount of heat in a body. The correct answer is (A) One litre of water at 30o C is mixed with one litre of water at 60o C, the temperature of the mixture will be between 30oC & 60oC. The correct answer is (D) A steel cup at 400oC is dropped in a container of water at 40o C, then heat will flow from cup to water. The correct answer is (A) Normal temperature of human body is 37oC or 98.4oF. The correct answer is (C) (D) (C) (B) (D) A, B, C, D A, B, C, D A, B, C, D A, D (D) (A) (A) (A) (B) (C) 1 kilo calorie = 1000 calories Ice point in Fahrenheit is 32°F A –(s); B – (p); C – (q) A –(p),(r); B – (s); C – (q) A –(q); B – (p); C – (r) A –(s); B – (r); C – (q)

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