Science 7

e. Which of the following is the density of water?

i. 1000 kg/m3 ii. 800 kg/m3

iii. 7800 kg/m3 iv. 13600 kg/m3

2. Answer these questions in short.

a. What is pressure? Mention its unit.

b. How does pressure depend upon force applied?

c. How does area affect the pressure?

d. What is density? Mention its unit.

e. Differentiate between force and pressure.

f. Which one-either iron or wood has more density?

3. Give reasons.

a. It is easier to cut vegetables with sharp knife.

b. It is easier to sew clothes with pointed needle than with a blunt one.

c. Base of wall of water dam is made wider.

d. Shoes of football players have studs.

H &RUN ÁRDWV RQ ZDWHU ZKHUHDV LURQ VLQNV LQ ZDWHU

4. Numerical problems

a. An object applies force of 500N on an area of 10m2. Find the
pressure exerted. $QV 3D

b. The weight of a body is 200N and its base is 2m2. Calculate the
pressure exerted by it. $QV 3D

c. A box has mass of 300kg. The area of the base is 150m2. Find the
SUHVVXUH H[HUWHG E\ WKH ER[ RQ WKH ÁRRU $QV 3D

d. An object has mass of 300kg. Its volume is 2m3. Find its density.
$QV NJ P3

e. A wooden block has mass of 50kg and it has dimensions

1m × 0.5m × 0.1m. Find its density. $QV NJ P3

Project Work

Collect some materials such as plastic, paper, cork, stone, iron
pieces, etc. Put all these things in the water of a beaker one by one.
What do you observe?
:K\ GR VRPH PDWHULDOV ÁRDW RQ ZDWHU DQG VRPH VLQN" ([SODLQ

Times' Crucial Science and Environment 47 Book 7

Chapter

5 :RUN (QHUJ\ DQG
3RZHU
ƐƟŵĂƚĞĚ ƉĞƌŝŽĚƐ͗ϱ

OBJECTIVES
At the end of the lesson, the students will be able to :
 ĚĞĮŶĞ ǁŽƌŬ͕ ĞŶĞƌŐLJ ĂŶĚ ƉŽǁĞƌ ĂŶĚ ƚĞůů ƚŚĞŝƌ ƵŶŝƚƐ͘
 ĞdžƉůĂŝŶ ƚŚĞ ĚŝīĞƌĞŶƚ ĨŽƌŵƐ ŽĨ ĞŶĞƌŐLJ͘
 solve the numerical problems related with work and power.

MIND OPENERS
 What is work?
 Why do we do work?
 How do we become able to do works?
 What gives us energy? Discuss.

Introduction

We perform different works in our daily life. Work is done only if the applied
force is able to move the load through some distance. Hence, the amount of
work done depends upon force. The force comes from the energy stored in
the body. Thus, force, work and energy are interrelated.

Work

We regard any of our tasks as works in our daily life. For example, reading,
writing, cooking rice, caring children, looking after a house, etc are also
regarded as works. But in science, they are not works. Then, what are
these if not work?
All the activities discussed above are the jobs of people. Hence, it is the
job (but not work) of a student to read and write, it is the job of a cook to
prepare foods, it is the job of a watchman to look after a building, etc. Work
is done only if a force applied upon a body moves it through some distance.
Thus, ZRUN LV VDLG WR EH GRQH LI D IRUFH DSSOLHG XSRQ D ERG\ PRYHV WKH ERG\
WKURXJK VRPH GLVWDQFH LQ WKH GLUHFWLRQ RI IRUFH

Times' Crucial Science and Environment 48 Book 7

Mathematically,
Work = Force × Distance
W=F×d

The SI unit of work is Joule (J).
Its CGS unit is erg. Here,

W=F×d
If a force of 1N moves a body through 1 metre distance, then

W = 1N × 1m
?1J = 1N × 1m
Hence, WKH ZRUN GRQH LV VDLG WR EH RQH MRXOH LI D IRUFH RI 1HZWRQ PRYHV D
ERG\ WKURXJK PHWUH GLVWDQFH LQ WKH GLUHFWLRQ RI IRUFH
1 Joule = 1 Newton × 1 metre

Conditions for the work done
The amount of work done depends upon the following essential
conditions:

a) The magnitude and direction of the applied force.
b) Distance moved by the body due to the effect of force.

Types of Work Done

The amount of work done is affected by two forces, gravity and friction.
Thus, work done can be categorized into two types:
1. Work Done Against Gravity
The earth pulls every object towards its centre
with a force. This force is called JUDYLW\. If we
try to lift a stone from the surface of earth,
ZH IHHO GLIÀFXOW\ EHFDXVH WKH VWRQH LV SXOOHG
downward by the gravity of the earth. If we
lift the stone through some height, force is
applied against gravity and the work is done
against the gravity.
Thus, LI WKH IRUFH LV DSSOLHG LQ RSSRVLWH
GLUHFWLRQ RI JUDYLW\ ZKLOH GRLQJ ZRUN LW LV
FDOOHG ZRUN GRQH DJDLQVW JUDYLW\

Times' Crucial Science and Environment 49 Book 7

Worked out example (1) "

If a person lifts a load of 150N through 3 metres, what is the work done?
Solution:

Given, 0
P
(QTEG
( !
&KUVCPEG
F
9QTM
WY

7UKPI VJG HQTOWNC
9 ( ª F 0 ª P
,
*GPEG VJG YQTM FQPG KU ,QWNGU

2. Work Done Against Friction

When we try to push a heavy wooden log Force Gravity
on the surface of the earth, we experience
GLIÀFXOW\ ,W LV GXH WR DQ RSSRVLQJ IRUFH Friction
produced between the two surfaces in
contact. This force which opposes the
motion of one body on the surface of another
is called IULFWLRQDO IRUFH. The frictional force
acts in opposite direction of the motion of
the body.

Hence, LI WKH IRUFH LV DSSOLHG LQ WKH RSSRVLWH GLUHFWLRQ RI IULFWLRQ ZKLOH GRLQJ
ZRUN LW LV FDOOHG ZRUN GRQH DJDLQVW IULFWLRQ

Worked out example (2) "

A person pulls a load of 300N through 4 meter, what is the amount
of work done?

Solution:

Given,

Force (F) = 300N
Distance (d) = 4m
Work done (w) = ?
Using formula, W = F×d = 300×4
= 1200J
Hence, the work done is 1200 Joule.

Times' Crucial Science and Environment 50 Book 7

Energy

We must apply force to do work. But we can apply force only if we have
energy in our body. Thus, energy is directly related to our work.

(QHUJ\ LV GHÀQHG DV WKH FDSDFLW\ RI D ERG\ WR GR ZRUN

The SI unit of energy is joule. The work and energy
have same units. One joule energy is that amount of
energy which can move a body of weight 1N through
1 metre.

Forms of Energy

Energy exists in different forms. They are:
1. Mechanical Energy
It is the energy of a body due to its state of rest or of uniform motion.
The mechanical energy can be divided into two types: potential energy and
kinetic energy.

%XOOHW ¿UHG IURP JXQ Stretched rubber

a. Potential energy

7KH HQHUJ\ RI D ERG\ GXH WR LWV SRVLWLRQ RU FRQÀJXUDWLRQ LV FDOOHG SRWHQWLDO
HQHUJ\. For example, a stretched rubber in a catapult, a stone raised above
the ground, water stored in a dam, a compressed spring, etc have potential
energy.

b. Kinetic energy

7KH HQHUJ\ RI D ERG\ GXH WR LWV VWDWH RI PRWLRQ LV FDOOHG NLQHWLF HQHUJ\. For
H[DPSOH HQHUJ\ RI D EXOOHW ÀUHG IURP D JXQ HQHUJ\ RI ÁRZLQJ ZDWHU
energy of wind, energy of a stone rolling down a cliff, etc.
2. Heat Energy

Heat energy arises due to the motion of molecules
in a body. ,W LV GHÀQHG DV WKH WRWDO NLQHWLF HQHUJ\ RI
WKH PROHFXOHV FRQWDLQHG LQ D ERG\. The sun is the
main source of heat energy for the earth. The other
VRXUFHV RI KHDW DUH ÀUH HOHFWULF KHDWHU EXUQLQJ
coal, etc.

Times' Crucial Science and Environment 51 Book 7

3. Light Energy
Generally, light is given off by the hot bodies. The light is the form of
energy which makes the things visible. The sun, the moon, glowing electric
lamps, lantern, burning candle, tuki, etc are the sources of light.

Candle Bulb Lantern Diyo Sun Moon

4. Electrical Energy
7KH HQHUJ\ RI D ERG\ GXH WR WKH ÁRZ RI HOHFWULFLW\ LV FDOOHG HOHFWULFDO HQHUJ\.
Actually, the electrical energy is caused due to the electrons in a conductor.
The electricity is of two types on the basis of motion or rest state of the
electrons. They are current electricity and static electricity. 7KH HOHFWULFLW\
GXH WR WKH ÁRZ RI HOHFWURQV LQ D FRQGXFWRU LV FDOOHG FXUUHQW HOHFWULFLW\.
Similarly, the electricity due to the deposition of electrons in a body is
called static electricity. Mainly, the current electricity is used in our daily
life for different purposes.

Television Iron Heater Computer
Current electricity is used to run different devices
such as radio, television, computer, electric heater,
electric iron, etc.

5. Chemical Energy

Gas cylinder Bread Rice Fire wood
Book 7
Times' Crucial Science and Environment 52

7KH HQHUJ\ VWRUHG LQ FKHPLFDO FRPSRXQGV LV NQRZQ DV FKHPLFDO HQHUJ\. Such
energy is obtained as a result of chemical reaction. The energy contained
in our foods such as rice, bread, cake, pulses, fats, oil, etc is regarded as
chemical energy. Similarly, the fuels such as diesel, petrol, coal, etc and
the electric cells also have chemical energy.

6. Nuclear Energy
The nucleus of an atom possesses a large amount of energy. This energy can
be released when a large nucleus breaks down into smaller ones or when
smaller nuclei combine to form a heavier nucleus. The nuclear energy can
be used to generate electricity.

The process of breaking down of a heavy nucleus into lighter ones is called
QXFOHDU ÀVVLRQ. Similarly, the process of combining lighter nuclei into heavy
nucleus is called QXFOHDU IXVLRQ. 7KH QXFOHDU ÀVVLRQ DQG IXVLRQ SURGXFH
huge amount of energy.
7. Magnetic Energy
7KH HQHUJ\ SRVVHVVHG E\ D PDJQHW LV FDOOHG PDJQHWLF
HQHUJ\. The magnetic energy is used in iron industries
and mines to lift heavy loads.
8. Sound Energy
7KH HQHUJ\ SURGXFHG GXH WR WKH YLEUDWLRQ RI ERGLHV LV FDOOHG VRXQG HQHUJ\.
We hear different kinds of sounds with our ears. Sound is produced by
the vocal cords present inside our neck. Similarly, a ringing bell, guitar,
madal, radio, etc are the sources of sound.

Guitar Madal Radio Bell

Transformation of Energy

A candle has chemical energy. When we burn a candle, the chemical energy
of candle is transformed into light and heat energy. 7KLV SURFHVV RI FKDQJH
RI RQH IRUP RI HQHUJ\ LQWR DQRWKHU LV FDOOHG WUDQVIRUPDWLRQ RI HQHUJ\.

Transformation of energy occurs inside our body also. In our body, the
chemical energy of the digested food is turned into heat energy during
respiration. This energy keeps us warm and helps us to do different works.

Times' Crucial Science and Environment 53 Book 7

Bulb Radio Microphone

There are different devices to convert one form of energy into another. For
example, an electric cell converts chemical energy into electrical energy, a
bulb converts electrical energy into heat and light energy, a radio converts
electrical energy into sound energy, etc.

Power
3RZHU LV GHÀQHG DV WKH UDWH RI GRLQJ ZRUN ,W PD\ DOVR EH GHÀQHG DV WKH
rate of transformation of energy.

Power = Work done
Time taken

?P = W
t

The SI unit of power is Watt (W). Power is also measured in the unit horse
power (h.p.).

1 H. P. = 746 watt

= 750 watt (approximately)

Here, P =Wt

If a work of 1 Joule is done in the time of 1 second, then

P =11Js ? 1W = 1J/1s

7KXV SRZHU RI D ERG\ LV VDLG WR EH RQH ZDWW LI LW SHUIRUPV WKH ZRUN RI MRXOH
LQ RQH VHFRQG

You must have seen electrical lamps labelled with 25W, 40W, 60W, 100W,
etc. If a bulb is labelled with 100 watt, it means that the bulb can convert
100 joules of electrical energy into light and heat energy in one second.

Worked out example (3) "

An electric lamp converts 4800 joules of electrical energy into heat
and light energy in 1 minute. What is the power of the heater?

Times' Crucial Science and Environment 54 Book 7

Solution:
Given,

9QTM
WY 'NGEVTKECN GPGTI[
,
OKPWVG
6KOG
V UGEQPFU
!


2QYGT
2

0QY 9 ,
V U
2 9

+HQFH WKH SRZHU RI WKH HOHFWULF ODPS LV ZDWW

ŽŶĮŐƵƌĂƟŽŶ ͗ ĂŶ ĂƌƌĂŶŐĞŵĞŶƚ ŽĨ ƉĂƌƚƐ Žƌ ĞůĞŵĞŶƚƐ ŝŶ Ă ƉĂƌƟĐƵůĂƌ ĨŽƌŵ

&ƌŝĐƟŽŶ : the opposing force created when one body rubs or drags over
another body

Lantern ͗ Ă ůĂŵƉ ǁŝƚŚ ƚƌĂŶƐƉĂƌĞŶƚ ĐŽǀĞƌŝŶŐ ƉƌŽƚĞĐƟŶŐ ƚŚĞ ŇĂŵĞ

Vocal cord : an organ in the throat which produces sound

Main Points to Remember

1. Work is said to be done if a force applied upon a body moves it
through some distance in the direction of force.

2. Work is measured by the formula:

Work = Force × Distance

The SI unit of force is Joule.
3. The capacity of a body to do work is called energy.
4. Energy exists in several forms. They are: mechanical energy,

heat energy, light energy, electrical energy, chemical energy,
nuclear energy, magnetic energy, sound energy, etc.
5. The rate of doing work by a body is called its power.

Exercise

1. Choose the best alternative in each case.
a. In which of the following case is work done?
i. Looking after a business house by sitting on a chair in front of it
ii. Caring children and teaching them
iii. Reading a book by sitting on a chair
iv. Pushing a stone through some distance by applying force

Times' Crucial Science and Environment 55 Book 7

b. Which of the following sentences is correct about work?

i. You do some work if you push a wall of a house but the wall
does not move.

ii. If you lift a stone from the surface of the earth, you do work
against friction.

iii. Work done is measured by the product of force applied and
distance travelled.

iv. The SI unit of work is Newton per metre.

c. Which of the following has kinetic energy?

i. Stretched rubber ii. Water stored in a dam

iii. None of these iv. A raised stone above the ground

d. A burning candle possesses

i. Chemical energy ii. Light energy

iii. Heat energy iv. All of these

e. Which of the following is a unit of power?

i. Watt ii. Megawatt

iii. Horse power iv. All of these

2. Answer these questions in very short.

a. If a man stands in front of a building to look after it, does he do work?

b. What is 1 Joule work?

c What is 1 Joule energy?

G 'HÀQH ZDWW SRZHU

H :KDW LV QXFOHDU ÀVVLRQ"

f. Write down the relation between watt and H. P.

'HÀQH

a. Work b. Energy c. Power d. Kinetic energy

e. Potential energy f. Nuclear fusion

g. Transformation of energy

4. Name the form of energy possessed by the following:

a. Electric cells b. Bread c. Rice d. Nucleus of an atom

e. Burning candle f. The sun g. Stretched rubber h. Wind

5. What kind of energy transformation occurs in the following?

a. Electric bulb b. Human body c. Radio

d. Microphone e. Electric fan f. Kerosene lamp

Times' Crucial Science and Environment 56 Book 7

6. Write down differences between:

a. Kinetic and potential energy

b. Work and energy

F 1XFOHDU ÀVVLRQ DQG QXFOHDU IXVLRQ

d. Work and power

7. Write down short notes on:

a. Electrical energy b. Nuclear energy

c. Power d. Mechanical energy

8. Answer these questions:

a. What factors affect work? Explain.

b. Write down the conditions necessary for the work done.

c. What is current electricity? Write down its use in our daily life.

d. Make a list of different forms of energy.

e. What is transformation of energy? Explain with examples.

9. Solve the following numerical problems.

a. A man moves a load of 30 kg through 9 metres. What is the work
done by him? $QV -

b. Deepta’s weight is 30 kg. If he climbs up 12 metres carrying a
load of 15 kg, what is the work done by him? $QV -

c. Nabina throws a stone 30 m away using a force of 150 N in
3 seconds. What is her power? $QV :DWW

d. The power of a crane is 36,000 watts. What will be the work done
by the crane in 2 seconds? $QV -

e. What magnitude of force is needed to perform a work of 2400
Joule while moving a load through 6 metres? $QV 1

Project Work

Mark two points A and B in a plane surface at a distance of 10m
from each other. Take a stone of mass about 3/4 kg and tie it to the
hook at a spring balance. Pull the stone on the plane surface with
the spring balance and note the reading. Pull the stone faster and
note the reading on the spring balance. Also note the time take n by
the stone to cross the distance of 10m. Then calculate your work and
power in both cases.

Times' Crucial Science and Environment 57 Book 7

Chapter

6 +HDW

ƐƟŵĂƚĞĚ ƉĞƌŝŽĚƐ͗ϱ

OBJECTIVES
At the end of the lesson, the students will be able to :
 ĚĞĮŶĞ ŚĞĂƚ͘
 tell the meaning of transmission of heat.
 explain the methods of transmission of heat.
 ĚĞĮŶĞ ƚĞŵƉĞƌĂƚƵƌĞ͘
 ĚĞĮŶĞ ƚŚĞƌŵŽŵĞƚĞƌ͘
 explain the structure and uses of the thermometer.

MIND OPENERS
 tŚĂƚ ĚŽ LJŽƵ ĨĞĞů ǁŚĞŶ LJŽƵ Ɛŝƚ ŶĞĂƌ ƚŚĞ ĮƌĞ͍ tŚLJ͍
 Why do you feel hot when you touch hot utensils?
 Which one has more temperature: either ice or boiling water?
 Have you ever used thermometer? What does it do? Explain.

Introduction

When we feel cold in winter season, we sit in the sunlight or near heater.
The sun and the heater give heat to us and make us feel warm. In the same
ZD\ ZH XVH JDV VWRYH RU ÀUHZRRG WR FRRN IRRG 7KH JDV VWRYH DQG ÀUHZRRG
give heat to the food and it gets cooked. Thus, KHDW LV GHÀQHG DV D IRUP RI
energy which gives sensation of warmth. The SI unit of heat is Joule. It is
also measured in calorie.
Every object is composed of molecules. These molecules are in constant
vibrating condition. Thus, each molecule contains kinetic energy. The
kinetic energy of these molecules makes the object hot. When molecules
vibrate with more speed, the molecules will have more kinetic energy. The
objects having more kinetic energy of molecules have more heat energy.
The heat energy of an object is equal to the sum of kinetic energy of all
molecules contained in that object.

Times' Crucial Science and Environment 58 Book 7

Transmission of Heat

+HDW ÁRZV RU WUDQVIHUV IURP KRWWHU VXEVWDQFH WR WKH FROGHU VXEVWDQFH 7KH
SURFHVV RI WUDQVIHU RI KHDW IURP KRWWHU ERG\ WR WKH FROGHU ERG\ LV FDOOHG
WUDQVPLVVLRQ RI KHDW

Transmission of heat occurs by three methods. They are:
a. Conduction
b. Convection and
c. Radiation.

1. Conduction
,W LV WKH SURFHVV RI WUDQVPLVVLRQ RI KHDW IURP KRWWHU HQG WR WKH FROGHU HQG RI
D VROLG VXEVWDQFH WKURXJK PROHFXOHV ZLWKRXW WKHLU DFWXDO PRYHPHQW
When you heat one end of a metal rod, the other end also becomes hot after
sometime. Here, heat transmits through conduction process.
All solid substances are composed of tightly arranged molecules. When
heat is applied to one end, the molecules of that region obtain heat and
transmit to the neighboring molecules of that region. They also transfer
the heat to the other molecules and so on. At last, the heat gets transferred
to the other end.

&RQGXFWLRQ RI KHDW

In this process, heat passes through molecules but molecules do not move
from one place to another.

Activity 6.1 To show conduction process

Material required: A pen
Procedure:

1. Make a group of ten students and tell them to stand in a
straight line.

2. Tell the front student to take a pen and pass it to the next
student.

Times' Crucial Science and Environment 59 Book 7

3. Tell the second student to pass the pen to the third student
and so on. At last, the pen gets transferred to the last student.

([SODQDWLRQ
Here, students are like molecules and pen is like heat. Pen passes
IURP WKH ÀUVW WR WKH ODVW WKURXJK VWXGHQWV EXW VWXGHQWV GR QRW PRYH
from one place to another.

Activity 6.2 To show that an iron rod is a good conductor

Materials required: Iron rod and wax

Procedure:
1. Take a long iron rod and put melted
wax at one end of the rod.
/HW WKH ZD[ JHW VROLGLÀHG
3. Suspend the rod through a thread
and heat the other end of the rod.

What do you see?

Observation

After sometime, you can see melting of the wax.

Conclusion: Iron is a good conductor of heat. Therefore, heat
passed from one end to the other end of the rod through conduction
process. Due to this, the wax gets melted.

Good and Bad Conductors

7KH VXEVWDQFHV ZKLFK FRQGXFW KHDW WKURXJK WKHP DUH FDOOHG JRRG
FRQGXFWRUV. Most of the metals like iron, copper, aluminum, zinc, etc are
good conductors.
7KH VXEVWDQFHV ZKLFK GR QRW FRQGXFW KHDW WKURXJK WKHP DUH FDOOHG EDG
FRQGXFWRUV 'U\ ZRRG SODVWLF JODVV HERQLWH ZDWHU HWF DUH EDG FRQGXFWRUV

Times' Crucial Science and Environment 60 Book 7

Activity 6.3 To identify whether a given substance is a
good conductor or not

Materials required:

A glass rod, a spoon, a long nail, a piece of wood and a beaker.

Procedure:
1. Take a glass rod, spoon, long nail and
a long piece of wood.

2. Take a beaker and keep hot water in
it.

3. Keep the glass rod, the spoon, the
long nail and the piece of wood in the
hot water of the beaker in such a way
that the upper end of each is above the level of the water.

4. Touch the upper end of each of them separately after 10
minutes. What do you feel?

Observation
<RX ZLOO ÀQG WKDW WKH HQG RI WKH JODVV URG DQG ZRRG GR QRW EHFRPH
hot whereas the end of the iron spoon and long nail get heated. This
is because the iron nail and iron spoon conduct heat whereas the
wood and glass rod do not conduct heat, i.e. iron nail and spoon
are good conductors of heat and wood and the glass rod are bad
conductors of heat.

Activity 6.4 To prove that water is a bad conductor of heat

Materials required:

A test tube, an ice piece and a piece of
copper wire

Procedure:
1. Take a test tube and put some water
in it.
2. Take a piece of ice and wrap it with a
copper wire. Place the ice piece in the
test tube so that it sinks to the bottom.
3. Hold the test tube with a test tube
holder and heat the upper part of the test tube as shown in
WKH ÀJXUH :KDW GR \RX ÀQG"

Times' Crucial Science and Environment 61 Book 7

Observation

You will see the boiling of the water at the upper part. But the ice at
the bottom does not melt. The reason behind this is: water is the bad
conductor of heat. Therefore, heat is not conducted towards the bottom.

Convection

&RQYHFWLRQ LV WKH SURFHVV RI WUDQVPLVVLRQ RI KHDW LQ ZKLFK KHDW WUDYHOV IURP
RQH SODFH WR DQRWKHU SODFH E\ WKH DFWXDO PRYHPHQW RI PROHFXOHV
Generally, convection process takes place in gases and liquids.

Activity 6.5 To demonstrate convection process

Materials required: A beaker and ink.

Procedure:
1. Take a beaker and put some water in it.
2. Put few drops of ink to the water to
make it coloured.
3. Put the beaker on the tripod stand and
heat the water.
:KDW GR \RX ÀQG"

2EVHUYDWLRQ DQG H[SODQDWLRQ
When you observe carefully, you will see that the hot water molecules
rise up from the bottom and cold water molecules from upper part move
down to occupy the place of hot molecules. When water molecules get
heat from the burner, they become lighter and move up whereas the
water molecules at the upper part are colder and heavier. They move
down to occupy the place of those hot water molecules. This process
continues and water gets heated.
The movement of hot and cold molecules of water make a current
named FRQYHFWLRQDO FXUUHQW.
Here, the heating of the water is due to convection process. In this
process, hot molecules move up carrying heat and actual movement of
hot and cold molecules takes place.

Activity 6.6 To show convection in air

Materials required: A cuboid paper box (biscuit carton), a candle,
incense sticks, a match box, etc.

Times' Crucial Science and Environment 62 Book 7

Procedure:
a. Take a cuboid paper box or a
carton of biscuit and make two
holes on its top.
b. Fix cylindrical paper chimney
in each hole as shown in the
ÀJXUH
c. Put a burning candle just below
one chimney inside the box.
d. Take an incense stick and burn
it and put the smoking incense near the mouth of other
chimney.

Observation:
You will see that the smoke of the incense stick comes out from the
chimney which lies just above the candle.

Conclusion:
The air above the burning candle gets heated and expands. The hot
air comes out from the chimney.
To occupy the place of the hot air, the air with smoke of incense stick comes
inside the box, from another chimney. It also gets heated and comes out
from the previous chimney. Thus, a convection current is formed.

Radiation

5DGLDWLRQ LV WKH SURFHVV RI WUDQVPLVVLRQ RI KHDW IURP RQH SODFH WR DQRWKHU
SODFH ZLWKRXW DQ\ PDWHULDO PHGLXP
To transmit heat by radiation process, there is no need of any material
medium. The heat from the sun comes to the earth by the radiation process.
There is no any material medium between the sun and the earth.
The speed of heat in the radiation process is equal to the speed of the light
in vacuum. It is 30,00,00,000 meter per second.

:KHQ ZH VLW QHDU WKH ÀUH ZH IHHO KRW +HUH WKH KHDW SDVVHV WKURXJK radiation
process. Conduction is not possible because air is bad conductor of heat.
Heat is not transmitted sidewise or downward by convection process. Heat
is transmitted only in upward direction by convection process. Therefore,
only method of transmission of heat here is convection.

Times' Crucial Science and Environment 63 Book 7

Temperature

When you touch cold water, you feel cold. When you touch warm water, you
feel warm. But we cannot say how much warm water is hotter than the cold
water. To say this in numerical value, we need a quantity. This quantity
is temperature. The temperature shows degree of hotness or coldness of a
substance in numerical value.
7HPSHUDWXUH LV GHÀQHG DV WKH GHJUHH RI KRWQHVV RU FROGQHVV RI D VXEVWDQFH
Its SI unit is Kelvin (K). It is also measured in degree Celsius (°C) and
degree Fahrenheit (°F). It is measured by thermometer.
The substances having high temperature are hotter than the substances
having low temperature. The substances with high temperature have the
molecules vibrating with high speed whereas the substances with low
temperature have the molecules vibrating with low speed.
Thus, the temperature shows the degree of speed of vibration of molecules
of a substance, too.

'L൵HUHQFHV EHWZHHQ +HDW DQG 7HPSHUDWXUH

Heat Temperature

1. It is a form of energy which is produced due 1. It is the degree of hotness or

to vibration of molecules of a substance. coldness of a substance.

2. It is measured in Joule or Calorie. 2. It is measured in Kelvin, °Celcius

or °Fahrenheit.

3. It is measured by calorimeter. 3. It is measured by thermometer.

Thermometer

When we touch a hot object, we feel it hotter. We can estimate the
temperature of the object by feeling the level of hotness. But, our estimation
is not accurate. We need special instrument named thermometer to measure
the actual temperature of the object. Thus, WKHUPRPHWHU LV GHÀQHG DV DQ
instrument which is used to measure temperature of an object.

It works on the principle that a liquid expands on heating and contracts on cooling. A
liquid is used in a thermometer. Such liquid is called thermometric liquid. Mercury
and alcohol are thermometric liquids which are mostly used in thermometer.
Thermometer is of two types:

a) Laboratory thermometer
b) Clinical thermometer

Characteristics of thermometric liquids

Mercury and alcohol are the common thermometric liquids.

Times' Crucial Science and Environment 64 Book 7

a. Mercury
1. It is good conductor of heat. Therefore, it easily passes heat from
the object.
2. It is a sliver coloured shiny liquid. Therefore, it can be seen clearly
in the glass. There is no need to colour it.
3. It does not wet the wall of capillary tube. Thus, rise or fall of
mercury in the tube is clear.
4. It has uniform rate of expansion and contraction.
5. It freezes at -39°C and boils at 357°C. Therefore, it measures the
high range of temperature from -39°C to 357°C.

b. Alcohol
1. It is transparent colourless liquid. Therefore, it is to be coloured
before use.
2. Its expansion rate is six times more than that of mercury. Therefore,
it is more sensitive.
3. It freezes at -115°C and boils at 78°C. Therefore, alcohol thermometer
is used to measure very low temperature. But it cannot measure
the temperature higher than 78°C.

Laboratory Thermometer

We use a thermometer to measure temperature of various substances. It is
also used to measure temperature of various substances in laboratories. Such
thermometer is called laboratory thermometer. Thus, laboratory thermometer
LV GHÀQHG DV D WKHUPRPHWHU ZKLFK LV XVHG WR PHDVXUH WHPSHUDWXUH RI
substances. Generally, it measures temperature from -10°C to 110°C.

Alcohol Capillary tube Glass tube

Bulb /DERUDWRU\ WKHUPRPHWHU Temperature scale

It consists of a capillary tube in the middle. The capillary tube has a narrow
bore. It is swollen to a bulb at one end and is sealed at the other end. The
EXOE LV ÀOOHG ZLWK PHUFXU\ RU DOFRKRO 7KHUH LV D FRYHU RI JODVV RXWVLGH
the capillary tube. The outside cover is graduated in degree from -10°C to
110°C. When the bulb comes in contact with an object whose temperature
is to be measured, there may be rise or fall of the level of mercury or alcohol
LQ WKH FDSLOODU\ WXEH $IWHU VRPHWLPH WKH OHYHO RI WKH OLTXLG UHPDLQV À[HG
The level of the liquid shows the temperature of the object.

Times' Crucial Science and Environment 65 Book 7

Clinical Thermometer

When we suffer from fever, our parents measure our temperature. They
use special type of thermometer named clinical thermometer.

Thus, FOLQLFDO WKHUPRPHWHU LV GHÀQHG DV D WKHUPRPHWHU ZKLFK LV XVHG WR
measure temperature of human body. A clinical thermometer consists of a
FDSLOODU\ WXEH ZLWK D ÀQH ERUH 2QH HQG RI WKH FDSLOODU\ WXEH LV VZROOHQ WR
D EXOE 7KH EXOE LV ÀOOHG ZLWK PHUFXU\ $QRWKHU HQG RI WKH WXEH LV VHDOHG
There is a narrow constriction in the capillary tube near the bulb. The
capillary tube is covered by a glass tube. It is graduated in degree from
35°C to 42°C or 94°F to 108°F.

Mercury Capillary tube Glass tube

Bulb Constriction &OLQLFDO WKHUPRPHWHU Temperature scale
We have to keep the bulb of thermometer in the armpit or mouth when
we want to measure the temperature of our body. There occurs rise in
WKH OHYHO RI PHUFXU\ LQ WKH FDSLOODU\ WXEH DQG LW EHFRPHV À[HG DIWHU VRPH
time. This level of mercury shows temperature of human body. When the
thermometer is taken out from the body, the temperature decreases. But,
the level of mercury does not return to the bulb due to the constriction
present in the capillary tube. This helps to take correct reading even after
sometime. The thermometer is jerked to return the mercury to the bulb.
Then, it can be used again to measure temperature of human body.

Activity 6.7 To study the structure and use of a thermometer

Materials required: A clinical and laboratory thermometer

Procedure:

1. Take a clinical and a laboratory thermometer.

2. Observe them carefully and identify the bulb, capillary tube,
thermometric liquid alcohol or mercury.

3. Find the temperature of your body by using clinical
thermometer. Find the temperature of water, ice, etc by using
laboratory thermometer

/RZHU À[HG SRLQW

It is the temperature of pure melting ice at normal atmospheric pressure.
Its value is 0°C.

Times' Crucial Science and Environment 66 Book 7

8SSHU À[HG SRLQW
It is the temperature of pure boiling water at normal atmospheric pressure.
Its value is 100°C.

Calibration of thermometer

Glass cover of a thermometer is graduated with a scale. It is done to do
correct measurement of temperature of an object. The process of putting
temperature scales in the thermometer is called calibration.
7R FDOLEUDWH WKH WKHUPRPHWHU ZH KDYH WR ÀQG RXW XSSHU DQG ORZHU À[HG SRLQWV

Activity 6.8 7R ¿QG ORZHU ¿[HG SRLQW RI D WKHUPRPHWHU

Materials required:

Uncalibrated thermometer, ice cubes, a beaker, standand a funnel.

Procedure: Stand

1. Put some ice cubes in a funnel Funnel Thermometer
and hold the funnel with a Melting ice
VWDQG DV VKRZQ LQ WKH ÀJXUH

2. Keep a beaker just below the
stem of the funnel.

3. Insert the bulb of an Beaker
uncalibrated thermometer
into the ice pieces.

4. Observe the level of the Water
mercury in the tube.

Observation:

You will notice falling of mercury level in the capillary tube. After
VRPHWLPH WKH OHYHO RI WKH PHUFXU\ UHPDLQV À[HG 7KLV SRLQW LV WKH
ORZHU À[HG SRLQW RI WKH WKHUPRPHWHU ,WV YDOXH LV ƒ& 0DUN WKH
OHYHO RI PHUFXU\ DV ORZHU À[HG SRLQW

Conclusion: 7KH ORZHU À[HG SRLQW RI D WKHUPRPHWHU LV ƒ&

Activity 6.9 7R ¿QG XSSHU ¿[HG SRLQW RI WKHUPRPHWHU

Materials required
$ URXQG ERWWRPHG ÁDVN XQFDOLEUDWHG WKHUPRPHWHU FRUN ZLWK
two holes, tripod stand, a retort stand, L-shaped glass tube, a
wire gauge, a burner, etc.

Times' Crucial Science and Environment 67 Book 7

Procedure
7DNH D URXQG ERWWRPHG ÁDVN DQG SXW VRPH ZDWHU LQ LW
2. Fix the thermometer of activity 6.8 and L-shaped glass tube
RQ WKH PRXWK RI WKH ÁDVN ZLWK WKH KHOS RI D FRUN DV VKRZQ LQ
WKH ÀJXUH
3XW WKH ÁDVN RQ WKH WULSRG VWDQG DQG À[ WKH VWHP RI WKH ÁDVN
with a retort stand.
+HDW WKH ZDWHU RI WKH ÁDVN ZLWK D EXUQHU

Thermometer L-shaped glass tube

Stand 5RXQG ERWWRP ÀDVN
Pure water
Wire gauze
Burner Tripod stand

Precaution
The bulb of the thermometer should be little above the level of water.
Observation
You will notice rising of the mercury level in the capillary tube.
When the water boils, the level of the mercury will be the maximum
DQG UHPDLQV À[HG ,W LV WKH XSSHU À[HG SRLQW RI WKHUPRPHWHU ,WV
YDOXH LV ƒ& 0DUN WKH OHYHO RI PHUFXU\ DV WKH XSSHU À[HG SRLQW
Conclusion: 7KH XSSHU À[HG SRLQW RI D WKHUPRPHWHU LV ƒ&
$IWHU À[LQJ WKH ORZHU DQG XSSHU À[HG SRLQWV GLYLGH WKH GLVWDQFH
between them into hundred equal parts. Each part is 1°C. In this
way, calibration of the thermometer is done.

^ĞŶƐĂƟŽŶ : physical feeling
sŝďƌĂƟŽŶ ͗ ĂĐƚ ŽĨ ǀŝďƌĂƟŶŐ
ŽŶƐƚƌŝĐƟŽŶ : a place with narrow opening

Times' Crucial Science and Environment 68 Book 7

Main Points to Remember

1. Heat is a form of energy which gives the sensation of warmth.
2. The heat of a substance is equal to the sum of kinetic energy of all

molecules of the substance.
3. The process of transfer of heat from hotter body to colder body is

called transmission of heat.
4. Conduction is the process of transmission of heat from hotter end

to colder end of a solid substance through molecules without their
actual movement.
5. Convection is the process of transmission of heat in which heat
travels from one place to another place by the actual movement
of molecules.
6. The substances which conduct heat through them are called good
conductors.
7. The substances which do not conduct heat through them are
called bad conductors.
8. Radiation is the process of transmission of heat from one place to
another place without any material medium.
9. Temperature is the degree of hotness or coldness of a substance.
10. Thermometer is a device which measures temperature of a substance.
11. Clinical thermometer is the thermometer which is used to
measure temperature of human body.
12. Laboratory thermometer is the thermometer which is used in lab
to measure temperature of a substances.
/RZHU À[HG SRLQW LV WKH WHPSHUDWXUH DW ZKLFK SXUH LFH PHOWV
8SSHU À[HG SRLQW LV WKH WHPSHUDWXUH DW ZKLFK SXUH ZDWHU ERLOV
15. Calibration is the process of marking temperature scale in a
thermometer.

Exercise

1. Choose the best alternative in each case.

a. By which process is heat transmitted in a vacuum?

i. Conduction ii. Convection

LLL 5DGLDWLRQ LY 5HÁHFWLRQ

Times' Crucial Science and Environment 69 Book 7

b. What instrument measures heat content of an object?

L 7KHUPRPHWHU LL 7KHUPRV ÁDVN

iii. Calorimeter iv. All

c. What is the process of transfer of heat by the actual motion of
the molecules called?

i. Conduction ii. Convection

iii. Radiation iv. None of these

d. What is the freezing point of mercury?

i. 0°C ii. -39°C iii. 357°C iv. 78°C

e. What is the temperature range of a clinical thermometer?

i. 35 to 42°C ii. 35 to 45°C

iii. 94 to 108°F iv. Both i and iii

2. Match the following:

Good conductor Human body temperature

Bad conductor Plastic
Clinical thermometer Convection

/RZHU À[HG SRLQW ,URQ

Transmission of heat in water 0°C

3. Copy the correct statements and correct the false statements if any.

a. The SI unit of heat is calorie.

b. Conduction is the process of transmission of heat from one place
to another through molecules without their actual movement.

c. Heat passes in solid substances by convection process.

d. Plastic, Iron, water and mercury are good conductors of heat.

e. Clinical thermometer measures the temperature of human body.

f. Laboratory thermometer contains constriction in the capillary
tube near the bulb.

J /RZHU À[HG SRLQW LV WKH WHPSHUDWXUH DW ZKLFK SXUH ZDWHU ERLOV

4. Answer these questions in short.

a. What is heat? Mention its S. I. unit.

b. What is transmission of heat? Mention the various methods of
transmission of heat.

Times' Crucial Science and Environment 70 Book 7

c. What is good conductor? Give some examples.

d. What is bad conductor? Give some examples.

H 'HÀQH

i. Conduction ii Convection iii. Radiation

f. What is temperature? Mention its unit.

g. What is a clinical thermometer? Draw a clinical thermometer.

h. What is laboratory thermometer? Draw its labelled diagram.

i. What is calibration of thermometer?

5. Differentiate between:

a. Heat and temperature

b. Alcohol and mercury

c. Clinical thermometer and laboratory thermometer

6. Give reasons:

a. Even though there is no any material medium between the sun
and the earth, the heat comes to the earth from the sun.

b. Constriction is kept in the capillary tube near the bulb of clinical
theromometer.

c. Metal utensils are used to cook foods.

d. Alcohol thermometer is used to measure temperature of very
cold substances.

7. Answer these questions in detail.

a. How is calibration of a thermometer done?

b. Explain the characteristics of mercury as thermometric liquid.

c. Explain an activity which shows that convection process occurs
in water.

Project Work

7DNH D ÀQH JODVV WXEH DQG FORVH LWV RQH HQG E\ KHDWLQJ LW RYHU WKH EXUQHU
)LOO VRPH FRORXUHG DOFRKRO LQWR LW DQG GHWHUPLQH WKH ORZHU À[HG SRLQW
7KHQ GHWHUPLQH WKH XSSHU À[HG SRLQW ZLWK WKH KHOS RI ERLOLQJ ZDWHU 1RZ
calibrate your thermometer and show it to your friends and teacher.

Times' Crucial Science and Environment 71 Book 7

Chapter

7 /LJKW

ƐƟŵĂƚĞĚ ƉĞƌŝŽĚƐ͗ϱ

OBJECTIVES
At the end of the lesson, the students will be able to :
 ĚĞĮŶĞ ƌĞŇĞĐƟŽŶ ŽĨ ůŝŐŚƚ ĂŶĚ ĞdžƉůĂŝŶ ŝƚƐ ƚLJƉĞƐ͘
 ĞdžƉůĂŝŶ ƚŚĞ ĐŽŶƐĞƋƵĞŶĐĞƐ ŽĨ ƌĞŇĞĐƟŽŶ ŽĨ ůŝŐŚƚ͘
 ƚĞůů ĂŶĚ ĚĞŵŽŶƐƚƌĂƚĞ ƚŚĞ ůĂǁƐ ŽĨ ƌĞŇĞĐƟŽŶ͘

MIND OPENERS
 Can you see in the dark?
 tŚLJ ŝƐ ŝƚ ĚŝĸĐƵůƚ ƚŽ ǁĂůŬ Ăƚ ŶŝŐŚƚ͍
 What helps us to see things?
 What things give us light? Discuss.

Introduction

:H FDQQRW VHH LQ WKH GDUN ,W LV GLIÀFXOW WR ZDON DW QLJKW EHFDXVH ZH FDQQRW
see the things properly. Light helps us to see the things. We can see the
WKLQJV RQO\ DIWHU UHFHLYLQJ WKH UHÁHFWHG OLJKW IURP WKH WKLQJV :H JHW OLJKW
from different sources such as the sun, the moon, electric lamps, candle,
ODQWHUQ ÀUH HWF

/LJKW LV D IRUP RI HQHUJ\ ZKLFK JLYHV WKH VHQVDWLRQ RI YLVLRQ. The light is
invisible itself but it makes the things visible on which it falls. All things
are seen only in the presence of light.

Ray of Light

$ QDUURZ VWUDLJKW OLQH DORQJ ZKLFK OLJKW WUDYHOV LV NQRZQ DV D UD\ RI OLJKW.
It is represented by a straight line with an arrowhead. The arrowhead
shows the direction of propagation of light.

A ray of light

Beam of Light

$ FROOHFWLRQ RI UD\V RI OLJKW LV NQRZQ DV EHDP RI OLJKW The beam of light
can be categorized into three types- parallel beam, convergent beam and
divergent beam.

Times' Crucial Science and Environment 72 Book 7

a. Parallel beam # RCTCNNGN DGCO QH NKIJV
$ EHDP RI OLJKW LQ ZKLFK WKH UD\V DUH SDUDOOHO
WR HDFK RWKHU LV FDOOHG SDUDOOHO EHDP. For
example, beams of light coming from the sun,
rays of light from a distant source, etc are
parallel beams.

b. Convergent beam # EQPXGTIGPV DGCO QH NKIJV
$ EHDP RI OLJKW LQ ZKLFK WKH VFDWWHUHG UD\V
RI OLJKW FRPH WR PHHW DW D SRLQW LV FDOOHG
FRQYHUJHQW EHDP )RU H[DPSOH UD\V RI OLJKW
FRQYHUJHG E\ D KDQG OHQV RQ D SLHFH RI SDSHU
LV D FRQYHUJHQW EHDP

c. Divergent beam

$ EHDP RI OLJKW LQ ZKLFK WKH UD\V RI OLJKW # FKXGTIGPV DGCO QH NKIJV
VSUHDG DZD\ IURP D SRLQW LV FDOOHG GLYHUJHQW
beam. For example, rays of light coming
out from a glowing bulb, burning lantern,
burning candle, etc.

5HÀHFWLRQ RI /LJKW

When light falls on the surface of an opaque object, some part of light is
absorbed by the body, very less part of it is transmitted through the object
DQG PRVW RI WKH OLJKW LV UHÁHFWHG 7KH SURFHVV RI UHWXUQLQJ RI OLJKW LQWR WKH
VDPH PHGLXP DIWHU VWULNLQJ D VXUIDFH LV FDOOHG UHÁHFWLRQ RI OLJKW. We see the
WKLQJV GXH WR UHÁHFWLRQ RI OLJKW

%DOO LV YLVLEOH GXH WR UHÀHFWLRQ RI OLJKW
$W ÀUVW WKH OLJKW IURP WKH VRXUFH IDOOV XSRQ WKH REMHFWV 7KHVH REMHFWV UHÁHFW
PRVW SDUW RI WKH OLJKW :KHQ WKH UHÁHFWHG OLJKW IDOOV RQ RXU H\HV ZH VHH WKH
WKLQJV ZKLFK KDYH UHÁHFWHG WKH OLJKW 6PRRWK DQG RSDTXH VXUIDFH DFWV DV
D JRRG UHÁHFWRU RI OLJKW

Times' Crucial Science and Environment 73 Book 7

5HJXODU DQG 'L൵XVH 5HÀHFWLRQ

5HJXODU UHÁHFWLRQ RFFXUV IURP D VPRRWK VXUIDFH VXFK DV PLUURU :KHQ D
SDUDOOHO EHDP RI OLJKW LV LQFLGHQW RQ D VPRRWK VXUIDFH WKH UHÁHFWHG UD\V DUH
SDUDOOHO WR HDFK RWKHU 6XFK UHÁHFWLRQ RI OLJKW LV NQRZQ DV UHJXODU UHÁHFWLRQ

5HJXODU UHIOHFWLRQ ,UUHJXODU UHIOHFWLRQ

When a parallel beam of light is incident on a rough surface, the rays of
OLJKW DUH UHÁHFWHG LQ VFDWWHUHG PDQQHU 6XFK UHÁHFWLRQ RI OLJKW LV NQRZQ
DV GLIIXVH UHÁHFWLRQ 7KH GLIIXVH UHÁHFWLRQ LV DOVR NQRZQ DV LUUHJXODU
UHÁHFWLRQ 7KH GLIIXVH RU LUUHJXODU UHÁHFWLRQ RI OLJKW RFFXUV RQ URXJK
VXUIDFHV OLNH ZRRG SDSHU ZDOO FORWKHV HWF 7KH GLIIXVH UHÁHFWLRQ RI OLJKW
is very important in our daily life. The rooms of our home or school become
EULJKW GXULQJ GD\ WLPH GXH WR GLIIXVH UHÁHFWLRQ RI OLJKW

5HÀHFWLRQ )URP $ 3ODQH 0LUURU

A mirror is a regular piece of glass which 1
is polished on one side and can return the $%
rays of light into the same medium.
LU
Suppose a ray of light AO is incident on
D SODQH PLUURU ;< DV VKRZQ LQ WKH ÀJXUH 2
Let the ray of light AO is sent back into
the same medium along OB after striking )LJ 5HIOHFWLRQ RI OLJKW
the mirror. Let ON be the normal at the ; <

point of incidence.

Incident ray

7KH UD\ RI OLJKW WKDW FRPHV IURP D VRXUFH DQG VWULNHV RQ WKH UHÁHFWLQJ VXUIDFH
LV FDOOHG LQFLGHQW UD\ ,Q WKH ÀJXUH $2 IDOOV RQ WKH VXUIDFH ;< 6R WKH UD\
AO is an incident ray.

Times' Crucial Science and Environment 74 Book 7

5HÁHFWHG UD\

7KH UD\ DORQJ ZKLFK WKH LQFLGHQW UD\ UHWXUQV LQWR WKH VDPH PHGLXP DIWHU
VWULNLQJ WKH VXUIDFH LV FDOOHG UHÁHFWHG UD\ ,Q WKH ÀJXUH 2% LV WKH UHÁHFWHG
ray.

Point of Incidence

7KH SRLQW DW ZKLFK WKH LQFLGHQW UD\ VWULNHV RQ D UHÁHFWLQJ VXUIDFH LV FDOOHG
SRLQW RI LQFLGHQFH ,Q WKH ÀJXUH 2 LV WKH SRLQW RI LQFLGHQFH

Normal

An imaginary perpendicular line which is drawn at the point of incidence
LV FDOOHG QRUPDO ,Q WKH ÀJXUH 21 LV WKH QRUPDO

Angle of Incidence

The angle between the incident ray and the normal is called angle of
LQFLGHQFH ,Q WKH ÀJXUH ‘AON = ‘L is the angle of incidence. It is also
called incident angle.
$QJOH RI 5HÁHFWLRQ
7KH DQJOH EHWZHHQ WKH UHÁHFWHG UD\ DQG QRUPDO LV FDOOHG DQJOH RI UHÁHFWLRQ
,Q WKH ÀJXUH ‘BON = ‘U LV WKH DQJOH RI UHÁHFWLRQ ,W LV DOVR FDOOHG UHÁHFWHG
angle.

/DZV RI 5HÀHFWLRQ

:KHQHYHU D UD\ RI OLJKW JHWV UHÁHFWHG LW REH\V WKH IROORZLQJ ODZV
7KH LQFLGHQW UD\ WKH UHÁHFWHG UD\ DQG WKH QRUPDO DW WKH SRLQW RI
incidence all lie in the same plane.
7KH DQJOH RI LQFLGHQFH LV DOZD\V HTXDO WR WKH DQJOH RI UHÁHFWLRQ

Activity 7.1 7R YHULI\ WKH ODZV RI UHÀHFWLRQ

Materials required
A plane mirror, a drawing board, a sheet of white paper, mirror

holders, thumb pins, pins, pencil, etc.
Procedure

1. Spread a sheet of paper uniformly over a drawing board and
À[ LW ZLWK WKXPE SLQV

2. Draw a straight line XY in the middle of the paper and draw
a normal ON at the middle of the line. (The normal is drawn
by making 90° on one side of the line).

Times' Crucial Science and Environment 75 Book 7

'UDZ D VODQWHG OLQH $2 DV VKRZQ LQ WKH ÀJXUH DQG SODFH D
plane mirror over the line and erect it with the help of mirror
stand.

2
;<
% %
$ L U
$

$%
1

)LJ 9HULILFDWLRQ RI ODZV RI UHIOHFWLRQ

4. tEhreecstlatnwtoedpilninseAin1 afrnodnBt o1 fvtehreticpalallnyeamt icrerrotra.in distance along

5. Observe the image of the pins from other side of normal and
tehreecitmtwagoeps ionfspBin2saAnd1 aAn2dinB1s.uch a way that the pins overlap

6. Remove the pins and mark the positions of the pins A1, B1, B2
and A2 with pencil.

7. Draw a line OB which meets the points B2 and A2.

8. Measure the angles ‘AON and ‘BON.

Observation

You will observe that ‘AON = ‘BON. Also AO, OB and ON all lie
at the same plane.

Conclusion

From the above activity, it can be concluded that:

D 7KH DQJOH RI LQFLGHQFH LV HTXDO WR WKH DQJOH RI UHÁHFWLRQ

E 7KH LQFLGHQW UD\ WKH UHÁHFWHG UD\ DQG QRUPDO DW WKH SRLQW RI
incidence all lie in the same plane.

7KXV WKH ODZV RI UHÁHFWLRQ DUH YHULÀHG

Image

If you place a ball on the table in front of a plane mirror, you will see
another ball inside the mirror. If you remove the ball from the table, no
ball will be seen in the mirror. In this activity, the ball that appears in the
mirror is the ‘image’ of the ball placed on the table. A plane mirror forms
the image of the object similar to that of the ball.

Times' Crucial Science and Environment 76 Book 7

0LUURU

2EMHFW ,PDJH

,PDJH IRUPHG E\ D PLUURU

7KH VWUXFWXUH RI DQ REMHFW IRUPHG E\ WKH UHÁHFWLQJ VXUIDFH GXH WR UHÁHFWLRQ
RI OLJKW LV FDOOHG LPDJH. :KHQ WKH UHÁHFWHG UD\V IURP D PLUURU RU DQ\
UHÁHFWLQJ VXUIDFH PHHW RU DSSHDU WR PHHW DQ LPDJH RI WKH REMHFW LV IRUPHG
An image may be real or virtual.
A UHDO LPDJH LV IRUPHG E\ WKH DFWXDO LQWHUVHFWLRQ PHHWLQJ RI WKH UHÁHFWHG
rays whereas a YLUWXDO LPDJH LV IRUPHG ZKHQ WKH UHÁHFWHG UD\V DSSHDU WR
intersect at a point. A real image can be obtained on a screen but a virtual
image cannot be obtained on a screen.

Image formed by a plane mirror 2EMHFW LPDJH
3ODQH PLUURU
A plane mirror forms a YLUWXDO LPDJH of
an object. It is because the image formed
by plane mirror cannot be obtained on
a screen. The image formed by plane
mirror is of same size as that of object.
Furthermore, the distance between the
object and mirror is equal to the distance
between the mirror and image. It means
that the object distance is equal to the
image distance.

Optical Instruments

7KH LQVWUXPHQWV ZKLFK RSHUDWH E\ XVLQJ OLJKW DUH NQRZQ DV RSWLFDO
LQVWUXPHQWV Periscope, kaleidoscope, microscope, camera, etc are optical
instruments.

Periscope Kaleidoscope Microscope Camera
Book 7
Times' Crucial Science and Environment 77

Periscope

3HULVFRSH LV DQ RSWLFDO LQVWUXPHQW ZKLFK
FDQ JLYH WKH YLHZ RI RXWVLGH REMHFWV IURP D
URRP A periscope consists of a cylindrical
case with sharp bends. A plane mirror is
placed at each bend in a simple periscope.
But in complex periscopes, a prism is
used instead of plane mirror.
In a periscope, the view of the outside is
UHFHLYHG E\ WKH ÀUVW PLUURU 7KLV PLUURU
UHÁHFWV WKH LPDJH RI WKH RXWVLGH REMHFWV
and sends to the second mirror. The
VHFRQG PLUURU LQ WXUQ UHÁHFWV WKH LPDJH
again and sends it to the observer’s eye.
You can make a periscope of your own by using cardboard paper and
regular pieces of mirror.

Kaleidoscope

$ NDOHLGRVFRSH LV D WR\ RSWLFDO LQVWUXPHQW LQ ZKLFK WKUHH SODQH PLUURUV
DUH KHOG DW ƒ LQFOLQDWLRQ WR HDFK RWKHU. These mirrors produce multiple
UHÁHFWLRQV LQVLGH WKH LQVWUXPHQW ,I FRORXUHG REMHFWV VXFK DV EHDGV
coloured paper bits, pebbles, etc are placed inside the kaleidoscope, the
PLUURUV SURGXFH PXOWLSOH UHÁHFWLRQV $V \RX ORRN LQVLGH IURP RQH HQG WKH
light entering from other end produces a colourful pattern. Thus formed
patterns entertain the observers.

Patterns formed in
Kaleidoscope

Tape Mirror
Cardboard

You can make your own kaleidoscope by joining three mirrors at an angle
of 60° with each other. Wrap both ends with transparent plastic after
placing some coloured matter inside it. Your kaleidoscope is ready. Enjoy
it by observing from one end.

Times' Crucial Science and Environment 78 Book 7

^ĐĂƩĞƌĞĚ : distributed at irregular interval
KƉĂƋƵĞ : an object through which light cannot pass
Real image : an image which can be got on the screen
Virtual image : an image which cannot be got on the screen

Main Points to Remember

1. A narrow straight line along which the light travels is called a ray
of light.

2. A collection of rays of light is known as beam of light.
3. A beam of light can be parallel, divergent or convergent.
4. The process of returning of light into the same medium after

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7KH SURFHVV RI UHÁHFWLRQ LQ ZKLFK WKH UD\V RI D SDUDOOHO EHDP RI

light incident on a smooth surface return parallel to each other is
FDOOHG UHJXODU UHÁHFWLRQ
7KH SURFHVV RI UHÁHFWLRQ LQ ZKLFK WKH UD\V RI D SDUDOOHO EHDP RI
light incident on a rough surface return scattered from each other
LV FDOOHG GLIIXVH UHÁHFWLRQ ,W LV DOVR NQRZQ DV LUUHJXODU UHÁHFWLRQ
7KH UHÁHFWLRQ RI OLJKW REH\V WKH IROORZLQJ ODZV
D 7KH LQFLGHQW UD\ UHÁHFWHG UD\ DQG WKH QRUPDO DW WKH SRLQW RI

incidence all lie at the same plane.
E 7KH DQJOH RI LQFLGHQFH LV HTXDO WR WKH DQJOH RI UHÁHFWLRQ
7KH VWUXFWXUH RI DQ REMHFW IRUPHG E\ WKH UHÁHFWLQJ VXUIDFH GXH WR
WKH UHÁHFWLRQ RI OLJKW LV NQRZQ DV LPDJH

Exercise

1. Choose the best alternative in each case.

a. A narrow straight line along which light travels is called

i. Medium ii. Beam iii. Ray iv. Path

E :KLFK RI WKH IROORZLQJ REMHFWV RIIHUV UHJXODU UHÁHFWLRQ RI OLJKW"

i. White paper ii. Table

iii. Blackboard iv. Looking mirror

Times' Crucial Science and Environment 79 Book 7

c. How are we able to see the things?

L :H VHH WKH WKLQJV GXH WR UHÁHFWLRQ RI OLJKW E\ WKH WKLQJV

ii. We see the things due to their colour.

iii. We are able to see the things due to blockade of light by the
things.

iv. We see things due to rectilinear propagation of light.

d. A beam of light in which the rays of light spread away from a
point is called

i. Convergent beam ii. Divergent beam

iii. Parallel beam iv. None

e. Image formed by a plane mirror is

i. Virtual ii. Laterally inverted

iii. Same size as object iv. All of these

2. Answer these questions in very short.
a. What is light?
b. What do you mean by a ray of light?
F 'HÀQH D GLYHUJHQW EHDP
d. Write down a use of a periscope.
e. Why do we use a plane mirror?
f. Name the two kinds of images.
J ,I WKH DQJOH RI LQFLGHQFH LV ƒ ZKDW ZLOO EH WKH DQJOH RI UHÁHFWLRQ"
h. What is meant by incident ray?
i. What is a normal?
j. Give any two examples of optical instruments.

3. Give reasons:
a. The image formed by a plane mirror is virtual.
b. We need a source of light to see the things.
c. Rooms of our school become bright at day even if no sunlight
reaches the room directly.
d. The moon is seen shining at night although it does not have its
own light.

Times' Crucial Science and Environment 80 Book 7

4. Write down differences between:
a. Convergent beam and divergent beam

E 5HJXODU DQG GLIIXVH UHÁHFWLRQ

c. Real image and virtual image

G ,QFLGHQW UD\ DQG UHÁHFWHG UD\
'HÀQH WKH WHUPV

D $QJOH RI LQFLGHQFH E $QJOH RI UHÁHFWLRQ

c. Normal d. Optical instrument

e. Kaleidoscope f. Periscope

6. Write down one use of:

D 'LIIXVH UHÁHFWLRQ E 3ODQH PLUURU

c. Periscope d. Kaleidoscope

7. Answer these questions:

D :KDW LV UHÁHFWLRQ RI OLJKW" ([SODLQ WKH UHÁHFWLRQ RI OLJKW LQ D
plane mirror.

E :KDW NLQG RI VXUIDFH FDXVHV UHJXODU UHÁHFWLRQ" ([SODLQ

F :ULWH GRZQ WKH ODZV RI UHÁHFWLRQ RI OLJKW

d. What are the characteristics of image formed by a plane mirror?

e. Describe an experiment to verify the lDZV RI UHÁHFWLRQ RI OLJKW
8. Diagrammatic questions:

D 'UDZ D GLDJUDP WR VKRZ D UHJXODU UHÁHFWLRQ

E 'UDZ D GLDJUDP WR VKRZ D GLIIXVH UHÁHFWLRQ

F &RPSOHWH WKH IROORZLQJ GLDJUDP ODEHO LW DQG GHÀQH WKH WHUPV
involved.

$ 1 <
; L

2

Times' Crucial Science and Environment 81 Book 7

G 6WXG\ WKH ÀJXUHV DQG DQVZHU WKH TXHVWLRQV

, ,,

L :KDW DUH VKRZQ LQ WKH ÀJXUHV"
ii. Write down two differences between them.
iii. Write down one use of each of these processes.

e. What is shown in the diagram? Explain its structure and
working.

Project Work

Prepare a periscope or kaleidoscope of your own using plane mirrors,
cardboard paper, tape, etc. Compare your work with that of your
friends and show to your teacher.

Times' Crucial Science and Environment 82 Book 7

Chapter

8 VRXQG

ƐƟŵĂƚĞĚ ƉĞƌŝŽĚƐ͗ϱ

OBJECTIVES
At the end of the lesson, the students will be able to :
 ĚĞĮŶĞ ƐŽƵŶĚ͘
 explain the meaning of longitudinal wave and its origin.
 ĞdžƉůĂŝŶ ƚŚĞ ƉƌŽƉĂŐĂƟŽŶ ŽĨ ƐŽƵŶĚ ǁĂǀĞ͘
 ĞdžƉůĂŝŶ ƚŚĞ ĨĂĐƚŽƌƐ ǁŚŝĐŚ ĂīĞĐƚ ƉƌŽƉĂŐĂƟŽŶ ŽĨ ƐŽƵŶĚ͘
 ĞdžƉůĂŝŶ ƚŚĞ ƐƉĞĞĚ ŽĨ ƐŽƵŶĚ ŝŶ ĚŝīĞƌĞŶƚ ŵĞĚŝĂ͘

MIND OPENERS
 Can you say how sound is produced?
 How does sound propagate from one place to another?
 Ž LJŽƵ ƚŚŝŶŬ ƚŚĂƚ ƐŽƵŶĚ ŚĂƐ ĞƋƵĂů ƐƉĞĞĚ ŝŶ Ăůů ŵĞĚŝĂ͍
 ĂŶ LJŽƵ ƐĂLJ ǁŚĂƚ ĨĂĐƚŽƌƐ ĂīĞĐƚ ƚŚĞ ƉƌŽƉĂŐĂƟŽŶ ŽĨ ƐŽƵŶĚ͍

Introduction

6RXQG LV D IRUP RI HQHUJ\ ZKLFK LV SURGXFHG GXH WR YLEUDWLRQ RI DQ REMHFW
When a string of a guitar is plucked, we observe that the string starts
vibrating and sound is produced. When you observe a ringing bell, you will
see the vibration of the bell. When you touch it, you can feel the vibration.
When you catch it, production of sound stops after sometime. It is because
the bell stops vibrating. When you touch a speaker in the front part when it
LV SURGXFLQJ VRXQG \RX FDQ IHHO WKH YLEUDWLRQV :KHQ \RX SXW \RXU ÀQJHUV
against your throat while speaking, you can feel vibrations.
Thus, DQ\ REMHFW ZKLFK YLEUDWHV LV D VRXUFH RI VRXQG. When vibration of the
object stops, the production of the sound also stops.

Madal Guitar Drum Radio
83 Book 7
Times' Crucial Science and Environment

Propagation of sound and sound wave

Activity 8.1 To produce and observe the sound waves

Materials required
Tuning fork, rubber pad, a bowl with water, etc.

Procedure
a. Take a bowl with water and put it on a table.
b. Take a tuning fork and hit its forks on a rubber pad.
c. Bring the vibrating tuning fork close to the water and touch
the water.

What do you see?
You can see ripples produced in water surface spreading out from
the point where you touch the water with the prong. These ripples
are waves. They carry sound from the source to other place.
As in water, sound wave is produced in the air also. When an object vibrates
in air, the molecules of the air move in to and fro motion. Due to this,
the air molecules are tightly packed in some regions and loosely arranged
in some regions. The region where the air molecules are tightly packed
is called FRPSUHVVLRQ and the region where the air molecules are loosely
arranged is called UDUHIDFWLRQ. The compressions and rarefactions are
formed alternatively in air. The compressions and rarefactions together
form a complete wave. This wave carries sound energy from the source to
other place.

ŽŵƉƌĞƐƐŝŽŶ %
$

ZĂƌĞĨĂĐƚŝŽŶ

&ŝŐ͗ ^ŽƵŶĚ ǁĂǀĞ

Times' Crucial Science and Environment 84 Book 7

Sound wave is a longitudinal wave. $ ORQJLWXGLQDO ZDYH LV RQH LQ ZKLFK
WKH SDUWLFOHV RI WKH PHGLXP PRYH LQ WR DQG IUR PRWLRQ LQ WKH GLUHFWLRQ RI
WKH ZDYH. In sound wave, the particles of the medium move in to and fro
motion forming compressions and rarefactions.
6RXQG HQHUJ\ WUDQVIHUV IURP WKH VRXUFH WR WKH RWKHU SODFHV LQ WKH IRUP RI
ZDYH 7KH SURFHVV RI WUDQVPLVVLRQ RI VRXQG IURP RQH SODFH WR DQRWKHU SODFH
LV FDOOHG SURSDJDWLRQ RI VRXQG
Sound wave travels through material medium only. It cannot travel in
vacuum. Therefore, it is called PHFKDQLFDO ZDYH.

Activity 8.2 Material medium is required for the propagation
of sound

Materials required
Bell jar, an electric bell, vacuum pump, vaseline, etc.

Procedure &RSSHU ZLUH

D 7DNH D EHOO MDU DQG À[ DQ HOHFWULF &RUN 6ZLWFK
bell inside the bell jar as shown in %DWWHU\
WKH ÀJXUH
%HOO MDU
b. Put the apparatus on a table and (OHFWURPDJQHW
connect a vacuum pump to the +DPPHU
lower part of the jar. *RQJ
9DFXXP SXPS

c. Connect the electric bell to battery 5XEEHU EDVH
with a switch.

d. Make the apparatus air tight by putting vaseline in the
connections.

e. Switch the circuit on and remove air from the jar by opening
YDFXXP SXPS :KDW GR \RX ÀQG"

Observation
In the beginning, you can hear the sound of the bell distinctly. Slowly
the sound becomes fainter and it gets vanished at last even though
the hammer is hitting the gong.

Conclusion

In the beginning, there was air inside the bell jar, and the sound
travelled through the air. When the air was removed from the jar
the sound could not come outside due to lack of material medium.
Thus, sound cannot be heard. This activity proves that material
medium is required for the transmission of sound.

Times' Crucial Science and Environment 85 Book 7

Propagation of Sound in Solid

Sound can travel through solids. Speed of sound in solid medium is the
highest of all.

Activity 8.3 Sound can travel through solid

Materials required
A bench or desk

Procedure
3XW \RXU HDU RQ D GHVN DV VKRZQ LQ WKH ÀJXUH
2. Tell one of your friend to tap the desk gently at another end.

Observation: You can hear the sound of tapping.
Conclusion: This activity shows that sound can travel through solid.

Propagation of Sound Through Liquid

Activity 8.4 Sound can travel through liquid

Materials required
A bucket full of water, a bell, a hammer, etc.

Procedure
1. Take a bucket full of water.
2. Hold a bell with a wire so that the
bell lies inside the water of bucket
DV VKRZQ LQ WKH ÀJXUH
3. Hit the bell with a hammer inside
the water. Can you hear the sound
of bell?

Observation: You can hear the sound of the bell.
Conclusion: Sound can travel through liquid.

Times' Crucial Science and Environment 86 Book 7

Transmission of Sound Through Gases

Sound can pass through gases also. Air is the mixture of different gases. We
hear the sound of human beings, birds, different animals, vehicles, radios,
televisions, etc. These sounds travel through the air from the sources and
reach to our ears. Air is the main medium for the transmission of sound.

6SHHG RI 6RXQG LQ 'L൵HUHQW 0HGLD

7KH GLVWDQFH WUDYHOOHG E\ VRXQG LQ XQLW WLPH LV FDOOHG LWV VSHHG ,WV XQLW LV
PHWHU SHU VHFRQG ZKLFK LV ZULWWHQ DV P V
The speed of sound in different media is different. The speed of sound is
maximum in solids and minimum in gases. This is because the molecules
are tightly arranged in solids than in liquids and gases. Since, sound energy
is carried by the molecules, it can be carried easily when the molecules are
closer with each other.

Speed of sound in some media

Sn. Medium Speed of Sound

1. ƌLJ ŝƌ 330m/s

2. DŽŝƐƚ ŝƌ 400m/s

3. Hydrogen 1280m/s

4. Water 1500m/s

5. dƵƌƉĞŶƟŶĞ 1325m/s

6. ůƵŵŝŶŝƵŵ 5100m/s

7. Copper 3560m/s

8. 'ůĂƐƐ 5500m/s

Activity 8.5 To show that sound travels faster in solid than
in gas

Materials required: Hammer

Procedure

Tell one of your friends to stand at one end of a wall of your class
room. Keep your ear at another end of the same wall. Tell the friend
to hit the wall gently with a hammer. Can you hear the sound? Take
out your ear from the wall. Tell the friend to hit the wall again.
:KDW GLIIHUHQFHV GR \RX ÀQG LQ WKH VRXQG"

Times' Crucial Science and Environment 87 Book 7

Observation:
You can hear the sound of the hammer distinctly, when your ear is
on the wall. It will be less distinct when your ear is in the air.
Conclusion:
The sound is distinct when your ear is on the wall. This is because
the sound travels faster in solid than in air.

Activity 8.6 Sound travels faster in solid than in liquid

Materials required
A bucket full of water, a bell, a hammer, etc.
Procedure
Take a bucket full of water. Hold a bell with a wire so that the bell
remains inside the water. Hit the bell inside water with a hammer
DV VKRZQ LQ WKH ÀJXUH &DQ \RX KHDU WKH VRXQG RI WKH EHOO"

Let the bell be touched with the wall of the bucket and hit it with
the hammer again. Is there any difference in the sound of the bell?
Observation: You can hear the sound of the bell distinctly in the
VHFRQG FDVH WKDQ LQ WKH ÀUVW FDVH
Conclusion: When the bell touches the wall of the bucket, the sound
travels faster through the solid wall of bucket than through the liquid.
Therefore, the sound is distinct when the bell touches the wall.

)DFWRUV $൵HFWLQJ 6SHHG RI 6RXQG LQ *DVHV

Speed of sound in gaseous medium is affected by various factors. They are:
1. Density
The speed of sound decreases when density of the gas increases. Carbon
dioxide is heavier than nitrogen. Therefore, speed of sound is less in carbon
dioxide than in nitrogen.

Times' Crucial Science and Environment 88 Book 7

2. Temperature
When temperature increases, speed of sound also increases.
3. Humidity
The amount of water vapour present in air is called humidity. The air
containing water vapour is called humid air or moist air. Speed of sound
increases when humidity of air increases.
4. Direction of motion of air
The speed of sound is more in the direction of air. Suppose, air is moving
from the east to the west, the sound travels faster towards the west
direction than in the east direction.

Propagate : pass

Vaseline : a type of jelly used in lubricants

Vacuum pump : a pump which creates vacuum

dƵƌƉĞŶƟŶĞ ͗ ǀŽůĂƟůĞ ƉƵŶŐĞŶƚ Žŝů ƵƐĞĚ ĨŽƌ ŵŝdžŝŶŐ ƉĂŝŶƚ

Density : heaviness of a substance

Main Points to Remember

1. Sound is a form of energy which is produced due to vibration of an
object.

2. Sound travels from one place to another in the form of wave.
3. Longitudinal wave is the wave in which the particles of the medium

move in to and fro motion.
4. The process of transmission of sound from one place to another

place is called propagation of sound.
5. Sound travels faster in solid than in liquid than in gas.
6. The factors which affect speed of sound in gas are humidity, density,

temperature and the direction of wind.

Exercise

1. Choose the best alternative in each case.

a. What is necessary for the production of sound?

i. Length of object ii. Solid state

iii. Vibration of object iv. Gas state

Times' Crucial Science and Environment 89 Book 7

b. In which medium does sound not travel?

i. Solid ii. Liquid

iii. Gas iv. Vacuum

c. The region where the air molecules are tightly packed due to
vibration is called

i. Compression ii. Rarefaction

iii. Crest iv. Amplitude

d. Sound wave is a ii. Mechanical wave
i. Longitudinal wave iv. None
iii. Both

e. What is the speed of sound in air?

i. 232 m/s ii. 1500 m/s

iii. 5000 m/s iv. 332 m/s

2. Fill in the blanks with suitable words.

a. Sound is a form of energy which is produced due to ……………
of an object.

b. Sound is a …………… wave.

c. …………… is the part of wave in which the molecules of the
medium are tightly packed.

d. The speed of sound is the highest in ………….. medium.

e. …………….. is the amount of water vapour present in air.
3. Answer these questions in short.

a. What is sound? How is it produced?
b. What is longitudinal wave? Mention the parts of longitudinal wave.
c. What is propagation of sound?
d. What is speed of sound? In which medium does a sound have the

highest speed?
4. Answer these questions.

a. How can you prove that material medium is required for the
transmission of sound?

Times' Crucial Science and Environment 90 Book 7

b. How can you prove that sound travels faster in solid than in air?

c. What are the factors which affect speed of sound in gases? How
do they affect? Explain.

5. Give reasons.

a. Sound is a mechanical wave.

b. Sound becomes louder in rainy season.

c. Speed of sound in carbon dioxide is less than that in air.

6. Speeds of sound in three different Medium Speed of sound
media i.e., solid, liquid and gas
are given in the table. Study the A 330 m/s

table and answer the following B 1500 m/s
questions: C 5100 m/s
a. Name the media A, B and C.

b. Which medium has greatest density?

7. Diagrammatic questions:

a. What is shown in the diagram?

b. Draw the diagram and label its parts.

c. What happens if all the gas is removed
from the vessel? Why?

Project Work

Take two empty cans of cold drink and remove their upper lids.
Make a small hole on the bottom of each can and join these cans
with a long piece of thread. Tell your friend to place the open end of
one can to the ear and talk through the thread.

Fig: Tin cans joined with a thread

Times' Crucial Science and Environment 91 Book 7

Chapter

9 0DJQHW

ƐƟŵĂƚĞĚ ƉĞƌŝŽĚƐ͗ϰ

OBJECTIVES
At the end of the lesson, the students will be able to :
 ĚŝīĞƌĞŶƟĂƚĞ ďĞƚǁĞĞŶ ŶĂƚƵƌĂů ĂŶĚ ĂƌƟĮĐŝĂů ŵĂŐŶĞƚƐ͘
 ĞdžƉůĂŝŶ ĂŶĚ ĚĞŵŽŶƐƚƌĂƚĞ ƚŚĞ ŵĞƚŚŽĚƐ ŽĨ ŵĂŐŶĞƟnjĂƟŽŶ͘
 explain the uses of magnets.

MIND OPENERS
 What is a magnet?
 How was a magnet discovered?
 tŚĂƚ ǁĂƐ ƚŚĞ ŶĂŵĞ ŽĨ ĮƌƐƚ ŶŽƟĐĞĚ ŶĂƚƵƌĂů ŵĂŐŶĞƚ͍
 Can we make magnets as per our needs?

Introduction

The word ‘magnet’ is a Greek word which means “a stone from magnesia”.
Magnesia is a part of ancient Greece where loadstones were found.
Loadstones are naturally occurring magnetized piece of iron ores found
in stones.The load stone is also called magnetite. They are the naturally
created magnets, which attract pieces of iron. It is claimed that magnets
were discovered and were in use in Greece and China since 2500 years
ago. Ancient Chinese navigators used magnetite to know the directions,
especially during the cloudy weather.
A magnet looks like iron but it has a power of attracting magnetic
VXEVWDQFHV WRZDUGV LWVHOI 'XH WR WKLV SURSHUW\ LW LV XVHG LQ YDULRXV ÀHOGV
A magnet is used in radio, transistor, television, speaker, motor pump,
electric generator, etc. A magnet can be used to collect the pieces of iron
spread on the ground. Doctors can use magnet to remove iron pieces if they
enter into our eyes.
A freely suspended magnet always comes to rest showing the north-south
direction. Hence, D PDJQHW LV GHÀQHG DV D VXEVWDQFH ZKLFK DWWUDFWV
magnetic substances and always comes to rest showing N-S directions
when suspended freely.

Times' Crucial Science and Environment 92 Book 7

Magnetism

Properties of a magnet is collectively called magnetism. A magnet has
following properties:

a) A freely suspended magnet always comes to rest pointing
north-south directions.

b) A magnet attracts magnetic substances.
c) The like poles of a magnet repel each other while the unlike poles

attract.
d) A magnet has strong attracting force at the poles and weak force in

the middle.
e) The magnetic poles always exist in pair. These poles can never be

separated.
f) The magnetic force is transferred to the magnetic substance which

is in contact to the magnet
J $ PDJQHW KDV D PDJQHWLF ÀHOG The space around a magnet up to

ZKLFK LWV HIIHFW FDQ EH H[SHULHQFHG LV FDOOHG PDJQHWLF ÀHOG

Care of Magnets

A magnet can lose its magnetic properties and turn into a magnetic
substance if it is not handled carefully. A magnet can lose its magnetic
properties due to the following activities.

a) Dropping a magnet from a height many times.

b) Bringing together or rubbing the similar poles of two magnets.

c) Hammering or heating a magnet.

d) Storing a magnet without using magnetic keepers. A piece of soft
iron which is used to keep magnets safely is called PDJQHWLF NHHSHU.

Magnetic keepers

7KH SURFHVV RI ORVLQJ WKH PDJQHWLF SURSHUW\ E\ D PDJQHW LV FDOOHG GHPDJQHWL]DWLRQ
A magnet turns into magnetic substance after demagnetization.

Magnetization

7KH SURFHVV RI WXUQLQJ D PDJQHWLF VXEVWDQFH LQWR D PDJQHW LV FDOOHG
PDJQHWL]DWLRQ. The non-magnetic substances cannot be magnetized. A
magnetic substance is mainly magnetized by the following two methods:

Times' Crucial Science and Environment 93 Book 7

1. Stroking method 1
2. Electrical method 6

1. Stroking Method
The process of turning a magnetic substance into a magnet by systematic
rubbing of magnet over it is called VWURNLQJ PHWKRG. It can be carried out in
two ways- single touch method and double touch method.

Activity 9.1 To make a magnet by single touch method

Materials required:
A bar magnet and a piece of an iron bar
Procedure

a. Place a piece of soft iron on the
table facing north-south directions.

E +ROG WKH LURQ SLHFH ÀUPO\ RQ
table with one hand and stroke
north pole of a bar magnet from
its one end to another.

c. Then lift the bar magnet swiftly and stroke again as before.
d. Repeat the process for about 60-70 times.
Observation
The iron piece now attracts the magnetic substances. It points the
north-south directions if suspended freely.
Conclusion
The iron piece has been magnetized by single touch method.

Activity 9.2 To make a magnet by double touch method

Materials required
Two bar magnets and a piece of an iron bar.

Procedure
3ODFH DQ LURQ EDU RQ WKH WDEOH DQG KROG LW ÀUPO\ LQ SRVLWLRQ
2. Stroke the north pole of one magnet and the south pole of another

magnet at the middle of the iron bar.
3. Rub the magnets towards opposite ends of the iron bar. Repeat

the process for about 60-70 times.

Times' Crucial Science and Environment 94 Book 7

11
6

6

Observation
The iron bar shows the magnetic properties.

Conclusion:
A magnetic substance can be magnetized by double touch method.

Electrical Method

If the magnetic substance is magnetized by using electricity, the process is
regarded as HOHFWULFDO PHWKRG. This method produces a temporary magnet.
But a very powerful magnet can be made by this method.
$ WHPSRUDU\ PDJQHW ZKLFK LV PDGH E\ PDJQHWL]LQJ D PDJQHWLF VXEVWDQFH
E\ XVLQJ HOHFWULFLW\ LV NQRZQ DV HOHFWURPDJQHW. Electromagnets are used
in the devices such as electric bells, telephone, electric fan, generator,
telegraph, etc.

Activity 9.3 To make a magnet by electrical method

Materials required

Battery, an iron nail, conducting wire, pins, etc.

Procedure

1. Take an iron nail and long insulated copper wire.

Coil (Solenoid) Pins attracted by
Iron nail temporary magnet
Copper wire

Battery

2. Wind up the iron nail with the insulated wire in about 100 turns.
The turning makes a continuous coil which is known as VROHQRLG.

3. Join the two ends of the coil with the battery; one end with +ve
end of battery while the other with -ve end.

4. Make a switch near the battery.
5. Switch the electric circuit on and observe.

Times' Crucial Science and Environment 95 Book 7

Observation
The iron nail attracts other nails or pins when the switch is on. But
the attracted pins drop down when the switch is off.

Conclusion
When electricity is passed through the coil, the iron nail becomes a
magnet. The iron nail acts as a magnet till the supply of electricity
is continued. Hence, the magnet is WHPSRUDU\.
The electromagnets are very useful magnets in several electrical
devices.
We should be careful while making an electromagnet. Following
points should be remembered while making an electromagnet:
a) Insulated wire should be used to make a coil. If a naked wire is

used, the battery will not work longer due to short circuit.
b) The iron nail should be made of soft iron.
c) A switch should be made in the electric circuit.

Making a Strong Electromagnet

We can make electromagnets according to the required strength. Following
are the methods of increasing the strength of an electromagnet:

a) Increasing the number of turns in the coil.
b) Passing more electricity through the coil.
c) Using soft iron piece for making an electromagnet.

Uses of magnet

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construction of advanced machines. Some of the important uses of magnets
can be summarized as follows:

a) Magnet is used in electric motor, dynamo, telephone, electric bell,
radio, television, etc.

b) It is used in factories for lifting the heavy pieces of iron, steel, etc.
c) It is also used in factories to separate magnetic substances from

non-magnetic substances.
d) Magnets are used in hospitals to remove iron or steel pieces from

wounds of the patients. They are also used to remove the tiny pieces
of magnetic substances entered in the eyes.
e) Magnets are used for making magnetic compasses, magnetic toys,
recording tapes, etc.

Times' Crucial Science and Environment 96 Book 7