Modern Graded Science 8

i. Which of them helps to do the work faster? Why?

ii. Identify the levers having the fulcrum in the middle.

iii. Identify the levers having the effort in the middle.

iv. Identify the levers having the load in the middle.

b. Sketch the labelled diagrams of the three types of lever.

c. Sketch the diagrams of a single movable pulley, a single fixed pulley and a
block and tackle of 3 pulleys.

7. Numerical Problems:

a. A crowbar having the length of 1.75 m is used to balance a load of 500 N. If the
distance between the fulcrum and the load is 0.5 m, calculate: [Friction is neglected]

i. effort applied to balance the load ii. MA iii. VR iv. efficiency

b. A load of 400 N is lifted by a first class liver in which the load is at the
distance of 20 cm and the effort is at the distance of 60 cm from the
fulcrum. If 150 N effort is required to lift the load, what is its efficiency?

c. In a lever, a load of 600 N is lifted by using 400 N effort. If the load is at the
distance of 20 cm and the effort at the distance of 40 cm from the fulcrum,
calculate its efficiency.

d. Study the given diagram and calculate the following:

i. work done by load ii. work done by effort

iii. M.A iv. V.R 15 cm 30 cm

v. efficiency [Friction is neglected]

e. A load of 600 N is lifted using a first class lever E
applying an effort of 350 N. If the distance
between the fulcrum and the effort is 60 cm and 600 N 2.6 m
the distance between the load and the fulcrum is
30 cm, calculate its efficiency. 0.6 m

f. Study the diagram and calculate the effort required
to balance the load.

Answers

7. a. i. 200 N ii. 2.5 iii. 2.5 iv. 100% b. 88.89% c. 75%

d. i. 37.5 J ii. 45 J iii. 1.5 iv. 2 v. 75% e. 85.71% f. 138.5 N.

GLOSSARY

Dhiki : a device used to separate husk from rice
Efficient : making careful use of resources; not consuming extra
Rigid : stiff or difficult to bend
Fulcrum : pivot; the point at which a lever moves up and down

SImple Machine 47

4Lesson PRESSURE

Total Estimated Pds : 6 [Th. 5 + Pr. 1]

On completion of this lesson, the students will be able to:

introduce atmospheric pressure and describe its importance.
introduce liquid pressure.
derive the formula for calculating liquid pressure and solve numerical

problems related to it.
introduce density and relative density and give their formulae.
explain the process of sinking and floating on the basis of density.
solve simple numerical problems related to density.

Atmospheric pressure

We have studied that pressure is force applied per unit area and its SI unit is pascal.
Pressure is given by all the stats of matter. The pressure given by air and that given by
liquid will be described here. The relation of pressure with sinking and floating of bodies
also will be analyzed and understood in this lesson. On the basis of sinking and floating
different means of transportation like boats, ships, road vehicles, aircraft and submarines
are constructed. Pressure is use for many different useful works in our daily life.

The earth is surrounded by air approximately up to the height of 9600 km from the surface
of the earth. This region of the air, which surrounds the earth, is called atmosphere. The
atmosphere of the earth is a mixture of different gases. Near the surface of the earth,
the air consists of about 78% of nitrogen, 21% of oxygen and the rest, 1% consists of
argon, tiny amount of carbon dioxide, water vapour and traces of other gases. Because
of gravity, these gases tend to accumulate near the surface of the earth. Therefore, air
is denser at sea level whereas it becomes thinner and thinner as the height increases
from sea level. Air has weight hence, it exerts pressure. This pressure is exerted on all
the bodies on the earth in the form of atmospheric pressure.

The pressure exerted by the atmosphere is called atmospheric pressure.
The atmosphere exerts pressure of about 105 Nm-2 or 760 mm of Hg at sea level. It is
also known as standard atmospheric pressure. The atmospheric pressure decreases as
we go up from sea level. Therefore, it is less at the top of Mt. Everest than in Janakpur
and Nepalgunj.

50 Modern Graded Science and Environment Book 8

Atmospheric pressure acts not only on the surface of the earth but also on all the objects
including living beings. The surface area of an average man is 2m2. The total force
exerted on his body by the atmosphere is given by:

F =PxA

= (105) x (2)

= 2 x 105 N

This is a great force but we do not feel such a great force on our body. This is due to the
fact that the blood in our body also exerts pressure nearly equal to it. Thus, the pressure
inside our body is about equal to the atmospheric pressure.

When the atmospheric pressure varies, we feel uneasy. At a high altitude, the pressure
of the air decreases. At such places nose-bleeding and sometimes eye-bleeding may
occur due to the low pressure of the atmosphere. It makes the blood pressure inside the
body greater than the atmospheric pressure. Similarly, if a living organism is placed in a
vacuum, the cells of the organism will burst because of its high internal blood pressure.

Due to the difference in pressure at different altitudes, the air blows from place to place.
Aeroplanes and jet planes fly at very high altitudes. The air pressure in these planes is
adjusted to make the passengers comfortable.

The atmosphere exerts pressure

Activity 4.1

To demonstrate the occurance of atmospheric pressure
Materials required

a tin can with lid, a burner, a tripod stand and some water
Procedure

steam lid atmospheric
tin-can pressure
water atmospheric
pressure
tripod
stand
burner

Fig 4.1 (a) heating water in can (b) after cooling the can

Pressure 51

1. Take a tin-can with less than one third of water in it.

2. Boil the water for a few minutes as shown in fig 'a'.

3. Close the lid of the can and simultaneously turn off the flame beneath the can.

4. Now, cool the can and observe it.
Before boiling the water in the tin-can, the air pressure inside and outside the can is
equal. When the water is boiled in the tin-can, the steam drives out the air inside it.
On cooling it, the steam inside it condenses. A partial vacuum is formed in the can and
consequently the excess atmospheric pressure outside causes it to collapse inwards.

Activity 4.2

To prove that atmospheric pressure occurs
Materials required

glass tumbler, some water and a postcard atmospheric
Procedure
atmospheric
1. Take a glass filled with water and
cover it with a postcard. glass
tumbler
2. Invert the glass tumbler supporting Fig 4.2 (a)
the postcard against the glass with your palm. atmospheric pressure
(b)

3. Now, hold the glass with your hand and remove the supporting palm slowly.

You will find that the water does not fall. It is due to the atmospheric pressure, exerted on
the postcard which holds it in the place. This activity confirms that the atmosphere exerts
pressure on a body from its all sides.

Mercury barometer A toricellian vacuum
glass tube
A barometer is an instrument for measuring
atmospheric pressure. About the middle of the mercury
seventeenth century, an Italian scientist named
Torricelli invented this instrument. A mercury
barometer is made in the following ways:

A one metre long glass tube closed at its one end B
is taken and filled completely with mercury. The air container
bubbles present in the glass tube should be removed
from the tube. Then by closing the mouth of the tube, it Fig 4.3 mercury barometer
is inverted and placed in a trough containing mercury
(Hg). We see that the level of Hg in the glass tube
decreases at first and finally it becomes constant.
The empty space above the surface of Hg in the tube
is called torricellian vacuum.

52 Modern Graded Science and Environment Book 8

The difference in the mercury level between the surface of Hg in the tube and the trough
measures the atmospheric pressure at a place. In the figure, pressure is shown by the
height, AB of mercury. At sea level, the atmospheric pressure is 760 mm of Hg. As the
height from the sea level increases, the height of mercury level in the tube decreases. It
means, the atmospheric pressure decreases along with increase in altitude.

Be careful that mercury is poisonous. It should not be spilled anywhere on the floor as it
is difficult to recover. Therefore, the experiment should be performed over a wooden or
plastic tray under the guidance of your science teacher.

Water cannot be used instead of mercury in a barometer because of the following
reasons:

1. The density of water is less than that of mercury. Thus, if water is used
in a barometer, the height of the tube should be about 11 m. It makes the
barometer inconvenient to handle.

2. Water is sticky to the walls of a barometer. Therefore, it is difficult to note
the exact reading.

3. Water vaporizes easily. This gives some errors in the reading of the
barometer.

4. Water is transparent, therefore, it is not clearly seen in the column.

Properties of air

Air has the following properties:
1. Air has weight and it can be compressed.

Acti vity 4.3

To prove that air has weight
Materials required

a physical balance, an air pump and two identical footballs

Procedure inflated deflated
1. Keep two identical deflated footballs football football
at the two pans of a physical balance.

2. Find the difference in weight.

3. Now, pump air in one of the footballs.

4. Place each of them on the pans of Fig 4.4 weight flat and in flatted balls
the physical balance again.

5. Now, find the change.

We find that the inflated football has more weight than the deflated one. This proves that
air has weight. The pressure of the compressed air in the football is higher than the air
pressure before pumping it.

Pressure 53

If a deflated football is kicked, it will cover some distance. If the inflated football is similarly
kicked, it will cover a longer distance than before. The compressed air inside the football
helps it to roll easily. Therefore, the wheels of vehicles such as bicycle, bus, van, aircraft,
etc. are filled with air.

2. Air occupies space air bubbles

Activity 4.4 glass
tumbler
To prove that air occupies space
Materials required trough

trough, some water and a glass tumbler water
Procedure
Fig 4.5

1. Take a trough filled with water.
2. Dip an empty glass tumbler into the trough into inverted position by tilting it

gradually.
3. Now, observe it.
You will notice that bubbles of air come out of the glass tumbler as the water rises in it.
This activity confirms that the glass tumbler which seems empty, actually contains air.

Activity 4.5

To study that air covers space

Materials required

a beaker, some water and a straw

Procedure straw pipe

1. Take some potable water in a beaker. water
beaker

2. Suck it by using a straw pipe. Does the Fig 4.6
water come into your mouth ?

How is it possible for the water to come into your mouth?

It is because before sucking there was air in the straw. When it is sucked, the air inside the
straw comes out of it and a partial vacuum is created there. Because of the reduced pressure
inside it and the effect of atmospheric pressure on it, the water rises up to the mouth.

Similarly, when the ink chamber of a fountain pen is pressed, the air inside it comes out
and a partial vacuum is created there. When the given pressure is removed, the ink fills
up in the chamber of the pen.

54 Modern Graded Science and Environment Book 8

Importance of atmospheric pressure

The importance of atmospheric pressure is presented below:
1. Atmospheric pressure balances the pressure in and out of our body. We

cannot survive in the lack of atmospheric pressure.
2. Due to atmospheric pressure, air pumps can fill air tubes of vehicles, balls, etc.
3. Due to atmospheric pressure, water pumps work.
4. Due to atmospheric pressure, a syringe can work and ink can be filled in pens.
5. Due to atmospheric pressure, we can suck juices, cold drinks, etc. from

their containers.
6. Change in atmospheric pressure causes wind to blow and it also helps to rain.

Liquid pressure

Like air, a liquid exerts pressure. This can be shown with the help of a simple experiment
as described below.

Activity 4.6

To demonstrate liquid pressure glass h 2
Materials required tube glass

two glass tubes opened at both ends, a rubber h1 tube
balloon, some thread and some water

Procedure

1. Take two glass tubes A and B, which are open at water water
their both ends.

2. Fix a balloon tight to its one end. (A) balloon (B)

3. Pour water in the tube through the open end. balloon

We will notice that the balloon begins to extend. (i) (ii)
The extension of the balloon is due to the pressure Fig 4.7
exerted by the water contained in it. It shows that
liquids also exert pressure.

In figure (i) the height of water h1 is less which exerts less pressure on the balloon.
But in figure (ii), the height of water h2 is more, which exerts more pressure on the
balloon. The experiment also proves that the pressure exerted by liquid increases if its
depth is increased.

Pressure 55

Characteristics of liquids

All the liquids have some common characteristics. They are explained as below:
1. Pressure in a liquid increases with depth.

Activity 4.7

To prove that liquid pressure increases with depth
Materials required

a can with three holes in the vertical line, can with
some water and a tripod stand holes

Procedure

1. Take a can with three side-holes A, B water
and C at different heights as shown
in the figure.

2. Fill it with water. Make sure that all stand
these side-holes are kept closed.

3. Open all these holes A, B and C Fig 4.8 a can with holes
simultaneously.

You will find that the water coming out from hole A covers the least horizontal distance
and the water from hole C covers the maximum distance. This experiment confirms that
liquid pressure increases with depth.

This principle is used in construction of dams. The base of a dam is built broader than its
upper part so that it can withstand greater pressure of water.

2. A liquid finds its own level

Activity 4.8

To prove that a liquid maintains its own level Fig 4.9

Materials required

a pa scal tube and some water

Procedure

1. Connect any three different-
shaped vessels A, B and C as
shown in the figure. It is called
Pascal tube.

2. Now, pour water in any one tube
and observe.

56 Modern Graded Science and Environment Book 8

You will find that the water remains at the same level in each of the tubes, whatever be
the shape of the tubes. It proves that a liquid maintains its own level.

This principle is used in the water supply system. For it, water is stored in a high tower
tank so that it can be lifted in multi-storeyed buildings through pipes without using electric
motors.

3. Liquid transmits pressure equally in all the directions.

Activity 4.9

To prove that a liquid transmits pressure equally in all the directions
Materials required

plastic bottle, a compass and some water.
Procedure

1. Take a plastic bottle and make numerous holes in it
with the help of a compass.

2. Fill it with water and then tighten the lid.
3. Press the bottle at any point and observe.

You will find that the force of water coming out from each Fig 4.10
hole increases. This is because the pressure applied to the

bottle is transmitted equally in all the directions by the water inside it. This is also called

Pascal's law.

4. The pressure at any point in a liquid depends on its density

The density of liquid also affects the pressure given by it. It is found that higher the
density of a liquid, more pressure it exerts.

Relation of liquid pressure with its factors

Suppose, a liquid (say water) having weight W is filled up to height h in a beaker having
its cross-sectional area A as shown in the figure. The pressure exerted by the liquid at
the base of the container is given by:

P = Weight of the liquid
base area of the container

or, P = W ………… (i) [... w = mg]
A

or, P = M.g ………… (ii)
A

or, P= dvg .................... (iii) [... d = m or m = d x v]
A v

Pressure 57

or, P = d(A.h)g Do you know?
[A... v = A x h]
Hydraulic machines are very advantageous to
\ P = dhg .............. (iv) us. They are based on liquid pressure. There
are four components in a hydraulic machine.
Here, P = liquid pressure They are: a reservoir to contain the hydraulic

d = density of liquid

h = depth of liquid system, valves to govern the pressure and
g = acceleration due flow and a cylinder to transform the circulating
fluid into energy.
to gravity

Thus, the pressure exerted by a

liquid depends on (i) density of the liquid (d), (ii) depth of the liquid (h) and (iii) acceleration

due to gravity (g). Liquid pressure is directly proportional to the factors mentioned above.

If any one factor among them increases, liquid pressure also increases.

Density

Take two tin cans of about 1 kg capacity each and fill one of them with saw dust and the
other with sand. Now weigh them. Which one is heavier? Why? It is because of their
different densities. The body having more density has more mass in unit volume. Thus,
density is defined as the amount of mass contained in unit volume of a body. Its SI unit
is kg/m3.

Or, density (d) = mass (m)
volume (v)

m
\ d = v
Different substances have different densities. Some of them are given below:

S.No. Substance Density S.N. Substance Density
1 Gold 19300 kg/m3 6 Pure milk 1030 kg/m3
2 13600 kg/m3 7 Water (at 4ºC) 1000 kg/m3
3 Mercury 11000 kg/m3 8 920 kg/m3
4 Lead 8000 kg/m3 9 Ice 600- 800 kg/m3
5 Iron 2700 kg/m3 10 Wood 800 kg/m3
Kerosene
Aluminum

The substances whose density is more than the density of a liquid sink in the liquid. On
the contrary, the substances whose density is less than the density of a liquid float on it.

Relative density

When the density of a substance is compared with the density of pure water at 4ºC, it is
called relative density of that substance. It is also called specific gravity.

58 Modern Graded Science and Environment Book 8

Thus, relative density of a substance is defined as the ratio of density of that substance
to the density of pure water at 4ºC. It has no unit.

R. density (dR) = density of a substance
density of pure water at 4°C

Relative densities of some substances are given below:

S.No. Substance Density S.N. Substance Density
1.03
1 Gold 19 6 Pure milk
1
2 Mercury 13.6 7 Water (at 4ºC) 0.92
0.6 - 0.8
3 Lead 11 8 Ice 0.8

4 Iron 8 9 Wood

5 Aluminum 2.7 10 Kerosene

Relative density of gold is 19 which means, the density of gold is 19 times more than the
density of pure water at 4ºC.

Activity 4.10

To measure density and relative density of the given materials

Materials required
a beam balance, a cylinder, a piece of thread, sone water, standard weights and given
materials such as stone, iron pieces, eraser.

Procedure
1. Weigh the mass of the given materials using a beam balance and standard

weights.
2. Now, measure the volume of each of them separately by using the cylinder method.
3. Divide the mass of each material with its volume to get their densities.
4. Now, the density of the materials is divided by the density of pure water at 4ºC to

know the relative density.

Sinking and floating

Fig 4.11 (a) a floating ship (b) a sinking ship

Pressure 59

The process of sinking and floating of bodies is directly related to their density. The

substances whose density is more than the density of a liquid sink in the liquid. On the

contrary, the substances whose density is less than the density of a liquid float on it. In

this condition, only a body can displace the liquid equal to its weight and the body floats

on the liquid. egg

Thus, when a body displaces water
the liquid equal to its weight,
the body floats on the liquid. water
It is known as the law of
floatation. beaker

beaker egg

Fig 4.12 (a) egg in normal water (b) egg in salt solution

Activity 4.11

To demonstrate that density of liquid affects sinking and floating

Materials required
a beaker, some water, a raw egg and some salt

Procedure

1. Take a beaker with some water and put an egg gently in it. Does the egg float or
sink ? Why ?

2. Now, dissolve enough salt in the water to make it saturated.
3. Put the egg into the prepared salt solution.
4. What happens now ? Why ?

Water has less density than the Do you know?
density of the egg, so the egg
sinks in water. But the saturated Pumice rock is a volcanic rock that has pores in
salt solution has more density than it. The holes that hold air. It makes the density
the density of the egg so the egg of the rock lesser than the density of water.
floats on it. Try the same activity Due to it pumice rock floats on water.
by taking a potato, a tomato or
any small fruit. You will get the
same result.

Solved numericals problems:

1. Density of iron is 8000 kg/m3. Calculate the mass of iron contained by a 5
m3 of iron block. Also calculate its relative density.
Solution:
Here,
Density of iron (d) = 8000 kg/m3
Volume of iron block (v) = 5 m3

60 Modern Graded Science and Environment Book 8

Relative density of iron (m) = ?
We have,
d= m
or, m = dvv = 8000 x 5 = 40000 kg

Again,

dR = density of iron = 8000 = 8
density of water at 4°C 1000

Thus, the iron block contains 40,000 kg of iron in it and its relative density is 8.

2. The volume of an iceberg is 2 m3. If it has 1840 kg mass, calculate the
density of the iceberg. Also calculate its relative density.
Solution:

Here,
Volume of iceberg (v) = 2 m3

Mass of iceberg (m) = 1840 kg

Density of iceberg (d) = ?

We have,

d = m = 1840 = 920 kg/m3
Again, dR = v2
density of iceberg
density of pure water at 4°C = 920 = 0.92
1000

Thus, density of iceberg is 920 kg/m3 and its relative density is 0.92.

3. Find the pressure exerted by a liquid at a depth of 8m. Density of the liquid
is 1000 kg m- 3. (Take g = 10 ms- 2).

Solution:

Depth of the liquid column (h) = 8 m
Density of the liquid (d) = 1000 kg m- 3
Acceleration due to gravity (g) = 10 ms- 2
We have,
P = hdg
= 8m x 1000 x 10m = 8 x 104 Nm- 2
Hence, the pressure exerted by the liquid is 8 x 104 Nm- 2 or 8 x 104 pa.

THINGS TO KNOW

1. Air surrounds the earth. Air has weight and it exerts pressure on the earth's
surface including all living and non-living things. This is called atmospheric
pressure.

2. Air has weight and it can be compressed.
3. Air occupies space.

Pressure 61

4. The pressure exerted by a liquid has the following characteristics:
a. The pressure in a liquid increases with depth.

b. A liquid finds its own level.

c. The pressure at any point in a liquid increases with its density.

d. A liquid transmits the given pressure equally in all the directions.

5. Liquid pressure is directly proportional to d, g and h or p = dgh

6. The amount of mass per unit volume of a body is called density of that body.

or d = m
v

7. The ratio of the density of a substance to the density of pure water at 4ºC is
called relative density of that substance.

or, dR = density of substance
density of pure water at 4°C

8. When a body displaces the fluid equal to its weight, the body floats on the

fluid.

THINGS TO DO plywood piece

Make Your Own Monemeter.

1. Take a piece of plywood polythene tube
finnel
whose size is 30 cm x 45 coloured
cm, about one metre long water ballon skin
base
thin and a transparent

polythene tube and a piece

of wood for its base.

2. Fix the plywood on the
base as shown in the figure.

3. Adjust the transparent tube on the plywood with the help of wires.

4. Adjust a small funnel at one end of the tube.

5. Fill the tube partially with coloured water and cover the open part of the
funnel with the skin of the balloon.

6. Press the balloon's layer gently and find the difference in the level of
coloured water in the tube. It is your own manometer to measure liquid
pressure.

62 Modern Graded Science and Environment Book 8

TEST YOURSELF

1. Fill in the blanks.
a. A liquid finds its own ......................... .
b. Pressure in a liquid increases with its ......................... .
c. The SI unit of density is ......................... .
d. The value of atmospheric pressure at sea level is ......................... .
e. Bicycle tubes burst if .............. air is filled.

2. Tick () the correct statements and cross () the incorrect ones.
a. Atmospheric pressure decreases with the increase in altitude.
b. Dams are made broader at the base.
c. Density and relative density are the same thing.
d. Air exerts pressure on all the bodies on the earth.
e. Atmospheric pressure at sea level is 86 cm of Hg.

3. Tick () the correct answer (MCQs):

a. The SI unit of pressure is:
ii. Nm-2
i. J iii. N iv. Nm

b. The pressure exerted by a liquid: Mass x Volume
i. is directly proportional to its density. Volume/Mass
ii. is inversely proportional to its density.
iii. does not depend on its density.

iv. equal to its density.

c. Density equals: ii.
i. Mass/volume iv.
iii. Force x Displacement

d. The pressure exerted by a liquid column of depth 2m on its container is:
(Take g = 10 ms- 2 and density = 800 kg m- 3)

i. 1.6 x 103 Nm-2 ii. 1.6 x 105 Nm-2
iii. 1.6 x 104 Nm-2 iv. 760 Nm-2
e. Atmospheric pressure:
i. increases as we go up from sea level.
ii. decreases as we go up sea level.
iii. remains constant as we go up sea level.
iv. sometimes increases but does not decrease.

Pressure 63

4. Give reasons.
a. The Level of mercury falls in a barometer while taking it to a mountain.
b. A gas balloon bursts when it comes at a high altitude.
c. Dams are made wider at the bottom than at the top.
d. We do not use water instead of mercury in a barometer.
e. When the piston of a fountain pen with a nib is dipped into ink and the air is
released by pressing it, the ink fills in the pen.
f. Ice floats on water but iron nails sink in it.
g. An iron nail sinks in water but floats on mercury.
h. Cotton is lighter than iron.
i. An egg sinks in pure water but floats on a saturated salt solution.
j. Relative density has no unit.
k. A balloon bursts when more air is filled in it.

5. Answer the following questions.
a. What is density? Write a formula by showing the relation among density,
mass and volume.
b. Define atmospheric pressure. Prove the presence of atmospheric pressure
with the help of an activity.
c. How can you prove that air has weight?
d. Explain the activity which proves that air occupies space.

e. Prove that P = dgh

f. Write about the structure and working method of a mercury barometer briefly.

g. In what condition does a body float?

h. Define density with a suitable formula.

i. Define relative density with a suitable formula.

6. Differentiate between:
a. air pressure and liquid pressure.

b. density and relative density.

c. sinking and floating.

7. Diagrammatic questions:
[Take g = 10ms-2]


a. Study the diagrams and answer the following questions.

i. In which tube does the liquid have more depth?

ii. In which tube does the liquid exert more pressure?

iii. How do you identify about the difference in pressure

acted by the liquid?

64 Modern Graded Science and Environment Book 8

b. Study the given diagram and answer the following
questions:

i. What does the figure prove?

ii. What may be the reason for passing out water in
the maximum force through hole C.

8. Numerical problems:
Take g = 10 ms–2

a. In a well, water is filled up to the height of 6 meters. Calculate the pressure
given by the water at the bottom of the well.

b. The pressure exerted by a liquid
column of depth 0.5 m on the base of
its container is 5000 Nm-2. Find the
density of the liquid.

c. Density of water is 1000 kg/m3.
What will be the volume of 35000 kg
water?

d. Study the diagram and calculate the
liquid pressure at A, B and C.

e. Density of lead is 11000 kg/m3. Calculate the amount of the mass of the lead
in the volume of 10 m3.

f. Calculate the density and relative density of gold if 96500 kg mass is adjusted
in the volume of 5m3.

Answers

8. a. 6×104 pa b. 1000 kg m-3
c. 35 m3 d. At A = 6×103 pa, At B = 3×103 pa and at C = 0
e. 110000 kg, 11 f. 19300 kg m3, 19.3e. 57. 6 kg.

GLOSSARY

Accumulate : to increase the quantity over a period of time

Simultaneously : happening at the same time

Consequently : as a result

Deflate : to let air out of a tyre, balloon, etc.

Dam : a barrier built across a river in order to stop the water from
flowing

Pressure 65

5Lesson ENERGY, WORK AND
POWER

Total Estimated Pds: 6 [Th. 5 + Pr. 1]

On completion of this lesson, the students will be able to:

tell the relation and differences between energy, work and power.
describe transformation of energy and demonstrate it.
solve simple numerical problems related to work, energy and power by

writing related formulae.

We need energy to do different types of work. Energy is obtained from different sources.
We get energy from our food. Similarly, machines get energy from the fuel used in them.
Humans, animals and machines do different types of work by using the energy. Different
persons and different machines have different capacity and rate of doing work. The rate of
doing work is power. In this lesson, we are going to study about energy, work and power.

Energy

We eat food to get energy and this energy helps us to do different kinds of work. When
a student works hard, he/she gets tired and feels that he/she has no capacity to do any
more work. Generally, we say that young people are more energetic than old people.
This is because, usually, young people have more capacity to do work than old people.

The capacity of a body to do work is called energy of the body. The energy of a body is
numerically equal to the amount of work which the body can do. The SI unit of energy is
joule which is also the SI unit of work. For example, if a body can do 10 joules work, we
say that the body possesses 10 joule of energy. Whenever work is done, equal amount
of energy is spent.

There are several ways of acquiring energy. The energy acquired by a body has different
names as follows depending on how this energy has been acquired.

1. Mechanical energy 2. Electrical energy
3. Heat energy 4. Light energy
5. Sound energy 6. Magnetic energy
7. Chemical energy 8. Nuclear energy

1. Mechanical energy

It is defined as the energy possessed by a moving body or the body resting at a certain height
above the surface of the earth. It has two forms: kinetic energy and potential energy.

Energy, Work and Power 1

a. Kinetic energy

Kinetic energy is the energy possessed by a body by virtue of its motion. A moving
hammer drives a nail into wood because of its kinetic energy. For example, a bullet fired,
a stone left from a catapult, wind, flowing water, rolling stone, etc. have kinetic energy.

If a body of mass m is moving with a velocity v, its kinetic energy is given by:
K.E. = 21mv2

From the above relation, we conclude that:

i. The kinetic energy of a body is directly proportional to the mass of the
moving body. It means that if the mass of a moving body increases by any
times, the kinetic energy increases by the same times.

ii. The kinetic energy of the body is directly proportional to the square of

velocity of the body. It means if velocity increases by any times, the kinetic

energy increases by the square times of the velocity change.

Some useful conversion of kinetic energy into work

i. Wind has enormous kinetic energy. The kinetic energy of the

wind can be uses in order to drive windmills, generate electricity,

lift water from a well, run a wind mill for electricity etc. Wind also

propels sailboats. This means, moving air or the wind has Fig 5.1 wind mill

capacity to do work i.e. it possesses kinetic energy.

ii. The flowing water of a river possesses kinetic energy which
can run water turbines to grind grains and produce electricity.
This shows that the flowing water of a river has capacity to do
work and thus possesses energy.

b. Potential energy Fig 5.2

Potential energy is the energy possessed by a body by virtue of its position or configuration.
For example, a stretched bow, a stretched catapult, a compressed spring, a stone raised
up to a certain height, the water stored in a dam, etc. have potential energy.

lifted block

h

Fig 5.3 (a) stretched catapult (b) a body at height

The gravitational potential energy of a body of mass m when placed at a height h from
the ground level is given by:

P.E. = mgh Where, m = mass
g = acceleration due to the gravity.
h = height from the earth surface.
From the above equation, we can conclude that potential energy is directly proportional

2 Modern Graded Science and Environment Book 8

to mass (m), acceleration due to gravity (g) and height (h) lifted from the surface. If any
one factor among them increases by any time, the potential energy also increases by
the same time.

Some useful conversion of potential energy into work

If the string of a bow is stretched, it possesses potential energy. If the string is released,
the potential energy is used to shoot the arrow forward. It means, the stretched string of
a bow has capacity to work.

The sum of kinetic energy and potential energy of a body is called its mechanical energy.
The kinetic and potential energy of a stone lying on the ground is zero.

Solved numerical problems:

1. A 2 kg stone is lifted up to a height of 8m from the ground. Calculate the
potential energy stored in it.

Here,
Mass (m) = 2 kg
Height (h) = 8 m
Potential energy (PE) = ?
We have,
PE = mgh

=2×9×8×8
= 156.8 J

Thus, the stone stores 156.8 J energy.
2. A 5 kg stone contains 1500 J potential energy at a height. Find out its height.

Here,

mass (m) = 5 kg

Potential energy (PE) = 1500 J

Height (h) = ?

We have,

PE = mgh

1500 = 5 × 9 × 8 × h

1500 = 49 h

h = 1500 = 30.61 m
49

Thus, the stone is at the height of 30.61m.

3. A bullet of 120g is fired from a gun. If the velocity of the bullet is 200 km/h,
calculate the kinetic energy of the bullet.

Here,

Mass (m) = 120g = 0.12kg

Velocity (v) = 200 km/h

Energy, Work and Power 3

= 200 × 1000
60 × 60

= 2000 = 555.56m/s2
36

Kinetic Energy (KE) = ?

We have,

KE = 1 mv2
2

= 1 × 0.12 × 555.56 × 555.56
2

= 0.6 × 555.56 × 555.56 = 185188.15 J

Thus, the bullet contains 185188.15 J kinetic energy.

4. An arrow thrown from a bow has 6350 J energy. If its mass is 50 g, calculate
its velocity.

Here,

Kinetic energy (KE) = 6350 J

mass (m) = 50 g = 0.05 kg

Velocity (v) = ?

We have, = 1 mv2
KE 2

or, 6350 = 1 × 0.05 × v2
2

or, 0.025 v2 = 6350

or, v2 = 6350
0.025

or, v = 254000 = 503.98 m/s

Thus, the velocity of the bow is 503.98 m/s.

2. Electrical energy

Electrical energy is possessed due to the continuous flow of electrons or the change
in the number of electrons. Some major sources of electrical energy are hydropower
stations, atomic power stations, thermal plants, battery, dynamo, etc.

We use electrical energy in order to:

a. run machines in factories, electric trains, lift, trolley buses, etc.
b. run fans in summer and heaters in winter.
c. run motors to pump out water from a well.

Modern life is not possible without electricity. Electricity is also used for many other
purposes in our daily life.

4 Modern Graded Science and Environment Book 8

3. Heat energy

Heat is a form of energy. It flows from a body at a higher temperature to a body at a lower
temperature. It gives the sensation of warmth.

The sun is the main source of heat. Heat energy is used for many jobs. Heat is produced
due to the combustion of inflammable products like petrol, diesel, kerosene, wood, gas,
coal, etc. Heat can be converted into useful mechanical energy. For example, steam is
generated when water is heated in a boiler by the combustion of coal. The steam runs
the engine and hence, the steam engine moves.

Note: We know that all the substances are composed of tiny particles called molecules.
These atoms or molecules keep vibrating about their mean position. Due to such motion,
each molecule possesses kinetic energy. The sum total of kinetic energy of all constituent
molecules of the body is a measure of heat energy.

If a body is heated, the molecular vibration increases and consequently, the body
possesses more heat energy.

4. Light energy

Light is a form of energy which produces the sensation of sight in the eye. If a body is
heated, it is capable of emitting light. The sun is the main source of light. Plants prepare
their food by photosynthesis, in which sunlight plays a vital role. Solar heaters, solar
cookers, solar cells, etc. use solar energy. When we light a candle, we get light. This is
the conversion of chemical energy of the candle into light energy.

5. Sound energy

Sound is a form of energy possessed by the vibration of molecules in a sounding body.
For example, if we touch a ringing bell, it feels vibrating. Our ear is not sensitive to all
vibrations produced by a vibrating body. Our range of hearing i.e. audible range is from
20 Hz to 20,000 Hz frequency of sound. 1Hz means 1 cycle per second. Sound requires
a material medium for its propagation. It cannot travel in a vacuum.

6. Magnetic energy

The energy stored in a magnet whose effects can be felt on the magnetic bodies is
called magnetic energy. A magnet can easily pull the pieces of iron nails towards it
when placed in the magnetic field. The motion of these small pieces of iron towards the
magnet is due to its magnetic energy.

Magnet is used for many purposes. Big electromagnets are used in factories to lift heavy
magnetic machines and separate iron pieces (magnetic bodies) from a heap of waste
materials. Besides this, it is used in radio, telephone, telegraph, dynamo, etc.

7. Chemical energy

The energy stored in a body, which is released from it, when it undergoes a chemical
change is called chemical energy. A chemical change is that process in which new

Energy, Work and Power 5

substances are formed by the process of chemical reaction. This energy can be released
in different forms. For example, if a matchstick is struck against a matchbox, it burns and
gives us heat and light energy. Here, the chemical energy stored in the matchstick is
changed into heat and light energy while burning.

The chemical energy of petrol, diesel, etc. is used to run vehicles. When a torch is
switched on, the chemical energy stored in the cell is converted into electrical energy and
then into light energy. Similarly, food eaten by us has chemical energy, which is changed
into other forms of energy during respiration.

8. Nuclear energy

The energy which is released by the splitting of a heavier nucleus of atom into lighter
nuclei (fission) or forming a heavier nucleus of an atom from the fusion of simpler nuclei
(fusion), is called nuclear energy. Nuclear energy is used in atomic power stations to
produce electrical energy. It can also be used for destructive purpose i.e. for making atom
bombs, hydrogen bombs, etc. Only some developed countries are using nuclear energy.

Transformation of energy

When we tune our radio on, a sound is heard. Have you ever thought where the sound
energy comes from? This is transformed from electrical energy. Thus, the conversion
of energy from one form to another is called transformation of energy. The principle of
conservation of energy sates that energy can neither be created nor destroyed but it can
be changed from one form to another. The total energy in a system remains constant.
Some examples of transformation of energy are given below:
1. While burning, the chemical energy stored in coal or wood changes into heat and

light energy.
2. A solar cell changes light energy into electrical energy. These cells are commonly

used in electronic calculators, photographic cameras, etc.
3. When two flint stones are struck against each other, sparks of light are produced along

with sound. Thus, the kinetic energy changes into light, heat and sound energy.
4. In a diesel engine, chemical energy is converted into heat energy and then into

kinetic energy.
5. In trolley buses, electric motors, fans, etc., electrical energy changes into mechanical

energy.

Work

Generally, the term work refers to any kind of physical or mental activity such as reading
a book, sitting on a chair, throwing a ball, etc. In science, the term work has a special
meaning i.e. work is done by a force when it produces motion in the direction of force.

The following are some common examples of work:

6 Modern Graded Science and Environment Book 8

1. When we throw a ball, we say that the force
exerted by our hand has done work on the ball.

2. When we push a book on a table, we apply
force on the book and hence, this force does
work on the book.

But reading a book by sitting on a chair is not a work Fig 5.4 carrying load
scientifically because there is no motion, even though the
person may get very tired while reading.

The work done by a force acting on a body is defined as the product of force and
displacement of the body in the direction of force.

Mathematically,

Work = Force × Displacement [in the direction of the force]

i.e., W = F × d d

Two conditions for work to be done are: F
a. force should be applied and
b. the body should move in the direction of initial position final position

force. Fig 5.5
In the SI system, force (F) is measured in newton
(N) and displacement (d) in metre (m).

Hence, the unit of work is Nm (or joule).

If, Force (F) = 1N and Displacement (d) = 1m

Then, Work done (W) = F × d

= 1N × 1m = 1 Nm = 1 joule
One joule of work is said to be done if a force of 1N displaces a body by 1m in the
direction of force.

Some larger units of work are kilo joule (kJ) and mega joule (MJ).

1 kJ = 103 joule friction
1 MJ = 106 joule
motion
Types of work ground

In general, work is done against friction and against Fig 5.6
gravity.

a. Work against friction Do you know?

Friction is the opposing force that One hour's worth of energy from the sun could
comes into action which tends to power the earth for a year. The sun is our single
oppose the motion of the body while greatest source of energy.
pushing or pulling another body. It
acts in the opposite direction to the

Energy, Work and Power 7

motion of the body. If a body is moved horizontally against the frictional force, this type of
work is called work against friction. For example, sliding a box and rolling a drum.

Mathematically,
Work against friction = Force applied × Distance travelled i.e. W = F × d

Activity 5.1

To measure work against friction

Materials required

a spring balance and a wooden block fixed with a hook

Procedure

1. Take a spring balance and attach it to the hook of a wooden block.

2. Drag it on a horizontal floor up to a distance of 5 m.

3. Consider, if the pointer of force wooden block
the spring balance shows 2 2 kg
kg, how much work is done
against the frictional force Fig 5.7
between the floor and the
wooden block? Calculate.

Here,

Distance moved (d) = 5m

Mass of the wooden box (m) = 2 kg.

We have, Weight of the wooden block (w) = m × g

or, F = 2 × 10 = 20 N

Again, Work done = F × d = 20 × 5 = 100 joule.
Therefore, work done against friction is 100 joule.

b. Work against gravity

When a stone is lifted upward, gravitational force acts vertically downwards and hence,
work is to be done against the force of gravity.

Therefore, if a body is moved vertically upwards against the gravity of the earth, this
type of work is called work against gravity. Lifting and throwing of a body upward are
examples of this type of work.

Mathematically,

Work against gravity = force applied x Height covered.
or, W = F x h

or, W = mgh

8 Modern Graded Science and Environment Book 8

Note: The acceleration produced in a body due to gravity is called acceleration due to
gravity. It is denoted by g. Average value of g is 9.8ms-2 For simplicity, we take
g = 10 ms-2.
Force = Mass × Acceleration
If a force on a body of mass 1 kg produces an acceleration of 1 ms-2, the force is called
one newton.

or, 1N = 1 kg × 1 ms-2

The force with which a body is attracted towards the centre of the earth is called its
weight.

or, Weight = force due to gravity

or, W = m × g

or, Weight of a body of mass 1 kg = 1 kg × 10 ms -2 =10N

Measurement of work against gravity

Activity 5.2

To measure work against gravity force
Materials required
a spring balance and a wooden block fixed with with a hook

Procedure 2 kg
1. Take a wooden block of mass 2 kg and attach it to a
spring balance. 4 cm wooden
block
2. Raise it vertically upwards.
Fig 5.8
3. How much work is done against gravity if it is raised by
4 m? Calculate.

Here, Height covered (h) = 4 m

Mass of the box (m) = 2 kg
Acceleration due to gravity (g) = 10 ms-2

We have, W = m × g × h

i.e., = 2 ×10 × 4 = 80 Joule
∴ Therefore, work done is 80 joule.

Solved numerical problems

1. Find the work done by a person if he uses 20 N force to carry a body up to 15
m away. If the time taken to do that work is 2 s, find his power in kilowatt.

Energy, Work and Power 9

Solution:

Here,

Force applied (F) = 20N

Distance covered (d) = 15m

Time taken (t) =2s

Work done (W) =?

Power (P) = ?

We have,
W = F × d = 20 × 15 = 300 J

Again,

P = W = 300 = 150 W
t 2

= 150 = 0.15 kW [ 1000 W = 1 kW]
1000

Therefore, the work done by of the person is 300 J and his power is 0.15 kW.

2. How much work is done by a person if he stands carrying a load of 50 kg at
the height of 1 m?

Solution:
Here,
As the person is not covering any distance, the work done is zero because:
W =F×d
or, W = F × 0 = 0

Power

We know that one type of energy can be changed into another by different devices.
The conversion of energy from one type to another is also work. Different persons and
machines have a different rate of doing work. The rate of doing work is called power.

Mathematically, ... W = F x s
F = m.g and
Power = Work done s=h
Time

i.e. P = w = F×s = m×g×h Work done
t t t

In the SI system, work is measured in joule (J) and time in second (s). Hence, the unit of
power is Js-1. Js-1 is also called watt (W).

If, Work (W) = 1 Joule

10 Modern Graded Science and Environment Book 8

Time (t) = 1 second

Then, = Work done = 1 s1ejcuolend= 1 Js -1 = 1 watt
Power (P) Time

If one joule of work is done in 1 second, power is said to be 1 watt.

Other common units of power are Kilowatt (KW), Megawatt (MW) and Horse Power (HP).

∴ kW = 1000 W (103 W)
1000000 W (106 W)
1 MW =

1 HP = 746 W [approx. 750 W]

Higher the power, faster the rate of doing work. For example, suppose coolie A takes
one minute to raise a box of mass (m) through a height of 2 metres and coolie B takes 30
seconds for the same job. Then coolie B has worked faster than coolie A. Thus, coolie
B has more power than coolie A.

Solved numerical problems

1. If a crane lifts a load of 75 N to a height of 20 m in 100 s, what is the power of
the crane?

Solution:

Here,

Weight lifted by the crane (F) = 75 N

Height raised (h) = 20 m

Time taken (t) = 100 s

Power (P) =?

We have, Do you know?
W = F × d

= 75 × 20 = 1500 J There are 2 types of energy: renewable and

∴ P = W = 1500 = 15 W non-renewable. 75% of the energy we use
t 100 comes from non-renewable sources like fossil
Therefore, the power of the fuel, coal, natural gases and nuclear energy.

crane is 15 W.

2. Calculate the power of a porter if he can carry 20 bricks to a distance of
75 m in 50 s. The weight of each brick is 20 N ?

Solution

Here,

Number of bricks (N) = 20
75 m
Distance covered (d) = 50 s
20 N
Time taken (t) = ?

Weight of each brick(w) =

Power (P) =

Now,

Energy, Work and Power 11

Total weight of bricks (F) = N × W

= 20 × 20 = 400 N
Again,
We have

W = F × d

= 400 × 75 = 30000 J
Again,

P = W
t

= 30000 = 600 W
50

Hence, the power of the porter is 600 W.

3. Study the diagram and calculate:

(i) Work done when he reaches the top.

(ii) Power if the person takes 10 s to do the work. final position
Solution

Here, = 60 kg 10
mass (m) = 12 cm 9
height of step (hs) = 10 8
Number of steps (N)
= 12 ×10 = 120 cm = 1.2 m 7
Total height (h)
6
5

Time (t) = 10 s 60 kg 4
Work done (W) = ? 2 3
Power (P) = ?
We have, 1 Fig 5.9
12 cm
W=F× d initial position
= m × g × h [... F = m × g]

= 60 × 9.8 × 1.2
= 705.6 J
Again,

P= W
t

= 705.6
10

= 70.56 W

Thus, the person does 705.6 J work and his power is 70.56 W.

12 Modern Graded Science and Environment Book 8

Interrelation between work, energy and power

We have studied that energy is the capacity to do work and work is transformation of
energy. Similarly, the rate of doing work is known as power. In this way, energy, work and
power are interrelated closely.

When a person does any work on a body, the brick on the table

energy of that body increases. The work done on

the body is equal to the loss of muscular energy of

that person. For example, when we keep a brick on

a table by lifting from the ground, the brick gains

potential energy. In fact, the energy gained by the brick on the ground
brick is the muscular energy lost by the person. Fig 5.10

How fast a person can convert his muscular energy

into another form of energy shows his/her power. Thus, energy, work and power are

interlinked together. They have a very close relationship.

Difference between work, energy and power

Even though energy, work and power are closely related, they are not the same thing. They
differ from each other in many aspects. Some differences between them are given below:

Energy Work Power

1. It is the capacity to do work. It is the product of force and It is the rate of doing work.
displacement.

2. Its SI unit is joule. Its SI unit is Joule. Its SI unit is watt.

3. It is not affected by time. It is not affected by time. It is affected by time.

THINGS TO KNOW

1. The work done by a force acting on a body is defined as the product of force and
displacement of the body in the direction of force.

or, W = F × d

2. Work is a scalar quantity. This is because work does not require any direction for
its description.

3. If a force of 1 N displaces a body by 1 m in the direction of force then 1 joule of
work is done.

4. In general, work is done either against friction or against gravity.

5. The capacity of a body to do work is called energy of that body.

6. Different forms of energy are as follows:

a. Mechanical energy b. Electrical energy c. Heat energy
f. Magnetic energy
d. Sound energy e. Light energy

g. Chemical energy h. Nuclear energy.

Energy, Work and Power 13

7. The energy possessed by a body due to its motion is called kinetic energy.

or, bodKyEd=ue21tomivts2 position, structure or configuration is
8. The energy possessed by a

called potential energy.

or, PE = mgh

9. The energy stored in the nucleus of an atom is called nuclear energy.

10. The conversion of energy from one form to another is called transformation of energy.

11. The rate of doing work is called power.

or, P= W = F×s = m×g×h
t t t
12. When one joule work is done in one second, the power is called one watt.

13. Energy, work and power are interrelated.

THINGS TO DO

Make two slanting surfaces of cardboard as
shown in the figure. Now, release a toy car
or a marble freely from the top of the surface
one by one.

Which one travels to a longer distance and why?
Discuss among your friends and try to know the
final reason from your science teacher.

TEST YOURSELF

1. Fill in the blanks.
a. The unit of work in terms of newton and metre is ......... .
b. The sum of kinetic and potential energy is called ......... .
c. A speeding car has ......... energy.
d. When force and displacement are in perpendicular directions, the work is ...

2. Tick () the correct answer (MCQs):

a. If a body of 1N force covers a distance of 1 m, it does .................. work.

(i) 1 W (ii) 1 N (iii) 1 J (iv) 1 HP

b. 1 HP is equivalent to:

(i) 570 W (ii) 670 W (iii) 750 W (iv) 600 W

c. Work is a:

(i) scalar quantity (ii) vector quantity

(iii) unknown quantity (iv) standard quantity

14 Modern Graded Science and Environment Book 8

d. Our food contains: (ii) heat energy
(i) muscular energy (iv) chemical energy
(iii) kinetic energy

3. Differentiate between:
a. kinetic energy and potential energy b. work and power
c. magnetic energy and chemical energy d. joule and watt
e. force and work

4. Define:

a. work b. nuclear energy
d. transformation of energy
c. power

5. What kind of energy transformation takes place in the following devices?

a. hydro-electric power station b. a bullet fired from a gun

c. a battery d. an arrow on a bow

e. electric bulb f. a wound spring of a toy car

g. heater h. water in spring

i. a green plant j. a stone resting at the top of a hill

k. human body l. a stretched rubber

6. Answer following questions.

a. A man has been guarding a house for one hour. Why is it not considered
work in science?

b. State a situation in which force is applied on a body, but no work is done.
c. Write the mathematical relation between work, force and displacement.
d. What factors does the kinetic energy of a body depend on?
e. What is one watt? Write the relation of watt with kilowatt, megawatt and

horsepower.
f. Ram has power of 550 watt. What does it mean?
g. Write any five forms of energy.
h. What is difference between work done by a fuel and work done by a moving object?
i. While climbing a ladder, what is the energy used by us?

j. What is the inter relation among work, energy and power?

7. Diagrammatic Questions:

a. Answer the following questions on the basis of
the given diagram.

i. Identify the type of work shown in fig. A and A
fig. B.

ii. Write the formula which can be used for B
energy of the body in fig. A.

iii. Write the formula that can be used for energy
of the body in fig. B.

Energy, Work and Power 15

8. Numerical problems:
a. Find the force required to do 25 joule work when the force causes a
displacement of 0.5 m.

b. A boy pushes a book by applying force of 5 N. Find the work done by this
force as the book is displaced through 20 cm while pushing.

c. Find the kinetic energy of a ball of mass 200 grams moving at a speed of 20 m/s.

d. A stone 'X' of mass 2 kg is at the height of 2 m from the ground level.
Another similar stone 'Y' of mass 4 kg is at the height of 4 m from the ground
level. What is the difference between their potential energy?

e. A 60 kg person climbs stairs of total height of 20 m in two minutes, calculate
the power delivered. [g = 10 ms-2]

f. How much time will it take to perform 440 joule of work at a rate of 11 W?

g. A 100 watt electric bulb is used for five hours. How much energy is consumed
by the bulb?

h. A coolie of 40 kg weight carries 20 kg salt and climbs up 20 steps of 20
centimeters each in 2 minutes. Calculate the work done by him and his power.
[ g= 10 m/s2 ]

Answers c. 40 J d. 120 J
8. a. 50 N b. 1 J g. 1.8 × 106 J h. 20 J, 2400 J, 20 W

e. 100 W f. 40 s

GLOSSARY

Gravity : the force that attracts objects towards the centre of the planet

Drag : to pull something/somebody along with effort

Energetic : having a lot of energy

Enormous : extremely large, huge

Windmills : a machine that changes kinetic energy of wind into other
form of energy to do different types of work

Propel : to move, drive or push something forward

Inflammable : that can burn easily

Magnetic field : the space around the magnet up to which its effect is felt

Atom bomb : a bomb that explodes using the energy that is produced
when an atom or atoms are split/ nuclear weapon

16 Modern Graded Science and Environment Book 8

6Lesson HEAT

Total Estimated Pds: 5 [Th. 4 + Pr. 1]

On completion of this lesson, the students will be able to:

define heat and temperature and differentiate between them.
describe the structure and working method of simple and clinical thermometers.
explain the ways of determining the units of temperature (Celsius, Fahrenheit

and Kelvin).
tell about the calibration of a newly constructed thermometer.
show the relation of one type of unit of temperature into other type and their

working methods.

If you stand near burning firewood, what do you feel? You feel warm. This is because you
are receiving heat from the burning wood. If you touch a piece of ice, you feel cold. This
is because you are losing heat to the ice. Thus, heat is a form of energy which flows from
a hotter body to a colder body. It gives the sensation of warmth. Heat in a body is due to
its molecular motion. Higher the molecular motion, the more the heat is felt.

A matter is made up of many molecules of the same or different kinds. The molecules
are at random motion. They are attracted together by intermolecular force of attraction.
The heat energy of a body made up of matter is the internal energy of the molecules of
the body. When we rub our palms together, the molecules of the palms get vibrated and
heat is produced.

Heat produces a kind of vibration in the molecules of the body. Kinetic energy is produced
on vibration of molecules. If the kinetic energy of the molecules of the body increases,
heat produced from it will also increase. In this way, heat is produced because of the
increase in the kinetic energy of the molecules of a body. Thus, the sum total of kinetic
energy of all the molecules of a given body is called heat.

As heat is a form of energy, its SI unit is joule (J). CGS unit of heat is calorie (cal). One
calorie heat is the amount of heat required to raise the temperature of one gram of pure
water by 1° C. One calorie is equal to 4.2 joules. Heat of a body is measured by a device
called calorimeter.

80 Modern Graded Science and Environment Book 8

Activity 6.1

To show heat transfer from a hot body to a colder body.

Materials required
three beakers, hot water, cold water and a thermometer

Procedure
1. Take two beakers and pour 100 ml hot water in one and 100 ml cold water in another.
2. Measure the temperature of both hot and cold water using thermometer and note

it down.
3. Thereafter, take a large sized beaker and mix the water from both the beakers.
4. Note its temperature as well. What is the temperature of the mixed water?

Observation
When you mix hot and cold water, the temperature of the mixture changes. The
temperature of the mixture is less than the temperature of hot water but more than the
temperature of cold water. This is because hot water looses heat whereas cold water
gains heat.

Conclusion

Thus, you can conclude that heat always transfers from a hotter body to a colder body.

Temperature

Activity 6.2 ABC

1. Take three beakers: A,

B and C filled with equal

amount of lukewarm water,

tap water and ice water Fig 6.1 (a) lukewarm water (b) tap water (c) ice water

respectively.

2. Dip your finger into beaker A containing lukewarm water for some time and
then in beaker B containing tap water. What do you feel? You feel cold.

3. Now, dip your finger into beaker C containing ice water for one minute and
then into the tap water.

How does the tap water feel, now? It is warm because the water of beaker B is hotter
than the water in beaker C. But it is colder than the water in beaker A as it feels cold.
Here, the sense of touch is the simplest way to distinguish hot water from cold water.

Heat 81

Our judgement regarding hotness or coldness of a body is our sense of temperature.
Therefore, the degree of hotness or coldness of a body is called temperature. The SI unit
of temperature is kelvin (K) and it is measured by using a thermometer.

When a body is heated, its temperature rises and when the body is cooled, its temperature
falls. When two bodies, one hotter and the other colder, are brought in a thermal contact,
the heat flows from the hot body to the cold body till they are at the same temperature.
Heat flows from a body at a high temperature to a body at a low temperature.

Difference between heat and temperature

The temperature of a body and the amount of heat it contains are different things. When a

body is heated, its temperature rises. This is because the molecules of the body begin to

vibrate faster, so the average kinetic energy of each molecule increases. Hence, the average

kinetic energy of the molecules of a hotter body is larger than that of the molecules of a colder

body. That's why, the temperature of Do you know?
a body is an indicator of the average

kinetic energy of the molecules of The highest temperature ever recorded in
that body. the shade was 57.8 °C (136 °F) in Libya on

Activity 6.3 September 13, 1922.

Take a beaker of water at a room temperature and a spoonful of hot water.

The temperature of the spoonful of hot water is higher than that of the

beaker of water, so the average kinetic energy of water molecules in

the former case is higher than that of the later case. The number of

water molecules is considerably more in the beaker than the molecules

in the water in the spoon. Fig. 6.2 beaker with

Thus, the total kinetic energy of water molecules in the beaker has tap water

more heat energy than that of the spoonful of water. Hence, more heat energy in a body

does not necessarily mean that it is at a higher temperature. Higher the average kinetic

energy, higher is the temperature of the body.

The difference between heat and temperature can be given as below:

Heat Temperature

1. It is a form of energy that gives the 1. It is the degree of hotness and coldness

sensation of warmth. of a body.

2. It measures the total kinetic energy of 2. It measures the average kinetic energy

all the molecules of a body. of all the molecules of a body.

3. It is cause of change in temperature. 3. It is the effect of heat.

4. Heat flows from a hot body to the cold 4. Temperature indicates the direction of

body. flow of heat.

5. Its SI unit is joule. 5. Its SI unit is kelvin.

82 Modern Graded Science and Environment Book 8

Thermometer

We can get a rough idea about the degree of hotness of hot water or coldness of ice by
touching. But our sense of touching does not say exactly how hot the water or how cold
the ice is. It also depends on the thermal condition of our body. Therefore, we need a
certain device to find the temperature of a substance.

A device which is used for measuring the temperature of a body is called a thermometer.
The working of thermometer is based on the principle that “when a body is heated, it
expands and it contracts on cooling." Generally, a liquid or a gas is used in a thermometer.
The volume of the liquid or the gas increases on heating. Solids are not used because
they expand very little with the rise in temperature. Some commonly used thermometers
are liquid thermometer, gas thermometer, radiation thermometer, thermoelectric
thermometer and resistance thermometer. Out of these thermometers, the commonly
used thermometer is a liquid thermometer.

Liquid thermometers are used to measure the temperature of human body, room
temperature, temperature of different bodies, maximum and minimum temperatures of
a given region, etc.

Liquid thermometers

Liquid thermometers are based on the principle that "increase in the volume of a
liquid is directly proportional to the rise in temperature" (thermal expansion of liquids).
The commonly used liquids in the liquid thermometer are alcohol and mercury.
So, alcohol and mercury are called thermometric liquids.

a. Mercury

Mercury is a thermometric liquid. It is used in thermometers because it has the following
properties:

1. It is a good conductor of heat.
2. It has a uniform rate of expansion and contraction.
3. It is silvery white in colour, therefore, it can be seen easily in glass.
4. Mercury does not wet glass. So, the rise and the fall of the mercury in a tube

are clean and smooth.
5. Mercury requires only a small amount of heat to expand.
6. Mercury remains in a liquid state over a quite wide range of temperature

because it freezes at –39 °C and boils at 357 °C.

A mercury thermometer cannot be used for measuring the temperature below –39 °C
because it freezes at this temperature. Therefore, mercury thermometers cannot be
used to measure the temperature in very cold regions. It is the disadvantage of the use
of mercury in a thermometer.

Heat 83

b. Alcohol

Alcohol is also used as a thermometric liquid as it has the following properties:
1. The freezing point of alcohol is –117 °C. Thus, it remains liquid to a very
low temperature. Due to this reason, an alcohol thermometer is used for
measuring the temperature in very cold regions.
2. Its expansion rate is about seven times more than that of mercury, which
makes the thermometer more sensitive.

The disadvantages of the use of alcohol as a thermometric liquid in a thermometer are
as follows:

1. It is colourless. Hence, it should be coloured before use in order to see it easily.
2. It is a bad conductor of heat.
3. It wets the wall of the capillary of the thermometer.
4. Its expansion rate is non-uniform.
5. Its boiling point is 78 °C. So, it cannot be used for measuring the temperature

above 78 °C. Due to this reason, an alcohol thermometer is not used to
measure temperature in very hot places.

Why is water not used as a thermometric liquid?

Theoretically, water can be used in a thermometer as it expands on heating and contracts
on cooling. It has the following drawbacks:

1. It is colourless, so it should be coloured before use in order to see it easily.
2. Pure water is a bad conductor of heat.
3. It wets the wall of the capillary of the thermometer.
4. Its expansion rate is non-uniform.
5. It vaporizes easily.
Because of these reasons, water is not used as a thermometric liquid.

Construction of liquid thermometer

Constructing a mercury thermometer:
1. A glass tube having a uniform fine bore is taken.
2. A bulb is formed at one end of the glass tube by heating and blowing gently
while its other end is left open.
3. The bulb and the fine bore are filled with mercury and the open end is sealed.
4. The lower fixed point and the upper fixed point are determined to make a
scale on it. The process is called calibration of thermometer.

Calibration of thermometer

After the construction of a thermometer, we have to make a scale on it. To make a scale on a
newly constructed thermometer, lower fixed point and upper fixed points have to be determined.

84 Modern Graded Science and Environment Book 8

The process is called calibration of thermometer. The distance between the upper fixed point
and the lower fixed point is devided into a number of equal parts, depending upon the type of
the chosen temperature scale. In this way, a scale is made on a newly constructed thermometer.

Lower fixed point: It is the temperature of the pure melting ice at normal pressure
( 760 mm of Hg). It is 0 °C in a celsius scale.

Upper fixed point: It is the temperature of the pure boiling water or steam at normal
pressure (760 mm of Hg). It is 100 °C in a celsius scale.

a. Lower fixed point Do you know?

The temperature at which water freezes According to NASA, when the
at sea level is called the lower fixed point. temperature reaches 95 °F our work
Its value is 0 ºC. output drops by 45 percent.

Activity 6.4

To determine the lower fixed point

Materials required

a retort stand, a funnel, a thermometer, a beaker, a stopwatch and some pieces of ice

Procedure stand

1. Take some pieces of ice in a funnel. thermometer

2. Fix the funnel in a stand as shown in the figure. lower fixed
point (0°C)
3. Insert the thermometer in the ice so that its
bulb is covered with ice. funnel
melting ice
4. Place an empty beaker below the funnel.

Observation beaker
water
The level of the mercury in the thermometer begins
to decrease. After some time, the level of the mercury
becomes constant.

Conclusion Fig 6.3

The constant temperature at a point on the scale of the thermometer gives the lower fixed

point. It means the level of the mercury in the thermometer is corresponding to 0°C ice.

Precautions

The bulb of the thermometer must touch the ice otherwise the temperature recorded by
the thermometer is higher than the temperature of the ice.

Heat 85

b. Upper fixed point

The temperature at which water boils at sea level is called the upper fixed point. At sea
level, its value is 100ºC.

Activity 6.5

To determine the upper fixed point

Materials required

a retort stand, a round-bottomed flask, a tripod stand, burner or spirit lamp, a wire gauze,
a thermometer, an L-shaped glass tube, some water and a cork

Procedure thermometer
upper fixed
1. Fill about half of the round-bottomed point (100°C)
flask with water. steam

2. Fix a thermometer and an L-shaped round bottom
glass tube on the mouth of the flask
round-bottomed flask by using a
cork as shown in the figure.

3. Keep the flask over a tripod stand.

4. Heat the flask with a spirit lamp till water
the steam is generated.

5. Heat the water for few minutes even wire gauze
after boiling. tripod stand

Observation spirit lamp

The level of the mercury in the

thermometer begins to rise. It is due to Fig 6.4
the expansion of the mercury in the bulb.

After some time, the level of the mercury becomes steady and fixed on further heating.

The point is marked.

Conclusion
The point of the thermometer gives the upper fixed point i.e. the level of mercury in the
thermometer is corresponding to 100 ºC water at the standard atmospheric pressure.

Precaution

The bulb of the thermometer should be above the level of the water in the flask, otherwise,
it will not give the temperature of the steam.

86 Modern Graded Science and Environment Book 8

Temperature scale

In general, there are three scales of temperature as given
below:

1. Centigrade scale 2. Fahrenheit scale
3. Kelvin scale

1. Centigrade scale

In this scale, the lower fixed point is 0 °C and the upper
fixed point is 100 °C. The interval between these two fixed
points is divided into 100 equal parts. Each part is called
one degree centigrade. It is denoted by 1 °C.

2. Fahrenheit scale

In this scale, the lower fixed point is

32 °F and the upper fixed point is Lower fixed point : 0 ºC = 32 ºF = 273 K

212 °F. The interval between these Upper fixed point : 100 ºC = 212 ºF = 373 K

two upper and lower fixed points is Fig 6.5

divided into 180 equal divisions. Each of the divisions is called one degree fahrenheit. It

is denoted by 1 °F.

3. Kelvin scale

In this scale, the lower fixed point is 273 K and the upper fixed point is 373 K. The interval
between these upper and lower fixed points is divided into 100 equal divisions. Each of
these divisions is called one kelvin. It is denoted by 1 K.

Relation between different temperature scales

We have noticed that the upper fixed point and the lower fixed point are equal to each of
the temperature scales such as centigrade scale, kelvin scale and fahrenheit scale. The
only difference is that the interval between these two fixed points is different for different
temperature scales.

If C, F and K are the readings on centigrade, fahrenheit and kelvin scales, the following

rule holds : C–0 F – 32 K – 273
100 180 100
= =

Types of liquid thermometer

Liquid thermometers are of many types. Some of the main types of them are
described below:

1. Clinical thermometer 2. Laboratory thermometer


Heat 87

1. Clinical thermometer

The mercury thermometer used to measure the temperature of the human body is called
a clinical thermometer. It is also called doctor's thermometer.

A clinical thermometer consists of a thick-walled glass capillary tube called a stem. A
thin-walled cylindrical bulb filled with mercury is provided at one end of the stem. The
capillary tube is sealed at the other end. The stem is graduated from 35 °C to 42 °C in
degree Celsius and from 94 °F to 108 °F in the Fahrenheit scale. There is a mark at
37 °C in the Celsius scale and 98.6 °F in the Fahrenheit scale which indicates the normal
human body temperature. Just above the bulb, there is a small constriction or kink in
the stem. The shape of the stem of the thermometer is made prismatic. This gives a
magnified view of the mercury inside the capillary.

bulb constriction/kink scale

°C
35 36 37 38 39 40 41 42

Fig 6.6 clinical thermometer

To measure the temperature of the human body, the bulb of the thermometer is placed
either under the tongue or in the armpit for about two minutes. Then the mercury in the
bulb expands and the level of mercury rises up in the stem through the constriction.
When the mercury in the stem cools, it cannot run back into the bulb because of the kink
or constriction. Due to this reason, the level of mercury in the column remains constant
and a reading can be noted accurately even after some time. To take the mercury back
into the bulb, it has to be jerked gently. Then it is kept safely for future use.

Nowadays, a digital thermometer is also used to measure the temperature. In a digital
thermometer, thermometric liquids such as mercury and alcohol are not used.

2. Laboratory thermometer

It is a simple type of mercury thermometer used for measuring the temperature of different
bodies in a laboratory. It is also called a simple thermometer. The temperature scale has
a wide range from -10 °C to 110 °C in Celsius scale. It consists of a thick walled glass
capillary tube called stem. A thin walled cylindrical bulb filled with mercury or alcohol is
provided at one end of the stem.

bulb mercury scale

-10 0 10 20 30 40 50 60 70 80 90 100 110°C

Fig 6.7 laboratory thermometer

The stem is graduated in degree from -10 °C to 110 °C. When the bulb is placed in contact
with a hot body, the volume of the mercury in the bulb expands and hence, the level of
mercury begins to rise because of the expansion of the mercury. When the bulb is placed in
contact with a cold body, the level of the mercury in the thermometer begins to fall because
of the contraction of the mercury. Thus, the temperature of a body can be measured by
keeping it in contact with the bulb of the thermometer. It is cylindrical in shape.

88 Modern Graded Science and Environment Book 8

Difference between clinical thermometer and laboratory thermometer

Clinical thermometer Laboratory thermometer

1. It is used to measure the temperature 1. It is used to measure the temperature of

of human body. different objects.

2. It is short with a prismatic structure. 2. It is long with a round and cylindrical structure.

3. The temperature scale ranges between 3. The temperature scale ranges between

35 °C to 42 °C in celsius scale. -10 °C to 110 °C in celsius scale.

4. It has a kink near the bulb. 4. It does not have a kink near the bulb.

5. Its bulb is filled with mercury. 5. Its bulb may have mercury or alcohol.

Temperature of human body

The temperature of a healthy person is 98.6 °F or 37 °C. If you sit in a room at
45 °C, what do you feel? You feel warmer. This is because you are receiving heat from
the surroundings continuously. Although you are receiving heat continuously, our body
temperature is fixed at 37 °C. This implies that there are some mechanisms occurring
inside our body which are responsible for the constant temperature of our body. Certainly,
we are losing heat by sweating.

Similarly, if we sit in a room at temperature 8 °C, what do we feel? We feel cold. This is
because we are losing heat continuously to the surroundings. Our clothes play a vital role
to produce heat and minimize the loss of heat from our body. Heat is always generated
inside our body. For example, if we run or do physical work like digging a field or physical
exercises, etc. we feel warmer.

Solved numerical problems

1. On a summer day, the maximum temperature recorded at your school is
42 °C. Convert it into the fahrenheit scale.

We have,

C–0 = F – 32
100 180

If C = 42 °C, then

42 = F – 32
100 180

or, F - 32 = 42 ×180
100

or, F = ( 42 ×18) + 32 = 107.6 ºF.
10

Thus, 42 °C is equivalent to 107.6 °F

Heat 89

2. The temperature of the Kathmandu valley varies from 18 °C to 32 °C. Express
the temperature into the kelvin scale.

We have,

C–0 = K – 273
100 100

or, C = K - 273
or K = C + 273, then

For C = 18 °C, K = 18 + 273 = 291

For C = 32 °C, K = 32 + 273 = 305

Thus, kelvin temperature corresponding to 18 °C and 32 °C are 291 K and 305 K respectively.

3. At what temperature do the centigrade and the fahrenheit thermometers
record the equal temperature?

We have,

or, C–0 = F – 32
100 180

or, C = F – 32
10 180

We assume, C = F = x

 1X0 = x – 32
18

or, 18 x = 10 x - 320

or, 8 x = -320

or, x = - 40

Hence, the centigrade and the fahrenheit thermometer record the same temperature
at - 40 °C or - 40 °F.

THINGS TO KNOW

1. Heat is a form of energy which flows from a hotter body to a colder body. It
gives the sensation of warmth.

2. One calorie is the amount of heat required to raise the temperature of one
gram of water by 1 °C.

3. Temperature is the degree of hotness or coldness of a body.

4. An instrument which is used to measure the temperature of a body is called
a thermometer.

5. Alcohol and mercury are the commonly used thermometric liquids.

90 Modern Graded Science and Environment Book 8

6. The main temperature scales are centigrade scale, fahrenheit scale, and
Kelvin scale. Generally, celsius and fahrenheit scales are used to measure
temperature.

7. Relation between different temperature scales is:

C–0 = F – 32 = K – 273
100 180 100

8. A clinical thermometer is graduated from 35 °C to 42 °C.

9. Normal human body temperature is 37 ºC or 98.6 ºF.

10. A human body loses heat by sweating and generates heat by different
mechanisms inside the body. The rate of heat loss and generation is adjusted is
such a way that the body temperature remains constant at 98.6 °F (or, 37 °C).

THINGS TO DO

1. Take two tumblers identical in size: one of steel and other of glass. Put equal
amount of hot water in these tumblers. Record the temperature of the water
in both glasses after five minutes. Is there any difference in the temperature?
Discuss the change in temperature and its reason with your friends.

2. Take a clinical thermometer and measure the temperature of your body.

TEST YOURSELF

1. Fill in the blanks.
a. Normal human body temperature is ..................... .
b. Heat energy flows from a hotter body to a ............. body.
c. The temperature of pure melting ice at sea level is ..................... .
d. At sea-level, the upper fixed point of a thermometer is ..................... ºC.
e. Alcohol and mercury are ..................... liquids.

2. Tick (√) the correct statements and cross (X) the incorrect ones.

a. The SI unit of temperature is kelvin.

b. Mercury is used as thermometric liquid in clinical thermometer.

c. The boiling point of the alcohol is 100 °C.

d. One jule is equal to 4.2 calorie.

e. Centigrade and fahrenheit are equal at 0 °C.

3. Tick () the correct answer (MCQs).

a. Generally, an alcohol thermometer is used to measure temperature:

i. in cold places ii. in hot places iii. of solid bodies iv. of gases

b. The melting point of pure ice at standard atmospheric pressure is:

i. 100 °C ii. 0 °C iii. 32 °C iv. 212 °C

Heat 91

c. The boiling point of pure water at standard atmospheric pressure is:

i. 100 °C ii. 0 °C iii. 32 °C iv. 212 °C

d. The boiling point of mercury is:

i. 100 °C ii. 78 °C iii. 212 °C iv. 37 °C

e. The normal human body temperature in fahrenheit scale is:

i. 78 ii. 98.6 iii. 100 iv. 273

f. What is the SI unit of heat?

i. fahrenheit ii. calorie iii. joule iv. kelvin

g. Heat produces the sensation of:

i. coldness ii. vision iii. hearing iv. warmth

4. Give reasons.

a. An alcohol thermometer is preferred for a very cold region.
b. Temperature of boiling water cannot be measured using an alcohol

thermometer.
c. Constriction is made in a clinical thermometer.
d. A clinical thermometer is jerked before using it.
e. Water is not used as a thermometric liquid.

5. Differentiate between: b. laboratory and clinical thermometers
a. heat and temperature

6. Answer these questions.
a. What is 1 calorie heat?

b. What are the melting and boiling points of mercury?

c. Write the function of alcohol in a alcohol thermometer.

d. Where is a alcohol thermometer used for?

e. On which principle is a thermometer based?

f. If you have to measure the temperature above 80 ºC, do you use an alcohol
or mercury thermometer? Why?

g. What are the advantages of using alcohol instead of mercury in a thermometer?

h. What are the disadvantages of using alcohol instead of mercury in a thermometer?

i. What is the difference between a centigrade thermometer and a fahrenheit
thermometer?

j. What are the advantages of the use of mercury in a thermometer?
k. Give the relationship among centigrade, fahrenheit and kelvin scales.
l. Write the importance of thermometers.
m. A mercury thermometer is used to measure the temperature of boiling water.

But, why do you think mountaineers travelling to cold regions use alcohol
thermometer?

92 Modern Graded Science and Environment Book 8

n. Among one litre boiling water and a red hot glowing nail, which one has more
amount of heat and why?

o. On what factors does the quantity of heat of an object depend?

7. Diagrammatic Questions:
a. Describe a clinical thermometer with a labelled diagram.
b. Draw a neat and labelled diagram of a laboratory thermometer.
c. Write the names of the given thermometers: A and B. Also answer the
following questions.

°C
35 36 37 38 39 40 41 42

A

-10 0 10 20 30 40 50 60 70 80 90 100 110°C

B

i. Which thermometer do you prefer to measure the temperature of
boiling water?

ii. Which thermometer do you prefer to measure the temperature of your
body?

8. Numerical problems:

a. Convert 300 K into the celsius scale.
b. Convert 220 °C into kelvin scale.
c. Convert 30 °C into fahrenheit scale.
d. Convert 260 ºF into ºC.

Answers

8. a. 27 °C b. 493 K c. 86 °F d. 12.67 °C

GLOSSARY

Judgement : the ability to make a sensible decision after consideration
Vibrate : to move or make something move from side to side very

Kinetic energy quickly
Density : the energy possessed by a body by virtue of its motion
Conductor : the mass per unit volume
Prismatic : the substance that conducts electricity
: using or containing the shape of a prism

Heat 93

7Lesson LIGHT

Total Estimated Pds: 6 [Th. 5 + Pr. 1]

On completion of this lesson, the students will be able to:

define mirror and describe its types, properties and uses.
draw the ray diagrams of an image formed by spherical mirrors.
introduce real and virtual images and differentiate between them.
tell the uses of concave and convex mirrors.
define refraction of light and state its laws.

Light is a form of energy which produces the sensation of sight. We can see any object
only when the light from the object enters our eyes. This is how reading a book is possible.
The objects kept in a dark room cannot be seen because no light from these objects
enters our eyes. If an iron rod is heated, it becomes red hot and emits light. This shows
that heat energy is converted into light. Thus, light is a form of energy.

Ray and beam of light

Light travels in a straight line. The term 'ray' refers to a single Fig 7.1 a ray of light
narrow path of light. It is represented by a straight line with an
arrowhead as shown in the figure. The arrowhead shows the
direction in which the light is travelling.

A collection of the rays of light is called a beam of light. It is of three types: parallel,
convergent and divergent.

The beam of light in which all the rays are parallel to each other is called a parallel beam.

The beam of light coming from distant sources such as the sun and stars is a parallel

beam.

The beam of light in which A

the rays of light meet at a FC P Q
point is called convergent focus
beam. The beam of light
reflected by a concave F
O

mirror is a convergent B
(c) divergent beam
beam. Fig 7.2 (a) parallel beam (b) convergent beam

94 Modern Graded Science and Environment Book 8

The beam of light in which the rays of light scatter from a point is called a divergent beam.
The beam of light reflected from a convex mirror is a divergent beam. The beam of light from
an electric bulb, tube light, candle, torch, etc. are other examples of divergent beam.

Real and virtual image

An image that can be obtained on a screen is called a real image. It is formed by the
actual intersection of the reflected or refracted rays. A concave mirror usually forms a
real image.

An image that cannot be obtained on a screen is called a virtual image. Generally, a
plane mirror and a convex mirror form a virtual image. It is formed by the apparent
intersection of rays.

Activity 7.1

Take a concave mirror and turn it out of the window. Place a thick white paper in front
of the mirror at the distance of about 25 cm. Adjust the paper and the mirror in such a
way that all the reflected rays are incident in the paper. Move the mirror back and forth
until the image of everything outside is seen clearly in the paper. At this point, measure
the distance between the mirror and the paper. This distance is called focal length. In
this case, the image formed by the concave mirror in the paper is a real image. Now,
swap the concave mirror with a convex mirror and a plane mirror and repeat the same
procedure. Is it possible to form the image?

If an image can be seen on the screen, it is a real image. This image is formed due to
the intersection of reflected rays. If an image cannot be seen on the screen, it is a virtual
image. Now, it seems as if the reflected rays intersected but in reality they do not.

The image formed by a concave mirror can be real. The image formed by convex and
plane mirrors cannot be real but it can be virtual.

Difference between real image and virtual image

Real image Virtual image

1. It is formed at the point where the 1. It is formed at the point where the

reflected rays meet. reflected rays appear to meet after

diverging.

2. It is always inverted. 2. It is always erect.

3. It is always formed in front of the mirror. 3. It is always formed behind the mirror.

4. Its size depends on the distance of the 4. Its size is larger in a concave mirror and

object from the mirror. smaller in a convex mirror.

5. It can be obtained on the screen. 5. It cannot be obtained on the screen.

Light 95

Mirror Do you know?

A Mirror has a surface which reflects a large fraction Optics is the study of light.
of incident light and forms the image of an object.

Mirrors are made up of glass having a polished

surface. The quality of the image formed depends on the smoothness of the surface.

For example, we can see our image in a new steel utensil. In general, mirrors are of two

types: plane mirror and spherical mirror.

A. Plane mirror

A plane mirror is a type of mirror whose reflecting surface is
flat. The shaded part in the figure shows the polished surface
of the mirror from which no reflection takes place.

It forms a virtual erect image in side the mirror at an equal reflecting polished
distance. Its size is equal to the size of the object. surface surface

Activity 7.2 Fig 7.3 plane mirror

Take a plane mirror and hang it on the wall. Stand in front of it and look at yourself in
the mirror. How does your image look like? When you move aside from the mirror, does
the image in the mirror move? Then move a little forward. Where does the image move?
Now, rise up your right hand and look at the mirror. Which hand is seen to be raised in
the mirror and why does it happen?

Characteristics of image formed by a plane mirror

1. The image is erect and virtual.

Let an object PO be placed in front of a plane mirror as Plane mirror
shown in the figure. A ray of light OA is incident normally
on the surface of the plane mirror and then it returns O A I
along AO. Another ray OB coming from the same point Object B Image
O is reflected along BC. When these two reflected rays
AO and BC, are produced backwards, they meet at P P'
I at the back of the mirror. The reflected rays AO and
BC do not actually meet, but only appear to meet when C Fig 7.4
produced. Hence, I is the virtual image of the point O
and P'I is the virtual image of the object PO.

2. The image is of the same size as the object.

As in the figure, if the height of the object PO is 4 cm, the height of its image P'I is also 4 cm.

3. The object distance is equal to the image distance.

The distance between a plane mirror and the object is called object distance. The
distance between the plane mirror and the image is called image distance. In the figure,

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