HPE-book-7-final-for-press

Think and Discuss

1. Discuss why scissors for cutting cloth may have blades longer than the handles,

but scissors for cutting metals have short blades and longer handles.

2. In the figure alongside (i) and

(ii), which one will you prefer to hh

roll the load upto the height and

why?

3. Explain why "steeper the inclined (i) (ii)

plane, greater is the effort required to push a load up.

Project work

• Collect the picture of ten different simple machine used in your house and

classify them into different types of simple machine.

• Make a model of pulley by using the lid of jam bottle, some paper, thread, a

thin nail to acts as axle.

• Take a pair of scissors, a pair of tin cutter and think of a sea saw. How are their effort arms

different from each other? Write a reform about it, taking help of your teachers.

• Draw pictures of various appliances you come across to work as lever. Mark the positions of

fulcrum, effort point and load point in each. Draw at least 10 to 12 such appliances.

Science and Environment Book - 7 47

4Lesson Pressure

Specific After the completion of the lesson, students will be able to:
Objectives
• define pressure and state its unit.
• give some applications of pressure.
• explore the components that affects the pressure.
• solve numerical problems related to pressure.

Preliminary Syllabus
Concept
• Pressure and its unit
Till now, we discussed the effect of application of force
on an object. But, can we measure these effect? The • Simple numerical
effect of the force on any object depends on two factors:
the amount of force applied and the area on which it problem related to
is applied. More the force, the more will be the effect. F
Lesser the area of contact, more will be the effect. P= A

Have you ever thought why the sharpened edge of a • Difference between
pencil hurts you more than a blunt one? Why we use
a sharp knife to cut vegetables? In case of the sharp force and pressure.
pointed end of a pencil or a sharp knife, force is applied
on a very small area and therefore the effects become • Application of pressure
more severe. To measure the effect of the force, we
measure the pressure. in daily life.

48 Science and Environment Book - 7

Do you know?Introduction

Try to press your wrist with a nib of pen, now press it with the opposite end
or the blunt end of the same pen. What difference did you feel? You felt more
pain while pressing with the nib of the pen than by the blunt side, isn't it?

Similarly, you can easily cut the vegetables with a sharp knife but it is difficult
to cut vegetables with a blunt knife. Why is it so? The effect of the applied force
depends upon the amount of force and the area of the application of the force.
The effect increases with the increase of force but decreases with the increase
of area. Since, the blunt end of a pen and a blunt knife have greater area than
the sharp one. So they have less effect even if we apply same amount of force.
In other words, they exert less pressure.

Let us perform an activity to see the relationship between the effect of force
and the area of contact.

Activity 1

To observe the relationship between the effect of force and the area of
contact.
Material required: Pot, soil and sharpened pencil.
Procedures:
1. Take a pot filled with soil.
2. Take a pencil and push it in the soil from its unsharpened end.
3. Now take a sharpened pencil and push it in the soil from its sharpened

end.
4. What do you observe? Which one is easier to push into the soil?
Observation: It is much easier to push the sharpened pencil than the blunt
one.

High heels concentrate a large amount of force
into a small area. The great pressure transmitted
through such heel can make you feel uncomfortable
while walking on soft ground.

Science and Environment Book - 7 49

Thus, pressure is defined as the force acting per unit area.

Mathematically,

Pressure (P) = Force (F)
Area (A)

\ P= F
A

Since, the unit of force is newton and the unit of area is m2. So, the unit of
pressure comes to be newton per square meter (N/m2), which we called

pascal (Pa) named after a French polymath Blaise Pascal. Thus, one pascal is

the pressure applied when force of one newton acts on an area of one square
metre (1 Pa = 1N/m2).

Some other units of pressure are mm of Hg, atmospheric pressure, etc.

Factors affecting the pressure

1. Force: The pressure is directly proportional to the force applied. i.e. as
you increase the force, the pressure also increases in the same proportion.
If you doubled the force, the pressure is also doubled and if you reduce
force by half, the pressure will also reduce by half.

2. Area: Pressure is inversely proportional to the area i.e. the pressure
decreases if you increase the area and vice-versa. The same amount of
force exerts more pressure if applied in a small area than in a large area.

Solved Numerical Problems

Example 1: If an object of 500 N occupies the area of 10 m2. Calculate the
pressure exerted by the object.

Solution:

Force (F) = 500 N
Area (A) = 10 m2

Pressure (P) = ?

Now, we know

Pressure (P) = Force (F)
Area (A)

= 500
10

= 50Pa

Thus, the pressure exerted by an object is 50 Pa.

50 Science and Environment Book - 7

Example 2: If a boy applies a force of 16N on an area of 16 cm2, what will be
the pressure exerted by him?

Solution:

Force (F) = 16 N

Area (A) = 16 cm2 = 16 = 0.0016m2
10,000
Pressure (P) = ?

Now, we have,

Pressure (P) = Force (F) = 16
Area (A) = 10,000 Pa.

0.0016

Thus, the pressure excreted by a boy is 10,000 Pa.

Example 3: A man exerts a pressure of 14 Pa by applying a force of 42 N on a
box. Calculate the area over which the force is applied.

Solution:

Pressure (P) = 14 Pa

Force (A) = 42 N

Area (A) = ?

Now, we have,

Pressure (P) = Force (F)
Area (A)

or, Area = Force = 42

Pressure 14

Area = 3 m2
So, the pressure is applied on 3 m2 area.

Example 4: Calculate the force applied on an area of 1360 cm2 to exert a
pressure of 17 Pa.

Solution:

Pressure (P) = 17 Pa

Area (A) = 1360 cm2 = 1360 = 0.136m2
10,000
Force (F) = ?

Pressure (P) = Force (F)
Area (A)
We have,

Science and Environment Book - 7 51

Force = Pressure × Area
= 17 × 0.136
= 2.312 N
Thus, the force applied is 2.312 N.

Difference between Force and Pressure

Force Pressure
1. It is an external agency that
1. It is the force acting per unit
changes or tries to change the area.
state of a body.
2. Force is measured in newton. 2. Pressure is measured in Newton
per square metre (N/m2) or
3. It is measured by the relationship,
Force (F) = Mass (m) ×acceleration (a) pascal (Pa).

3. It is measured by the

relationship,

Pressure (P) = Force (F)
Area (A)

Applications of pressure in our daily life

1. We use sharp objects for cutting. For example; knife to cut vegetable, axe
to cut wood, etc. The sharp edge of these articles exert more pressure on
the things and make it easier to cut.

2. The heavy trucks are fitted with double wheels so that they cover large
area and thus exert less pressure on the road. Similarly, the tractors have
wider and bigger wheels to prevent them from being stuck in the mud due
to high pressure.

3. The foundation of the building is made wider than the walls because it is
the one which supports the load of the whole building. Because of its wide
area, it exerts less pressure and hence building is stable.

4. The feet of the camel are flat and broad, so they occupy more area. Hence,
they exert low pressure and can walk easily on the sand.

5. Footballers have studs in their boot which increases the pressure due to
less low surface area. So, the players do not slip in the slippery field.

6. School bags and shopping bags have broad straps or belts so that the area
of contact increases and thus, the pressure on the hand is reduced.

52 Science and Environment Book - 7

Points to Remember

1. The force acting per unit area is called pressure.
2. The SI unit of pressure is N/m2 or pascal (Pa).
3. Pressure increases with the increase in force applied.
4. Pressure decrease with the increase in surface area.
5. 1 pascal pressure can be defined as the pressure exerted by 1 N force

in 1m2 surface area.
6. Pressure plays a significant role in our day-to-day activities.

Exercise

1. Choose the best answer from the given alternatives

a. What is the SI unit of pressure?

i. newton ii. watt iii. pascal iv. joule

b. Which of the following statement is true?

i. Pressure increases with increase in force.

ii. Pressure decreases with increase in force.

iii. Pressure increases with increase in area.

iv. None of them.

c. As the area increase, the pressure______.

i. increases ii. decreases

iii. remains constant iv. may increase or decrease.

d. What pressure is exerted by 50 N body in 5 m2 area ?

i. 10 Pa ii. 5 Pa iii. 250 Pa iii. 15 N

Science and Environment Book - 7 53

2. Fill in the blanks with correct words.
a. The_______ acting per unit area is called pressure.
b. The pressure is inversely proportional to________.
c. The______ increases with the increase in force.
d. Football boots have studs to________ pressure on the ground.
e. Heavy trucks are fitted with double wheel to________ pressure on the
road.
f. The tip of a nail is made _____ to make it easy to drill into the wall.

3. Put a tick (√) for the correct statement and a cross (×) for the incorrect
one.
a. Pressure does not have any unit.
b. The SI unit of pressure is newton.
c. Pressure depends only on the force applied.
d. A sharp knife cuts easily than a blunt one.
e. Tractors have wider wheels to increase the pressure on the ground.
f. A thin strap exerts more pressure on the shoulder of the carrier.

4. Answer the following questions.
a. Define pressure and write its SI unit.
b. Write the formula to calculate pressure.
b. Name the factors on which the pressure depends.
c. Give two examples in which pressure is increased by reducing the
surface area of contact.
d. When the force applied on an object is doubled, how does the pressure
exerted on the object change?
e. How is pressure related to the area?
f. Write the differences between force and pressure.
g. Write any three applications of the pressure in our daily life.

5. Give reasons of the followings:
a. It is easier to cut vegetables with a sharp knife than a blunt one.
b. The boots of footballers have studs in it.

54 Science and Environment Book - 7

c. The tractors have wide tyres.
d. A camel can walk easily in desert.

6. Now compare a lady standing on the pointed heel and the elephant on
one foot. Find out

a. Who exerts greater force on the
ground, the lady or the elephant?

b. Who is likely to leave a hole in the
ground, the lady standing or the
elephant's foot and why?

7. Solve the following numerical problems.

a. A force of 350 N is acting upon a 50 m2 surface area. Calculate the

pressure on the ground. [Ans: 7 Pa]

b. A solid of weight 80 N when placed on a surface, the area of contact
is found to be 1.6m2. Find the pressure exerted by the solid on the

surface. [Ans: 50 Pa]

c. If an object of 650 N exerts 13Pa of pressure on the ground. What is the

surface area covered by the object? [Ans: 50 m2]

d. A force of 100 N can produce a pressure of 100,000 Pa. Calculate the

area in cm2 on which the force acts. [Ans: 10 cm2]

e. If an object covering 9m2 surface area exerts 70Pa pressure on the

ground. Find out the force. [Ans: 630 N]

f. A solid weigh 200N. What will be the pressure exerted by the solid
when it is kept on a glass surface having an area of contact 500 cm2.

[Ans: 4000 Pa]

Project work

• Collect different objects of different mass and area and keep them on your

hand. Observe the pressure exerted by different objects.

Science and Environment Book - 7 55

5Lesson Work, Energy
and Power

Specific After the completion of the lesson, students will able to:
Objectives
• define work, energy and power with their units.
• define transformation of energy.
• explain in short about the different forms of energy.
• solve numerical problems related to work, energy and power.

Preliminary Syllabus
Concept
• Energy and its types
We do different types of work in our everyday life. We play, carry our school • Mechanical energy
bags, pedal a cycle, go for jogging and so on. These are few examples of • Chemical energy
physical work. We may read, think, solve problems, etc. These are few • Heat energy
examples of mental work. • Light energy
• Sound energy
In physics, we recognize only physical or mechanical work and that too if • Electrical energy
there is some displacement i.e. the body has to be moved from one place • Magnetic energy
to another place. In science, we do not recognise mental work as work • Nuclear energy
done, although it also consumes energy.
• Work and its type
If a boy, while playing football runs about all over the field or if he cycles to • Work done against
some distance, he gets tired. He has used something to do these things. If gravity
he had stayed at home and sat down at a place, he would not have felt so • Work done against
tired. What has he used to do these things? He has used his energy. friction

To do more of work, we need more energy. After doing sufficient amount
of work, one feels tired because a lot of energy has been spent during this
process. Hence, we say, there is a direct relationship between work and
energy. The energy is the cause and work is its effect. The unit of energy is
joule (J) which is the same as that of work.

Suppose an old man takes one hour to do a particular work whereas a
young man takes half an hour to do the same work. So, we can conclude
that the power of the young man is double than that of old man. Ordinarily,
a person who has great physical strength is said to have great power. The
term "power" and "work" have its scientific meaning quite different from
what we take it to mean during our daily conversation.

56 Science and Environment Book - 7

Introduction

Work

Suppose you wish to move a wall by pushing it and
you apply a lot of force on it. However it does not
move an inch even after pushing it for hours. You get
exhausted and think that you have done lot of work on
the wall. But in terms of science, you have not done any work.

In science, work is said to be done only if the force applied on a body moves
it through some distance. For examples; moving a cart, kicking a football,
carrying a load and moving a certain distance, hitting a ball with a bat are
considered as work done.

Work done is defined as the product of force and displacement.

Mathematically,
Work done = Force × displacement

W=F×d

As SI unit of force is newton and Think and Discuss
displacement is metre. So, the SI unit
of work done is newton metre (Nm) 1. Is it work done when leaves fly in
which is simply called as joule. Its the wind?
CGS unit is erg.
2. Is it work done when you close
If you displace a body through 1m your eyelids?
distance applying 1 N force, then

Work done = F × d
= 1 N × 1m
= 1 Nm
= 1 Joule

Hence, 1 joule work is done if a force of 1 N displaces a body through 1 metre
distance in the direction of the force.

Science and Environment Book - 7 57

Types of work
There are two types of work.
1. Work done against friction

When a work is done by pulling or pushing against the
surface, it is called work done against friction. Pushing
a cart to a certain distance, walking, riding bicycle,
pulling a wooden log are some examples of work done
against friction.

2. Work done against gravity

The work done by displacing a body to opposite
direction of the gravity is called work done against
the gravity. When you are lifting something vertically
above the ground, you are doing the work against the
gravity. For example, lifting a load from the ground,
pulling water from the well, crane lifting a wooden log.
Work done against gravity (W) = Force × displacement
or, W = weight × height [Therefore, Force = weight, displacement = height]
Since, weight = mg; m = mass, g = acceleration due to gravity
Thus, W = mgh.
Thus, whenever work is done against gravity, the amount of work done is
equal to the product of weight of the body and the vertical distance through
which the body is lifted.

Solved Numerical Problems

Example 1: A force of 250 N is applied to displace a body through a distance
of 15m. Calculate the work done.
Solution:
Force applied (F) = 250 N
Displacement (d) = 15m
Work done (W) = ?

58 Science and Environment Book - 7

Here, we know = Force (F) × displacement (d)
Work done (W) = 250 × 15
= 3750 J


Thus, the amount of work done is 3750 J.

Example 2: A crane lifted a wooden log of 200 kg to a height of 25 m from the
earth surface. Calculate the work done against gravity. [g = 9.8m/s2]

Solution:

Mass of the log (m) = 200 kg

Height (h) = 25 m

Work done against gravity (W) = ?

Now, we have

W = mgh

= 200 × 9.8 × 25

= 490000 N

Hence, the work done against gravity is 490000N.

Energy

We need energy to do different work. We get the required energy from the
food we eat. Similarly, the vehicles also need energy to run. In this case, the
fuel we keep in it, burns to provide the necessary energy. The capacity to do
work is known as energy. It is a scalar quantity and is measured in joule in SI
system.

Types of energy

The most striking fact about energy is its diversity. Energy appears in many
forms. We will be discussing about few here.

Mechanical energy: The energy possessed by a body by virtue of its state of
rest or of motion is known as mechanical energy. It is basically the sum of two
other types of energy i.e. kinetic energy and potential energy.

Science and Environment Book - 7 59

a. Kinetic energy

The energy possessed by a body by the virtue of its motion is known as kinetic
energy. For example; a bullet fired from a gun, the flowing water, wind, moving
car, rolling football have kinetic energy. The kinetic energy depend upon the
mass (m) and velocity of the moving (v) body. For instance, an army throws a
bullet by his hands and asks his staff to catch it, he'll catch it easily, won't he?
But what if he fired it with a gun and asked him again to catch it? Will he be
able to catch it? Obviously no, because the bullet fired from the gun has more
velocity and thus possess more kinetic energy which can penetrate the body.

Kinetic energy can be calculated as K.E = 1 mv2
2

Where, m = mass of the moving object
v = velocity of the moving object

In the game of carrom, all the plastic coins are placed in the middle of the
carrom board and they are hit by a plastic striker. The coins move to some
distance due to the force of the striker. If more force is applied to the striker,
the coins move to a larger distance. So, this moving striker has kinetic energy.
From this we conclude that if the moving body has more speed, it will have
more kinetic energy.

You regularly play and watch cricket match. You observe that when a
batsman hits the ball with a little force, the ball goes near about to fetch him
one or two runs. But when he hits the ball with full force, it goes for a boundary
i.e. four or six runs.

So, we conclude that more the speed of hitting the ball, the higher is its kinetic
energy.

b. Potential energy

The energy possessed by a body by the virtue of its position is known as
potential energy. For example; water collected in dam, a stone at a height,
parked vehicle, stretched spring, leg lifted to kick football have the potential
energy which can perform work. The water stored in a dam when released can
rotate the turbine because of the potential energy stored in it.

60 Science and Environment Book - 7

The formula to calculate potential energy is P.E. = mgh

where, m = mass of a body

g = acceleration due to gravity

h = height from the surface of the ground

Activity

Place an ordinary coil spring on a table
and fix its lower end with the table top.
Now, place a small ball on it. You will
see that nothing happen to the ball. Now
press the ball and the spring with your
finger. As soon as you release the spring,
the ball will bounce away. Why ? Because
when the compressed spring is released,
its potential energy converts into kinetic energy of the ball. Thus, we say
that the compressed spring has potential energy.

Hence, we conclude that potential energy depends on the change in the
configuration or shape of the body.

Solved Numerical Problems

Example 3: Calculate the kinetic energy possessed by a body having mass of 5
kg moving with the velocity of 30 m/s.
Solution:
Mass of the body (m) = 5 kg
Velocity of the body (v) = 30 m/s

Science and Environment Book - 7 61

Kinetic energy (K.E) =?
Now, we have
Kinetic energy (K.E) = 1 mv2
2

= 1 × 5 × (30)2
2

= 1 × 5 × 900
2

= 2250 joule

\ The kinetic energy possessed by the body is 2250 joule.

Example 4: Calculate the potential energy of a body having a mass of 25 kg at
the height of 10 m from the ground. [g = 9.8 m/s2]

Solution

Mass of the body (m) = 25 kg

height (h) = 10 m

acceleration due to gravity (g) = 9.8 m/s2

Potential energy (P.E) = ?

We know,

Potential energy (P.E.) = mgh
= 25 × 9.8 × 10
= 2450 joule

\ The potential energy possessed by the body is 2450 joule

Chemical energy

The energy released due to the chemical reactions is known as chemical energy.
The food we eat contains chemical energy. So, it undergoes chemical reaction
called oxidation and releases energy in the form of heat energy which we can
utilize to perform various tasks. Similarly, the fuel used in the vehicles, dry
cells, wood, oil, coal, etc. have chemical energy stored in them.

Heat energy

Heat is the form of energy that gives us sensation of warmth. It is produced
due to vibration of the molecules of a body. Sun, burning coal, electric heater
are the sources of heat energy. The heat energy produced by burning fire is

62 Science and Environment Book - 7

used to cook food. The burning of petrol and diesel provides energy to run the
vehicles.

Light energy

Light is the form of energy that gives the sensation of vision and makes the
things visible. Sun is the major source of heat and light in earth. Beside candles,
lanterns, electric bulbs are the sources of the light energy.

Sound energy

Sound energy is produced due to vibration of a material medium. It gives us
sensation of hearing which is perceived by our ears. When sound is produced
by a vibrating body, our ear drum vibrate. Speaker, radio, television, musical
instruments, bell, horns of vehicles are some sources of sound energy.

Electric energy

The energy produced due to the continuous flow of electrons is known as
electric energy. Electrical energy can be converted into various types of energy
through various devices. A heater can convert it into heat energy, bulbs can
convert it into light energy, fan can convert it into mechanical energy. The
sources of electrical energy are cell, photocell, battery, generator, etc.

Magnetic energy

The energy possessed by a magnet is called magnetic energy. A magnet
attracts the magnetic substance by the virtue of its magnetic energy. It is used
to separate magnetic substance from the non-magnetic substances. It is also
used in the electrical appliances like radio, speaker, etc.

Nuclear energy

Nuclear energy is the energy released during nuclear fission or fusion. Atoms
are the tiny particles that make up every objects in the universe. There is
enormous energy in the bond that hold atoms together. The nuclear energy
can be released in two ways, nuclear fission and nuclear fusion. It can
produce large amount of heat and light energy. The sun produces energy by
nuclear fusion. It is also used to produce electricity and make atom bomb and
hydrogen bomb.

Science and Environment Book - 7 63

Transformation of energy

Transformation of energy is the process of changing energy from one form to
another form. Energy transformation occurs everywhere, every second of the
day. According to the law of conservation of energy, the energy can neither be
created nor be destroyed but it can be transformed from one form to another.
An electric bulb transforms electric energy into light and heat energy. A natural
gas converts chemical energy into heat energy. Our body converts the chemical
energy of food into mechanical energy to allow us to work.

When a toy car is operated, the chemical energy of the battery is converted into
electrical energy and again the electrical energy is transformed into kinetic
energy.

Chemical energy Electrical energy Kinetic energy

(in battery) (When switched on) (Motion of toy car)

Similarly,

Potential energy Kinetic energy electric energy

(water stored in dam) (when released rotates the turbine) (Used in electric appliances)

Power

Work is said to be done when a force causes a displacement. Work does not have
any relation with the amount of time that the force acts to cause displacement.
Sometimes the work is done quickly and sometime rather slowly.

To complete the same amount of work, one can take more time than the other.
Thus, power is the rate at which a certain amount of work is done.

Mathematically,

Power (P) = Work done (W)
Time taken (t)

Work is measured in joule and time is measured in second. So, the unit of

power is joule per second and is called watt. Thus, the SI unit of power is watt

which is denoted by the symbol W.

1 watt = 1 joule
1 second

So, a body is said to have power of 1 watt if it does work at the rate of 1 joule

in 1 second.

64 Science and Environment Book - 7

The larger amount of power is measured in terms of horse power. One horse
power is equivalent to approximatly 750 watt.

We can see an electric bulb marked 60 W or 20 W and so on. It denotes the
power of the bulb. If an electric bulb is labelled with 60W, it means that the
electric bulb converts 60 joule of electrical energy into heat and light energy in
1 second.

i.e. 60 W = 60 J
1sec

The power is measured in kilowatt and megawatt as well.

1000 watt = 1 kilowatt (kW)

1000 kW = 1 Megawatt (MW)

Solved Numerical Problems

Example 5: A man can cover a distance of 20m in 10 second carrying a load of
50 kg. Calculate his power.

Solution:

Mass (m) = 50 kg

Weight or force (F) = mg = 60 × 9.8 = 490 N

Distance (d) = 20 m

Time (t) = 10 sec

Power (P) =?

Now, we have = work done
Power (P) time taken

= F×d
t

= 490 × 20
10

= 980 watt

\ The power of the man is 980 watt.

Science and Environment Book - 7 65

Example 6: Shreya weighing 50 kg carries a bag weighing 10 kg to the top of a
tower 20m high in 30 second. Calculate her power. (g = 10 m/s2).

Solution:

Total mass (m) = 50 kg + 10 kg = 60 kg

height (h) = 20 m

acceleration due to gravity (g) = 10 m/s2

Then, we have

Power = W = mgh = 60 × 10 × 20 = 40 W
tt 30

Thus, the power of Shreya is 40 W.

Points to Remember

1. Work is said to be done if a force applied on a body moves it through
some distance in the direction of the force. It is measured by the
formula: W = F × d.

2. One joule work is said to be done if a force of one newton displaces a
body through a distance of one metre in its own direction.

3. Work done by pushing or pulling a body over a surface is known as
work done against friction.

4. Work done by lifting a body in opposite direction of the gravity is
called work done against gravity.

5. The capacity to do work is called energy.
6. The energy possessed by a body by the virtue of its motion is called

kinetic energy.
7. The energy possessed by a body by the virtue of its position is called

potential energy.
8. The energy can neither be created nor be destroyed but it can be

transformed from one form to another. This is known as 'Principle of
conservation of energy'.
9. The rate of doing work is called power. Its SI unit is watt.
10. A body is said to have to have 1 watt power if it does 1 joule work in
1 second.

66 Science and Environment Book - 7

Exercise

1. Choose the best answer from the given alternatives.

a. Work is measured by _______.

i. mass × velocity ii. force × distance

iii. mass × acceleration iv. force × velocity

b. According to science, which of the following is a work?

i. reading book ii. cooking food in standing position

iii. kicking a football iv. watching TV

c. Which of the following is the work done against gravity?

i. dragging a log ii. drawing water from well

iii. pushing a cart iv. cooking food

d. What is the SI unit of energy?

i. Newton ii. Joule iii. Watt iv. Dyne

e. What sort of energy is possessed by moving vehicle ?

i. kinetic energy ii. potential energy

iii. magnetic energy iv. nuclear energy

f. Our food consists _________.

i. kinetic energy ii. chemical energy

iii. light energy iv. magnetic energy

g. Power of automobiles are related in terms of_________.

i. horsepower ii. watt iii. joule iv. newton

2. Fill in the blanks with correct words.

a. The SI unit of work is ___________.

b. Pulling a cart through a certain distance is the work done against
__________ .

c. The capacity to do work is called __________ .

d. Kinetic energy is possessed by a body due to its state of __________ .

e. The water stored in a dam has __________ energy.

f. Battery and food consist of _______ energy.

g. The SI unit of power is __________ .

Science and Environment Book - 7 67

3. Tick (√) the correct statement and cross (×) the incorrect one.

a. Work is not said to be done unless you displace a body by applying
force on it.

b. Pulling water from the well is work done against gravity.

c. The rate of doing work is called energy.

d. The SI unit energy is watt.

e. Kinetic energy depends upon the velocity of the moving body.

f. Energy can be created and destroyed.

g. Power can be measured in Horse power as well.

4. Match the following:

Work done bread and cell

Kinetic energy tuning fork

Potential energy product of force and displacement

Sound energy rate of doing work

Chemical energy gun fired from bullet

Power a compressed string

5. Give one word.

1. Product of force and displacement

2. Capacity to do work

3. Rate of doing work

6. Answer the following questions:

a. Define work done and write its SI unit.

b. Define 1 joule work.

c. What is work done against friction? Give an example.

d. What is work done against gravity? Give an example.

e. Define energy and write its SI unit.

f. What is meant by mechanical energy?

g. What are the factors on which the kinetic energy and potential energy
depend?

h. What is chemical energy? Give an example.

i. What is the law of transformation of energy?

j. What is power? Write the formula to calculate it.

k. Define 1 watt power.

68 Science and Environment Book - 7

7. Give reason of the following.

a. No work is done if a man is pushing against a wall.

b. A horse and a dog are running with the same speed but a horse has
more kinetic energy.

c. A teacher moving around the class is doing work but a child standing
and reading a book is not doing any work.

d. A bullet thrown with hand does not hurt you but the one fired from a
gun can kill you.

8. Differentiate between.

a. Work and power
b. Kinetic and potential energy

9. Show how does the energy transform on the following.

a. Electric heater.
b. Toy car
c. Television

10. Solve the following numerical problems

a. Calculate the work done when a body is displaced by 15 m applying

the force of 8 N. [Ans: 120 J]

b. A force of 600 N displaces a body through 30m. Calculate the work

done. [Ans: 18000 J]

c. A crane lifts a wooden log of 200 kg upto a height 5m from the earth
surface. Calculate the work done against gravity (Take g = 10 m/s2)

[Ans: 10,000 J]

d. Calculate the kinetic energy of a body having mass 1 kg travelling

with the velocity of 20m/s2. [Ans: 200 J]

e. A moving body is brought to rest in 2m by a force of friction of 20N.

What is the energy of the moving body? [Ans: 40 J]

f. Calculate the potential energy of a stored water mass 5 kg at the height

of 15 m from the surface. (take g = 10 m/s2) [Ans: 750 J]

g. A man pulls a box of 40 kg and covers a distance of 10m in 5 seconds.

Calculate the power. (take g = 10 m/s2) [Ans: 800 W]

Science and Environment Book - 7 69

h. Calculate the power of a person who can travel a distance of 45 m in
15 sec carrying a load of 65 kg. (take g = 10 m/s2) [Ans: 1950 watt]

i. A power of a water pump is 200 W. How much time does it require to

complete 800 joule work? [Ans: 4 s]

Project work

• Go to the school ground with your teacher and measure the distance of 100m.

Take the weight of some of the students using weighing machine and note
them. Let to run the 100m distance and also note the time taken by them in
doing so. Calculate the power of each student using following table.

S.N. Name of Mass Acceleration due Distance Power Remarks

student (m) to gravity (g) (d)

• Make a list of 10 objects you see around yourself and note down the energy transformation

in them.

70 Science and Environment Book - 7

6Lesson Heat

Specific After the completion of the lesson, students will be able to:
Objectives
• define heat and state the effects of heat.
• write short notes on the various mode of transmission of heat.
• explain about temperature and thermometer.
• describe in brief about the structure of a thermos and a thermometer and

their working principle.

Preliminary Syllabus
Concept
• Heat and its
You use the word heat so often. You may say: It's a hot day. 'The heat is too transmission
much'. 'Let's heat up the food'. Do you ever stop to think about what heat really • conduction
means in all these cases? • convection
• radiation
It is easy to take the heat that we have on the earth for granted because much
of it comes to us naturally from the sun. The sun is constantly giving the earth • Construction of
light and heat. We may often grumble about the heat, especially on hot summer thermos and its
days, but there would be no life on earth without heat. application

We need heat to keep ourselves and our surroundings warm, to cook food, iron • Temperature
clothes, drive the engines of vehicles and trains, run factories and for thousands • Thermometer
of other useful activities. All warm blooded animals need a definite amount of
heat for the various systems to function inside their bodies. and its working
principle.
Even in the early days, people realised that heat could be used to cook food
and keep themselves and their home warm. At that time, they did not try to
understand what this heat was and where it came from. Even today, it is difficult
to describe heat because it cannot be seen, smelt or tasted. It can only be felt
and we can see the effect it has on things.

Much after people realised that heat could be used to cook food and warm
things, scientists came to understand that heat can also be used to do work.
For example, heat is used to run steam engines. To do this, coal is burnt and
the heat produced is used to turn water into steam. Under pressure, this steam
is used to push the piston of an engine, which turns the wheels and finally the
train moves in a particular direction. Thus, the heat produced by the burning
coal is converted into energy which is used to turn the wheels of the train. This
proves that heat is a form of energy.

Heat may be considered one of nature's gifts to earth and human beings have
clearly made use of this gift to improve conditions of life on earth.

Science and Environment Book - 7 71

Introduction

We feel hot when we touch the fire and we feel cold when we touch the ice
cubes. We feel such a sensation due to the flow of heat. Heat always flows
from a hot body to a cold body. When we touch a hot object like a candle, the
heat from the candle flows to our body and we feel hot. When we touch an ice-
cube, the heat flows from our body to the ice and we feel cold. Similarly, we
feel hot when we sit in front of fire or a heater. We stay outside in sunlight to
keep ourselves warm in winter. We can cook our food by heating. Thus, heat
is a form of energy that gives us sensation of warmth. Heat is produced due to
the movement of molecules. It is actually the sum of kinetic energy of all the
molecules of the substance. It is measured in Joule (J) or Calorie (Cal) by using
a device called caloriemeter.

Sun, electric heater, bulb, burner of stove, fuel are some sources of heat energy.
We use the heat energy to cook food, boil water, stay warm, dry clothes.

Effects of heat

Heat can bring following effects in a body.
1. Heat can change the temperature of a body.
2. Heat can change the state of body.
3. Heat can cause expansion of a body.
4. Heat can change the solubility of a substance.

Activity 1

To show heat can be transferred
Materials Required: a beaker, water, bunsen burner
Procedures:
Pour some tap water into a beaker or into any suitable
vessel. Heat this over a flame. Observe what happen?
After sometime the water becomes so hot that you cannot
touch the water of the container.
This experiment helps us to realise that heat is a form of energy that can
get transferred from one object to another. In this case, the flame heated
the container which get hot and at the same time the water in the container
also got heated.

72 Science and Environment Book - 7

Activity 2

To show that heat has energy
Materials Required: a beaker, water,
bunsen burner
Use the same beaker as in activity 1. This
time, as the water is getting heated, cover
the beaker with a lid. What happens? You
will notice that as the water begins to boil, the lid of the container starts
moving up and down.
Conclusion: This experiment shows that heat has energy. When the water
boils, it gives out steam (water vapour) which produces enough pressure
to push the lid up.

Transmission of heat

Sun is the main source of heat energy on the earth. The sun is million miles
far away from the earth. So, how does the heat of the sun reach the earth?
Why does a boiled water cool down slowly once it is taken off from the flame?
Where does the heat go? All these phenomena are because of the transmission
of heat. Heat flows from a body at higher temperature to the body at lower
temperature. Transmission of heat takes place by three different methods
which are mentioned here.

Conduction

Conduction is the transfer of heat between
substance that are in direct contact with each
other. The heat transfers through the molecules
of the substance but without the actual
movement of the molecules. For example, if we
heat one end of a metal rod, the other end also
becomes hot after some time. It is due to the
conduction of heat. Better the conductor, more rapid will be the transmission
of heat.

Metal is a good conductor of heat which are composed of compactly arranged
molecules. When heat is applied at one end, the molecules of that end obtain
heat and transfer it to the other molecules in contact. These molecules further
transfer the heat to other neighbouring molecules and so on.

Science and Environment Book - 7 73

In this way, heat gets transferred to the other end. Thus, the process of
transmission of heat in which heat passes from particle to particle in the
direction of fall of temperature of the body is called conduction.

Activity 3

To demonstrate the transmission of heat in solid by conduction.
Materials Required: iron rod, stand, iron nails, wax, candle
Procedure:
1. Take a knitting needle and six thumb

pins. Fix the pins on the knitting needle at
a regular distance with molten wax on the
head of each pin.
2. Fix the set up of the experiment as shown
in the figure.
3. Heat one end of the knitting needle and
note your observation.
4. You will notice that as soon as the knitting needle starts getting heated,
the pins start falling one after the other. The first one to fall is the one
closest to the candle and the last to fall is the one farthest from the flame.
Conclusion : This experiment proves that the heat can flow from the
region of higher temperature to the region of lower temperature by the
method of conduction.

Conduction is the flow of heat through matter from higher temperature to
lower temperature without movement of matter as a whole.

Condition for conduction of heat from one body to another body are:
1. The two bodies should be in contact with each other.
2. The two bodies should have different temperatures.

Some substances are good conductor of heat and some are poor conductor of
heat. Materials that allow heat to pass through them easily are called good
conductors of heat. Materials that do not allow heat to pass through them
easily are called insulators or poor conductors of heat. Insulators are useful
in reducing the loss of heat energy. Materials like wood, paper, leather, cotton
cloth are some examples of insulators.

74 Science and Environment Book - 7

Metals are good conductor. Silver is the best conductor of heat and among
the common metals; copper, aluminium and then iron are next in order. In
general, both liquids and gases are poor conductor of heat. Air, wood, paper,
cork, thermocol etc are poor conductors of heat.

Activity 4

To show water and air are poor conductor of heat.
Materials Required: test tube, water, bunsen
burner, match stick
Procedure:
a. For water

• Hold a large test tube of water at the bottom.

Place a gentle bunsen flame just below the
water surface until the water at the top boils, without heating your
hand.

• Can you still hold the test tube? Is water a good conductor or a poor

conductor of heat?

• Most liquids are poor conductors of heat.

b. For air

• Take a very hot bunsen flame.
• Hold a live matchstick about 1 cm away from

the bunsen flame. The matchstick doesnot get
enough heat to brust into flame.
Conclusion: This experiments show that water and air are poor conductor
of heat.

Uses of good and bad conductions of heat in our daily life:

1. Cooking utensils are made of good conductors (metals or alloys). In these
vessels, heat from the flame is conducted to the food inside quickly and
efficiently. It helps in faster cooking of food items.

2. The handles of cooking utensils are made of wood or plastic (poor
conductors). It is because the wood or plastic is a poor conductor of heat.
Hence we can hold the handle with both hands and remove the utensils
from the flame.

Science and Environment Book - 7 75

3. The air trapped in and between our clothes and blankets keep us wam.
Similarly, the air trapped in fur or feathers keep an animal warm. Birds fluff
up their feathers in winter to trap more air. Since air is a poor conductor of
heat, it does not allow body heat to flow out.

4. Mercury is used in thermometer because it is a good conductor of heat.

5. Insulators like bricks and mud are used in the construction of houses.
During summer, the heat outside is not conducted into the rooms while
in winter warm rooms do not loose their heat soon.

6. Slabs of ice are covered with saw dust or gunny bags to prevent them
from melting because saw dust or gunny bags contain a large amount of
air trapped which acts as an insulator.

7. We wear woolen clothes in winter because wool is a poor conductor of
heat. The wool fibres trap air in between them which prevents loss of
body heat.

8. Two thin blankets are warmer than one thick blanket because a layer of air
is trapped in the former.

Convection

Heat energy is transferred from hot place to cold place by the process of
convection too. Convection occurs when warmer areas of liquid or gas rise to
cooler areas of the liquid or gas, cooler liquid or gas then takes the place of the
warmer areas. This result in a continuous circulation pattern. Thus, convection
can be defined as the process of transmission of heat in liquid or gas from one
place to another due to the actual movement of the molecules.

Convection is possible only in liquid and gas because their molecules are
loosely arranged and can move from one place to another easily which is not
possible in the solid particles where the molecules are tightly arranged. Water
boiling in a vessel is good example of convection. When the water is heated
in a vessel, the water at the bottom gets heated first. The heated molecules get
lighter and move up while the cooler molecules of water comes down to take
place of the warm water. This process continues in the vessel and finally the
water boils. During this process, there sets the flow of water molecules. This
process is known as convectional current.

As in liquid, air also transmits the heat by the actual movement of particles or
molecules by convection method. The air molecules nearby land surface get

76 Science and Environment Book - 7

heated by the heat of the sun and get expanded. Thus expanded molecules of
air become lighter and rise up creating the vacuum. The vacuum is filled by
the cold molecules of air nearby it. In such way, the blow of hot air molecules
and cold air molecules takes place which is generally known as blowing of air
or wind.

Activity 5

To show the convection of heat
Materials Required: water, crystals of copper sulphate,
beaker, tripod stand, stand, wire gauge, bursen burner.
Procedure:
1. Take a beaker and fill it partially with water.
2. Put some crystal of potassium permanganate and let

it settle for few minutes.
3. Heat the beaker using a bunsen burner.
Observation
The coloured water rises from the bottom of the beaker.
Conclusion
This shows the transmission of heat takes place by convection in the liquid.

Black marks often appear on the wall or ceiling
above a lamp or a radiator. They are caused by
dust being carried upwards in air, by convection
currents produced by the hot lamp or radiator.

A laboratory demonstration of convection currents
in air can be shown using the apparatus as shown
in figure alongside. The air around the candle
flame gets hot and rises. Cooler air is then drawn
in through the other chimney to replace the hot air, which has escaped. Using
smoke, you can see how the air moves. The movements of the hot and cold
air set up convection current. In houses, the air we breathe out is warmer and
hence lighter. It rises up and goes out from ventilators. This is why ventilators
are made near the ceiling. Convection currents set up by electric, gas and oil
heaters help to warm our homes.

Science and Environment Book - 7 77

Activity 6

Studying convection in air Thread
Materials Required: Sheet of paper, scissors,

pen, candle.

Procedure: Spiral
paper
1. Cut a round sheet of paper of diameter
about 20 cm. Draw a spiral on it as shown

in figure.

2. Cut the paper along the marked line using a pair of scissors. Suspend

the paper from its central point vertically using a thread.

3. Now light a candle and bring it below the paper spiral. Ensure that the
flame is kept at a distance so that the paper will not burn.

4. Observe what happens? Will the paper spiral start rotating? Why does
the paper spiral start rotating.

Radiation

Radiation is the method of heat transfer
that does not rely upon any contact
between the heat source and the heated
object as in the case of conduction
and convection. In radiation, heat
transfers from one place to another without any material medium. Heat can be
transmitted through empty space by thermal radiation, often called infrared
radiation. No mass is exchanged and no medium is required in the process of
radiation. Example of radiation is the heat from the sun. Though there is no
medium between sun and earth, the heat of sun reaches to earth. We can dry
our clothes on the sunlight because of heat from the sun. The heat from an
electric heater to the surrounding is also an example of the radiation of heat.

Here is a model to give a clear concept of heat transfer in various ways. See
the difference between the three ways of heat transfer. Three ways of getting a
book to the back of the class are given as follows:

1. Conduction: A book can be passed from student to student - just as heat is
transferred from particle to particle.

2. Convection: A student can walk to back of class carrying the book. This is
the way, hot air move in convection, taking the energy (heat) with it.

78 Science and Environment Book - 7

3. Radiation: A book can be thrown to the back of the class like the same way
energy is radiated from a hot object.

Thermos flask

Thermos flask is an insulating vessel which
is often used for keeping liquids such as
water, milk etc cold or hot for a long time.
It keeps the hot liquid hot for a long time
by preventing the flow of heat through
conduction, convection or radiation. A
thermos flask consists of a double walled
glass vessel, outer case made of plastic or
metal, cork and lid. In the space between two
walls, both pieces of glass are coated with shiny bright "silvering". The air is
pumped out so that a vacuum is formed between the two walls. A vacuum is
used because it stops heat transfer by stopping conduction and convection.

Shiny surfaces are poor radiators of heat. Radiation is reduced by silvering
both walls on the vacuum side. The silvering on one glass wall reduces
radiation of heat and the silvering on the other glass wall reflects back any
heat that may have been radiated. For example, if a hot liquid is stored inside
the bottle, a small amount of radiation from the hot liquid inside wall is
reflected back across the vacuum by the silvering on the outer wall. The
mouth of the flask is closed by a wooden cork which is a bad conductor of
heat and does not let the heat flow through it. A thermos flask is generally
used to store tea and coffee.

Activity 7

To determine the temperature of liquid of the thermos flask after every
interval of time
Apparatus required: thermos flask, water, thermometer.
Procedure:
1. Fill a thermos flask with boiled water.
2. Cap the flask.
3. Now, measure the temperature of the water every hour, using a

thermometer.
4. Note down the observations. What do you conclude?

Science and Environment Book - 7 79

Do you know ?
In some hot countries, most of the buildings are painted white to reflect the radiation
well and keep the buildings cool.

Temperature

Temperature is the degree of hotness or coldness of a body. It determines how
hot or cold a body is. The temperature of a body is measured in kelvin (K) but
it can also be measured in term of degree Celsius (oC) or degree Fahrenheit
(oF). The device used to measure the temperature is called thermometer.

The body with higher temperature are hotter than the body with lower
temperature. The molecules of the body with high temperature vibrate with
higher speed than the molecules of the body with lower temperature of a
substance. So, the temperature is also defined as the average kinetic energy of
the vibrating molecules of the body.

The normal temperature of human body is 37oC (98.6oF).

Difference between heat and temperature

Heat Temperature

1. Heat is a form of energy that gives 1. Temperature is the degree of

us sensation of warmth. hotness or coldness of a body.

2. It is measured in joule or Calorie. 2. It is measured in Kelvin (K),
Celsius (°C) or Fahrenheit (oF).

3. It is measured by caloriemeter. 3. It is measured by thermometer.

4. Heat is the cause of temperature. 4. Temperature is the effect of heat.

Thermometer

Thermometer is a device which is used to measure the temperature of a
body. A thermometer works on the principle that "the materials expand on
heating and contract on cooling". A thermometer consists a calibrated glass
stem. The stem consists of a thin capillary tube at the end of which there is a
bulb containing thermometric liquid like mercury or alcohol. When the bulb
of the thermometer comes in contact with a hot body, the liquid in the bulb
expands and rises towards the capillary tube. The marking up to which the
liquid expands show the temperature of the body. The capillary tube has a

80 Science and Environment Book - 7

kink near the bulb which prevents the liquid from flowing back into the bulb
while measuring the temperature (in clinical thermometer). There are different
types of thermometers. But in this lesson, we will study only two types of
thermometer.

a. Laboratory thermometer

A laboratory thermometer is used in laboratory to Laboratory thermometer
measure temperature of various laboratory reagents
like boiling water, sand, the air, etc. The stem of a
laboratory thermometer is graduated in the degree
from -10oC to 110oC that means it can measure the
temperature of a body ranging from -10oC to 110oC.

b. Clinical thermometer

A clinical thermometer is a thermometer used to measure human body

temperature. It is short but has finely calibrated

range. It is also known as doctor's thermometer. It

is used to measure the body temperature when we

suffer from fever. A clinical thermometer is graduated
from 35oC to 42oC or 94oF to 108oF in celsius scale and

fahrenheit scale respectively. It has very short range

of calibration because the normal temperature of

human being is 37oC which does not go below 35oC Clinical thermometer
and above 42oC.

Precautions to be taken while reading a clinical thermometer

1. Wash a thermometer with water or an antiseptic solution before and after
use.

2. Before use, the mercury level in the thermometer should be below 35oC.
If it is not so, it can be lowered down by giving repeated jerks to the
thermometer.

3. The reading in the thermometer should be taken by keeping the level of
mercury along the line of sight.

4. Never hold the thermometer's bulb while reading it.

5. Handle the thermometer with care, it can break if hit against hard object.

Science and Environment Book - 7 81

Thermometric liquids

The liquids used in the thermometer are called thermometric liquids. Mercury
or alcohol is generally used in the thermometer as thermometric liquid.

Advantages of mercury as thermometric liquid
1. It is clearly visible in the capillary tube because of its shining silvery

appearance.
2. It has uniform rate of expansion.
3. It does not wet the wall of the capillary tube.
4. It is a good conductor of heat.
5. It can measure a wide range of temperature because of its very low freezing

point of -39oC and high boiling point of 357oC.
Disadvantages of mercury as thermometric liquid
1. It cannot measure very low temperatures as its freezing point is -39oC.
2. Mercury is a poisonous, so it is very harmful if its tube is broken.
3. It shows slow response.
4. It is expensive.
Advantages of alcohol as thermometric liquid
1. It expands uniformly.
2. It has a low freezing point of -115oC. So, it can measure very low

temperature.
3. It is easily available, cheap liquid and safe to use.
Disadvantages of alcohol as thermometric liquid
1. It wets the wall of capillary tube.
2. It has low boiling point. So, it cannot be used to measure high

temperature.
3. Since it is colourless liquid, it needs to be dyed.
4. It does not react quickly to change in temperature.

82 Science and Environment Book - 7

Points to Remember

1. Heat is the form of energy that gives us sensation of warmth.
2. Heat is measured by using caloriemeter. It is measured in joule or

calorie.
3. The process of transfer of heat is called transmission of heat.
4. Transmission of heat takes place by three methods - conduction,

convection and radiation.
5. Conduction is the process of heat transfer in which heat flows from

one point to another point without actual movement of the molecule
or particles. It occurs in solid.
6. Convection is the process of heat transfer in which heat flows from
one place to another by the actual movement of the molecules. It takes
place in liquid and gas.
7. Radiation is the process of transmission of heat without any material
medium.
8. A thermos flask is an insulating device which is used to keep hot or
cold liquid for a long time.
9. The degree of hotness or coldness of a body is called temperature.
10. SI unit of temperature is kelvin (K). It is also measured in degree
Celsius (oC) or degree Fahrenheit (oF).
11. A device that is used to measure the temperature of a body is called
thermometer.
12. Thermometer works on the principle that "substance expands on
heating and contracts on cooling".
13. The liquid used in thermometer is known as thermometric liquid.
14. Mercury and alcohol are two thermometric liquid.

Science and Environment Book - 7 83

Exercise

1. Choose the best answer from the given alternatives.

a. What is the SI unit of heat?

i. Kelvin ii. Calorie iii. joule iv. Celsius

b. Which of the following is not the effect of heat?

i. Increase in temperature ii. change in state of body.

iii. increase in volume iv. increase in mass

c. Which of the following is not an insulator?

i. air ii. wood iii. plastic iv. copper

d. Which of the following process does not need any material medium
for transfer of heat?

i. conduction ii. convection iii. radiation iv. All of them

e. Convection takes place in___________.

i. Solid ii. Liquid only iii. gas only iv. both liquid and gas

f. Heat from the sun reaches to us by__________.

i. conduction ii. convection iii. radiation iv. evaporation

g. Thermometer is used to measure___________.

i. heat ii. temperature iii. length iv. time

2. Fill in the blanks with suitable words.

a. Heat flows from _________ to _________ body.

b. Heat flows in metal by _________ .

c. In a vacuum, heat is transferred by _________.

d. Brick, wood, glasses are the examples of _________ conductor of heat.

e. _________ is the degree of hotness or coldness of a body.
f. Normal temperature of our body is __________ oF.

g. Mercury and alcohol are _________ liquid.

3. Put a tick (√) for the correct statement and a cross (×) for the incorrect
one.

a. The hot water remains hot if it is left free.

b. Heat is measured using a caloriemeter.

84 Science and Environment Book - 7

c. Heat can flow from body of low temperature to the body of high
temperature.

d. Convection takes place in a solid.

e. The heat of sun reaches the earth by the process of radiation.

f. A thermos flask can keep a hot substance hot for a long time.

g. Laboratory thermometer has kink near its bulb.

h. Mercury is suitable to measure very low temperature.

i. Alcohol is an insulating substance.

4. Match the following:

Heat solid

Temperature liquid and gas

Conduction caloriemeter

Convection thermometric liquid

Radiation thermometer

Mercury vacuum

5. Give one word

a. Form of energy that gives us sensation of warmth.

b. Degree of hotness and coldness.

c. Device to measure heat.

d. Device to measure temperature of a body.

e. Materials that allow heat to pass through them easily.

f. Materials that do not allow heat to pass through them.

6. Answer the following question:

a. Define heat. Write its unit.

b. What is meant by transmission of heat? What are the ways through
which heat can be transferred?

c. What do you mean by conduction? Give an example.

d. What is convection?

e. Describe the structure of a thermos flask.

f. What is the reason behind silvering the walls of thermos flask?

g. What is thermometer? What are different types of thermometer.

h. How is laboratory thermometer different from a clinical thermometer?
Give diagram to support your answer.

Science and Environment Book - 7 85

i. What is the average body temperature of a healthy person? Also
mention the units of temperature.

j. What is the advantage of using alcohol instead of mercury in a
thermometer?

7. Give reasons of the following:
a. Convection cannot take place in solid material.
b. We feel hot when we touch a boiling water.
c. Alcohol cannot be used to measure very high temperature.

8. Differentiate between.
a. Heat and Temperature
b. Conduction and Convection
c. Alcohol and Mercury

Think and Discuss

1. In a vacuum, there is no matter. Do you think conduction can take place in vacuum?
2. Why is the handle of a metallic kettle covered with strips of cane?
3. One litre of water at 40oC is mixed with one litre of water at 60oC. What will

happen to the temperature of the mixture?

Project work

Use a thermometer to measure the temperature of different objects like
ice, water, hot, water, room, your own body, etc and show them in a table.

86 Science and Environment Book - 7

7Lesson Light

Specific After the completion of the lesson, students will be able to:
Objectives
• explain about light and its sources with examples.
• define reflection of light and its types.
• state the laws of reflection of light and demonstrate them.
• mention some application of reflection of light.
• explain the construction and working of simple optical instrument like

periscope and kaleidoscope.

Preliminary Syllabus
Concept • Reflection of light
• Types of reflection of
Light is a form of energy that we can detect
with our eyes. We are able to see the beautiful light
and colourful world around us only because of • Regular reflection
light. We see pictures in books and magazines, • Irregular reflection
motion pictures in a cinema hall and in television. • laws of reflection of
You see your own image in the mirror everyday. light
All these observations are dependent on light. • Construction of some
When it is dark, you cannot see very well. Objects optical instruments
that produce their own light are called luminous • periscope
objects. For example, the sun, other stars, electric • kaleidoscope
lamps, candles, glow worms, etc. Other objects
are illuminated by this light and reflect it into our
eyes. They are non-luminous objects. This page,
you and the moon are some examples of non
luminous objects.

Science and Environment Book - 7 87

Introduction

We can see everything in the day time but we cannot see anything at the
darkness of the night. It is because there is presence of sunlight in the day time
which is absent at night. We need some other sources of light at night to see
the things around us.

Light is something that enables us to see the things around us. We cannot see
anything in the absence of light. When the light falls on an object, some part
of the light is absorbed by the substance, some parts pass through it and the
remaining part of it is reflected back. When the reflected light falls in our eyes,
we can see that objects. So, light is the form of energy that gives us sensation
of vision.

Light always travels in a straight path as long as it propagates in a same
medium. The speed of light is 3 × 108 m/s. The sunlight takes about 8 minutes
and 20 seconds to reach the earth.

Sources of light

The objects that emit light are called sources of light. There are different
sources of light. Some of them are natural while other are artificial or man
made. The naturally occurring sun, moon, stars, fire flies, etc are natural
sources and devices like torch, lanterns, electric bulb etc are artificial sources
of light.

The objects which emit their own light are called luminous objects. Sun, stars,
torch light, candle, etc are some examples of luminous objects. Similarly, the
objects which do not emit their own light but are visible in the present of light
are known as non-luminous objects. Objects like plastics, glass, rocks, paper,
soil, moon, planets are some non-luminous objects.

Sun Burning candle Stars
Fig. Luminious body

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Stones Paper Wood
Ray and beam of light Fig. Non-luminious body

The direction of the path in which light is travelling is called a ray. It is
represented by a straight line with an arrow on it.

Ray

A beam is a stream of light or the collection of the rays. The beam may be
parallel, diverging (spreading out) or converging (getting narrower).

Parallel beam of light

Divergent beam of light Convergent beam of light

Reflection of light

As we have studied earlier, when Light
light falls on the surface, some part source

of the light is absorbed. Some part

of it passes through the surface Eye
and remaining part is reflected

back to the same medium. The

phenomenon of returning back

of the light in the same medium

after stricking the surface is called Object
reflection of light. A plain and Fig. An object is visible due to the reflection of light
smooth surface reflects maximum

light and rough surface reflects lesser amount of light. We can see the tin of the

roof of the house shining in the sunlight. This is due to the reflection of light.

Science and Environment Book - 7 89

Types of reflection of light

There are two types of reflection of light.

a. Regular reflection of light

When the parallel beam of light is incident on a

smooth and plain surface, the reflected rays are

also parallel, this type of reflection is called regular Regular reflection
reflection of light. Light reflected from a mirror is

a perfect example of a regular reflection of light. Similarly, the reflection of

light from any smooth or polished surface is also a regular reflection of light.

b. Irregular reflection of light

When a parallel beam of light is incident on

a rough surface, the reflected rays are not

parallel but scattered irregularly. This type

of reflection of light is known as irregular

reflection of light or diffuse reflection. The irregular reflection
light reflected from the rough and irregular

objects like paper, wood, clothes etc are some example of irregular reflection

of light.

Activity 1

Observation of reflection of light

Materials Required: Book, mirror, two cardboard, tubes

Procedure: Light source
1. Use the book to prop up a mirror

upright.

2. Take two cardboard tubes of equal

diameters of approximately 5cm.

3. Hold one tube at an angle with the

end touching the mirror. Reflecting surface
4. Ask a friend to hold the second

tube at a matching angle.

5. Shine the flashlight into the tube you are holding.

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6 What do you observe?

7. When the tubes are at the correct angle, the light will bounce off the
mirror and down to the end of the second tube.

8. If your friends holds their hand at the end of the tube, a circle of
reflected light will be seen.

9. On a rough surface, light is not reflected like this. It is scattered back in
several different directions.

Some terms related to reflection of light IN R
Y
a. Incident ray ∠i ∠r
b. Point of incidence. XO
c. Normal
d. Reflected ray Reflection of light
e. Angle of incidence
f. Angle of reflection

a. Incident ray: The ray of light coming from the source that strikes the surface
of an object is called incident ray. In the given figure, IO is an incident ray.

b. Point of incidence: The point of incidence is the point where the incident
ray strikes the reflecting surface. 'O' is the point of incidence in the given
figure.

c. Normal: A perpendicular drawn from the point of incidence is known as
normal. In the given figure, ON is a normal.

d. Reflected ray : The ray obtained after the reflection of light from the point
of incidence is called reflected ray. In the given figure OR is the reflected
ray.

e. Angle of incidence: The angle between the incident ray and the normal
is called angle of incidence. In the given figure, ∠ION is the angle of
incidence. It is denoted by ∠i.

f. Angle of reflection: The angle between the reflected ray and normal
is called angle of reflection. In the given figure, ∠RON is the angle of
reflection. It is denoted by ∠r.

Science and Environment Book - 7 91

Laws of reflection of light

The laws of reflection of light states that.
1. The incident ray, the reflected ray and the normal to the surface always lie

on the same plane at the point of incidence.
2. The angle of incidence is always equal to the angle of reflection.

i.e. ∠i = ∠r

Activity 2

To verify the laws of reflection

Materials Required: a plane mirror with N
stand, a drawing board, sheet of white paper,
AP
pins, ruler, pencil. C

Procedures: B ∠i ∠r
1. Fix a white sheet of paper in a drawing
X Y
board with the help of the thumb pins. O

2. Draw a straight line XY in the middle of B' C'
the paper. A' D'
Figure: Verification of laws of reflection
3. Fix two pins A and B vertically in front of
the mirror as shown in the figure.

4. Try to locate image of the two pins A and B and fix two more pins C and

D on the paper in front of the mirror, in such a way that the both lie along

the line 'A' and 'B'.

5. Mark the position of A and B with a pencil and remove the pins and the

mirror.

6. Draw the line joining A and B which meets on the point 'O' on the line

XY.

7. Similarly join O, C and D as well. Also draw a normal ON at O.

8. Measure the angle of incidence i.e. ∠AON and the angle of reflection

∠DON.

Observation:

∠AON was found to be equal to ∠DON i.e. ∠i = ∠r

Conclusion:

Thus, the angle of incidence is equal to the angle of reflection. Similarly, the

incident ray AO normal ON and the reflected ray OD are also in same plane.

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Reflection of light on a plane mirror

A plane mirror is a flat reflective surface. It reflects light at the same angle
as it strikes the mirror. The mirror that we generally use at home is a plane
mirror. It is coated with silver in its back which produces image by reflection.
The surface of the mirror is very smooth. The smoothness of the surface of the
mirror determines the quality of the image formed. Beside the ordinary mirror,
highly polished metal surface, clean still water, oil and polished furniture can
also act as a plane mirror.

A plane mirror is represented by a straight line
with shaded lines in its opposite side.

We use plane mirror to look our own image while Coloured surface
combing hair, washing face or dressing up.

Periscope

Periscope is an optical instrument. It is based on
the principle of reflection of light. It was invented
to observe things from a hidden position or the
object beyond the sight of an observer. It was
used to see above water from submarines in
all direction from inside it for different other
purpose.

A periscope is a long tube with a mirror at each
ends. The mirrors are fitted into each ends of
the tube at angle of 45° so that their reflection
surface face each other.

In the periscope, light hits the top mirror at 45º and reflects away at the same
angle. The light then bounces down to the bottom mirror which again reflects
the light at 45°, right into eyes so that the object can be seen.

A periscope lets you see over the top of a things, such as fence or wall that you
aren't tall enough to look over. It can also be used to look around the corner.

It is also used in some gun turrets and in armored vehicles. It can be used in a
stadium to see above the heads of a crowds.

Kaleidoscope

A kaleidoscope is an optical instrument with two or more reflecting surfaces

Science and Environment Book - 7 93

inclined to each other in an angle so that one or more objects on one end of the
mirror are seen as regular symmetrical pattern when viewed from the other
end. This instrument is also based on the reflection of light.

A kaleidoscope is usually tube shaped but it can be made in other shapes as
well. A viewer looks through a small hole at one end while the light enters
through the other end covered with glass or plastic. This reflects the image of
mirror inside the tubes and form colorful pattern.

Construction of kaleidoscope

Take three mirror strips of same shape and size. Join them to form a triangular
prism. You can use rubber band or cello tape to hold the strip in the prism
shape. Wrap it with a black paper. Now, keep some colourful pieces of glass or
broken pieces of bangles inside the tube. Cover both the end of the tube with
white tissue papers. Now, observe the kaleidoscope from one end pointing
towards the light. You can observe the beautiful patterns of colour inside the
tube.

Points to Remember

1. Light is the form of energy that gives us sensation of vision.
2. The straight path along which a light travels is called ray.
3. The collection of rays of light is called beam.
4. The phenomenon of returning back of the light in the same medium after

striking an object is called reflection of light.
5. The ray of light that strikes the surface of an object is called incident ray.
6. The ray obtained after the reflection of the light from the point of incidence is

called reflected ray.
7. The perpendicular drawn on the surface at the point of incidence is called

normal.
8. When the parallel beam of light strikes on a smooth surface, the reflected rays

are also parallel to each other, this reflection is called regular reflection.
9. When the parallel beam of light strikes on a rough surface, the reflected

rays are not parallel but scattered irregularly, this type of reflection is called
irregular reflection.
10. Image is formed due to regular reflection of light.
11. Periscope and kaleidoscope are optical instruments that are based on the
principle of reflection of light.

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Exercise

1. Choose the best answer from the given alternatives.

a. Light is a form of energy that gives us sensation of ________.

i. hearing iii. warmth iii. vision iv. taste

b. Which of the following is not a luminous body?

i. sun ii. star iii. moon iv. fire flies

c. Which of the following surface cause a regular reflection of light?

i. wall ii. clothes iii. book iv. mirror

d. If the angle of incidence is 45o, what is the angle of reflection?

i. 45o b. 90o c. 0o d. 180o

e. Angle of incidence is always equal to the _________.

i. point of incidence ii. angle of reflection

iii. normal iv. angle of refraction

f. The angle made by incident ray with normal is ________.

i. incident angle ii. refracting angle

iii. angle of reflection iv. critical angle

g. How many pieces of mirror are used in a periscope?

i. 1 ii. 2 iii. 3 iv. none

2. Fill in the blanks with correct words.

a. The speed of light in vacuum is __________ m/sec.

b. The sun is __________ body because it has its own light.

c. The collection of rays of light is called __________ .

d. Returning of light ray from the surface of an object is called _______.

e. Plane surfaces causes __________ reflection.

f. Perpendicular drawn from the surface at a point of incidence is called
__________ .

3. Put a tick (√) for the correct statement and a cross (×) for the incorrect
one.

a. Light needs medium to travel.

b. Luminous body emits its own light.

Science and Environment Book - 7 95

c. The collection of beam is called ray.
d. We can see the object due to reflection of light.

e. A smooth surface shows a regular reflection of light.
f. The angle of incidence is equal to the angle of reflection.
g. Normal is a ray of light.
h. Periscope is made on the principle of reflection.

4. Match the following

Star collection of rays

Stone regular reflection

Smooth surface irregular reflection

Rough surface luminous body

Beam non luminous body

5. Give one word.

a. Form of energy that gives sensation of sight.
b. Phenomenon of returning back of light to same medium.
c. Perpendicular drawn on the surface at point of incidence.

d. The angle which is made by the normal and the incident ray.

6. Answer the following question:

a. What is light? What are the sources of light?
b. Explain the reflection of light with an experiment.

c. What are regular and irregular reflection of light? Explain with the

figure.

d. Define the following:

i. incident ray ii. reflected ray iii. normal

iv. angle of incidence iv. angle of reflection

e. State the laws of reflection of light.
f. What is a plane mirror? Write its uses.
g. What is periscope? Write its uses.
h. What is kaleidoscope? Mention its uses.

7. Give reasons of the following.

a. Objects are visible in day light but not in dark.
b. The utensils of steel shines but that of wool does not.

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