New Creative Science and Environment Book 7

5 Work, Energy and Power

After the completion of this unit, students will be able to:
Æ define work, energy and power.
Æ describe the types of work with their application.
Æ explain the types of energy with their application.
Æ explain power and its application.

Introduction
The concept of work is closely related with the concept of energy. In fact, the

concept of work provides a link between force and energy. The general ideas of
work and energy can be applied to a wide range of phenomena in different fields
of physics. Further from the practical point of view, it is important to know not
only the work done on an object but also the rate at which it is being done. This
involves the concept of power. We shall discuss these concepts in this chapter in
detail.

Work

The traditional meaning of work is
quite different from its scientific meaning.
In everyday activity, the term ‘work’ is used
equally for mental and physical work. As we
say a watchman is continuously standing
at the gate and has become completely
exhausted. So, he has done work.

Similarly a girl is preparing for her
exam by continuously reading and practising
questions. So, we can say that she is working
hard.

But in the scientific concept, both of them have not done any work because
they have not changed their position in spite of the expenses of their force.

In physics, “work is said to be done if anybody changes its position by the
application of force on it.

Work,E nergy and Power 47

Thus, for the work to be done,
i) a force should be applied to the object and
ii) the object should be displaced from its original position.
Some examples of work are lifting an object from the floor and putting it
on the table, moving a cart due to the pulling of a bullock, pushing a van, etc.
Mathematically,
work is the product of force and displacement,
i.e. work (W) = Force (F) × Displacement (S)

\ W=F×S
Its SI unit is joule or (Nm).
From the above expression, it is clear that,

when force = 1N, displacement = 1m, then
work = 1 joule.

Hence, when a body covers the displacement of 1m by the application of 1N
force, then the work done is said to be 1 joule.

Work done by a constant force

The direction of the displacement of an object and the direction of the force
can have different relations with each other. The directions of the force and the
displacement may be:

i) Same
ii) Opposite
iii) Perpendicular to each other

i) Force acting in the same direction of displacement

If the displacement of a body is the same as the direction of
the force, then the work done by the body is positive, e.g. a body
falling freely under the effect of gravity.

ii) Force acting in the opposite direction of displacement

If the displacement of the body is in the opposite direction
of the force then the work done by the body is negative, e.g. a
ball thrown upwards against gravity, work done by frictional
force, etc.

48 New Creative Science and Environment; Book 7

iii) Force acting in the perpendicular direction of displacement

If the displacement of the body is perpendicular to the direction of force
then no work is said to be done, as there is no displacement in the direction to the
force, e.g. when a block is moving around a circular path, the work done by the
centripetal force is zero.

Memory Tips

The unit of work and energy i.e. joule is named after the British physicist
James Prescott Joule.

Questions

# State a situation in which force is applied to a body but no work is done?
# A person standing for a whole day feels tired even he does not

seem to do any work, why?
Types of work

There are two types of work done

i) Work against gravity, and

ii) Work against friction

i) Work against gravity

If the displacement of the body is opposite to the direction of the gravitational
force of the earth then the work is called work against gravity, e.g. lifting a load from
the ground by applying some force.

Work against gravity can be calculated by using the formula (W) = m × g ×
h. Here, ‘m’ denotes mass of the body, ‘g’ denotes acceleration due to gravity and
‘h’ denotes height from the earth surface.

Question

Calculate work done against gravity when 3 kg stone is raised by 3 m. 49

Solution:
Work done against the gravity = Force × displacement
= mass × acceleration due to gravity × height
[ Force = mass × acceleration due to gravity]
\ W =m×g×h
= 3kg × 10 ms–2 × 3m
= 90 joule
Hence, the work done against gravity is 90 joule.

Work,E nergy and Power

Work against friction

If the displacement of the body is opposite to the direction of the frictional force,
then the work is called work against friction, e.g. pushing a car to a certain distance,
pulling a wooden plank to a certain distance, etc.

Work against friction can be calculated by using the formula (w) = m × g ×
d. Here, ‘m’ indicates for mass of the body, ‘g’ indicates acceleration due to
gravity and ‘d’ indicates distance covered on the surface.

1. Calculate the work done agianst friction
when 5 kg mass is draged about 5 m
distance.

\ Work done against friction = Force × displacement
= mass × g × displacement ( F = m×g)

= 5 × 10 × 5 = 250 joule

Hence, the work done against friction is 250 joule.

2. Find the work done by a boy if he applies the force of 5N to displace a
book through 20 cm.

Solution: Given, force (F) = 5N

Displacement (S) = 20cm
Work (W) = ? 20

= 100 = 0.2 m

We have, W = F × d = 5 × 0.2 = 1 joule

\ The work done by the boy is 1 j.

Memory Tips

The acceleration produced on a body due to the gravity of the earth is
called acceleration due to gravity. Its average value is 9.8 m/s2. Here, for our
convenience, we take g = 10 m/s2

Energy

We use energy to do work and make all the movements. When we eat food,
our body transforms food into energy to do work. When we run or walk or do
some physical work, we use energy. Cars, planes, trains and various machines
transform energy into work. In general, to perform work anybody must have
energy.

50 New Creative Science and Environment; Book 7

Thus, energy of an object is defined as the capacity for doing work. Its S.I
unit is joule.

One joule energy is the energy required to do 1 joule work.

Bigger units of energy are,

1 kj = 1000 j

1 mj = 106 j

Forms of energy

There are various forms of energy. They are:

1) Mechanical energy 5) Magnetic energy

2) Heat energy 6) Electrical energy

3) Light energy 7) Chemical energy

4) Sound energy 8) Nuclear energy

1) Mechanical energy

The energy possessed by the state of the body (either rest or motion state) is
called mechanical energy. It is of two types. They are:

(a) Kinetic energy (b) Potential energy

(a) Kinetic energy

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

Mathematically,

kinetic energy of a moving object is defined as half the product of its mass
(m) and the square of the velocity of that object (v).

1
Therefore, kinetic energy (K.E) = 2 mv2

1
\ K.E = 2 mv2
Examples of bodies possessing kinetic energy

i) A ball rolling on a surface

ii) A bullet fired from a gun

iii) Water in motion

iv) A falling body

v) Moving car

vi) A running man etc.

Work,E nergy and Power 51

(b) Potential energy

The energy possessed by a body by the virtue of its position is called potential
energy. Mathematically, potential energy of a body is defined as the product of
its mass (m), acceleration due to gravity (g) and its height from the ground (h).

Therefore,
potential energy (P.E) = m × g × h

\ P.E. = mgh

The potential energy may be gravitational or elastic.

Examples of bodies possessing potential energy
i) A stretched rubber of a catapult
ii) A stretched bow
iii) Water stored in dam
iv) A stone at a height, etc.

2) Heat energy

Heat is a form of energy produced due to the
random vibration of the molecules of a body. It flows
from the body at a higher temperature to the body at
a lower temperature. It gives us the sensation of the
warmth.

The heat energy is used to do various work
such as to cook our food, to make our body warm,
to boil water, to run engines, etc. It is also produced due to the combustion of oil,
chemical reaction and nuclear reaction.

Memory Tips

The sun gives us enormous amount of heat and light. The energy in the sun is
produced due to the nuclear reaction in which hydrogen atoms are continuously
changing into helium atoms.

3) Light energy

Light is a form of energy which gives us the
sensation of vision.

We get light from the sun, a burning candle, a
glowing bulb, etc. The sunlight trapped by chlorophyll is
used by green plants to prepare their food. So, due to the
sunlight, all living things can exist on the earth.

52 New Creative Science and Environment; Book 7

4) Sound energy

Sound is a form of energy produced due to
the vibration of the molecules of a body. It gives us
the sensation of hearing to our ears. It can produce
disturbance in a medium through which it propagates.
So, it is also regarded as a form of energy.

5) Magnetic energy

The energy possessed by a magnet with the help of
which it can show the attractive and directive property is
called magnetic energy.

With the help of magnetic energy, a magnet can
attract iron, nickel, cobalt-like magnetic materials. So,
the magnetic energy is used to separate iron dust from
soil and iron dust from eyes. In factories, it is also used to lift heavy loads.

6) Electrical energy

It is a form of energy produced due to the change
in the number of electrons in a body or flow of electrons
through a conductor.

Electricity is the most versatile form of energy that
can be easily changed into other forms like heat, sound,
light etc. Nowadays electricity is used to run electrical
appliances like T.Vs., radios, refrigerators, bulbs, vehicles, cookers, etc.

7) Chemical energy

The energy released by a body when it undergoes chemical change or
reaction is called chemical energy.

Our food consists of chemical energy. When it oxidizes with air then it gets
converted into heat energy. It is used to perform various activities of our body.
Similarly, the chemical energy stored in fuel when burns with oxygen is released
in the form of heat, which is used to run vehicles.

8) Nuclear energy

The tremendous amount of energy released by changing the nucleus of one
atom into another is called nuclear energy.

The heavy atoms are unstable and they break into simpler nuclei. This
process is called nuclear fission. Similarly, the lighter nuclei get combined and
form a heavier nucleus and the process is called nuclear fusion. In both of these

Work,E neryg and Power 53

processes enormous energy is released and this energy is called nuclear energy.
Atom bombs and hydrogen bombs are based on nuclear energy. Nuclear energy
is also used to generate electricity.

Law of conservation of energy

It states that, “energy can neither be created nor be destroyed but can be
changed from one form to another’’.

Transformation of energy

The conversion of one form of energy into an other form is called
transformation of energy. For example, in a radio, when the switch is on, the
electrical energy is converted into sound energy. When we eat food, the chemical
energy stored in the food is converted into heat energy. That heat energy is used
to make our body warm and to perform different physical activities like running,
walking, etc. In all these activities, our heat energy is converted into the kinetic
energy.

The other examples of transformation of energy are as given below.

i) In an electrical fan, when the switch is on, the electrical energy is
converted into the kinetic energy.

ii) In vehicles, the chemical energy of fuel is converted into the heat
energy, which is again converted to the kinetic energy.

iii) In the process of generation of electricity, the potential energy of
the water stored in dams is converted into the kinetic energy. It is
again converted to the kinetic energy to run turbines and it is finally
converted into the electrical energy.

Memory Note

Lavoisier in 1774 stated that matter can neither be created nor be destroyed.
This is called the law of conservation of mass. But according to Albert Einstein,
mass can be converted into energy and vice-versa. Thus, the laws of conservation
of mass and energy have become one, which is called the law of conservation of
mass-energy.

Question

# How does the energy transformation take place in an electric
bulb, an electric heater and windmills?

# What would have happened if nature had not allowed the
transformation of energy?

54 New Creative Science and Environment; Book 7

Visit a nearby mill like a rice mill, oil mill, wind mill or water mill and
observe how the energy is being transformed in them.

Power

All of us cannot do the same work at the same time. For example, a stronger
man may do a certain type of work in relatively less time than a weaker person.
Similarly, a more powerful vehicle would complete a journey in a shorter period
of time than a less powerful one. Thus, power measures the speed of work done.
Thus, the rate of doing work is called power.

Mathematically,

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

Its SI unit is js–1 or watt.

1 watt power (1 js–1 power) :

If 1 joule work is done in one second then the power is called one watt power.
The bigger units of power are kilowatt (kw), megawatt (mw) and horsepower
(hp)

1000 watt = 1 kw

106 watt = 1 mw

746 watt = 1 hp

The power of engine is measured in horsepower.

Solved Numericals

1. Find the power of a crane if it lifts a load of 750N upto the height of 200m
in 100 seconds.

Solution: Given,
Force (F) = 750 N
Height (h) = 200 m
Time taken (t) = 100s
Power (P) = ?

Work,E nergy and Power 55

We have, P = w
t
F × d
or, P = t [ W = F × d]

750 × 200 \ P = 1500 watt
or, P = 100

Hence, the power of the crane is 1500 watt.

1. A boy standing for a whole day feels tired. Has he done any work? Why
does he feel tired?

Ü A boy standing for a whole day has not done any work because he hasn’t
changed his position but he feels tired because he is applying force against
the force of gravity.

2. Is it possible that a body is in an accelerated motion by the application of
force on it, yet no work is being done by that force? Give an example.

Ü Yes, it is possible when the force is perpendicular to the direction of motion.
For example, the moon revolves around the earth under the centripetal
force of attraction of the earth but the earth does no work to the moon.

3. State the conditions under which force does no work.

Ü Force does not work, when

i) the displacement is zero.

ii) the displacement is perpendicular to the direction of force.

4. Can the kinetic energy of the body be negative?

Ü No, because, K.E. = 1 mv2. Mass and velocity would never have negative
2
values.

5. Prove heat is a form of energy.

Ü Heat is used to warm water and steam thus generated is used to run steam
engines. Thus, heat has the capacity of doing work and hence it is a form of
energy.

6. An electric bulb is marked 60 watt. What do you mean by it?

Ü It means that the bulb can convert 60 joules of electric energy into heat and
light energy in 1 second.

56 New Creative Science and Environment; Book 7

F Work is said to be done if anybody changes its position by the application
of force to it.

F The SI unit of work is joule.

F The work may be i) work against gravity, and

ii) work against friction

F Energy is the capacity of doing work. Its SI unit is also joule.

F 1 joule energy is the energy capable of doing 1 joule work.

F The forms of energy are:

a) Mechanical energy b) Light energy

c) Heat energy d) Sound energy

e) Electrical energy f) Magnetic energy

g) Chemical energy h) Nuclear energy

F Mechanical energy is of two types a) Kinetic energy b) Potential energy.

F We get energy in different ways like:

a) heat and light energy from the sun.

b) kinetic energy from wind and running water.

c) magnetic energy from a magnet etc.

F The conversion of one form of energy into another is called transformation
of energy.

F The rate of doing work is called power. Its SI unit is watt.

1. Fill in the blanks.
a) The relation of work, force and displacement is ___________.
b) The SI unit of work is ___________.
c) ___________ is the capacity of doing work.
d) The rate of doing work is called __________.
e) A running man has ___________ energy.

Work,E nergy and Power 57

2. State ‘True’ or ‘False’.

a) When a person stands for a whole day, some work is said to be done
by him.

b) The SI unit of power is joule.

c) A stone at a certain height has potential energy.

d) Heat has not capacity of doing work.

e) A body having more power can do the work in less time.

3. Differentiate between:

a) Work and energy
b) Work and power
c) Kinetic energy and potential energy

d) Work from burning fuel and work from moving objects

4. Answer the following questions.

a) Define work. Write its SI unit. Is work scalar or vector quantity?

b) State a situation in which force is applied to a body, but no work is
done.

c) A man is standing for a whole day and he feels tired but no work is
said to be done by him, why?

d) On which factors does the kinetic energy depend?

e) What form of energy do the following objects contain?

(i) food (ii) a stone at height (iii) fuel

(iv) magnet (v) moving vehicle (vi) sun

f) A bulb is marked 100 watt. What do you mean by it?

g) Name the forms of energy.

h) Write the principle of conservation of energy.

i) What kind of energy transformation takes place in the following
devices?

(i) Hydro-electricity power station

(ii) Electric bulb

(iii) Battery

58 New Creative Science and Environment; Book 7

(iv) Heater
(v) Steam engine
(vi) Windmill

Numerical Problems

1. Find the amount of work done when a body moves 20m distance by the

application of 200 N force. (Ans: 4000 Joule)

2. Find the kinetic energy of the body of mass 40 kg moving with the velocity

of 20 m/s. (Ans: 8000 Joule)

3. How much work is done by a man if he moves through a distance of 15m

by applying the force of 20N? Find his power if he does the work in 2

seconds. (Ans: 300 J, 150 W)

4. A porter can carry 40 bricks at a time. If the weight of each brick is 10N, find
his power if he carries these bricks up-to 70m in 50 seconds. (Ans: 600 W)

5. In a room a 60 watt bulb is used for ten hours. How much energy is consumed

by it? (Ans: 2.16 × 106 J)

Energy – capacity of doing work

Displacement – distance covered in a fixed direction

Fission – to convert into small pieces

Drag – to pull something

Fusion – to combine to make big

Transformation – to change from one form to another

Conservation – to save

Work,E nergy and Power 59

6 Heat

After the completion of this unit, students will be able to:
Æ explain the effects of heat.
Æ explain the transfer of heat.
Æ explain about the measurement of temperature.

Introduction

As already discussed in the previous class, heat is a form of energy that
gives sensation of warmth. Heat is used for different purposes. The selection of
clothes according to seasons, human behaviour and action, all are dependent
on heat. In our home, we use heat to cook food, to keep room warm, to press
clothes and so on. Likewise industries, factories, vehicles, etc. require heat for
their operation.

Effects of heat

When an object is heated, various changes can be observed. The changes
thus occurred in an object is known as effects of heat. Some of the effects of heat
are as described here.

1. Change in temperature

When an object is heated, its temperature increases. Similarly, when heat is
withdrawn from an object, its temperature decreases. Temperature is the degree
of hotness and coldness of the body.

Thermometer Thermometer
Beaker Beaker
Water
Water

Temperature of water before heating Burner
Temperature of water after heating
60 New Creative Science and Environment; Book 7

2. Change in physical state
Heat causes change in physical state. When solid objects are heated above

their melting point, they convert into liquids. Similarly, liquids when heated
above their boiling point, convert into gases.

If the above mentioned process is reversed, gases convert into liquid and
liquids convert into solids.

3. Thermal expansion
Heat causes the expansion of substances. The expansion of any material on

heating is called thermal expansion.
Solid, liquid and gas expand when they are heated. The expansion of gas can

be easily observed even in the small increase in temperature but the expansion of
solids cannot be seen more clearly.

Heat 61

Questions

‰ What do you mean by effect of heat?
‰ What is thermal expansion?

Memory Note

Æ Heat is the total sum of kinetic
energy of vibrating molecules.

Æ When water is heated from 0°C
to 4°C, it contracts instead of
expanding and its maximum
density remains 1000 kg/m3
at 4°C. This phenomenon is
called anomalous expansion of
water.

Transfer of heat

As already discussed, heat is a form of energy that flows from a region of
higher temperature to a region of lower temperature. There are various modes
through which heat flows.

The method by which heat is transferred from one region to another is
called transfer of heat.

There are three methods of transfer of heat. They are:

1. Conduction 2. Convection 3. Radiation

Conduction of heat

If a spoon is left in a cup of hot tea for sometime, its edge becomes hot.
How? If we heat one end of the spoon over a flame, the part we are holding will
become hot after some time. Although that end is itself not in a direct contact
with the flame. How?

The heat transfers through the spoon due to the process of conduction.

Mechanism of conduction

A matter is made up of tiny particles called molecules. In solids, molecules
are tightly packed and cannot move freely. But, they can vibrate about their fixed
position.

To understand the mechanism of conduction, let’s take an example of
metallic rod.

62 New Creative Science and Environment; Book 7

The molecules of a metallic rod vibrate which are in contact with a flame.
These molecules collide with their neighbouring molecules and make them
vibrate. They in turn pass the vibration to their neighbours even further from the
flame and so on. In this way, heat is transferred from one molecule to another
although all molecules are in their original position.

Conduction is the process of heat transfer from one point to another in
solids without the actual movement of their molecules but due to the vibration
of molecules.

Objective
To demonstrate the conduction of heat

Materials required

Iron rods, stand, small metal nails, wax

Procedure

iron rod

metal nails Source of heat

Conduction of Heat
1. Take an iron rod and stick the metal nails to the rod at an equal

distance using wax.
2. Arrange the rod as shown in the figure.
3. Supply the heat and observe the effect.
Observation
As iron rod gets heated, metal nails start to fall one by one in order.
Conclusion
Hence, conduction is a method of transfer of heat in solids.

Heat 63

Memory Note

Æ Conduction only occurs in solids.
Æ During the conduction of heat, molecules do not move from one point

to another.
Questions

‰ What is conduction?
‰ Can we observe conduction in liquids?

Convection of heat

In the case of liquids and gases, the molecules themselves carry heat energy
from a hotter region to a colder region. This kind of heat transfer is known as
convection.
Mechanism of convection

If a beaker containing water is heated, the water at
the bottom becomes hot, and since hot water is lighter than
cold water, it rises carrying heat with it. The gap created
at the bottom is filled with the cold and denser water.

This process continues till the water in the beaker
gets heated equally.

The process of transfer of heat from one part to
another by the movement of the molecules is called
convection.

Objective
To show that the transfer of heat in liquid is

due to convection
Materials required

Beaker, burner, potassium permanganete
crystals
Procedure

1. Fill 2/3 part of the beaker with water.

64 New Creative Science and Environment; Book 7

2. Take a pinch of potassium permanganete crystals and drop it in the
beaker very gently.

3. Let it settle for few minutes.
4. Light the burner and heat the beaker. What do you observe?
Observation

When water inside the beaker gets heated, column of coloured water
rises up in the beaker.
Conclusion

Hence, heat transfers in the liquids by the process of convection of heat.

Memory Note
Æ Convection is the process of heat transfer by the movement of molecules
from one part to another.
Æ Convection occurs in liquids and gases.

Questions
‰ Why do water molecules rise up when they get heat?

Land breeze and sea breeze

Land breeze and sea breeze are the application of convection of heat.
During the day, the air above land gets heated and rises. This air is replaced
by the cold air flowing from the sea. This process is called sea breeze. At night,
since land cools down very fast, the hot air above the sea rises. This air is replaced
by the air flowing from the land. This process is called land breeze.

Heat 65

Radiation of heat

Radiation is the process of
transfer of heat without any medium.
Hence, it is a process of heat transfer
through a vacuum.

Heat energy from the sun
reaches the earth through radiation.
Objects get heated after absorbing
radiation.

Conductors and insulators

Substances that allow heat to pass through them are called conductors.
Metals are good conductors of heat.

Substances that do not allow heat to pass through them are called insulators.
Wood, straw, clay, rubber, etc. are insulators.

Memory Note

Æ Heat travels with the speed of light during radiation.
Æ Air and water are poor conductors of heat.

Tharmas

Tharmas is a device which works in the application
of heat. It can maintain the temperature of the substance
for a long time. In a tharmas hot substance remains hot
for a long time and similarly cold substance remains cold
for a long time.

Structure

Tharmas is made from glass. The inner side of the tharmas is made up of
two layers of glass and the outer side is made from the metal or wood or plastic.
Between two layers of the glasses there remains a vacuum. The wall of the glass
is made reflecting by using silver surface.

Working

Glass is a bed conductor of heat. It does not allow conduction of heat. The
vacuum present between the layers of the glass protects the convection of heat.

66 New Creative Science and Environment; Book 7

The silver surface of the glass reflects heat and protects it from radiation. Thus,
in a tharmas, hot substance remains hot and cold substance remains cold as it
protects the transmission of heat.

Measurement of temperature

The temperature of a body is the degree of its hotness and coldness. Our
sense organs cannot find the actual amount of heat that contains in an object. So,
we need to have a device that gives the actual amount of heat present in an object.

The device that is used to measure the temperature of a body is known as
the thermometer.

The working principle of thermometer is materials expand on heating and
contract on cooling.

Thermometer

A thermometer consists of a long, narrow, uniform glass tube. It has a bulb
at one end. This bulb contains mercury. Outside the bulb, a small shiny thread of
mercury can be seen.

According to its use thermometers are of two types. They are:
1. Laboratory thermometer
2. Clinical thermometers

Laboratory thermometer

Laboratory thermometers are used in scientific work. They may be used
to measure the temperature of any object, boiling water, air, etc. In a laboratory
thermometer, we just read the number on the scale at the tip of the red or silvery
grey line in the thermometer.

Clinical thermometer

The thermometer used to measure the human body temperature is a called
clinical thermometer. It measures the narrow range of temperature (35 to 42°C).
This is because the human body temperature doesn’t go below 35°C and above
42°C. In the clinical thermometer there is a narrow constriction near the bulb.

Heat 67

It is called kink. This construction protects sudden fall down of mercury in the

capillary.

Bulb Constriction (kink) Scale

Memory Note

Æ The presence of kink in clinical thermometer prevents the sudden
downfall of mercury. This helps to take the reading easily.

Æ Clinical thermometers measure a narrow range of temperature.

Thermometric substances

Substances that are used in thermometers are called thermometric
substances. Generally, mercury and alcohol are used as thermometric
substances.

Mercury

Mercury is used in thermometer due to the following reasons.
1. It remains in liquid state for a wide range of temperature.
2. Its freezing point is –38.87°C and the boiling point is 356.58°C.
3. It does not stick on the wall of the thermometer.
4. It is shiny and easy to see.
5. It has an uniform expansion.

Alcohol

Alcohol is used in a thermometer because its freezing point is –117°C. So, an
alcohol thermometer is very useful in cold places. It cannot be used to measure
the temperature of hot bodies because its boiling point is 78°C.

Lower fixed point and upper fixed point

The lower fixed point is the temperature at which ice melts.
The upper fixed point is the temperature at which water converts into
vapour.
The lower fixed point is taken as 0°C and the upper fixed point is taken as
100°C. During the construction of a thermometer, a scale is designed dividing the
distance between the lower and upper fixed points into 100 equal parts. Each part
is then called as 1°C.

68 New Creative Science and Environment; Book 7

Objective

To determine the lower fixed
point

Materials required

Funnel, stand, beaker, ice, a
thermometer

Procedure

1. Put some pieces of ice in the
funnel and set the apparatus
as shown in the diagram.

2. Clamp the thermometer in
such a way that the bulb remains within the ice.

3. The level of mercury in the thermometer falls and remains fixed at
a certain point.

4. As the level of mercury remains fixed at a certain point for more
than 10 minutes, mark that point.

5. The point thus marked is the lower fixed point.

Objective

To determine the upper fixed point

Materials required

Round bottom flask, tripod
stand, burner, thermometer, cork,
glass tube

1. Fill half of the round bottom
flask with water.

2. Fit the thermometer and
glass tube and fix the flask
on the tripod stand with the
help of stand as shown in
the diagram.

3. Heat the flask.

Heat 69

4. As water gets heated, the level of mercury in the thermometer rises.
5. At certain temperature, the water starts to evaporate and the level of

mercury remains constant at a point.
6. As the level of mercury remains constant at that point for more than

10 minutes, mark the point.
7. The point thus marked is upper fixed point.
One the basis of melting point of ice and boiling point of water, there are
three temperature scales. They are:
1. Celsius scale, (developed by Anders Celsius)
The melting point of ice is 0°C and the boiling point of water is 100°C.
2. Fahrenheit scale (developed by D.G. Fahrenheit)
The melting point of ice is 32°F and the boiling point of water is 212°F.
3. Kelvin scale (developed by Lord Kelvin)
The melting point of ice is 273 K and the boiling point of water is 373 K.
Kelvin (k) is the SI unit of the temperature.
The relation between the above temperature scales is given by the relations.

C F – 32 K – 273
100 = 180 = 100

Celsius scale Fahrenheit scale Kelvin scale

70 New Creative Science and Environment; Book 7

1. The upper part of a burning candle is hotter but its lower part is colder.
Why?

Ü According to the transmission of heat by convection, the air in contact with
the flame rises and the upper part of the burning candle is hotter but there
is no effect of convection at the lower part. So, the lower part feels colder.

2. A mercury thermometer cannot be used in cold places. Why?
Ü The freezing point of mercury is –38.87°C. Hence, a mercury thermometer

cannot be used in cold places.

F Heat is a form of energy that gives sensation of warmth.
F Change in temperature, change in state and thermal expansion are the

effects of heat.
F The method by which heat is transferred from one region to another is

called transfer of heat.
F Conduction, convection and radiation are the methods of heat transfer.
F Conduction is the process of heat transfer from one point to another in

solids without the actual movement of their molecules.
F Convection is the process of heat transfer from one part to another by the

movement of molecules.
F Radiation is the process of heat transfer without any medium.
F Substances that conduct heat are called conductors.
F Substances that do not conduct heat are called insulators.
F A temperature is the degree of hotness and coldness of a body.
F The thermometer is used to measure temperature.
F Clinical thermometers have kink to prevent sudden fall of mercury.
F The lower fixed point is the temperature at which ice melts.
F The upper fixed point is the temperature at which water boils.

Heat 71

A. Fill in the blanks.

1. The hotness of an object is determined by its _____________.

2. Temperature in cold regions cannot be measured by ___________
thermometers.

3. Heat transfers without medium due to the process of ___________.
4. In liquids, heat transfers by the process called _____________.
5. A cold spoon if dipped in a cup of hot tea becomes hot by the process

called ______________.
B. Match the following with the best choice.

1. Kelvin clinical thermometer

2. Heat thermometric liquid

3. Metals rubber

4. Mercury Joule

5. Convection good conductors

6. Kink in liquid

7. Insulator SI unit of temperature

C. Write true (T) for the correct and false (F) for the incorrect statement.

1. The mode of heat transfer in solid is radiation.
2. Heat causes change in temperature.

3. Heat flows from low temperature to high temperature.

4. Wood is a bad conductor.

5. Heat energy from the sun reaches us through radiation.

D. Answer the following questions.
1. What is temperature? Name the device used to measure temperature.
2. When you hold a metal rod over a flame, the end of the rod away from
the flame also gets hot. Explain how this happens?
3. What are the characteristics of mercury, so that it is used in
thermometers?

72 New Creative Science and Environment; Book 7

4. Explain the mechanism of convection.
5. What is radiation? Where does it occur?
6. Explain any three effects of heat in brief.
7. Draw a well labeled diagram of:

i. Laboratory thermometer
ii. Clinical thermometer
8. Write short note on temperature scale.
9. Explain the experiment to determine lower fixed point in a thermometer.
10. What do you mean by convection? How do land breeze and sea breeze
occur?
11. Write the differences between:
i) Conduction and convection
ii) Clinical and laboratory thermometer

Require - need
Expansion - growth, process of making larger
Actual - real

‰‰

Heat 73

7 Light

After the completion of this unit, students will be able to:
Æ describe reflection of light
Æ say about types of reflection.
Æ state the lateral inversion.
Æ describe the image formed by plane mirrors.

Introduction

Switch on light at night and look around your room. Can you imagine
how your room will look if the light is switched off? Would you be able to see
anything? Can you see objects when there is no light?

The only answer for all these questions is, we cannot see anything without
light. Light is a form of energy that enables us to see. In previous class, we have
discussed about the rectilinear propagation of light (light always travels in a
straight line). Now, we will be concerned on some basic properties of light.

Before we start, we must be familiar with the following terms.

i. Ray of light

A straight line along which light travels is known as a ray of light. It is
represented by a straight line with an arrowhead. Arrowhead denotes the
direction of light propagation.

A light ray

ii. Beam of light

The collection of rays of light is known as beam of light. There are three
types of beams of light.
a. Parallel beam of light

If the rays of light are parallel with each other then it is called parallel beam
of light.

74 New Creative Science and Environment; Book 7

A parallel beam of light

b. Divergent beam of light
If the rays of light spread away from a point, it is called divergent beam of light.

A divergent beam of light
c. Convergent beam of light

If the rays of light come to meet at a point, it is called convergent beam of light.

A convergent beam of light

Reflection of light

You see different sentences on the blackboard written by your teacher.
Think what makes possible to see those sentences? The answer is light. But how
light makes possible to see?

First of all light from the sun or any other sources comes and strikes on
the blackboard, then it rebounds and reaches our eyes. So, we see the sentences
written on the blackboard. This phenomenon is known as reflection of light.

The process of rebouncing of light in the same medium after striking on the

surface is known as reflection of light.

incident rays reflected rays

Mirror

Lithg 75

Regular and irregular reflection

All types of surfaces reflect light. But while talking of reflection, we refer to
a smooth surface as regular surface and a rough and wavy surface as an irregular
surface.

When parallel beams of light falling on a regular surface reflect parallel
to each other, then the reflection is called regular reflection. Regular reflection
occurs in plane mirrors.

When parallel beams of light falling on an irregular surface, reflect in all
direction, the reflection is called irregular reflection. Wood, wall, paper, soil, etc.
cause irregular reflection.

Reflection at plane mirror

Any smooth and polished surface that can return the rays of light into the
same medium is called a mirror.

Consider XX’ be a plane mirror. AO be a ray of light striking on the mirror
and OB a rebounded ray. Let’s draw ON perpendicular to XX’ at O.

76 New Creative Science and Environment; Book 7

The following terms are frequently used while reading reflection.

i) Incident ray
The ray of light striking at the plane mirror is called incident ray. In the

figure AO is an incident ray.
ii) Reflected ray

The ray of light rebouncing towards the same medium after striking at the
plane mirror is called reflected ray. In the figure, OB is a reflected ray.
iii) Point of incidence

The point at which incident ray strikes on the plain mirror is called point of
incidence. In the figure, O is point of incidence.
iv) Normal

The imaginary line perpendicular to plane mirror at the point of incidence
is called normal. In the figure, ON is a normal.
v) Angle of incidence (∠i)

The angle between the incident ray and the normal is called angle of
incidence. ∠AON is the angle of incidence.
vi) Angle of reflection (∠r)

The angle between reflected ray and normal is called angle of reflection.
∠BON is the angle of reflection.
Law of reflection

The law of reflection states that
i) The angle of incidence is equal to the angle of reflection i.e. ∠ i = ∠r.
ii) The incident ray, the reflected ray and the normal lie on the same plane

at a point of incidence.

Lihtg 77

Objective

To verify laws of reflection

Materials required

Plane mirror, pins, drawing paper, pencil, scale, wooden board

Procedure
1. Fix a drawing paper
on the wooden board
and draw a line XX’
at the middle of the
paper.
2. Mark a point O at
the middle of line
XX’ and draw ON perpendicular to XX’ at O.
3. Put the plane mirror coinciding its reflecting surface on the line XX’.
4. Draw a line AO and fix two pins P1 and P2 on the same line.
5. Look at the image of pins in the mirror and fix two other P3 and P4
in such a way that images of pins P1 and P2 in the mirror and P3 and
P4 coincide.
6. Draw a straight line along P3 and P4 touching O and name the line
obtained as OB.
7. Measure the angle between AON and BON. What do you find?

Observation

∠AON and ∠BON are equal and the line segments AO, ON and OB
meet at point O lying on XX’.

Conclusion

This verifies the laws of reflection i.e.

i. angle of incidence is equal to angle of reflection, and

ii. the incident ray, the reflected ray and the normal at the point of
incidence lie on the same plane.

Image

Place a burning candle infront of a plane mirror. It appears as a similar
candle is placed behind the mirror. The candle that appears behind the mirror is

78 New Creative Science and Environment; Book 7

the image of the candle formed by the mirror. The candle itself is the image.

Lateral inversion

Stand infront of a plane mirror and look your
image. Raise your left hand. Which hand does your
image raise? You will find that in the mirror left appears
right and right appear left.

Now write your name on a piece of paper and
show it infront of a plane mirror. How does it appear
in the mirror?

The phenomenon by which right appears left and left appears right in a
plane mirror is called lateral inversion.
Periscope

Periscope is an optical instrument. It is based on the
principle of reflection of light. It helps to see the objects
that are beyond the sight of the observer. Simply, it is an
instrument use for observing from a hidden position. It is
widely used in submarines to view the objects above the
water surface. Military personnel also use periscopes in
some weapons and armed vehicles.

How to make a simple periscope?
Materials required

Cylindrical pipes (pvc or metallic or paper), pipe joints, two pieces of
plane mirror, glue, wrapping paper, scissors, etc.
Procedure

Take two pipe-joints and glue two pieces of mirror on their corner making
an angle of 45°. Take a cylindrical pipe. Join the pipe using the joints in the form
of alphabet “Z” so all three pipes are 90° to each other. Cover the pipe using the
wrapping paper of your choice. Then, the periscope is ready to use. Look from
one hole of the periscope and observe the objects that are beyond your direct
sight. Try it to observe the visitors knocking your gate.

Lihtg 79

Kaleidoscope

Kaleidoscope is an optical device. It uses mirrors
or glass to create a variety of colourful patterns. It works
in the principle of reflection of light. Kaleidoscopes are
usually tube-shaped and look like a small telescope.
The viewer looks through a small hole in one end and
light enters through the other end which is covered by
translucent glass or plastic or paper. This reflects the
images of the mirrors inside the tube and forms colourful
pattern. It is popular toy for the kids.

How to make a Kaleidoscope?

Materials required

3 equal pieces of rectangular shaped glass or hard transparent plastic, a
piece of translucent paper, rubber strings, a piece of black chart paper, colourful
objects like small pieces of bangles in its edges.

Procedure

Take three equal pieces of rectangular glass or hard transparent plastic
and join to each other to make a triangular tube. Be careful to avoid cuts on
hand if the edges of glasses are sharp. Use the rubber string to tie it and wrap
black chart paper over its rectangular faces. Cover its one end with thin sheet of
paper. Put some small shiny objects or pieces of glass bangles inside the tube.
Take one piece of black paper and make a hole in it. Close its other end with
that black paper having hole in it. You can also use a hollow cylindrical cover to
fit your kaleidoscope. The kaleidoscope is ready now. You can view colourful
patterns when you look inside the tube. These colourful patterns are due to
reflection of light.

1. What is reflection of light?
Ü The process of rebouncing of light

into the same medium after striking a
surface is known as reflection of light.

Here AO is an incident ray which
strikes on the surface XY at O and
turns back to the same medium as OB.

80 New Creative Science and Environment; Book 7

2. What is regular reflection?
Ü When parallel beams of light falling on a

regular surface reflect parallel to each other,
the reflection is called regular reflection.

F Light is a form of energy that enables us to see.
F A straight line along which light travels is called the ray of light.
F The collection of rays of light is known as a beam of light.
F If the rays of light are parallel with each other, it is called parallel beam

of light.
F If the rays of light spread away from a point, it is called divergent beam

of light.
F If the rays of light come to meet at a point, it is called convergent beam

of light.
F The process of rebouncing of the light in the same medium after striking

a surface is known as reflection of light.
F When parallel beams of light falling on a regular surface reflect parallel

to each other then it is called regular reflection.
F When parallel beams of light falling on irregular surface, reflect in all

direction, then it is called irregular reflection.
F The angle between incident ray and normal is called angle of incidence.
F The angle between reflected ray and normal is called angle of reflection.
F The law of reflection states that

§ angle of incident is equal to angle of reflection.
§ the incident ray, the reflected ray and the normal at the point of

incident lie on the same plane.
F The phenomenon by which right appears left and left appears right in a

plane mirror is called lateral inversion.

Lithg 81

A. Fill in the blanks.

1. _____________ enables us to see.

2. A straight line along which light travels is known as ____________.

3. Smooth surface is also called ___________ surface.

4. Angle of incidence is equal to ____________.

5. In lateral inversion, left appears _______________.

B. Match the following:

1. Form of energy divergent beam

2. Heavy surface lateral inversion

3. Spread away from a point incident angle

4. Angle between incident ray and normal light

5. Right appears left irregular reflection

C. Write True or False.

1. Light always travels in a straight line.

2. The collection of beam is called ray.

3. In convergent beam, rays of light meet at a point.

4. Angle of incidence is equal to angle of reflection.

5. In lateral inversion, an inverted image is formed.

D. Write differences between:

1. Regular and irregular reflection.

2. Convergent beam and divergent beam of light.

3. Angle of incidence and angle of reflection.

4. Periscope and kaleidoscope.

E. Answer the following questions.

1. What is reflection of light?

2. How is it possible for us to see?

3. What is regular reflection?

82 New Creative Science and Environment; Book 7

4. What kind of surface causes regular reflection?
5. Explain the experimental verification of laws of reflection.
6. What do you mean by lateral inversion? Give an example.
7. Describe experimentally the image formed by a plane mirror.
8. Complete the following diagram and mention incident ray, reflected

ray, angle of incidence and angle of reflection.

A

9. State laws of reflection.
10. What is periscope? Why it is used?
11. Describe the structure of a Kaleidoscope.

Imagine - assume, think something probably true
Familiar - well known
Strike - to hit, to fall light etc. on something

‰‰

Light 83

8 Sound

After the completion of this unit, students will be able to:
Æ explain about the sources of sound.
Æ explain the propagation of sound.
Æ distinguish pleasant and unpleasant sounds.

Introduction

We listen to music in the radio and on television. On every aspect, we
produce and listen to sounds. We speak, cry and shout to others. When we are
producing sounds, others as well as ourselves listen them. Why do we like the
sound of a guitar? It is so because it produces sharp and soft music, which is a
form of sound. So sound is the natural need of organisms. As different organisms
have different organs for producing sounds. We have vocal cords to produce
sound. We are able to produce sound due to vibration of our vocal cords. In the
same way, the mosquito vibrates its wings and we hear the buzzing sound.

Thus, we can define sound as the form of energy that is produced from the
vibration of the matters.

Memory Note
Æ Sound is the form of energy which gives sensation of hearing.
Æ The vocal cord in males is larger than in females.

84 New Creative Science and Environment; Book 7

To show the vibration of an object producing sound
Take a shaving blade and fix it on a table
or a desk as shown in the figure. Bend the upper
end of the blade and leave it. The blade begins to
vibrate. You can see it vibrating and you can hear
sound as well. Touch the blade carefully with
your finger. It stops vibrating. Does it produce
any sound now?
When the blade was vibrating, the sound was
also produced. It proves that sound is produced
due the vibrating object. As you touch the blade, the vibration ceases and also
the sound is not heard.

Wave

Take a small piece of stone and throw into the
pond. It produces ripples. Those ripples are called
waves. Waves are produced due to the disturbance in
the medium.

There are two types of waves. They are
(i) Transverse waves
(ii) Longitudinal waves
Sound waves are the longitudinal waves. They move from one point to
another point by making compression and rarefaction.

Sound 85

Sources of sound

Those objects that produce sounds are known as the sources of the sound.
As we already discussed, sound is produced from the vibration of matters.
Vibration is the to and fro movement of the object. Thus the vibrating objects are
the sources of sound. Since sound is also a form of energy, it transfers from one
end to another end in the form of waves.

Take a guitar and pluck its string with your hand.
There occurs a vibration of the string. Then the sound
is heard. The sound is produced due to the vibration of
the string. When the string stops vibrating, the sound is
also not heard. This shows that the vibration of string
produces sound.

Take a tuning fork, a rubber pad and a tennis ball. Hit the tuning fork
prongs of the tuning fork in the rubber pad and bring it near to
your ear. Do you hear the sound produced by the fork? If you
touch the prongs of the tuning fork with your finger, the vibration
of the prongs can be recognized.

Again hit the tuning fork in
the rubber pad. The prongs of the
fork vibrate. Touch the tennis ball
with this fork. What do you see? The
buzzing sound is produced when
the fork touches the ball and the ball
moves backward. It is known that the
direction of vibration of the tuning fork
is forward and backward. It also shows
that the vibration in the fork is producing sound.

86 New Creative Science and Environment; Book 7

Pleasant and unpleasant sound

We like some kinds of sounds and hate others. When a radio is tuning,
people enjoy listening to its music. But when a vehicle blows a horn, people try
to close their ears with their hands. Why this is so?

Such type of sound that pleases us and changes our mood is known as
pleasant sound. Chirping of birds, moving streams, music, etc. are pleasant
sounds.

Sounds produced by vehicles, planes, industries and heavy machines
disturb our thoughts and irritate our brain. Such types of sounds that irritate us
when they are heard are known as unpleasant sounds.

Some people like heavy music while some like soft music. Thus, the degree
of pleasant and unpleasant sounds also depend on the nature of people.

Propagation of sound

Sound is produced when an object vibrates. The sound produced by a bell is
heard over a large distance. This is due to transmission of sound from one point
to another in the form of sound wave. Thus, propagation of sound is defined as
the process of transferring of the vibrational energy from one point to another
in the form of sound waves. Sound needs a medium to travel. So sound wave
is also known as mechanical wave. A series of compressions and rarefactions
are noticed during the propagation of sound. Sound travels in all directions. In
the same way, compression and rarefactions are formed during the travelling of
sound in the air.

Propagation of Sound in different mediums

It is already discussed that sound travels in the form of vibrational energy
of the molecules. The rate of travelling of sound is different in different mediums.

1. Propagation of sound through solids

Sound needs a medium to travel. The molecules in solids are highly
compressed. So less vibrational energy is used to transfer sound from one
molecule to another. Thus the speed of sound in the solids is the highest.

Sonud 87

Put your head on a
desk resting it with one
ear of its side. Tap under
the desk with your hand
or any object. What do
you hear? The sound
was loud and fast. In the
same way, remove your
ear from the desk and tap
the desk and listen to the sound. Compare the sounds when you were resting
your head on the desk and when you were far from it. Share your result with
your teacher and friends.

2. Propagation of sound in liquid

Liquids can also propagate the sound. The molecules of the liquids have
less intermolecular distances than those of the gases. So, travelling of sound in
liquids is faster than in gases.

Take a bucket full of water. Take two stones and dip
them into the bucket. Strike the stones inside the bucket by
dipping your hand into it. Do you hear any sound? Yes, the
liquid can also propagate the sound.

3. Propagation of sound in air (gases)

We speak to our friends and families. Do they hear? Yes they can hear.
Because our sound waves travel in the gaseous medium. But the speed of
propagation of sound in gaseous mediums is less than that in solids and liquids.

Memory Note

Æ Sound needs a material medium for propagation.
Æ Sound has the highest speed of propagation in solids and the least in gases.

88 New Creative Science and Environment; Book 7

To prove that sound cannot transmit in vacuum
Take a bell jar and suspend an
electric bell in it. Take out the electric
wires through an airtight cork and
connect them to a battery with a
switch as shown in the figure. Keep
the bell jar over a glass plate. So, the
air cannot enter it. Connect a vacuum
pump to the bell jar. Now, press the
switch, the hammer of the bell strikes
the gong and thus you can hear its
sound because there is still air inside
the bell jar.
Now, switch on your vacuum pump also. As, it draws air from the bell
jar, the sound of the bell becomes fainter and fainter and finally you cannot
hear the sound completely. It is because there is no medium through which
sound could travel.
This proves that sound cannot travel through a vacuum.

Shrillness and loudness of sound

Sharpness of the sound is known as shrillness of the sound. Shrillness of
the sound depends on the speed of vibration. The fast moving objects produce
shrilling sounds. Also it depends on the size of the object. Smaller the object,
shriller the sound is. Door bells, guitars etc. produce sharp sounds.

Loudness of the sound increases with force of vibration. When a molecule
is displaced to a greater extent then there is increase in the loudness of the sound.
Beating of a drum, explosions, etc. produce loud sounds.

1. What do you mean by the propagation of sound?
Ü Movement of sound energy from one point to another through the vibration

of molecules is known as the propagation of sound.

Sound 89

2. Why is sound known as mechanical wave?
Ü Sound needs material mediums like solids, liquids or gaseous state for its

propagation. So sound is known as mechanical wave.
3. What happens when an electric bell is adjusted in a vacuum and the

switch is on to produce the sound?
Ü The sound needs a medium for its propagation. Though the electric bell

is adjusted to the vacuum. The sound is produced by the vibration of the
hammer and gong. But it does not travel due to the lack of a medium. So an
electric bell adjusted to a vacuum does not produce sound.

F Sound is the form of energy that gives sensation of hearing.
F Sound is produced by vibrating matters.
F Tuning a fork is an instrument which is widely used in the practical

work on sounds.
F Irritating sounds produced from automobiles, heavy machines etc. are

unpleasant sounds.
F Pleasant sounds please us and are produced by the chirping of birds, the

movement of streams, rainfalls, etc.
F Travelling of sound from one point to another due to the vibration of

molecules of the medium is known as propagation of sound.
F Sound travels in solids with a high speed and it travels in gas at low

speed.
F Sharpness of the sound is shrillness of the sound.

A. Fill in the blanks.
1. __________ gives the sensation of hearing.
2. ___________ sound irritates us.
3. Sound travels fast in ___________ medium.
4. Sound cannot travel through ______________.
5. ____________ the object, shriller the sound is.

90 New Creative Science and Environment; Book 7

B. Match the following. unpleasant sound
1. Singing of birds

2. Madal sound cannot propagate

3. Solid objects pleasant sound

4. Sound of vehicles sound propagates fast

5. Vacuum source of sound

C. True or false.
1. Sound is produced when an object vibrates.
2. An unpleasant sound makes our mood fresh.
3. Sound travels very fast in gas.
4. Sound needs a medium for its propagation.
5. An explosion of bombs produces shrill sounds.

D. Differentiate between:
1. Pleasant and unpleasant sound
2. Shrillness and loudness of sound.

E. Answer the following questions.
1. What is sound? How is sound produced?
2. What do you mean by vibration?
3. What are the factors affecting the shrillness of sound?
4. Describe an experiment on how sound travels in solid mediums.
5. What do you mean by the sources of sound? Give some examples.
6. Describe an experiment on how sound travels in liquid mediums.
7. Name the instrument used frequently while doing experiments on
sound.

8. How can you distinguish whether a sound is produced by a boy or a girl?

Buzzing - a continuous sound made by bee, etc.
Propagate - to spread
Shrillness - very high and loudness of sound

‰‰

Sonud 91

9 Magnetism

After the completion of this unit, students will be able to:
Æ explain the properties of magnet.
Æ identify natural and artificial magnets.
Æ explain the methods to make artificial magnets.

Introduction

People discovered magnet about 2,000 years ago. At that time, the people
of Magnesia found a wonderful stone that could attract iron objects. The name
magnetite was given to that stone.

Chinese people of those days found that when this stone was suspended
freely tying with a thread, it always rested pointing north and south directions.
The stone was used by navigators to find the direction in the deep sea and was
named as leading stone. The name leading stone later became load stone.

Magnet is capable of attracting some metals like iron, cobalt, nickel and
steel. A freely suspended magnet comes at rest showing north-south directions.

Hence, the substance which attracts small pieces of iron and always rests
pointing north-south direction when suspended freely is known as a magnet.

What are natural and artificial magnets?

Magnets that are found in nature are called natural magnets. Load stone is
a natural magnet. Natural magnets are irregular in shape and have low attractive
power.

Human-made magnets are called artificial magnets. The attractive power of
an artificial magnets is very high in comparison to that of natural magnet. So, the
shape of artificial magnets can be designed according to their use. Bar magnet,
U-shaped magnets, horse –shoe magnets, etc. are some examples of artificial
magnets. Iron or steel are used to make artificial magnets.

92 New Creative Science and Environment; Book 7

Memory note

The earth also shows magnetic properties and such properties are defiend
under the earth’s magnetism.

Properties of magnet

1. Magnet attracts magnetic substances.

Magnet always attracts magnetic substances when they
are brought near to it. This property of the magnet is very
useful to separate magnetic substances.

During the process of extraction of different metals
from their ores, magnetic separation method is used. Which
separates iron and other magnetic substances from the mixture.

2. A freely suspended magnet comes at rest pointing north-south directions.

When a magnet tied with a thread is suspended
freely, it comes at rest pointing north-south directions.

The end of a magnet pointing towards north is
called the north pole and the end pointing towards south
is called the south pole.

3. Like poles of magnet repel and unlike poles attract.

When the north pole of one magnet is Attraction of opposite poles of magnets
brought close to the north pole of another, Repulsion of like poles of magnets
they repel each other. Likewise when the
north pole of one magnet is brought close to
the south pole of another, they attract each
other.

Hence, like poles of magnet repel and unlike poles attract.

Mange tism 93

4. The magnetic force is greater at the ends than at the middle.
Magnets have greater magnetic force at their ends in

comparison to the middle part.

5 Magnetic poles cannot be separated.
If we break a bar magnet into two pieces, each broken end becomes a pole of

the magnet. Simply we can say poles are formed at the broken ends of a magnet.
Hence, magnetic poles cannot be separated.

Methods of making magnet

Magnetic substances like iron, nickel, steel and cobalt can be converted into
magnet. The process by which magnetic substances are converted into magnet is
known as magnetization. The following are the methods of making magnet.

1) Stroking methods
2) Electrical method
3) Two way touching method

1. Stroking method

a. Single touch method

Objective
To make a magnet by single touch method

Materials required
A bar magnet, iron bar

Procedure
1. Place the iron bar on a table.
2. Take a north pole of a magnet near one end of the rod and rub it
along the bar upto the other end as shown in the diagram.
3. Again repeat the process from the first end of the bar magnet.
4. Repeate the process for 40 to 50 times.

Conclusion
Hence, iron bar becomes magnet through this process.

94 New Creative Science and Environment; Book 7

b. Double touch method

Objective
To make a magnet by double touch method

Materials required
Two bar magnets, an iron bar

Procedure

1. Place an iron bar on the table.
2. Take the north pole of one magnet and the south pole of another

magnet at the middle of the iron bar.
3. Start to rub the magnets towards each end.
4. Repeat the process upto 40 to 50 times.
Conclusion
An iron bar becomes a magnet.

Two way touching method

Objective
To make a magnet by two way touching method

Materials required
Two bar magnets, a rubber cork, an iron bar

Procedure
1. Place an iron bar on the table.
2. Keep two bar magnets on the iron bar having opposite poles.
3. Rub the magnets from one end to another without lifting.
4. Repeat the process for 40 to 50 times.

Conclusion
In this way we can get magnetic property on the iron bar.

Magne tism 95

Electrical method

Objective
To make a magnet through an electrical method

Materials required
An iron nail, conducting wire and a dry cell

and pins
Procedure

1. Take an iron nail and wind it with a
conducting wire.

2. Join two ends of the wire to each terminal of battery as shown in
the diagram.

3. Take small iron pins near the iron nail. What do you find?
Observation

The iron nail attracts the pins.
Conclusion

Hence, the iron nail becomes a magnet.
The iron nail remains magnet till the supply of electricity.
The magnet made with the use of electricity is known as electromagnet.

Magnetic induction

Objective
To observe magnetic induction

Materials required
A bar magnet, an iron nail, paper, pins

Procedure
1. Put the bar magnet near to one end of an iron nail.
2. Bring the paper pins in contact to the iron nail. What do you see?

Observation
The iron nail attracts the pins.

Conclusion
An iron nail becomes a magnet when it is kept near to the bar magnet.
The process by which magnetic materials become magnets if placed near

or in contact with a magnet is called magnetic induction.
During magnetic induction the end near to the magnet gains opposite

pole as shown in above diagram.

96 New Creative Science and Environment; Book 7