Green Science and Environment 7 Final (2076)

From the above relation, pressure depends upon the following two factors:

i. Force applied ii. Contact area

The same force can produce different pressure which depends upon the
contact area over which force acts. It means if force acts over a large area of
a surface, it produces a small pressure but if the same force acts over a small
area, it produces a larger pressure.

When force acting on a surface area increases, the pressure also increases and
vice-versa.

Worked out Example: 1

A metallic box of 2000N occupies 2m² surface area. Calculate the pressure
exerted by the box. If the box covers a surface area of 4m2, calculate the pressure
exerted.

Solution:

Given,

Weight of metallic box or force (F) = 2000N

Covered area (A) = 2m²

Pressure (P) = ?

According to the formula,

F
P = A

2000
= 2
= 1000 N/m²

\ The pressure exerted by the box (P) = 1000N/m² or (Pa).

Now,
Force (F) = 2000 N
Area (A) = 4m2
Pressure (P) = ?
We know,

F
P = A

GREEN Science and Environment Book-7 51

2000
= 4
= 500 N/m²
\ The pressure exerted (P) = 500 Pa.

Differences between force and pressure Pressure
Force

1. The pull or push which changes 1. The force acting per unit area of a
or tries to change the position of a surface is called pressure.
body is called force.

2. Force is measured in newton (N). 2. Pressure is measured in newton
per square meter (N/m²) or pascal
(Pa).

Activity 1

Take some coins, cardboards and two bricks. Place the coins uniformly on a
thin cardboard placed over two bricks as shown in figure (a) and pile up the
coins and place them on the cardboard at its middle as in figure (b) separately.
Observe the bending of cardboard by the coins in both cases. Also, comment
on your observation.

Coins Cardboard Coins
Cardboard

4.1 Bricks Fig.Bricks
Fig.
Application of pressure in our daily life

1. Sharp knives, khukuri, axe, etc. are used to
cut things. Cutting edges of these articles are
sharp and occupy less area which exerts more
pressure on the things and make it easy to cut.

4.2

52 GREEN Science and Environment Book-7

2. Pointed nail is used to hammer in wooden Fig. Fig.
blocks than a blunt one. Pointed nail occupies
less area and exerts more pressure to pierce in Fig. Fig.
the block.

4.3

3. Wooden slippers are kept below railway line.
Wooden sleepers cover more area and control
or decrease the pressure exerted by the train
and the railway line does not sink into the
ground.
4.4

4. Heavy trucks are fitted with double wheels
to cover more area of contact and control or
decrease the pressure exerted by the weight of
truck on the ground.

4.5

5. Studs are kept on football players’ boots to
increase the pressure by decreasing contact
area on the ground and make it easy to
perform activities.

4.6

6. The foundations of houses and buildings are Fig.
made wider to increase the area of contact and
control the pressure exerted by the weight of
house and building. It prevents houses and
buildings from sinking into the ground.

4.7

Key Concepts

1. The perpendicular force acting per unit area of a surface is called pressure.
2. The SI unit of pressure is N/m2 or Pa.
3. The same force can produce different pressures depending on the area over

which the force acts.

GREEN Science and Environment Book-7 53

4. One pascal pressure (1 Pa pressure) can be defined as the pressure exerted
when one newton force acts on 1 m2 surface area.

5. When the force acting on a surface increases, the pressure also increases and
vice-versa.

6. Pressure depends on the area of contact and force applied.
7. There are many applications of pressure in our daily life.

Exercise

1. Tick (√) the correct statement and cross (×) the incorrect one.
a. The perpendicular force acting per unit area of a surface is called
pressure.

b. In SI system, pressure is measured in Pa.

c. Pressure depends on the force and area of the surface.

d. A sharp knife can exert less pressure than a blunt one.

e. The thin string exerts more pressure on the shoulder of a school
bag carrier.

2. Fill in the blanks with appropriate words.
a. The force acting per unit ....................... is called pressure.
b. ....................... is the SI unit of thrust.
c. One end of nail is made pointed to increase .......................
d. When area decreases, the pressure .......................
e. The pressure exerted by a body depends on ....................... and
.......................

3. Answer the following questions.
a. What is pressure? Write its SI unit.
b. Which formula is used to calculate pressure?
c. Name the factors on which pressure depends on.
d. Why do we prefer a sharp knife to cut vegetables?
e. Write any three differences between pressure and force.
f. Write down the importance of pressure in our daily life.
g. What is the difference in pressure exerted on the ground while walking
on a single foot and both feet?

54 GREEN Science and Environment Book-7

4. Give reason.
a. One end of a nail is pointed.
b. Studs are made on the football player’s boot.
c. Foundations of tall buildings are made wide.
d. A camel can walk quickly on desert.
e. We prefer a sharp knife to chop vegetables.

5. Tick (√) the best answer from the given alternatives.
a. ................ is the force acting per unit area of a surface.

Volume Density

Pressure Pascal

b. In SI system, pressure is measured in ................

kg/m³ N/m²

Nm² N/m

c. The two factors that affect pressure are ................

Force and mass Force and area

Force and volume Area and density

d. When the force acting on a body decreases, the pressure ................

decreases increases

divides multiplies

6. Numerical problems:

a. A force of 500N acts on the area of 2.5m²; calculate the pressure

exerted. [Ans: 200 Pa]

b. Calculate the force applied when 400 Pa of pressure is exerted by a box

of surface area 2m². [Ans: 800 N]

c. The weight of a stone is 1400N. If it exerts a pressure of 700 Pa, calculate

the area. [Ans: 2m2]

d. A force of 2000 N acts on 5m2 surface area. Calculate the pressure

exerted. [Ans: 400 Pa]

GREEN Science and Environment Book-7 55

UNIT Energy, Work and

5 Power

Weighting Distribution (Approximate) Teaching periods : 3 Marks (in %): 1

Before You Begin

If a person can do a lot of work, we say the person is very energetic.
Energy of a body is the capacity or ability of a body to do work. Living
beings cannot survive in the absence of energy. Similarly, energy is
required to operate machines in industries, to run automobiles and so
on. Work is said to be done when the force acting on a body produces
motion in the direction of the force. In SI system, work is measured in
joule (J). The rate of doing work is called power. Its SI unit is watt (W).
Energy, work and power are interrelated to each other.

Learning Objectives Syllabus

After completing the study of this unit, students will be able to: • Introduction to energy
i. introduce energy, work and power. • Types of energy
• Introduction to work
ii. explain the types of energy. • Types of work
• Introduction to power
iii. write the formula to calculate energy, work and • Relationship among energy,
power.
work and power
iv. solve simple numerical problems related to energy, • Transformation of energy
work and power. • Simple numerical problems

related to energy, work and
power

Glossary: A dictionary of scientific/technical terms

energy : the capacity of doing work

work : the product of force and displacement

power : the rate of doing work

potential energy : the form of energy that an object gains as it is lifted

kinetic energy : the form of energy that an object gains when it moves

nuclear : related to nucleus of an atom

56 GREEN Science and Environment Book-7

Energy

We cannot operate automobiles without Fig.
fuel. The fuel burns in the engine of these
automobiles and provides energy to move 5.1
them. Similarly, we cannot work for a long
time without food. We get energy from the
food that we eat. The capacity or ability of
a body to do work is called energy. Living
beings get energy from the food. So plants
and animals cannot survive without food.
In the SI system, energy is measured in
joule (J). Energy is a scalar quantity.

Energy provides force to do work. The object having no energy cannot do
work. Whenever work is done, energy is consumed.

Types of Energy

There are different forms of energy in nature. They are as follows: (i) Mechanical
energy, (ii) Chemical energy, (iii) Sound energy, (iv) Heat energy, (v) Light
energy, (vi) Electrical energy, (vii) Nuclear energy and (viii) Magnetic energy.

1. Mechanical energy

Mechanical energy is the energy possessed by a body due to its state of motion
or of position. It is of two types:

a. Kinetic energy (KE)

b. Potential energy (PE)

a. Kinetic energy

A moving hammer has kinetic Do You Know
energy. So it can do work on a nail
it strikes. Similarly, running water Running water and blowing air have kinetic
has kinetic energy so it can rotate a energy. So we can rotate a turbine with the
turbine. The energy possessed by a help of running water and blowing air to
generate electricity.

body by virtue of its motion is called

kinetic energy. Running water, blowing air, the bullet fired from a gun, moving
vehicle, rolling ball, etc. possess kinetic energy.

GREEN Science and Environment Book-7 57

Fig.

5.2

Bullet fired from a gun Moving car Moving wind mill

Following formula is used to calculate the kinetic energy of a moving body.

Kinetic energy (KE) = 1 mv2
2

Where, m = mass of a moving body

v = velocity of a moving body

From the above relation, it becomes clear that kinetic energy of a moving body

is equal to the product of half of its mass and square of its velocity.

Activity 1

Take a volleyball. Throw it slowly and ask your friend to catch the ball. Repeat
this activity by increasing the speed of the ball while throwing. Ask your friend
to say the difference while catching the ball in both cases. More force is required
to catch the ball thrown at a high speed than that in low speed.

Repeat above activity with a tennis ball. The mass of a volleyball is more
than that of a tennis ball. So less force is required to catch a tennis ball than
that to catch a volleyball thrown at the same speed.

This activity proves that kinetic energy increases with increase in mass and
velocity of the moving body and vice-versa.

Worked out Numerical: 1

A metal ball of a mass of 5 kg is moving with the velocity of 25 m/s. Calculate
the kinetic energy.

Given,
Mass of the metal ball (m) = 5 kg

Velocity (v) = 25 m/s

Kinetic energy (KE) = ?
1

KE = 2 mv2

58 GREEN Science and Environment Book-7

1
= 2 × 5 × (25)2
= 1562.5 J
\ Kinetic energy (KE) = 1562.5 J

b. Potential energy Football
Mass (m)
The water stored in a dam possesses potential
energy. When this energy is released, it can rotate Height (h)
the turbine. The potential energy of a body is defined
as the energy possessed by the body by virtue of its Fig.
position or configuration (change in shape or size).
The energy stored in the stone lifted from the ground, 5.3
stored water in a dam, stretched spring, stretched
elastic, etc. are some examples of the objects having
potential energy. Football at a certain height from
the ground possesses potential energy.

Following formula is used to calculate the potential energy (PE) of a body.

\ PE = mgh

Where, m = mass of a body
g = acceleration due to gravity (g = 9.8 m/s2)
h = height from the surface of the ground

Worked out Numerical: 2

A stone of a mass of 25 kg is located at a height of 15 m from the ground.
Calculate the potential energy stored in the stone. [Take g = 9.8 m/s2.]
Given,
Mass (m) = 25 kg
Height (h) = 15 m
Acceleration due to gravity (g) = 9.8 m/s2
Potential energy (PE) = ?
We know,
PE = mgh

GREEN Science and Environment Book-7 59

= 25 × 9.8 × 15

= 3675 J

\ Potential energy (PE) = 3675 J

Activity 2
Take a catapult. Stretch the elastic of the catapult and throw a pebble in

an open place. Be careful while throwing the pebble as it may hit birds,
animals or people. Which energy helps to throw the pebble? Name the type
of energy present in the stretched elastic of the catapult.

2. Chemical energy

When petrol is burnt in the engine of a car, the chemical energy stored in petrol
is used to run the car. The energy stored in a matter is called chemical energy.
Bread, coal, petrol, diesel, battery, wood, oil, etc. have chemical energy stored
in them. Chemical energy is released when chemical change takes place. Some
sources of chemical energy are given below:

Fig.5.4KeroseneCell
Fig. Bread

3. Sound energy

Sound energy is a form of energy which is
produced due to the vibration of a material
medium. A vibrating body possesses sound
energy. Loudspeaker, radio, television, horn of
vehicles, temple bell, etc. are some sources of
sound energy.

Sound can be experienced as a form of energy 5.5
when the window panes shatter due to an
explosion or loud sound produced by a low- Loudspeaker produces
flying aeroplane. sound

60 GREEN Science and Environment Book-7

4. Light energy Fig. Fig. Fig.5.6
Fig.
Light is a form of energy which makes things
visible. Light is produced by extremely hot
objects. The sun, lantern, torch light, electric
bulb, kerosene lamp, etc. are some sources of
light energy. The sun is the main source of light
energy for the earth.

The sun

5. Electrical energy 5.7

The form of energy which is produced due to Lighting CFL
continuous flow of electrons is called electrical
energy. Cell, photocell, battery, generator, etc. are
the sources of electrical energy. Electrical energy
is used to rotate fans, drive trains, light bulbs,
operate equipment like television, computer,
camera, mobile phone, etc.

6. Heat energy 5.8

The form of energy which gives the sensation Burning coal
of warmth is called heat energy. Electric heater,
sun, burning coal, etc. are some sources of
heat energy. The burning of diesel in a truck
engine provides the energy needed to run the
truck. Similarly the heat energy produced from
burning fire is used to cook food, etc.

7. Magnetic energy

The energy obtained from a magnet
is called magnetic energy. It is used in
electric bell, loudspeaker, mobile phone,
television, radio, etc. Similarly, magnetic
energy is used to generate electricity.

8. Nuclear energy 5.9
Magnet attracting iron

The energy obtained from the nucleus of nails

an atom is called nuclear energy. This energy can produce a large amount

of heat and light energy. Nuclear energy is used in atomic power plants to

produce electricity. Similarly, nuclear energy is used for making atom bomb,

hydrogen bomb, etc.

GREEN Science and Environment Book-7 61

Work

Work is said to be done when the force acting on a body produces motion
in it in the direction of the force. In other words, work done is defined as the
product of force and displacement. In the SI system, work is measured in joule
(J) or newton-metre (Nm).

Formula of work done
Work done (W) = Force (F) × Displacement (s) \ W = F × s

No mechanical work is done when a student studies for 2-3 hours sitting on
a chair. Similarly, no work is done by a teacher teaching a class. Work is said
to be done only if the force applied to a body succeeds in moving it. When a
person pushes a wall, no work is done because the wall does not cover distance.
But work is done when a person pushes a cart because the cart covers some
distance. Therefore, the work done by a body depends on:
a. The magnitude of force applied (F)
b. The distance covered in the direction of the force (s) applied

Fig.

5.10 A man is pushing a cart (Some
work is done)
A man is pushing a wall (No work
is done)

One joule work

When one newton force displaces a body through a distance of one metre, the
work done is called one joule.
In short,

1 joule = 1 newton × 1 metre [ W = F × s]

Types of Work b. Work done against gravity

There are two types of work. They are:
a. Work done against friction

62 GREEN Science and Environment Book-7

a. Work done against friction

The work done by pushing or pulling an
object on a surface is called work done
against friction. For example, work done
by pulling a wooden log on the road,
work done by pushing a cart, etc.
Fig.
Fig. 5.11
b. Work done against gravity

The work done by lifting a body vertically upward from the earth's surface is

called work done against gravity. For example, work done by a crane by lifting

a wooden log vertically upward from the ground.

Do You Know

Friction is the force which opposes the motion
of a body moving on other body when they are
in contact.

5.12

Formula of work done against gravity
Work done against gravity (W) = Force × displacement
or, W = Weight × height [ Force = Weight (mg), Displacement = Height (h)]

\ W = mgh

Worked out Numerical: 3

A force of 300 N displaces a body through 20 m. Calculate the work done.
Given,
Force (F) = 300 N
Displacement (s) = 20 m
Work done (W) = ?
We know,
W = F × s = (300 × 20) Nm = 6000 J
\ Work done (W) = 6000 J.

GREEN Science and Environment Book-7 63

Worked out Numerical: 4

A crane lifts a wooden log of 150 kg upto a height of 10 m from the earth's
surface. Calculate the work done against gravity. [Take g = 9.8 m/s2.]
Given,
Mass of wooden log (m) = 150 kg
height (h) = 10 m
Acceleration due to gravity (g) = 9.8 m/s2
Work done (W) = ?
We know,
W = mgh
= 150 × 9.8 × 10
= 14700 J
\ The work done against gravity (W) =- 14700 J.

Power

Power can be defined as the rate of doing work. The SI unit of work done is
joule (J) and that of time is second (s). So the SI unit of power is J/s or W (watt).
Power is also measured in kilowatt (kW), megawatt (MW), horsepower (h.p.),
etc.

Formula to calculate power (P)
Work done (W)

Power (P) = Time taken (t)

\ W
P= t

From the above relation, it becomes clear that power (P) depends on two
factors, viz. amount of work done (W) and time taken (t).

Two persons that do the same amount of work can have different power. For
example, Ram completes 100 J work in 10 seconds. But Hari completes 100 J
work in 5 seconds.

W 100
Power of Ram (P) = t = 10 = 10 W

64 GREEN Science and Environment Book-7

W 100
Power of Hari (P) = t = 5 = 20 W

From the above calculation, it becomes clear that when a body or a person
takes lesser time to do a particular work, the power is said to be greater and
vice-versa.

One watt power

One watt power can be defined as the rate of doing one joule work in one
second time.

In short, 1 watt = 1 joule
1 second

Meaning 20 W written on an electric bulb: It means that the electric bulb
converts 20 J of electrical energy into heat energy and light energy in 1 second.

\ 20 J
20 W = 1s

Relation among watt (W), kilowatt (kW), megawatt (MW) and horse power
(h.p):

1000 W = 1 kW
1000 kW = 1 MW
746 W = 1 h.p.

Worked out Numerical: 5

A man pulls a cart of 40 kg and covers a distance of 10 m in 5 seconds. Calculate
the power.

Given,

Mass (m) = 40 kg

\ Weight or Force (F) = m × g

or, Force (F) = 40 × 9.8 [ g = 9.8 m/s2]

= 392 N

Distance (s) = 10 m

time (t) = 5 s

GREEN Science and Environment Book-7 65

We know,

W
P = t

F×s
= t

392 × 10
= 5
= 784 W

\ Power of the person (P) = 784 W

Worked out Numerical: 6

A crane lifts a load of 50000 N upto a height of 50 m in 10 seconds. Calculate
the power of the crane in horsepower.

Given,

Load (F) = 50000 N

Height (h) = 50 m

Time (t) = 10 s

Power (P) = ?

We know,
W

P = t
F×h

= t [ W = F × h]

50000 × 50
= 10
= 250000 W

250000 [ 1 h.p. = 746 W]
= 746 h.p.
= 335.12 h.p.

\ Power of the crane (P) = 335.12 h.p.

66 GREEN Science and Environment Book-7

Key Concepts
1. The capacity or ability of a body to do work is called energy.
2. There are different forms of energy. They are mechanical energy, heat energy,

light energy, sound energy, chemical energy, electrical energy and magnetic
energy.
3. Mechanical energy is the energy possessed by a body due to its state of
motion or of position. It is of two types, viz. kinetic energy and potential
energy.
4. The energy possessed by a body by virtue of its motion is called kinetic
energy.
5. The potential energy of a body is defined as the energy possessed by the
body by virtue of its position or configuration (change in shape or size).
6. The work done by pushing or pulling an object on a surface is called work
against friction.
7. The work done by lifting a body vertically upward from the earth's surface
is called work done against gravity.
8. The rate of doing work is called power. Its SI unit is watt (W).

Exercise

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

a. The SI unit of energy is joule.

b. The PE of a body kept on the earth's surface is zero.

c. The rate of doing work is called power.

d. Running water consists of potential energy.

e. Burning coal consists of heat energy.

2. Fill in the blanks using appropriate words.
a. The formula of kinetic energy is .......................
b. Battery and bread consists of ....................... energy.
c. The rate of doing work is called .......................
d. The SI unit of work done is .......................
e. The formula of power is .......................

GREEN Science and Environment Book-7 67

3. Tick (√) the best answer from the given alternatives.
a. Which of the following is the source of heat energy?

heater moon bread candle

b. ....................... energy is produced due to vibration of material medium.

heat light sound chemical

c. The SI unit of work done is .......................

N J Pa W

d. The formula of power is .......................

P = W P = W × t P = W × d P=W
t F

4. Answer the following questions:

a. What is energy? Write down its SI unit.
b. Name any five types of energy.

c. Define mechanical energy with any two examples.

d. What is a potential energy? Write down the formula to calculate
potential energy.

e. Define kinetic energy with any two examples.

f. What is chemical energy? Name any three objects having chemical
energy.

g. Define heat energy and light energy.

h. What is nuclear energy? Write its one use.

i. What is meant by work done? Define one joule work.

j. What is meant by work done against gravity? Give one example of
work done against friction.

k. What is power? Write its SI unit.

l. Define one watt power.

5. Differentiate between:
a. Kinetic energy and Potential energy
b. Work done against friction and Work done against gravity

68 GREEN Science and Environment Book-7

c. Work and Power
d. Heat energy and Sound energy

6. Name the form of energy present in the given objects/devices.

a. Fire b. Bread

c. Running water d. Water stored in a pond

e. Burning coal f. Bullet fired from a gun

g. Lighting bulb h. Ringing bell

7. Numerical Problems.

a. Calculate the potential energy stored in a metal ball of a mass of 80 kg
kept at the height of 15 m from the earth's surface. [Take g = 9.8 m/s2]
[Ans: 11760 J]

b. Study the given figure and calculate the potential 200 kg
energy. [Take g = 9.8 m/s2]
[Ans: 98000 J] 50 m

c. A ball of a mass of 1.5 kg is moving with the velocity of 20 m/s.

Calculate the kinetic energy of the bullet. [Ans: 300 J]

d. Sanu carries an object 20 m away by applying 500 N force. Calculate

the work done. [Ans: 16000 W]

e. A crane lifts a load of 4000 N to a height of 20 m in 5 seconds. Calculate

the power of the crane in horse power. [Ans: 32.17 h.p.]

GREEN Science and Environment Book-7 69

UNIT Heat

6

Weighting Distribution (Approximate) Teaching periods : 3 Marks (in %): 1

Before You Begin

When we touch a hot iron pot we feel hot and when we touch ice we
feel cold. How do we get such a sensation? It is due to the flow of heat
from a hot object to a cold one. When we touch a burning candle, heat
flows from the flame of candle to our body and we feel hot. When
we touch ice, heat flows from our body to the ice and we feel cold.
Heat is the form of energy which gives us the sensation of warmth,
i.e. hotness or coldness. The degree of hotness or coldness of a body
is called its temperature. Heat is measured in joule and temperature
is measured in kelvin.

Learning Objectives Syllabus
After completing the study of this unit, students will be able to:
i. introduce heat and state the effects of heat. • Introduce to heat
• Transmission of heat
ii. explain various methods of transmission of heat. - Conduction
- Convection
iii. introduce temperature and thermometer. - Radiation
• Thormos flask- structure and
iv. explain the structure of thermometer and measure
temperature by using it. utility
• Temperature
• Structure and working of

thermometer
• Thermometric liquids

Glossary: A dictionary of scientific/technical terms

heat : the form of energy which gives us the sensation of warmth
temperature
conduction : the degree of hotness or coldness of a body

convection : the process of transfer of heat from one particle to another without actual
movement of particles
radiation
: the process of transfer of heat by the actual movement of molecules of a
medium

: the transmission of heat from a hot body to a cold body without affecting
a medium

70 GREEN Science and Environment Book-7

Fig.Heat

Concept of Heat

We feel hot which we touch a burning coal and we feel cold while touching
ice cubes. When heat energy flows into our body, we get sensation of warmth
and when heat flows out from our body, we get the sensation of coldness. The
sensation of warmth or coldness is caused due to gain or loss of heat. So, heat
is a form of energy which gives sensation of warmth or coldness. When we rub
palm of our hands against each other, they become warm. Heat is produced
due to the movement of molecules. Heat is measured in joule (J) or calorie
(Cal.). It is measured by a device called caloriemeter. In our day to day life, we
use heat energy for various purposes such as cooking foods, drying clothes,
boiling tea, running engines, etc.

6.1
We get heat energy from different sources like the sun, burning substances,
electric heater, light bulb, running vehicles, etc. The sun is the main source of
heat energy.

Effect of Heat

The major effects of heat are as follows:
1. Expansion of the substances
2. Change in the state of substances
3. Change in the temperature of substances
4. Change in the solubility of substances

Transmission of Heat

Heat energy flows from a hot body to a cold one. Heat flows from one place to
another due to the difference in temperature between them. When we mix hot

GREEN Science and Environment Book-7 71

water (high temperature) and cold water (lower temperature), heat flows from
hot water to the cold water. The flow of heat energy from a body at a higher
temperature to another body at a lower temperature is called transmission of
heat. This process also takes place within a body of different temperature at
two parts.

There are three ways of transmission of heat. They are:

a. Conduction b. Convection c. Radiation

a. Conduction

Conduction is the process of transfer of heat from one particle to another
without actual movement of the particles. When one end of metallic (iron) rod
such as iron, copper, etc. is heated its another end also becomes hot. In solids
like iron, molecules are compactly arranged with each other. When one end
of the metallic rod is heated, the molecules of that end absorb heat and starts
vibrating. The vibrating molecules collide with their nearby molecules and
transfer a part of absorbed heat in each of them. In this way, transmission of heat
takes place in the solids till both ends of the body are at the same temperature.
This process of transfer of heat from one particle to another particle without
actual movement of the particles of a solid (conductor) is called conduction.
In this method, no physical movement of the particles within the material
medium takes place.

Experiment: 1

Aim: To demonstrate the transmission of heat in solids by conduction method

Materials required
Clamp stand, iron rod, iron nails, wax, candle

Procedure
i. Clamp the iron rod and the clamp stand.
ii. Fix some (4-5) iron nails on the iron stand with the help of wax.
iii. Heat the iron rod at one end with the help of a burning candle.

Fig. Wax Nail
(falling)
Stand

Iron
nails
6.2

72 GREEN Science and Environment Book-7

Observation
After few minutes of heating, it can be observed that the iron nails fall one by
one starting from the end of the rod nearby the flame of candle.

Conclusion
This experiment proves that the heat is transferred from higher temperature
(hot end) to the lower temperature (cold end) of the body by conduction
method.

Activity 1

Take two frying pans with handles. The handle of one pan should be covered
with bakelite (plastic) and that of other should be naked. Now heat both of
pans for the same time by a bunsen burner. Wait for a while and touch the
handles. What do you feel?

Good conductor and Bad conductor
The substance which conducts heat easily is called good conductor of heat.
Metallic substances like iron, cobalt, nickel, silver, aluminium, copper, etc. and
their alloys are good conductors of heat.

The substance which does not conduct heat easily is called bad conductor or
insulator of heat. Glass, plastic, paper, wood, cloth, etc. are bad conductor or
insulators of heat. All liquids except mercury and gases are poor conductor of
heat.

b. Convection

The process of transmission of heat through liquids and gases from one point
to another due to the actual movement of molecules is called convection. In
liquids and gases, molecules are loosely arranged and are free to move, so it is
possible for the actual movement of molecules. But in the solids, molecules are
tightly arranged and convection is not possible.

When liquid (water) is heated, the water molecules at the bottom of the vessel
get heated first. Heated molecules expand and become lighter. These light
molecules rise up whereas cold molecules come down as they are heavier. In
this way, molecules of bottom get heated and rise up and cold molecules fall
down at the bottom. This action sets the flow of water molecules. This process
is known as convectional current.

As in liquids, air also transmits the heat by the actual movement of particles or
molecules by convection method.

GREEN Science and Environment Book-7 73

The air molecules nearby land surface get heated at the day time due to the
heat of the sun. These molecules expand and become lighter and rise up. It
creates the vacuum and the vacuum is filled by the cold molecules from the
surroundings. In this way, the movement of hot air molecules and cold air
molecules take place, which is commonly known as blowing of air or wind.

Experiment: 2

Aim: To demonstrate the transmission of heat by convection method

Materials required
Water, crystals of copper sulphate, beaker, tripod stand, wire gauge, bunsen
burner

Procedure Burner

i. Take a beaker and Water
fill it partially
with water. Wire guaze
Potassium
ii. Put a crystal permangnate
of potassium Tripod stand
permangnate.

iii. Heat the beaker
by using a bunsen
burner. What do
you observe?

Observation Fig. Bunsen burner

It can be observed

that the coloured 6.3
water rises from the

bottom of the beaker where heat is applied and the coloured water spreads

and fall down at the side of the beaker. In this way, convectional current can be
observed. This current transfers heat to the entire molecules of water by actual
movement of heated molecules.

Conclusion

This experiment shows that the transmission of heat takes place by the actual
movement of molecules in liquids. This method of transmission of heat is
called convection.

74 GREEN Science and Environment Book-7

c. Radiation

Radiation is the transmission
of heat from a hot body to a

cold one without affecting the

medium. The transmission of
heat from the sun to the earth is
an example of radiation. We can
dry up a wet cloth in heat of the
sunlight. We know that there

is not any material medium

between the earth and the sun Fig.6.4
except the earth's atmosphere. Fig.
Then how does the heat travel
from the sun to the earth? It
is because of the third way of

transmission of heat which does not require a material medium. This mode
of heat transfer is called radiation. The transmission of heat by radiation does
not require any material medium. So, radiation also takes place in a vacuum.

Transmission of heat from an electric heater to its surrounding is possible due
to the radiation process.

Thermos flask Do You Know

A vessel which is used to keep hot Thermos flask was invented by James Dewar
liquids hot and cold liquids cold for in 1890 AD.
several hours is a thermos flask. It
preserves heat by reducing heat lost
and decreasing effect of conduction,

convection and radiation. A thermos Stopper
flask consists of double-walled Vacuum
glass vessel, outer case (plastic or Double glass shell
steel), cork support, stopper and lid.
There is a vacuum created between Silver coating
two glass walls to reduce the loss Casting
of heat due to the conduction and
convection process. 6.5
The outer surface of the inner wall
and the inner surface of outer wall
are silvered to reduce the loss of
heat due to radiation. The mouth of
the flask is closed by a wooden cork,

GREEN Science and Environment Book-7 75

which is a bad conductor of heat. The glass vessel is fitted by using cork pads at
the different points surrounded at outer with an outer case.

Concept of Temperature Do You Know

The temperature of a body is defined Heat always flows from the body at higher
as the degree of hotness or coldness temperature to the body at lower temperature.

of a body. It can also be defined as

the average kinetic energy of the

molecules present in a body. Temperature of a body is the measure of the

average kinetic energy or thermal energy of its molecules. Temperature is the

property of a substance which determines the direction of the flow of heat.

Temperature is measured by using thermometer.

Units of temperature

The SI unit of temperature is kelvin (K). But temperature is commonly
measured in degree Celsius (°C) and degree Fahrenheit (°F).

Differences between Heat and temperature

Heat Temperature

1. Heat is the sum of kinetic energy 1. Temperature is the average

present in the molecules of a kinetic energy present in the

body. molecules of a body.

2. It is measured by using 2. It is measured by using
calorimeter. thermometer.

3. Its SI unit is joule (J). 3. Its SI unit is kelvin (K).

Thermometer

A device used to measure the temperature of a body is
called thermometer. So, thermometer is a device which is
used to measure the temperature of a body.

Principle of construction of thermometer Fig.

When a body is heated, it expands and when it is cooled, it 6.6
contracts. This is the working principle of a thermometer. Thermometer
Thermometer can be made by using liquid or gas as they
expand more than the solids. However, we commonly use
liquid thermometers.

76 GREEN Science and Environment Book-7

Types of Thermometer

There are different types of thermometers. But in this unit, we will study two
types of thermometer, viz. laboratory thermometer and clinical thermometer.
And in this unit, we will study the structure of a laboratory thermometer.

Laboratory thermometer

The thermometer which is used to measure the temperature
of various objects in a laboratory is called a laboratory
thermometer. It consists of a capillary tube made of glass in
which one end is closed and the other end is connected to a
cylindrical bulb.

The cylindrical bulb has a small portion of the capillary Fig.
tube filled with mercury or coloured alcohol. The air from
the capillary tube is completely taken out before closing the 6.7
open end of the capillary tube. The scale in the laboratory
thermometer ranges from – 10° C to 110° C. Laboratory
thermometer

While measuring the temperature of a body, the bulb of the thermometer is
kept in close contact of the body. As the bulb comes in contact with the hot
body, the thermometric liquid inside the bulb expands and rises upwards
in the capillary tube. Finally, the liquid gives a constant level, which is the
temperature of the body. When the bulb of the thermometer is kept in a cold
body, the thermometric liquid contracts upto a constant level which shows the
temperature of the cold body.

Thermometric Liquids

Two liquids, i.e. mercury and alcohol are used in a thermometer. These liquids
are called thermometric liquids. The liquids which are used in a thermometer
are called thermometric liquids.

Advantages of mercury as a thermometric liquid
1. Mercury can be seen clearly in the capillary tube as it is a silvery white liquid.
2. It has a uniform rate of expansion and contraction.
3. It is a good conductor of heat as it is a liquid metal.
4. It can measure a wide range of temperature because the freezing point of

mercury is –39°C and boiling point is 357°C.
5. Mercury does not stick to the inner wall of a capillary tube.

GREEN Science and Environment Book-7 77

Disadvantage of mercury as a thermometric liquid
A mercury thermometer cannot measure very cold temperatures as the freezing
point of mercury is –39°C. Therefore, mercury thermometer is not suitable to
measure very low temperatures in cold countries.

Advantages of alcohol as a thermometric liquid
1. Alcohol is suitable to measure very low temperatures in cold countries as its

freezing point is –115°C.
2. It can measure temperatures accurately as its rate of expansion is six times

more than that of mercury.
3. It is not very expensive.

Disadvantages of alcohol as a thermometric liquid
1. Alcohol is not suitable to measure the temperature above 78°C because its

boiling point is 78°C.
2. It is a bad conductor of heat. So, it has no uniform rate of expansion and

contraction.
3. It cannot give accurate measurement of temperature because it sticks to the

inner wall of the capillary tube.

Activity 2
Go to your science laboratory with the science teacher. Study the structure

and working mechanism of a clinical and laboratory thermometer.
Measure the temperature of melting ice, hot water and boiling water using

a laboratory thermometer.

Activity 3
Take a clinical thermometer and measure your body temperature.
Compare your body temperature with your friend's body temperature.

Key Concepts

1. Heat is a form of energy which gives the sensation of warmth. In the SI
system, heat is measured in joule.

2. The process of transfer of heat is called transmission of heat.

3. There are three modes of transmission of heat: (a) Conduction (b) Convection
(c) Radiation.

78 GREEN Science and Environment Book-7

4. The process of transfer of heat from one particle to another particle without
actual movement of particles is called conduction. It commonly occurs in
solids.

5. The process of transfer of heat by the actual movement of molecules in
liquids and gases is called convection.

6. The process of transfer of heat from a hot body to a cold body without any
material medium is called radiation.

7. The vessel which keeps hot liquid hot and cold liquid cold for several hours
is called a Thermos flask.

8. The degree hotness or coldness of a body is called its temperature.
9. A device which is used to measure temperature is called a thermometer.
10. A thermometer is constructed on the principle that liquid expands on

heating and contracts on cooling.
11. The liquid, which is used inside a thermometer is called thermometric

liquid.

Exercise

1. Tick (√) the correct statement and cross (×) the incorrect one.
a. Heat is a form of energy which gives the sensation of warmth.

b. Convection takes place only in liquid, not in gas.

c. Radiation does not require any material medium.

d. The device which is used to measure heat energy is called a
calorimeter.

e. The degree of hotness or coldness of a body is called its temperature.


f. The SI unit of temperature is °C.

2. Fill in the blanks with appropriate words.
a. In solid, molecules are .................... arranged.
b. .................... is not required for radiation.

GREEN Science and Environment Book-7 79

c. The substances which conduct heat easily are called ....................
d. The degree of hotness or coldness of a body is called ....................
e. Mercury and .................... are thermometric liquids.

3. Tick (√) the best answer from the given alternatives.

a. The SI unit of heat is .................... kelvin
degree celcius

joule fahrenheit
b. Convection takes place in .................... gas
liquid

solid all of the above

c. The transmission of heat due to actual movement of molecules is called
....................

conduction convection

radiation non of the above

d. The freezing point of mercury is ....................

39 °C 100 °C

212 °C 357 °C

e. The transmission of heat which takes place is called ....................

conduction convection

radiation temperature

4. Answer the following questions.
a. Define heat and write any three effects of heat.
b. What do you mean by transmission of heat?
c. What is conduction? Give one example.
d. What are good conductors? Write any three examples.
e. What are insulators? Write any three examples.
f. What is convection? How does wind blow?

80 GREEN Science and Environment Book-7

g. What is radiation? Can this process take place in a vacuum? Why?
h. What is temperature? Write its SI unit.
i. What is a thermos flask? Write its use.
j. What is a thermometric liquid? Write examples.
k. What is a thermometer?

5. Differentiate between:
a. Heat and Temperature
b. Conduction and Convection
c. Thermometer and Calorimeter
d. Conductor and Insulator

6. Match the following:

A B

Conduction Thermometer

Heat Liquid

Convection Vacuum

Thermometer Solid

Radiation Calorimeter

7. Draw a neat and labelled figure of a thermos flask.

8. Describe the structure of a laboratory thermometer.

GREEN Science and Environment Book-7 81

UNIT Light

7

Weighting Distribution (Approximate) Teaching periods : 3 Marks (in %): 1

Before You Begin

In a dark room, we cannot see the things kept there although our eyes
are open. We need light to see the things kept there. So, light is a form
of energy which makes things visible. It is produced from extremely
hot objects. The objects which emit light are called the sources of light.
The sun is the main source of light on the earth. Some other sources of
light are light bulb, burning candle, kerosene lamp, etc.

Learning Objectives Syllabus

After completing the study of this unit, students will be able to: • Introduction to light
i. introduce light and its sources with examples. • Sources of light
• Ray and beam of light
ii. define reflection of light and its types. • Reflection of light and its types
• Laws of reflection of light
iii. state the laws of reflection of light and demonstrate • Reflection of light from a plane
them.
mirror
iv. construct some equipment like periscope and • Periscope and Kaleidoscope
kaleidoscope and describe their working mechanism.

Glossary: A dictionary of scientific/technical terms

light : the form of energy which makes things visible
luminous : shining in the dark due to its own light
beam : a collection of several rays of light
normal : a perpendicular drawn on the surface at the point of incidence
image : a picture of something/somebody seen in a mirror

82 GREEN Science and Environment Book-7

Fig. Fig.Light

Light is one of the important forms of energy essential for all living beings.
Animals are able to see things of their surroundings due to the presence of light.
We can easily experience and understand about the effect of light in our vision.
We cannot see anything in a dark place though we keep our eyes open. But
we can see things with the help of a torch light. We can see things only in the
presence of light. So, light is a form of energy which gives sensation of vision.
Light travels in a straight line as long as it propagates in the same medium. It
travels very fast with the speed of 3 lakh kilometer per second (300000 km/s.).
The sunlight takes 500 seconds to reach the earth from the sun.

Sources of Light

Those objects which emit light are called sources of light. For example, the
sun, star, torchlight, burning candle, glowing bulb, firefly, etc. The sun is the
main source of light. Some sources of light which can emit their own light
like the sun, star, glowing bulb, firefly, etc. are known as luminous objects. So,
luminous objects are the objects which emit light of their own.

7.1
Luminous objects

Some objects like plastics, glass, rock, paper, soil, etc. do no emit light of their
own. Those objects which do not emit light of their own are called non-luminous
objects.

7.2
Non-luminous objects

GREEN Science and Environment Book-7 83

Activity 1

Classify the given objects in terms of luminous and non-luminous sources of
light.

- star - sun - stone - soil - sand - plastic - pencil

- pen - paper - book - bulb - burning candle - black board

Ray of Light

A very narrow path of light is called a ray Fig. Fig.7.3
of light. It is represented by a line with an
arrowhead. The arrowhead shows the direction A ray
of propagation of light.

Beam of Light

The collection of several rays of light is called a
beam of light. It is broader and consists of many
rays.

7.4

A beam of light

Reflection of Light

When we stand in front of a plane mirror, we see our own image in it. It is
possible only due to the reflection of light by the mirror. When the rays of light
fall on a shiny or polished surface, the rays are returned to the same medium.
This phenomenon is called reflection of light. The phenomenon of returning
of the light to the same medium after striking a surface is called reflection of
light.

Incident rays

Reflected rays

Fig.

7.5 Mirror
Reflection of light

84 GREEN Science and Environment Book-7

We are able to see things when light from a luminous body falls on these things.
It is due to the reflection of light by these things. Non-luminous objects do not
absorb all light, they reflect some light which enters our eyes. As a result, we
see them. Therefore, the objects in our surroundings are visible due to the
reflection of light.

Types of Reflection of Light

There are two types of reflection of light on the basis of direction of reflected
rays. They are as follows:

a. Regular reflection of light b. Irregular reflection of light

a. Regular reflection of light

When a parallel beam of light strikes on a smooth surface, the reflected rays
are also parallel to each other. This type of reflection of light is called regular
reflection. For example, reflection of light from a plane mirror or any highly
polished surface. A plane mirror or polished surface appears bright due to the
regular reflection of light.

Fig. Fig.7.6 x y

Regular reflection of light

b. Irregular reflection of light

When a parallel beam of light strikes a rough surface, the reflected rays are
not parallel to each other. This type of reflection of light is called irregular
reflection. The reflected rays scatter in different directions. For example,
reflection of light from furniture, clothes and other rough surfaces.

7.7 x y
Irregular reflection of light

GREEN Science and Environment Book-7 85

Some Terms Related to Reflection

1. Mirror 2. Incident ray 3. Point of incidence
6. Angle of incidence
4. Reflected ray 5. Normal
R
7. Angle of reflection

IN

Fig. ∠i ∠r

X Y
7.8 O

In the figure 7.8, ON - Normal
XY - Mirror ∠ION - Angle of incidence
IO - Incident ray ∠RON - Angle of reflection
O - Point of incidence
OR - Reflected ray

1. Mirror

Any smooth or polished surface which can return back the rays of light into
the same medium is called mirror. For example, looking glass, highly polished
metal surface, still water or oil, polished furniture, etc.

2. Incident ray

A ray of light striking a reflecting surface is called the incident ray.

3. Point of incidence

The point at which the incident ray strikes the reflecting surface is called point
of incidence.

4. Reflected ray

The light ray obtained after reflection from the surface in the same medium in
which incident ray is travelling is called reflected ray.

86 GREEN Science and Environment Book-7

5. Normal

The perpendicular drawn to the surface at the point of incidence is called
normal.

6. Angle of incidence

The angle which the incident ray makes with the normal at the point of
incidence is called the angle of incidence. It is denoted by ∠i.

7. Angle of reflection

The angle which the reflected ray makes with the normal at the point of
incidence is called angle of reflection. It is denoted by ∠r.

Laws of Reflection of Light I N R

Laws of reflection of light state that:
Incident ray Reflected ray
i. the incident ray, the reflected ray Fig.
and the normal lie on the same 60° 60°
plane at the point of incidence. 7.9 O

ii. the angle of incidence is equal to
the angle of reflection.

Reflection of Light from a
Plane Mirror

A plane mirror consists of a

smooth or polished surface

which can return back the

rays of light falling on it into

the same medium. A looking Fig. x Reflected surface
glass is a mirror. One side of y

it is coated with silver due to 7.10 Shaded surface
which another side of it acts

as a reflecting surface. Similarly, highly polished metal surface, still water or

oil, highly polished furniture, etc. act as plane mirror.

While drawing the diagram of a mirror, the reflecting surface is represented by
a straight line and the silver coated side is represented by shaded lines.

GREEN Science and Environment Book-7 87

Activity 2

Fix a sheet of white paper firmly on a fixing board with the help of thumb
pins. Draw a straight line XY on the paper. Place the long edge of the mirror
along the line. Fix two pins A and B vertically in front of the mirror. Try to
locate image A' and B' of the two pins. Fix two more pins C and D on the
paper in front of the mirror, so that they both lie along the line A' and B'.

A'

B'

X Y
O

BD
Fig.
Fig.A C
7.11
Fig.
Mark the position of pins with a pencil and remove the pins and the mirror.
Draw a line joining the point A and B. It meets the mirror at point O on the
line XY. Now, join the O, D and C. Now, measure the angle of incidence and
angle of reflection.

Angle of incidence O Plane mirror
∠i ∠r Point of incidence

Angle of reflection

Incident ray Reflected ray
A
Normal C
7.12 N

Periscope 45° M1
45°
A periscope is a simple Plane mirror
device based on the principle
of successive reflections from 45°
two plane mirrors.
M2 45°
A periscope consists of
two plane mirrors M1 and 7.13

88 GREEN Science and Environment Book-7

M2 facing each other fixed at 45° to the frame-work of a tube (card board or
wooden) which is bent twice at right angle.

The beam of light entering the tube from the object strikes the mirror M1 at 45°.
That beam is reflected by the mirror M1 and travelled through the tube and
strike at the mirror M2 at 45°. That beam finally gets reflected from the mirror
M2 parallel to its original path and reaches to the eye of the viewer.
The main uses of a periscope are as follows:

1. It is used to see over the heads of a crowd.

2. It is used to observe the enemies activity above the ground by a soldier sitting
in a trench.

3. It is used to see the movement of ships over the surface of the sea from a sub-
marine.

Kaleidoscope

Kaleidoscope is a simple toy based Fig. Tape Mirror
on the principle of the formation of
several images by inclined mirrors.

Construction of a Kaleidoscope 7.14 Card board

Take three mirror strips and join them to form a triangular prism. You can join
the strips with the help of rubber bands or cello tape. Wrap them by a black
paper. Now, place a few pieces of coloured glass (broken bangles) inside the
tube. Cover both the open ends of the tube with white tissue papers. Now
hold and rotate the kaleidoscope pointing towards light. You can see beautiful
patterns of colours.

Activity 3

Prepare a periscope and a kaleidoscope and demonstrate in your school.

Key Concepts

1. Light is a form of energy which gives the sensation of vision.
2. The speed of light in a vacuum is about three lakh kilometer per second.
3. An object which emits lights of its own is called a luminous object.
4. An object which does not emit light of its own is called non-luminous object.
5. A very narrow path of light represented by a straight line with an arrowhead

is called a ray.

GREEN Science and Environment Book-7 89

6. The collection of several rays of light is called a beam of light.
7. Regular reflection of light means when a parallel beam of light strikes a

smooth surface and the reflected rays are parallel to each other.
8. Irregular reflection of light means when a parallel beam of light strikes on a

rough surface and the reflected rays are not parallel to each other.
9. A ray of light striking a reflecting surface is called an incident ray.
10. A ray of light which gets reflected after striking a reflecting surface is called

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

normal.
12. A periscope and kaleidoscope are constructed on the basis of reflection of

light.

Exercise

1. Tick (√) the correct statement and cross (×) the incorrect one.
a. Light travels in a straight line.

b. Light is a form of energy which makes things visible.

c. Reflection of light on a smooth surface is irregular reflection.

d. Smooth and highly polished surface is called a mirror.
e. The angle between a normal and an incident ray is called angle of

incidence.

2. Fill in the blanks with appropriate words.
a. Light is a form of energy which gives the sensation of ...................
b. The speed of light in a vacuum is ...................km/s.
c. An object which emits light of its own is called a ................... of light.
d. A ray of light which strikes a surface is called ................... ray.
e. ................... is the perpendicular drawn at a point of incidence.

90 GREEN Science and Environment Book-7

3. Answer the following questions:

a. Define light. Write down the speed of light in a vacuum.

b. What do you mean by the source of light? Give any four examples of
it.

c. What is reflection of light? Mention its types.

d. What is regular reflection of light? Draw a figure showing regular
reflection.

e. State the laws of reflection of light.

f. Write the name of devices which works on the principle of reflection
of light.

g. What is a periscope? Write its use.

h. What is a kaleidoscope? Write its use.

i. Define the following terms.

i. Incident ray ii. Reflected ray

iii. Normal iv. Angle of incidence

v. Angle of reflection

j. Draw diagrams showing regular and irregular reflection of light.

4. Differentiate between:
a. Luminous object and Non-luminous object
b. Regular reflection and Irregular reflection
c. Ray and Beam of light
d. Incident ray and Reflected ray

5. Tick (√) the best answer from the given alternatives.

a. Which of the following object is a luminous object?

wood water

star soil

b. Which one of the following is a living source of light?

oil-lamp torch light

firefly burning candle

GREEN Science and Environment Book-7 91

c. If the angle of incidence is 45°, the angle of reflection is ...................

35° 54°

45° 50°

d. The angle between normal and a reflected ray is ...................

acute angle reflected angle

incident angle right angle

e. The phenomenon of returning the light to the same medium after
striking a surface is called ................... of light.

dispersion refraction

reflection propagation

6. Study the given figure and answer the following questions.

a. Which phenomenon is shown in I N R

the given diagram?

b. Name the angle of incidence,
angle of reflection and normal.

c. What is the magnitude of ∠r? 45° ∠r
Why?

O

92 GREEN Science and Environment Book-7

UNIT Sound

8

Weighting Distribution (Approximate) Teaching periods : 3 Marks (in %): 1

Before You Begin

In our surroundings, we use different things that produce sound.
When materials vibrate, sound is produced. So, sound is the form
of energy which is produced due to vibration of a material medium.
Sound produces sensation of hearing. The substances that produce
sound are called sources of sound. Temple bell, horn of vehicles,
loudspeaker, guitar, television, etc. are some sources of sound. Sound
waves are produced when a material vibrates. Sound propagates in
the form of a wave.

Learning Objectives Syllabus

After completing the study of this unit, students will be able to: • Introduction to sound
i. introduce sound and sources of sound. • Sources of sound
• Wave motion
ii. define sound wave (longitudinal wave) and explain • Types of wave
its propagation. - Transverse wave
- Longitudinal wave
iii. explain the factors affecting the propagation of sound. • Propagation of sound wave
• Speed of sound in different
iv. demonstrate that the speed of sound in different
media is different. media

Glossary: A dictionary of scientific/technical terms

sound : the form of energy which is produced due to vibration of a body

wave motion : a periodic disturbance travelling through a medium which is produced by
a vibrating body

frequency : the number of complete cycles made in one second

transverse wave : a wave that vibrates at 90° to the direction in which it is moving

longitudinal wave : a wave that vibrates in the direction that it is moving

GREEN Science and Environment Book-7 93

Fig.Sound

We hear different types of sound around us everyday. We hear the sound of our
parents and friends, the sound of vehicles running on the roads, the barking
of dogs, sound of aeroplanes, the clatter of pans in the kitchen, the chirping of
birds and so on. In the night, most sounds cease but we can still hear the sound
of some insects. All these sounds produce a sensation of hearing in our ears.
So, sound is form of energy which produces a sensation of hearing in our ears.

Activity 1

Make a list of any eight types of sound heard by you. Write down the names
of the object, living or non-living, which sound the loudest and which the
faintest.

Sound Produced by Vibrating Bodies

When a school bell is hammered, it vibrates and produces sound. When it
stops vibrating, sound is ceased. Sound is produced by the vibration of bodies.
So, every vibrating body is a source of sound.

Activity 2

Take a scale and hold its one end firmly on the table with your one hand.
Flick the free end of the scale with your
another hand. What do you observe? The
scale is seen vibrating and a humming
sound is heard. The humming sound is
produced by the to and fro motion of the
ruler. The ruler produces sound as long
as it remains vibrating.

8.1
Vibrating ruler

Activity 3

Pluck the wire of a guitar or sitar. It produces sound. Touch the wire when it
is producing sound. What happens to the sound? Why?

From the above examples, it can be concluded that sound is produced from
vibrating bodies. When vibration stops, the sound also stops. So, we can
conclude that the main source of sound is a vibrating body.

94 GREEN Science and Environment Book-7

Sources of Sound

We have seen many things like bell,

guitar, loudspeaker, radio, television, Do You Know
etc. that produce sound. These things
are called sources of sound. An object Vibration is the rapid to and fro motion of a
which produces sound is called a particle or a body about its mean position.
source of sound. Radio, television, A body can be vibrated by hitting, stroking,
plucking, blowing, etc.

guitar, animals, vehicles, machines,

etc. are some examples of sources of

sound. Actually, every vibrating object is the source of sound.

Wave Motion

Sound requires a material medium for its propagation. It can travel through
solids, liquids and gases. But sound cannot travel through a vacuum. Sound
propagates in the form of waves. Wave motion is a periodic disturbance
travelling through a medium which is produced by a vibrating body. Wave
motion travels in all possible directions through the medium at a constant
speed. However, articles of the medium do not move from one place to another.
They only vibrate passing on energy they posses from one particle to another.

Wave motion can be easily understood by Fig.
the following example. If we drop a piece
of stone in a pond, ripples spread out in 8.2
all directions on the surface of water. If we
keep a piece of paper on the water surface, Wave motion does not carry
it does not move away along the wave. It matter with it.
moves up and down at the same place. It
means wave motion carries energy and not
matter.

From this experiment, the following
conclusion can be obtained:

i. The disturbance moves from one place to another but water is not carried
along with it.

ii. When a stone is dropped into a pond, the energy carried by the stone disturbs
the water molecules and they start to vibrate by gaining energy from the stone.
Vibrating molecules transfer some energy to the nearby water molecules and
move up and down in the form of ripples.

GREEN Science and Environment Book-7 95

Types of Wave ii. Longitudinal wave

There are two types of waves. They are:

i. Transverse wave

Transverse wave

Visit a nearby pond and drop a piece of stone at the centre of the pond. You can
see ripples on the surface of water in the pond. The ripples produced on the
surface of pond are called transverse waves. The wave in which the particles of
the medium vibrate up and down perpendicular to the direction of the wave
is called a transverse wave.

For example: ripples produced on the water surface in a pond, vibration of
stretched string in musical instruments, waves formed in a slinky when move
up and down. A transverse wave can propagate only in solids and at the
surface of liquids.

Crests

Fig. Troughs
Fig. 8.3

A transverse wave consists of crests and troughs. Crest is the point of maximum
positive displacement of a transverse wave. Similarly, trough is the point of
maximum negative displacement of a transverse wave.

Longitudinal wave
A wave in which particles of the medium vibrate to and fro in the direction
of the wave is called a longitudinal wave. For examples, sound wave, wave
formed in a slinky when pulled and pushed, etc.

Rarefaction Compression
8.4

A longitudinal wave consists of regions of compressions and rarefactions. A
compression is that part of a longitudinal wave in which the particles of the
medium are closer to one another and rarefaction is the part of the wave in
which the particles are farther apart.

96 GREEN Science and Environment Book-7

Differences between transverse wave and longitudinal wave

Transverse wave Longitudinal wave

1. In this wave the particles of the 1. In this wave, the particles of the
medium vibrate up and down medium vibrate to and fro in the
perpendicular to the direction of direction of the wave.
the wave.

2. It is produced only in solids and 2. It is produced in solid, liquid and

surface of liquids. gas.

3. It consists of crests and troughs. 3. It consists of compression and
rarefaction.

Propagation of sound wave Do You Know

Sound needs material medium for Propagation of sound is the transmission of
its propagation. The sound from a sound wave from one place to another.

vibrating body propagates through

air and reaches our ears and we hear

the sound. So the transmission of the sound from one place to another is called

propagation of sound. The propagation of sound takes place through solids,

liquids and gases. Sound cannot propagate through vacuum.

Air is one of the media for the propagation of sound. The speed of sound in air is
about 332 m/s. Sound wave cannot propagate in the absence of air. So, sound cannot
propagate on the surface of the moon as that happens on the surface of the earth.

Liquid is another medium for propagation of sound. The speed of sound in
water is about 1498 m/s. The sound produced by aquatic animals like dolphin,
whale, etc. propagates through water.

Similarly, solid is a medium through which sound propagates. The speed of
sound in the steel medium is about 5200 m/s. The sound wave travels farther
in solids than in liquids or gases.

Sound propagates in the form of longitudinal wave. When sound wave propagates
through air, the particles of air vibrates to and fro parallel to the directions of
propagation of sound wave. It forms regions of compressions and rarefactions
which are shown in the given figure.

Compression

Fig.

8.5 Rarefaction

GREEN Science and Environment Book-7 97

Speed of Sound in Different Media

The speed of sound is not the same in all material medium. It is maximum
in solids and minimum in gases. In solids, molecules are packed closer than
in gases. So, molecules of solids carry vibrations farther than by liquids and
gases. The speed of sound in some common media is given below:

Medium Speed
Water 1498 m/s
Wet air 400 m/s
Dry air 345 m/s
Copper 3700 m/s
Glass 5000 m/s
Steel 5200 m/s

Activity 1
To demonstrate the propagation of sound through a solid.

Button Thread

Fig.
Can
Hearing
8.6 Speaking

Make a toy telephone by using two empty cans, two small buttons and a
long thread. Make a hole at the bottom of each tin can, pass the thread
through the hole and tie its ends to a button. Use this telephone to talk
to your friends. This activity proves that sound can travel through a thread
(solid).

Activity 2

To demonstrate the propagation of sound through liquids.

Take a squeaking toy and keep it in a polythene bag. Hold the toys in a bucket
of water and squeeze it. Can you hear its squeak. Place your ear against the
side of the bucket and squeeze the toy again. Do you hear its squeak? In
which case did you hear the sound more clearly? This activity shows that the
sound travels through liquid.

98 GREEN Science and Environment Book-7

Key Concepts
1. Sound is a form of energy which gives the sensation of hearing in our ears.
2. Sound is produced due to the vibrating body, so vibrating body is the source

of sound.
3. Sound can travel in solid, liquid and gas but it cannot travel in a vacuum.
4. A periodic disturbance travelling through a medium which is produced by

a vibrating body is called wave motion.
5. There are two types of waves. They are transverse wave and longitudinal

wave.
6. Sound travels in the form of longitudinal wave.
7. A transverse wave is the wave in which particles of the medium vibrates

up and down perpendicular to the direction of the wave. A transverse wave
consists of crests and trough.
8. Crest is the point of maximum positive displacement of a transverse wave.
9. Trough is the point of maximum negative displacement of transverse wave.
10. Longitudinal wave is the wave in which the particles of the medium vibrate
to and fro in the direction of the wave.
11. Propagation of sound is the transmission of sound from one place to another.

Exercise

1. Tick (√) the correct statement and cross (×) the incorrect one.
a. Sound is a form of energy which produces sensation of sound.

b. Sound can propagate through a vacuum.

c. Sound travels faster in gas than in solid medium.

d. The speed of sound in the air is 332 m/s.

e. Longitudinal wave consists of crests and troughs.

2. Fill in the blanks with appropriate words.
a. Sound is produced from a ................... body.
b. The wave of sound is a ................... wave.

GREEN Science and Environment Book-7 99

c. ................... medium is required for the propagation of sound.
d. A ................... wave consists of crests and troughs.
e. Sound cannot travel in ...................

3. Choose the best answer from the given alternatives.

a. Sound is a form of energy which gives the sensation of ...................

vision warmth

hearing all of above

b. Sound cannot travel through a ...................

solid liquid

gas vacuum

c. The speed of sound in air is ...................

332 m/s 1500 m/s

5200 m/s 2500 m/s

d. The speed of sound is minimum in ...................

solid liquid

gas air

e. Which one of the following medium produces sound?

solid medium liquid medium

gas medium vibrating medium

4. Answer the following questions:
a. What is sound?
b. How is sound produced?
c. What is source of sound? Write any four examples.
d. What is a wave motion?

100 GREEN Science and Environment Book-7