Oasis Science and Technology 7

A car lifted by jack screw Screws
Figure 3.21: Some screws

6. Wedge
In our daily life, we use a number of simple machines like axe, knife, nail, sickle,
needle, etc. These objects have either sharp or pointed surfaces. Such types of simple
machines are called wedges. So, a wedge is a simple machine having two or more
sloping surfaces that taper either to form a sharp edge or pointed edge.

Effort

Load Load

Figure 3.22: Wedge

Wedges are used for splitting and cutting things, drilling holes, etc. One end of a
wedge is blunt whereas another end is sharp or pointed. An effort is applied to the
blunt end of the wedge to work with sharp edge.

Effort

Sharp edge Axe Khukuri Knife

Typical wedge
Figure 3.23

PHYSICS Oasis School Science and Environment - 7 43

3.3 Importance of Simple Machines

We perform different types of work in our daily life. Some works are easy whereas
some are difficult. In some works, we have to apply more effort. Such works can be
done easily by using simple machines. Such machines make our work easier and
faster by multiplying the effort applied and changing the direction of the force.

Activity 3

• Identify the simple machines given below. Classify them and write down
their names and uses in the given table. Also, draw their diagrams in your
exercise book.

(a) (b) (c)

(d) (e) (f) (g)

(h) (i) (j)

Figure 3.24

S. No. Name of simple machine Use

i.

ii.

iii.

iv.

v.

vi.

What do you learn from this activity? Write down the conclusion of this
activity.

44 Oasis School Science and Environment - 7 PHYSICS

Summary

• The machine which is simple in structure and makes work easier, faster and
more convenient is called a simple machine.

• We use simple machines to multiply the force applied, change the direction
of the force applied and to apply force at a convenient point.

• A lever is a rigid bar which moves freely about a fixed point called the
fulcrum.

• The lever in which the fulcrum is situated between the effort and the load is
called the first class lever.

• The lever in which the load is situated between the fulcrum and the effort is
called the second class lever.

• The lever in which the effort is situated between the fulcrum and the load is
called the third class lever.

• A pulley is a simple machine having a grooved circular disc over which a
rope passes.

• A wheel and axle is a simple machine having two cylinders of different
radius.

• Inclined plane is a sloping surface or a wooden plank which is used to push
things upward.

• A screw is a modified inclined plane with grooves cut in it.
• Wedge is a simple machine having two or more sloping surfaces that taper

either to form a sharp edge or pointed edge.

Exercise

1 Choose the best answer from the given alternatives.

a. Crowbar is an example of ..................

(i) first class lever (ii) second class lever
(iii) pulley (iv) third class lever

b. The distance between the fulcrum and the load is called .................

(i) load arm (ii) effort arm

(iii) fulcrum (iv) all of the above

c. A force applied to a machine to do work is called ..................

(i) output (ii) input
(iii) load (iv) effort

PHYSICS Oasis School Science and Environment - 7 45

d. A pulley is a simple machine used to .................

(i) multiply force (ii) apply a force in a convenient direction

(iii) multiply load (iv) none of the above

e. The doorknob of a house is an example of ................

(i) wedge (ii) pulley

(iii) wheel and axle (iv) screw

f. An axe is an example of ..................

(i) lever (ii) pulley

(iii) wedge (iv) screw

2 Tick (√) the correct statement and cross (x) the incorrect one.
a. Beam balance is an example of a simple machine.
b. Crowbar helps us to multiply the force.
c. Sewing machine is a complex machine.
d. Wheel-barrow is an example of first class lever.
e. In a fixed pulley, the pulley moves with the load.
f. A pulley helps to change the direction of the effort applied.
g. The work done by the machine is called input work.
h. In a wheel and axle, effort is applied on the wheel.

3 Fill in the blanks with appropriate words.

a. A simple machine makes our work easier, .................. and more convenient.
b. A complex machine consists of a number of .................. machines working

together.
c. The fixed point about which a lever can rotate is called ..................
d. Fishing rod is an example of .................. class lever.
e. A pulley is a simple machine which helps to change the .................. of the

force.
f. The distance between two screw threads is called a ..................
g. In the first class lever, .................. lies in between.
h. The work done on a machine is called .................. work.

4 Answer the following questions.

a. What is a simple machine? Give any three examples.
b. Write down any three advantages of using simple machines in our daily life.
c. Name any five types of simple machines. Also, give one example of each.
d. What is a lever ? Name the various classes of lever.
e. What is first class lever ? Give any two examples.
f. What is second class lever? Give any three examples.

46 Oasis School Science and Environment - 7 PHYSICS

g. Define third class lever with any two examples.
h. What is a pulley? Why is it used in our daily life ?
i. What is a wheel and axle? Give any three examples.
j. What is an inclined plane? Write down its uses.
k. What is a screw ? What is meant by pitch and thread of a screw ?
l. What is a wedge? Name any three wedges which are used in our daily life.
m. What is meant by input work? Write down the principle of lever.

5 Differentiate between:

a. Simple machine and Complex machine
b. First class lever and Third class lever
c. Input work and Output work
d. Fixed pulley and Movable pulley

6 Match the following:

A B
First class lever
Wedge Fishing rod
Complex machine Door knob
Inclined plane Wheel-barrow
Third class lever Pulley
Wheel and axle Beam balance
Second class lever Axe
Ramp
Crane

7 Give reason:

a. A bottle-opener is called the second class lever.
b. A pulley is used to lift water from the well.
c. A screw-driver is used to unscrew the rusted nut.
d. The steering of a car is called a wheel and axle.
e. The ramp is used to lift a heavy load.
f. The hill roads are built to have gradual slopes.

8 Classify the given levers. Draw their diagrams showing fulcrum, points of
application of effort and load.

i. Nut-cracker ii. Spade iii. Wheel-barrow iv. Fire tongs

v. Scissors vi. Broom vii. Human arm holding a load

viii. A knife used to cut an apple

PHYSICS Oasis School Science and Environment - 7 47

9 Describe the importance of simple machines in our daily life.

10 Numerical Problems

a. A load of 1000 N can be lifted by applying an effort of 250 N. If the load arm
is 25 cm, calculate the effort arm. [Ans: 100 cm]

b. An effort of 20 N is applied to lift a load. If the load arm and effort arm are
15 cm and 60 cm respectively, calculate the load. [Ans: 80 N]

c. Neha and Reha are playing see-saw. Neha is sitting 60 cm away from the
fulcrum and Reha is sitting 40 cm away from the fulcrum. Calculate the effort
that Reha should apply to lift Neha. The weight of Neha is 360 N.

[Ans: 540 N]

d. Study the given figure and calculate the load distance. [Ans: 20 cm]

40 cm 325 N

650 N

?

e. An effort of 4 N is applied to lift a load of 20 N. If the effort arm is 5m,

calculate the following:

(i) load arm (ii) input work

(iii) output work

[Ans: (i) 1m (ii) 20 Nm (iii) 20 Nm]

48 Oasis School Science and Environment - 7 PHYSICS

Unit Estimated teaching periods: Th Pr
3 1
4
Pressure

Pressure

Objectives

After completing the study of this unit, students will be able to :

• introduce pressure and explain the application of pressure in our daily life
with examples.

• solve some simple numerical problems related to pressure.

Course of Study

• Introduction to pressure
• Measurement of pressure
• Differences between force and pressure
• Application of pressure in our daily life

Points to be Focused/Questions to be Discussed

• What is pressure? What is its SI unit?
• How is pressure calculated?
• What is the relation between pressure, force and area?
• What are the applications of pressure in our daily life?

PHYSICS Oasis School Science and Environment - 7 49

4.1 Introduction

We prefer a sharp knife for cutting
vegetables to a blunt one. A sharp knife
cuts vegetables better due to its thin edge.
The force of our hand falls over a small area
of the object producing a large pressure
which cuts the object easily. But a blunt
knife does not cut an object easily due to
its thick edge. The force of our hand falls
over a large area of the object and produces Figure 4.1

less pressure which cuts the object with difficulty. Thus, the effect of the same force
on different areas is different. The force acting perpendicularly on a unit area of
a surface is called pressure. If 'P' is the pressure exerted when a force 'F' acts on a
F
surface area (A), then Pressure (P) = A .

In SI system, pressure is measured in newton per square metre (N/m²). It is also

called pascal (Pa). Pressure is a scalar quantity.

4.2 Measurement of Pressure

The value of pressure is obtained by dividing the force acting on an object by the
area of the object on which the force acts.
The total force acting perpendicularly on a given surface is called thrust. So pressure
can also be represented as follows:

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

From the above relation, it can be concluded that pressure depends on the following
two factors:

(i) Force applied or the thrust
(ii) Area over which the force acts

The same force can produce different pressures depending on the area over which
it acts. When a 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 large pressure. When the
force acting on a surface increases, the pressure also increases and vice-versa.

Example 1

A wooden block of 800N occupies 2m² surface area. Calculate the pressure
exerted.

Solution:
Given,

Force (F) = 800N

pressure /ˈpreʃə/ - the thrust acting per unit area of a surface

50 Oasis School Science and Environment - 7 PHYSICS

Area (A) = 2m²

Pressure (P) = ?

According to the formula,

P= F
A
800
P= 2

P = 400 N/m²

∴ The pressure exerted (P) = 400 Pa.

Differences between Force and Pressure

Force Pressure
The force acting per unit area of a
1. Force is the pull or push which 1. surface is called pressure.
changes or tries to change the
position of a body. It is measured in pascal (Pa).

2. It is measured in newton (N). 2.

3. Its formula is F = m × a. 3. Its formula is P= F .
A

Activity 1

• Take a brick and a piece of foam. Find the weight of the brick. Put the brick
on the foam with its broad surface as shown in fig. (a) and with its narrow
surface as shown in fig. (b) separately. Observe the depression formed by
the brick in both cases. Also, comment on your observation.

Brick Brick

Foam

(a) (b)

Figure 4.2

PHYSICS Oasis School Science and Environment - 7 51

4.3 Application of Pressure in Our Daily Life

• The cutting edge of knives, blades, axes, khukuris, etc.
are sharpened to increase the pressure by decreasing
the area of their edges.

Figure 4.3

• The rear wheels of tractors are made large and flat so
that there is less pressure on the ground and the tyres
do not sink into the ground.

• Studs are kept on the football player’s boot to increase Figure 4.4
the pressure by decreasing the surface area of the sole
so that the player does not slide due to less pressure. Figure 4.5
Figure 4.6
• Camels can walk easily on sand in desert as compared
to horses or donkeys because they have broad and flat Figure 4.7
soles which exert less pressure on the sand. Figure 4.8

• A drawing pin is kept broad at the thumb side but
sharp and pointed at the pin side. It is done to reduce
pressure at the thumb and to increase pressure on the
drawing board.

• A broad steel belt is provided over the wheels of army
tanks so that they exert less pressure on the ground and
do not sink into it.

• Heavy trucks are fitted with six to eight double wheels
to increase the area of contact on which their weight
acts and hence reduces the pressure on the ground.

Figure 4.9
rear /rɪə/ - the back part of something
stud /stʌd/ - a small round piece of sth which is attached to the surface of sth

52 Oasis School Science and Environment - 7 PHYSICS

• Wooden sleepers are kept below railway line so that Figure 4.10
there is less pressure of the train on the ground and
railway line does not sink into the ground.

• School bags are provided with wide straps so that the
weight of bag may fall over a large area of the shoulder
of the child producing less pressure on the shoulder.

Figure 4.11

Summary

• The force acting perpendicular on a unit area of a surface is called pressure.
• The total force acting perpendicular on a given surface is called thrust.
• The same force can produce different pressures depending on the area over

which it acts.
• When the force acting on a surface increases, the pressure also increases and

vice-versa.
• Pressure plays a significant role in our day to day activities. Sometimes we

should increase pressure and sometimes we should decrease it.

Exercise

1 Choose the best answer from the given alternatives.

a. The force acting per unit area of a surface is called __________

(i) force (ii) pressure
(ii) pascal (iv) volume

b. The SI unit of pressure is __________

(i) Nm² (ii) N/m³
(iii) N/m² (iv) N/m

c. The pressure exerted by a body depends on __________

(i) force and area (ii) area and length
(iii) force and volume (iv) force and mass

d. When the force acting on a body increases, the pressure __________

(i) decreases (ii) increases
(iii) remains the same (iv) multiplies

PHYSICS Oasis School Science and Environment - 7 53

2 Tick (√) the correct statement and cross (×) the incorrect one.
a. Pressure is the thrust acting per unit area of a surface.
b. The SI unit of pressure is newton square metre.
c. Pressure depends only on area.
d. A blunt knife exerts more pressure than a sharp one.

3 Fill in the blanks with appropriate words.
a. The ……….. acting per unit area is called pressure.
b. The SI unit of pressure is ………..
c. When area ……….., the pressure increases.
d. The tip of a sewing needle is made sharp to increase ………..
e. The SI unit of ……….. is pascal (Pa).

4 Answer the following questions.
a. What is pressure? Write down its SI unit.
b. Which factors do pressure depend on?
c. Which formula is used to calculate pressure?
d. Write any three applications of pressure in our daily life.
e. Why do we prefer a sharp knife to a blunt one to cut vegetables?
f. Why are studs made on the sole of football player's boots?
g. Write any two differences between force and pressure.

5 Give reason.
a. Studs or spikes are made on the football player's boots.
b. The rear wheels of a tractor are made large and flat.
c. The tip of a nail is made sharp.
d. Foundations of high buildings are made wide.
e. Skiers use long and flat skies to slide over snow.
f. A camel can walk easily on desert but a horse cannot.

6 Numerical Problems
a. A force of 300N acts on the area of 1.5m². Calculate the pressure exerted.
[Ans: 200 Pa]
b. When a box is kept on a surface of 2m², a pressure of 400 pascal is exerted.
Calculate the force.
[Ans 800N]
c. The weight of a box is 1200N. If it exerts a pressure of 800 Pa, calculate the area.
[Ans: 1.5m²]

54 Oasis School Science and Environment - 7 PHYSICS

Unit Estimated teaching periods: Th Pr
3 1
5
Crane

Energy, Work and Power

Objectives

After completing the study of this unit, students will be able to :

• introduce energy, work and power.
• explain the types of energy.
• explain the types of work.

Course of Study

• Introduction to energy
• Types of energy
• Introduction to work
• Types of work
• Power

Points to be Focused / Questions to be Discussed

• What is energy? What is its SI unit?
• What are the different types of energy?
• What is work? What is its SI unit?
• What are the different types of work?
• What is power? What is its SI unit?

PHYSICS Oasis School Science and Environment - 7 55

5.1 Introduction

We eat food to get energy. When we work for a long time, we get tired and cannot
work any more due to lack of energy. Thus, the ability or capacity to do work is
called energy.
We do a variety of works in our daily life. In Physics, work is said to be done when
a body moves through a distance when a force is applied to it. Energy and work are
measured in joule (J). The rate of doing work is called power. In SI system, power
is measured in watt (W).

5.2 Energy

Food provides necessary energy to our body. Energy is required to do various types
of work. The capacity to do work is called energy. It is a scalar quantity. Mechanical
energy, electrical energy, sound energy, heat energy, etc. are some examples
of energy. In SI system, energy is measured in joule (J) and in CGS system, it is
measured in erg.
The magnitude of energy is equal to the magnitude of work done. So, we use the
formula of work done to calculate the value of energy.

Types of Energy
There are different types of energy. Mechanical energy, heat energy, sound energy,
atomic energy, electrical energy, chemical energy, magnetic energy, etc. are some
major forms of energy. Let us discuss the various types of energy.
1. Mechanical Energy
The energy contained by a body at rest or in motion is called mechanical energy. It
is the sum of kinetic energy and potential energy. It has two types:
a. Kinetic Energy: The energy possessed by a body in motion is called kinetic energy.
All moving bodies have this type of energy. For example, a fired bullet, a flying
bird, a moving vehicle, water falling from a dam, etc. possess kinetic energy. If a
stone is thrown to a window pane, it breaks. This is due to the kinetic energy of the
stone.

Flying bird Flying aeroplane Running water
Figure 5.1: Objects having kinetic energy PHYSICS

56 Oasis School Science and Environment - 7

Kinetic energy is calculated by the following formula:

Kinetic energy (KE) = 1 mass × (velocity)2
2

i.e. KE = 1 mv²
2

Where,

m = mass of the moving body

v = velocity of the moving body

It is to be noted that the kinetic energy of a moving body is directly proportional to
the product of the mass and square of the velocity of the body.

b. Potential Energy: The energy possessed by a
body at rest is called potential energy. A stretched
rubber, a leg lifted to kick the ball, water stored
in a dam, etc. possess potential energy.

When a body of mass ‘m’ is raised to a height
‘h’, the energy contained by the body (PE) is
calculated by the following formula.

Potential Energy (PE) = m × g × h Figure 5.2: A leg lifted to kick the ball
∴ PE = mgh

Where, ‘g’ is acceleration due to gravity and its value is 9.8 m/s².

2. Heat Energy

The energy obtained from a hot object is called
heat energy. It always flows from a hot object to a
cold one. So, it gives the sensation of warmth. Coal,
petrol, diesel, gas, etc. burn and produce heat energy
which can be used to run engines. The heat obtained
from the sun can be used to dry clothes. It also helps
to maintain water cycle in nature. Heat energy is Sun Heater
Figure 5.3: Sources of heat energy

produced due to movement of molecules present in a body.

3. Sound Energy

Sound is a form of energy
which is produced due to
the vibration of a body.
Sound can travel through a
material medium. It cannot
travel through vacuum. Harmonium Guitar Loudspeakers
Loudspeaker, radio, tape
Figure 5.4: Sources of sound energy

recorder, guitar, harmonium, horns of vehicles, etc. are some sources of sound.

vibration /vaɪˈbreɪʃn/ - a continuous shaking movement
vacuum /ˈvækjuːm / - a space which is completely empty of all substance, including all air

PHYSICS Oasis School Science and Environment - 7 57

4. Light Energy
Light is a form of energy that makes things visible. Sunlight is used by green plants
to prepare food. Coal, gas, mineral oil, etc. burn and produce light energy. Light is
the main source of energy. The sun is the major source of light energy.

5. Magnetic Energy

The energy possessed by a magnet is called magnetic
energy. It is used to produce electricity and to lift heavy
loads of magnetic substances like iron. It is used in
loudspeaker, telephone, radio, telegraph, etc.

6. Chemical Energy Figure 5.5: Magnetic energy

The energy produced due to the chemical change of a substance is called chemical
energy. Coal, wood, gas, mineral oil, etc. store chemical energy. When these
substances are burnt, energy is produced. This energy is chemical energy. In a
chemical change, the state of a body is altered and the new substances are produced.
In our body, the digested food reacts with oxygen in mitochondria and releases
energy. Similarly, dry cell, bread, rice, etc. contain chemical energy.

7. Nuclear Energy

The energy which is obtained by splitting of heavy nucleus
of an atom or combining light nuclei together is called
nuclear energy. Such energy produces a large amount of
heat, light and sound. This energy is used to heat water
which helps to run steam engine to produce electricity.

8. Electrical energy Figure 5.6: Bomb explosion

The energy produced due to continuous flow of electrons through a conductor is
called electrical energy. It is used to rotate fans, operate televisions, light bulbs,
drive trains, run washing machines, etc. Electrical energy can produce heat, light,
sound, etc. Electrical energy can easily be converted into other forms of energy
like mechanical energy, sound energy, heat energy, light energy, etc. Therefore,
electrical energy is very useful to us.

Activity 1

• Take a matchbox and a candle.
• Light the candle and observe it carefully.
• A burning candle gives light and heat energy.

rotate /rəʊˈteɪt/ - to move or turn around a central fixed point
58 Oasis School Science and Environment - 7 PHYSICS

Activity 2

• Take a madal and place it vertically on the table.
• Hit on the skin of the madal and observe the skin carefully.
• The skin vibrates and produces sound.

It shows that sound is produced due to vibration.

Activity 3 Figure 5.7

• Take a bar magnet and place it near the heap of iron dust.
• What do you observe? Does the magnet attract iron

dust?
This type of energy is called magnetic energy.

5.3 Work

The term work is very common in our daily life. People do various activities in their
daily life which are generally called work. For example, writing, reading, running,
etc. In science, work is defined in a specific way. Work is said to be done if force is
applied on a body and the body moves in the direction of the force. But no work is
done if the body is not displaced even if a force is applied on it. So, when we read
a book, there is no displacement. Thus no work is done in this case. “Work is said
to be done if a force applied on a body can displace the body in the direction of the
force.” It is a scalar quantity. In SI system, work is measured in joule (J).
Formula of Work Done
Work done is the product of force and displacement.
Therefore,
Work done (w) = Force (F) × Displacement (s)

∴ w=F×s

One Joule Work

Work is said to be 1 joule if 1N force displaces a body through 1m in the direction
of the force.

Work Done

If the applied force is perpendicular to the displacement,
no work is done by the force. For example, when a
person moves on the ground by carrying a load of 20 kg,
the work done by gravity is zero.

In the above case, you need to remember that work done Figure 5.8:Showing no work done
by gravity is zero but the work done by the person is not
zero. It means that there is some work done by the person.

PHYSICS Oasis School Science and Environment - 7 59

Types of Work

Generally, there are two types of work:

(i) Work done against gravity
(ii) Work done against friction

(i) Work done against gravity: The force due to which a body is attracted towards the
earth is called gravity. The work done by lifting a body vertically upwards is called
work done against gravity. For example, lifting a bag from the ground, lifting water
from the well, etc.

(ii) Work done against friction: The force which opposes the motion of a body is called
friction. The work done by pulling or pushing a body horizontally on a surface is
called work done against friction. For example, work done by pulling a load on the
table, pushing a body horizontally on the road, etc.

Example 1

A cart is displaced to a distance of 25m by applying
500N effort. Calculate the work done.

Solution:

Given,

Force (F) = 500N

Displacement (s) = 25 m Figure 5.9:Work done against gravity
Work done (w) = ?

According to the formula,

w = F×s

= 500 × 25

= 12500J

∴ The work done (w) = 12500J.

5.4 Power

A person ‘A’ completes a particular work in 10 seconds but another person ‘B’
completes the same work in 5 seconds. We say that the 'rate of doing work' of
person ‘B’ is more than that of ‘A’.

Thus, the rate of doing work is called power. The SI unit of power is joule per
second (J/s) or watt (W). Power is a scalar quantity.

Power can be calculated by the given formula:

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

∴ P= w
t

60 Oasis School Science and Environment - 7 PHYSICS

Above formula shows the relation between power (P), work done (w) and time
taken (t). Thus, power depends on two factors (i) work done, and (ii) time taken.

When a body takes less time to do a particular work, its power is said to be more
and vice–versa. Similarly, when a body does more work in a particular time, its
power is said to be more and vice–versa.

One watt power

The rate of doing one joule work in one second time is called one watt (1W) power.

Horsepower

It is a popular unit of power mostly used in engineering. It is denoted by H.P.
1H.P. = 746W

Relationship between watt (W), kilowatt (kW) and megawatt (MW)

1000 W = 1 kW

1000 kW = 1 MW

1MW = 1000000W

Example 2

A person does 80J of work in 5 seconds. Calculate the power of the person.

Solution: 80J
Given,
Work done (w) =

Time taken (t) = 5s

Power (P) =?

According to the formula,

P= w = 80 = 16 J/s
t 5

∴ The power of the person is 16W.

Example 3

A crane lifts a load of 1000N to a height of 15 metre in 10 seconds. Calculate the
work done and power of the crane.

Solution:
Given,

Force or load (F) = 1000 N

Displacement (s) = 15 m
Time taken (t) = 10 s

PHYSICS Oasis School Science and Environment - 7 61

Work done (w) = ?

Power (P) =?

According to the formula,

w = F×s

= 1000 × 15 Nm

= 15000J

∴ Work done by the crane (w) = 15000J

Now,

P= w
t
15000
= 10

= 1500 J/s

∴ The power of the crane (P) = 1500 W

Summary

• The capacity to do work is called energy. It is a scalar quantity.

• The energy possessed by a body at rest is called potential energy.

• The energy obtained from a hot object is called heat energy.

• Sound is a form of energy which is produced due to the vibration of a body.

• Light is a form of energy that makes things visible.

• The energy possessed by a magnet is called magnetic energy.

• The energy produced due to the chemical change of a substance is called
chemical energy.

• The energy which is obtained by splitting of heavy nucleus of an atom or
combining light nuclei together is called nuclear energy.

• The energy produced due to the continuous flow of electrons through a
conductor is called electrical energy.

• Work is said to be done if a force applied on a body can displace the body in
the direction of the force.

• Work is said to be 1 joule if 1N force displaces a body through 1m in the
direction of the force.

• The rate of doing work is called power. Its SI unit is watt (W).

62 Oasis School Science and Environment - 7 PHYSICS

Exercise

1 Choose the best answer from the given alternatives.
a. The capacity of doing work is called _______

(i) work (ii) force

(iii) energy (iv) power

b. Flowing water and blowing air contain _______

(i) heat energy (ii) kinetic energy
(iii) light energy (iv) potential energy

c. Which form of energy is produced while playing a drum?

(i) light energy (ii) sound energy
(iii) electrical energy (iv) magnetic energy

d. The SI unit of work is _______

(i) joule (ii) newton

(iii) watt (iv) metre

e. The rate of doing work is called _______

(i) force (ii) energy
(iii) power (iv) work

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

a. The ability to do work is called energy.

b. A stretched rubber contains light energy.

c. Heat energy is produced due to kinetic energy.

d. Pulling water from a well is a work against friction.

e. The SI unit of power is joule.

3 Fill in the blanks using appropriate words.

a. The energy contained by a body in motion is called ………........
b. ………........ is a form of energy which makes things visible.
c. Dry cell, bread, rice, etc. contain ………........energy.
d. ………........ energy is used to rotate fans.
e. The rate of doing work is called ………........

PHYSICS Oasis School Science and Environment - 7 63

4 Answer the following questions.

a. What is energy? Write down its SI unit.

b. What is mechanical energy? Name two types of mechanical energy.
c. What is kinetic energy? Give an example.
d. What is potential energy? Give an example.
e. What is heat energy? Name any two sources of light energy.
f. What is meant by electrical energy?
g. What is work done? Write its SI unit. Define one joule work.
h. What is power? Write its SI unit.
i. What is meant by one watt power?

5 Differentiate between:

a. Potential energy and Kinetic energy
b. Sound energy and Magnetic energy
c. Work done against friction and Work done against gravity
d. Work and Power

6 Name the form of energy present in each of the given objects:

a. running water
b. water stored in a pond
c. burning candle
d. battery
e. ringing bell
f. magnet
g. rotating fan

7 Numerical Problems

a. A box is displaced to a distance of 25m by applying 200N effort. Calculate the
work done. [Ans: 5000J]

b. Calculate the amount of force applied when a body is displaced to a distance
of 15m. The amount of work done is 3000J. [Ans: 200N]

c. A person does 2500J of work in 10 seconds. Calculate the power of the
person. [Ans: 250W]

d. A wooden log is displaced to a distance of 20m in 10 seconds by applying
500N effort. Calculate the work done and power.

[Ans: w = 10000J, P = 1000W]

64 Oasis School Science and Environment - 7 PHYSICS

Unit Estimated teaching periods: Th Pr
3 1
6
Heat

Heat

Objectives

After completing the study of this unit, students will be able to :

• demonstrate various methods of transmission of heat and explain them.
• introduce temperature.
• explain the structure of thermometer and measure temperature by using it.

Course of Study

• Introduction to heat
• Transmission of heat
– Conduction
– Convection
– Radiation
• Structure and utility of thermos flask
• Temperature
• Structure of thermometer

Points to be Focused / Questions to be Discussed

• What is heat?
• What is meant by transmission of heat?
• What are the different modes of transmission of heat?
• What are conduction, convection and radiation?
• Why is thermos flask used?
• What is temperature?
• What is thermometer? Why is it used?

PHYSICS Oasis School Science and Environment - 7 65

6.1 Introduction

When we touch a burning candle, we feel hot and when we touch an ice-cube, we
feel cold. Similarly, when we rub our hands against each other, they become warm.
It is because the kinetic energy changes into heat energy and we feel hot. So, heat
is a form of energy which causes the sensation of warmth or coldness. When heat
flows into our body, we get the sensation of warmth and when it flows out from
our body, we get the sensation of coldness. Heat is produced due to the movement
of molecules. Heat is measured in joule (J), calorie (cal.), etc. In our daily life, we
use heat for various purposes, such as cooking, drying clothes, running engines,
heating water, etc. The sun is the main source of heat energy. We also obtain heat
energy from fire, electric heater, diesel, coal, etc.
When a body is heated, the following effects can be observed in the body:
i. increase in volume (expansion)
ii. increase in temperature
iii. change of state

6.2 Transmission of Heat

It is a well-known fact that water flows from a higher level to a lower level. Similarly,
heat flows from one place to another due to the difference in temperature between
them. When two bodies at different temperature are brought into contact, heat flows
from the body at higher temperature to the body at lower temperature. In our daily
life, we observe the transmission of heat. For example, the sun is extremely hot. The
heat of the sun flows to the earth as the sun is at higher temperature than the earth.
When a body gets heat, it becomes hot and if the body loses heat, it becomes cold.
The flow of heat energy from a body at a higher temperature to another body at
lower temperature is called transmission of heat. This process also takes place from
one part of a body to another part of the same body.
There are three distinct modes of transmission of heat. They are conduction,
convection and radiation.
(i) Conduction
When one end of an iron rod is heated, its another end also becomes hot. This method
of transmission of heat from one end of the rod to another is called conduction.
All solids get heated through conduction. The process of transfer of heat from one
particle to another without actual movement of the particles is called conduction.
In this method, no physical movement of the particles within the material medium
takes place.
In solids, molecules are very closely packed together. If we heat one end of a metallic
rod, the molecules at that end absorb heat and start vibrating. These vibrating
molecules collide with their neighbouring molecules and transfer a part of heat

sensation /senˈseɪʃən/ - the ability to feel through sense of touch

66 Oasis School Science and Environment - 7 PHYSICS

energy to them. These particles start vibrating more rapidly and transfer heat energy
to their neighbouring particles and so on. In this way, transmission of heat energy
takes place in solids. Such method of transmission of heat is called conduction. This
process continues till both ends of the rod are at the same temperature.

Experiment 1

Objective: To demonstrate the transmission of heat in solids by conduction

Materials required: Stand, iron rod, wire, iron nails, Bunsen burner

Procedure

• Bring a small piece of iron rod and clamp it to a stand. Fix 4-5 iron nails on the iron
rod with the help of molten wax as shown in the fig. 6.1 (a). Heat the iron rod at one
end with the help of a Bunsen burner. What do you observe after 2-3 minutes?

Iron rod Direction in which heat travels

Iron nails attached Iron nails
to the rod falling down
Stand
Stand Bunsen
(a) burner

Figure 6.1: (a) Before heating and (b) After heating (b)

Observation
After a few minutes, it can be observed that the iron nails fall one by one starting
from the end which is near the flame of the burner [fig. 6.1 (b)].
Conclusion
This experiment proves that the heat is transferred from the hot end to the cold end
of a body by conduction.

Activity 1

• Take a metal rod and a wooden stick. Hold one end of the metal rod in one
hand and the wooden stick in another. Now, heat the other end of the rod
and wooden stick by a Bunsen burner. Wait for a while. What do you feel?
After some time, it can be felt that the wooden stick can be held for a
longer time than the metal rod. It is because the metal rod becomes hot
very soon. So, we cannot hold it for a long time. In other words, the metal
rod conducts heat easily. So, it becomes hot very soon. But the wooden
stick does not conduct heat easily. So, it remains cold and we can hold it
for a long time easily.

PHYSICS Oasis School Science and Environment - 7 67

Good conductors and Bad conductors

The substances which conduct heat easily are called good conductors of heat. All
metals and their alloys conduct heat easily. So, they are good conductors. Copper,
aluminium, iron, silver, gold, etc. are some examples of good conductors. Silver is
the best conductor of heat.

The substances which do not conduct heat easily are called bad conductors or poor
conductors of heat. All liquids except mercury and gases are poor conductors of heat.
Cloth, glass rod, wood, paper, plastic, etc. are some examples of bad conductors.
Bad conductors are also called insulators.

(ii) Convection

Convection is the process by which heat is transmitted through liquids and gases
from one point to another due to the actual movement of molecules. In liquids and
gases, heat is transmitted by convection. It is because the molecules of liquids and
gases are free to move about. Convection is not possible in case of solids because
the molecules in solids are very closely packed. The process of transmission of heat
by the actual movement of molecules of a medium is called convection.

When water is heated, the water molecules at the bottom of the container get
heated first. These molecules expand and become lighter. Those hot molecules
have more kinetic energy and rise up whereas cold water molecules, being heavier,
come down. In this way, each water molecule at the bottom gets heated and rises
up and cold molecules fall down at the bottom. This action sets the flow of water
molecules which is known as convectional current. This current transfers heat to
the entire mass of water. Therefore, the transmission of heat takes place by the
actual movement of hot water molecules.

In coastal regions, breeze generally blows from the sea to the land during the day
and from the land to the sea during the night. This is an example of convection
current of air or gases.

Experiment 2

Objective: To demonstrate the transmission of heat (convection) in liquids

Materials required: Beakers, water, a few crystals of potassium permanganate,
tripod stand, Bunsen burner, wire gauze

Procedure

• Take a beaker and fill it Beaker Water
partially with water. Put Convection
a crystal of potassium Beaker Potassium
permanganate into it current permanganate
without disturbing the Water
Potassium Wire gauze Tripod stand
permanganate Burner

water. Keep the beaker (a) Figure 6.2 (b)

on a table and wait for
a while [fig. 6.2 (a)]. What do you observe?

68 Oasis School Science and Environment - 7 PHYSICS

• Take another beaker and fill it partially with water. Put a crystal of potassium
permanganate. Now, heat the beaker by using a Bunsen burner [fig. 6.2 (b)]. What
do you observe?

Observation

In the first beaker [Fig. 6.2 (a)], the crystal of potassium permanganate dissolves
in water and the colour of the crystal moves uniformly but convectional current
cannot be seen.

In the second beaker [fig. 6.2 (b)], it can be observed that the coloured water rises
from the place where the heat is being applied. After moving some distance, the
coloured water spreads and comes down along the sides of beaker. In this beaker,
convectional current can be observed. This current transfers heat to the entire mass
of water by actual movement of heated water molecules.

Conclusion

This experiment proves that the transmission of heat takes place by the actual
movement of heated molecules or convection in liquids.

(iii) Radiation

We feel very hot when we stand in the sun during
summer season. We know that there is no material
medium between the earth and the sun except the
earth’s atmosphere. Then how does the heat travel
from the sun to the earth? It is because of the third
mode of heat transmission which does not require
a material medium. This mode of heat transfer is
called radiation. So, radiation can be defined as the Figure 6.3

transmission of heat from a hot body to a cold body without affecting the medium.
A material medium is not essential for radiation. So, radiation also takes place in
vacuum. Transmission of heat energy from the sun to the earth, from an electric
heater to its surroundings, etc. are some examples of radiation. The heat energy
emitted by a hot body is called radiant heat. This heat energy propagates in a straight
line with the speed of light, i.e. three lakh kilo meters in one second.

6.3 Blowing of Air or Wind

During the day, the air on the surface of the earth gets heated due to the heat of the
sun. So, the hot air becomes lighter and moves upwards while its place is occupied
by cold air. This movement of air is called wind or blowing of air. The movement of
hot and light air upwards and heavy air downwards is called convection of air.

6.4 Thermos flask

The device which keeps hot liquids hot and cold liquids cold for several hours is
called a thermos flask. It was invented by James Dewar, a British scientist in 1890 AD.

radiant /ˈreɪdɪənt/ - giving a warm bright light

PHYSICS Oasis School Science and Environment - 7 69

Structure

A thermos flask consists of a double-walled glass vessel, outer case, cork support,
stopper and a lid. There is a vacuum in between the two walls of the thermos flask.
This vacuum reduces heat loss due to conduction and convection. The outer surface
of the inner wall and the inner surface of the outer wall are silvered. This silvered
surface reduces heat loss by radiation.

The mouth of the vessel is closed by a cork, which is a bad
conductor of heat. The glass vessel is kept on cork pad and
surrounded with an outer case. It is thus thermally insulated
and keeps hot liquids hot and cold liquids cold for a long
period of time.

Use Figure 6.4: Thermos flask

A thermos flask is used to keep hot liquids hot and cold liquids cold for several
hours.

6.5 Temperature

When we touch an object, we know whether it is hot or cold. When a body is hotter
than our body temperature, we feel it hot and when the temperature of a body is
less than that of our body temperature, we feel it cold. Thus, the degree of hotness
or coldness of a body is called temperature.
Thermometer is used to measure the temperature of a body. In SI system,
temperature is measured in kelvin (K). However, it is widely measured in degree
Celsius (ºC) and degree Fahrenheit (ºF).
Differences between Heat and Temperature

S. N. Heat S. N. Temperature
1. It is the degree of hotness or
It is a form of energy which 1. coldness of a body.
2. produces the sensation of
warmth. It is the effect of heat.

It is the cause of change in 2.
temperature.

3. Heat always flows from a hot 3. Temperature gives the direction

body to the cold body. of flow of heat.

4. The SI unit of heat is joule (J). 4. The SI unit of temperature is
kelvin (K).

6.6 Thermometer Figure 6.5: Thermometer

When we touch an object, we can get the idea of
temperature of that body. If the object is felt hot, we
say its temperature is high and if the object is felt cold,
we say its temperature is low. However, we cannot

70 Oasis School Science and Environment - 7 PHYSICS

depend on our sense of touch to estimate the temperature of a body. So, scientists
have designed a special device for measuring temperature of objects which is called
thermometer.
The word thermometer has been derived from two Latin words ‘thermo’ which
means heat and ‘meter’ which means a measuring device. So thermometer is a device
which is used for measuring the temperature of a body. The earliest thermometer
was developed by Galileo in 1592 AD. Thermometer is constructed on the principle
that fluids expand on heating and contract on cooling. It is also called the principle
of thermometer. Doctors use a small thermometer to measure our body temperature
when we get fever. This thermometer is called clinical thermometer.
In this unit, we will discuss the structure of a laboratory thermometer and
thermometric liquids.
Structure of a Thermometer
A laboratory thermometer consists of a long glass tube having a thin-walled
capillary tube. A glass bulb of suitable size is attached at one end of the capillary
tube. This bulb and some portion of the capillary tube is filled with a thermometric
liquid (i.e. mercury or alcohol). Near the other end of the capillary tube, there is an
expansion chamber to allow for excess expansion.

Bulb Mercury Capillary tube Glass stem

Figure 6.6: Structure of a laboratory thermometer

The air above the thermometric liquid in the capillary tube is removed by
evacuation and the other end of the glass tube is sealed. The entire length of the
glass tube is calibrated in degree Celsius (ºC) or degree Fahrenheit (ºF) scales to
read the temperature (fig. 6.6). A common laboratory thermometer can measure the
temperature from –10ºC to 110ºC. It is called the range of thermometer.
Working
To measure the temperature of a body, the bulb of a thermometer is kept in close
contact with the body for a few minutes. The thermometric liquid kept in the bulb
expands or contracts which shows the temperature of the body.
If the body whose temperature is to be measured has a temperature below the room
temperature, the level of thermometric liquid (say mercury) contracts. As a result,
the level of the mercury column in the capillary tube falls down to a constant value.
This value is read on the graduated scale which is the temperature of that body.
If the body whose temperature is to be measured has a higher temperature than
the room temperature, the mercury in the bulb expands. As a result, the level
of mercury in the capillary tube rises and finally becomes constant at a certain
value. This value is read in the graduated scale which determines the temperature
of that body.

PHYSICS Oasis School Science and Environment - 7 71

Activity 2

• Bring some hot water in a beaker and dip the bulb of a laboratory
thermometer into it. Observe the expansion of mercury inside the capillary
tube. Record the temperature of the hot water.

• Bring some cold water in a beaker and now dip the bulb of the same
thermometer into it. What do you observe? Is the level of mercury in the
capillary tube falling down?
What do you learn from this activity?

6.7 Thermometric Liquids

The liquids which are filled inside the bulb of a thermometer are called thermometric
liquids. Generally, mercury and alcohol are used as thermometric liquids.

Advantages of Mercury as a Thermometric Liquid
(i) Mercury is a silvery white liquid. So it can be seen easily in the capillary tube.
(ii) It is a good conductor of heat being a liquid metal.
(iii) Its rate of expansion and contraction is uniform.
(iv) It does not stick to the inner wall of the capillary tube.
(v) It remains in liquid state in a wide range of temperature. The boiling point of

mercury is 357 °C and its freezing point is –39 °C.
Disadvantage of Mercury as a Thermometric Liquid
The freezing point of mercury is –39°C. So, it cannot measure the temperature
below –39°C. Due to this reason, mercury thermometer cannot be used in very cold
regions to measure very low temperature.

Advantages of Alcohol as a Thermometric Liquid
(i) The freezing point of alcohol is –117°C. So, it can be used to measure very low

temperature in cold regions.
(ii) Its expansion rate is six times more than that of mercury. So, it is a more

sensitive liquid.
(iii) It is cheaper than mercury.
Disadvantage of Alcohol as a Thermometric Liquid
Since the boiling point of alcohol is 78°C, alcohol thermometer is not suitable to
measure the temperature above 78°C.

72 Oasis School Science and Environment - 7 PHYSICS

Activity 3

• Bring a mercury thermometer and an alcohol thermometer. Study their
structure carefully and draw a labelled diagram of each.

• Measure your body temperature by using both of them one by one. What
do you find?

Summary

• Heat is a form of energy which causes the sensation of warmth or coldness.
• The flow of heat energy from a body at higher temperature to another body

at lower temperature is called transmission of heat.
• The process of transfer of heat from one particle to another without the

actual movement of the particles is called conduction.
• The process of transmission of heat by the actual movement of molecules of

a medium is called convection.
• Radiation can be defined as the transmission of heat from a hot body to a

cold body without affecting the medium.
• The device which keeps hot liquids hot and cold liquids cold for several

hours is called thermos flask.
• Thermometer is a device which is used for measuring temperature. The

degree of hotness or coldness of a body is called its temperature.
• Thermometer is constructed on the principle that fluids expand on heating

and contract on cooling.
• The liquids which are filled inside the bulb of a thermometer are called

thermometric liquids.

Exercise

1 Choose the best answer from the given alternatives.

a. Heat is measured in ______.

(i) kelvin (ii) calorie
(iii) metre (iv) degree Celsius

b. Conduction takes place in ______.

(i) solid (ii) liquid
(iii) gas (iv) all of the above

c. The transmission of heat in the absence of a medium is called ______.

(i) conduction (ii) convection
(iii) radiation (iv) none of the above

PHYSICS Oasis School Science and Environment - 7 73

d. The boiling point of mercury is ______.

(i) 375°C (ii) 257°C

(iii) 357°C (iv) 735ºC

e. The freezing point of alcohol is ______.

(i) – 117°C (ii) –171°C

(iii) 171°C (iv) 271°C

2 Tick (√) the correct statement and cross (x) the incorrect one.

a. Solids expand on heating.
b. Gases have a fixed shape and volume.
c. Conduction does not take place in gases.
d. A material medium is not required for radiation.
e. Wood is a poor conductor of heat.
f. The device which is used for measuring heat is called thermometer.
g. In very cold regions, alcohol thermometer is suitable for measuring

temperature.

3 Fill in the blanks with appropriate words.

a. The SI unit of heat is ............
b. In liquids, the molecules are ............ packed.
c. The substances which do not conduct heat easily are called ............
d. The heat energy propagates in a ............ line with the speed of ............
e. The bulb of a thermometer is filled with ............

4 Answer the following questions.
a. What is heat? Write down its units.
b. Write down any two effects of heat.
c. What is meant by transmission of heat?
d. Name the three modes by which heat is transmitted.
e. What is conduction? Give one example.
f. What are good conductors? Give any two examples.
g. Define convection and convectional current.
h. What is radiation? Can this process take place in a vacuum? Why?
i. What is meant by convection of air?
j. What is temperature? Write down its units.
k. What is a thermos flask? Why is it used?
l. What are thermometric liquids? Name them.
m. Write down the advantages and disadvantages of using mercury as a
thermometric liquid.

74 Oasis School Science and Environment - 7 PHYSICS

5 Differentiate between:

a. Conduction and Convection
b. Convection and Radiation
c. Thermos flask and Thermometer
d. Heat and Temperature

6 Match the following:

A B
Convection Transfer of heat in solids
Alcohol Transfer of heat in vacuum
Calorie Temperature measuring device
Conduction Transfer of heat in gases
Thermometer Unit of heat energy
Radiation Thermometric liquid
Heat measuring device

7 Give reason.
a. Convection cannot take place in solids.
b. A balloon filled with hot air moves upward.
c. Mercury thermometer is preferred to measure high temperature.
d. Alcohol is coloured to keep in the bulb of a thermometer.

8 Describe an experiment to demonstrate the transmission of heat in solids.
9 Describe the structure of a thermos flask in brief.
10 Draw a neat and labelled figure showing the structure of a laboratory thermometer.
11 How is the temperature of a body measured? Describe in brief.

PHYSICS Oasis School Science and Environment - 7 75

Unit Estimated teaching periods: Th Pr
3 1
7
Sun

Light

Objectives

After completing the study of this unit, students will be able to :

• define reflection of light and explain its types.
• state and demonstrate the laws of reflection of light.
• cthoenisrtrwuocrtksionmg emeeqcuhiapnmisemn.t like Periscope and Kaleidoscope and explain

Course of Study

• Introduction to light
• Sources of light
• Reflection of light
• Types of reflection of light
• Laws of reflection of light
• Reflection of light from a plane mirror
• Periscope and Kaleidoscope

Points to be Focused / Questions to be Discussed

• What is light?
• What is reflection of light?
• What are the types of reflection of light?
• What are the laws of reflection of light?
• What are periscope and kaleidoscope? Why are they used?

76 Oasis School Science and Environment - 7 PHYSICS

7.1 Introduction

We are able to see things in our surroundings due to the presence of light. When
we enter a dark place, we cannot see anything even though we keep our eyes open.
But when we switch on a bulb or use a torch light, every object in the room can be
seen. We can see things only in the presence of light. So, light is a form of energy
which causes the sensation of sight. Light can also be defined as the form of energy
which makes things visible.
Light travels very fast. The speed of light in a vacuum is about 3 lakh kilometre per
second. No object can travel as fast as light. The distance between the sun and the
earth is about 15 crore kilometres. We can calculate that light takes only 8 minutes
and 20 seconds to reach the earth from the sun. Light travels in a straight line when
it propagates in the same medium.

7.2 Sources of Light

Objects like the sun, star, torch light, glowing bulb, burning candle, etc. emit light.
Such objects are called sources of light. Those objects which emit light are called
sources of light.

Sun Glowing bulb Star Burning candle
Figure 7.1: Some sources of light

Some objects emit their own source of light whereas some do not. Objects like the
sun, star, glowing bulb, firefly, etc. emit their own source of light. The objects which
emit light of their own are called luminous objects. On the other hand, objects like
soil, rock, plastic, glass, etc. do not give out light themselves. The objects which do
not emit light of their own are called non-luminous objects. These objects do not
absorb all the light and they reflect some light which enters our eyes. So, we are able
to see them.

Activity 1

• Classify the following objects into luminous and non-luminous sources of
light.

star, table, stone, firefly, sun, glowing bulb, book, soil, plastic, paper,
burning candle, pen, notebook, blackboard, moon, cloth, pencil

luminous /ˈluːmɪnəs/ - shining in the dark

PHYSICS Oasis School Science and Environment - 7 77

7.3 Ray and Beam of Light

A ray of light is a very narrow path (i) (ii)
of light represented by a line with Figure 7.2: A ray and beam of light
an arrowhead. The arrowhead
shows the direction of propagation
of light.

A beam of light is the collection of several rays of light. It is broader and consists of
many rays.

7.4 Reflection of Light

When the rays of light fall on a shiny surface or Incident rays Reflected rays
any polished surface, the rays are returned to
the same medium. This phenomenon is called Smooth reflecting surface
reflection of light. When we stand in front of a Figure 7.3: Reflection of light
plane mirror, we see our own image in it. It is
possible only due to the reflection of light. The
phenomenon of returning of the light to the same
medium after striking a surface is called reflection
of light.

In a dark room, we cannot see things. But those things become visible in the presence
of light. Non-luminous objects do not emit light of their own. So we cannot see
them in the absence of light. We are able to see them when light from a luminous
body falls on them. These objects do not absorb all light, they reflect some light
which enters our eyes. As a result, we see non-luminous objects. These objects are
visible due to the reflection of light.

Activity 2

• Take a plane mirror and go out of your classroom where there is sunlight.
• Let the sunlight fall on the surface of the mirror and adjust the mirror in

such a way that the reflected light rays fall on the wall.
• In this activity, the rays of light that come from the sun and strike the

mirror are called incident rays . Similarly, the rays of light that get reflected
from the mirror and strike the wall are called reflected rays (fig. 7.3).
• What do you learn from this activity? Draw a neat diagram showing this
phenomenon and label the incident rays and reflected rays.

All kinds of surfaces reflect light rays falling on them but all the surfaces do not
reflect light like the plane mirror. It is because the reflecting surface of the mirror
is smooth which reflects a parallel beam of light in one direction. Similarly, rough
surfaces reflect a parallel beam of light in various directions. On this basis, reflection
is of two types, viz. regular reflection and irregular reflection.

78 Oasis School Science and Environment - 7 PHYSICS

Regular Reflection of Light

When a parallel beam of light strikes a smooth Incidents rays Reflected rays
surface, the reflected rays are also parallel to each
other. Such type of reflection is called regular
reflection of light. Reflection of light from a
polished surface or reflection from a plane mirror
is an example of the regular reflection (fig. 7.4). A Figure 7.4: Regular reflection of light
polished surface appears bright due to the regular
reflection of light.

Irregular Reflection of Light Incident rays

When a parallel beam of light strikes a rough surface, Reflected rays
the reflected rays are not parallel to each other. Such Figure 7.5: Irregular reflection
type of reflection is called irregular reflection. In
this reflection, the reflected rays scatter in different
directions (fig. 7.5).
The reflection of light from wall, clothes, room,
furniture and other rough surfaces are some examples of the irregular reflection.

7.5 Terminology Incident ray O Reflected ray
∠r
Point of incidence: It is the point at which a ray of
light hits the surface (smooth or rough). In the given ∠i
diagram, O is the point of incidence.

Incident ray: The ray of light which comes from the A P Normal B
source and strikes the surface is called incident ray. In Figure 7.6: Reflection of light

the given diagram, AO is the incident ray.

Reflected ray: The ray of light which gets reflected after striking a surface is called
reflected ray. In the given diagram, OB is the reflected ray.

Normal: The perpendicular drawn on the surface at the point of incidence is called
normal. In the given diagram, OP is the normal.

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

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

7.6 Laws of Reflection of Light A PBNormal

Law 1: The incident ray, the reflected ray and the Incident ray 45° 45° Reflected rPinayociindteonfce
O
normal lie on the same plane at the point of incidence
Figure 7.7
(fig. 7.7).

Law 2: The angle of incidence is equal to the angle of

reflection.

PHYSICS Oasis School Science and Environment - 7 79

7.7 Reflection of Light from a Plane Mirror

We use a plane mirror or looking glass to see our face. Reflecting surface
The looking glass is a smooth and flat glass plate. One
side of this glass is coated with silver due to which Polished surface
another side of the glass acts as a reflecting surface. Figure 7.8: Plane mirror
While drawing, we represent the reflecting surface of
the mirror by a straight line and the silver coated side
of the mirror by shaded lines (fig. 7.8).

Polished surfaces of metals, glass, nickel, motionless water surface, etc. act as a
plane mirror. So we can see our image on the surfaces of those objects. Generally,
we fix a plane mirror on the frame of wooden board or hard paper board. It gives
support to the mirror. It also helps us to keep the mirror in erect position.

Activity 3

• Take a plane mirror and keep it erect. Fix two straight plastic pipes as
shown in the given diagram.

• Keep a burning candle near the Plane mirror

opening of a plastic pipe and observe
the candle from the opening of Plastic pipe Q Plastic pipe
another pipe. Can you see the candle
or not? If you cannot see the candle,
you can adjust the pipe by increasing P
or decreasing the angle between two Eye R
pipes till you see the candle clearly. Burning
Figure 7.9 candle

• Q When you see the candle clearly, don’t move
the pipes and draw a neat diagram on a
∠i card board paper showing such types of
∠r arrangement. (fig. 7.10).

PS R Draw a normal at the point of incidence.
You will get a diagram as shown in fig.
Figure 7.10 7.10. Now, measure the angle of incidence
and the angle of reflection.

In above activity, the rays of light coming from the burning candle pass through
the plastic pipe and strike the surface of the mirror. Those rays are reflected by the
mirror which pass through another pipe. As a result, we can see the burning candle
from the opening of another pipe.

In the above figure 7.10, RQ is the incident ray, QS is normal and PQ is the reflected

ray. Similarly, ∠RQS is the angle of incidence and ∠SQP is the angle of reflection.

image /ˈɪmɪdʒ/ - a picture of sb/sth seen in a mirror

80 Oasis School Science and Environment - 7 PHYSICS

Activity 4

• Take a piece of a plane mirror and break it into small pieces carefully.
Now, stick those pieces to the surface of a wooden board so that they form
a rough surface on the wooden board. Now, go out of your classroom and
reflect the sunlight on the wall of your classroom.
What can you observe in this activity? Can you see the rays of light scattered
on the wall? What is the reason behind it? Discuss among your friends.

Activity 5

• Fix a white paper on a wooden board with the help of thumb pins.

• Draw a line MN in the middle of the M X N
paper. Q2
Select a point X in the middle of the D P1 P2 Q1
? D'
• line MN and draw a perpendicular XY 30° ?
45° ? C'
?
on the line MN. C 60°
75°

• Draw four angles of 30º, 45º, 60º and 75º B B'
to the line XY as shown in the diagram. A A'
Y
• Now, fix a plane mirror vertically along Figure 7.11

the surface MN.

• Fix tpwinospiinntshPe1 manirdroPr2 vertically on the line DX. Now, locate the images of
the by viewing the reflection from side B of the mirror
and fix two more pins aQn1daQnd2. Q2 in such a way that they lie in a straight
line. Join the points Q1
• Remove the pins P1 and P2 and repeat this activity for the lines CX, BX and
AX respectively.

• Identify the angle of incidence and the corresponding angle of reflection.
Measure those angles with a protractor and fill in the table given below:

S.N. Angle of Angle of Result Conclusion

incidence reflection

1. ∠DXY = 75º ∠D' XY = 75º ∠DXY = D'XY

2. .............................. .............................. ..............................

3. .............................. .............................. ..............................

4. .............................. .............................. ..............................

From this activity, it can be concluded that the angle of incidence is equal
to the angle of reflection. Similarly, we can see that the incident ray, normal
and the reflected ray all lie on the same plane at the point of incidence.
This activity verifies laws of reflection of light.

PHYSICS Oasis School Science and Environment - 7 81

7.8 Periscope

Periscope is a simple device based on the principle of reflection of light at the two
parallel plane mirrors producing multiple reflections.

Structure

Periscope consists of a A 45° Plane mirror (M1)
cardboard or wooden tube
bent twice at right angles. It is Tree to be seen 45°
provided with two opening ‘A’ 45°
and ‘B’ as shown in the figure.
aTrweoadpjulasnteedmaitrrthoersbMen1dasnodf tMhe2 Plane mirror (M2) 45° B
tube so that they are parallel to 45°
one another and their reflection
surfaces face each other. Each 45°
plane makes an angle of 45º
with the side of the tube (Please Figure 7.12: Periscope
see the figure).
Working

arTpanahydresarlsalaterryleisktrooeeftfthllheiegecihtrmetodeirrnibrgtoyeirnrtiahMnlegp2 mtaahttiher4rta5ounºrb.deMTrfhe1reaaoscnmehdrttahthhyeeesovrabieerjfewelceeftcirstn’etsardlielkyyreaeryt.ehsfeltermcatvieredrlobrthyMrot1huaegt hm45tiºhr.reTohrtuoMbsee2

Uses

Periscope gives us a higher view than normal. So it is used
(i) to see the movement of ships over the surface of the sea from a submarine

submerged under water.
(ii) to see over the heads of a crowd.
(iii) to observe the enemy activities above the ground by a soldier sitting in a trench.

7.9 Kaleidoscope Patterns formed
in kaleidoscope
Kaleidoscope is a simple device based on the
principle of reflection of light. It consists of Tape Mirror
three rectangular mirror strips fitted together in
the form of a long prism. The shiny surface, i.e. Cardboard
reflecting surfaces of the mirrors are adjusted
facing inwards. The mirror prism is wrapped Figure 7.13
with a black cardboard paper with a cello tape.
Some coloured broken glass pieces are kept
inside the tube and both open ends of the tube are
covered with greased white tissue paper.

82 Oasis School Science and Environment - 7 PHYSICS

When the kaleidoscope is held pointing towards light and rotated, beautiful pattern
of colours can be seen.

Use
Kaleidoscope is used as a toy.

Activity 6

• Prepare a periscope and kaleidoscope and demonstrate them in your class.

Summary

• Light is a form of energy which causes the sensation of sight.
• The speed of light in a vacuum is about 3 lakh kilometers per second.
• Those objects which emit light are called sources of light.
• The objects which emit light of their own are called luminous objects.
• The objects which do not emit light of their own are called non-luminous

objects.
• A ray of light is a very narrow path of light represented by a line with an

arrowhead.
• A beam of light is the collection of several rays of light.
• When a parallel beam of light strikes a smooth surface, the reflected rays

are also parallel to each other. Such type of reflection is called regular
reflection of light.
• When a parallel beam of light strikes a rough surface, the reflected rays are
not parallel to each other. Such type of reflection is called irregular reflection
of light.
• The ray of light which comes from the source and strikes the surface is
called the incident ray.
• The ray of light which gets reflected after striking a surface is called the
reflected ray.
• The perpendicular drawn on the surface at the point of incidence is called
normal.
• Periscope and kaleidoscope are constructed on the basis of reflection of
light.

PHYSICS Oasis School Science and Environment - 7 83

Exercise

1 Choose the best answer from the given alternatives.

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

(i) refraction (ii) reflection

(iii) dispersion (iv) propagation

b. Which of the following objects is a non-luminous object?

(i) sun (ii) star
(iii) glowing bulb (iv) wood

c. Which of the following is a living source of light?

(i) burning candle (ii) oil-lamp
(iii) firefly (iv) torch-light

d. A ray of light falling on a mirror is _________.

(i) incident ray (ii) reflected ray
(iii) refracted ray (iv) none of the above

e. If the angle of reflection is 43º, the angle of incidence is _________.

(i) 53º (ii) 43º
(iii) 33º (iv) 90º

f. The angle between normal and the reflected ray is _________.

(i) reflected angle (ii) acute angle
(iii) reflex angle (iv) all of the above

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

a. Light is a form of energy which gives sensation of vision.
b. Burning candle is not a source of light.
c. Moon is an example of a non-luminous object.
d. The reflection of light from a rough surface is an irregular reflection.
e. The angle between the normal and the reflected ray is called an

angle of incidence.

3 Fill in the blanks with appropriate words.
a. The object which emits light is called a ________ of light.
b. A beam of light is the collection of several ________ of light.
c. The rays of light which strikes a surface is called ________ ray.
d. The perpendicular drawn at the point of incidence is called ________.
e. The angle of incidence is equal to the angle of ________.

84 Oasis School Science and Environment - 7 PHYSICS

4 Answer the following questions.

a. What is light? Write down the speed of light in a vacuum.

b. What does a source of light mean? Give any three examples.

c. What is reflection of light ? Write down its types.

d. What is regular reflection of light ? Draw a diagram showing the irregular
reflection of light.

e. Define the following terms:

(i) Point of incidence (ii) Incident ray
(iii) Reflected ray (iv) Angle of reflection
(v) Normal

f. State the laws of reflection of light.

g. What is periscope? Write its uses.

h. What is kaleidoscope? Write its use.

5 Differentiate between:

a. Luminous object and Non-luminous object
b. Ray of light and Beam of light
c. Regular reflection and Irregular reflection
d. Angle of incidence and Angle of reflection

6 Give reason.
a. The sun is called a source of light.
b. A polished surface appears bright.
c. We use a plane mirror as a looking glass.
d. Reflection of light from a plane mirror is called the regular reflection of light.

7 Study the given figure and answer the following questions.
a. Which phenomenon is shown in the given diagram?
b. Name the angle of reflection and the incident ray.
c. What is the magnitude of ∠POB? Why?

O

45° ?

A PB

PHYSICS Oasis School Science and Environment - 7 85

Unit Estimated teaching periods: Th Pr
3 1
8
Playing guitar

Sound

Objectives

After completing the study of this unit, students will be able to :

• define the sound wave (longitudinal wave) and explain its propagation.
• explain the factors that affect the propagation of sound.
• demonstrate that the speed of sound in different media is different.

Course of Study

• Introduction to sound
• Wave and its types
• Propagation of sound
• Speed of sound in different media

Points to be Focused / Questions to be Discussed

• What is sound?
• What is a wave?
• What are two types of wave?

• What is a transverse wave?

• What is a longitudinal wave?
• What is propagation of sound?
• Why is the speed of sound different in different media?

86 Oasis School Science and Environment - 7 PHYSICS

8.1 Introduction

In our surroundings, we hear a variety of sounds such as horn of vehicles, voices
of our family members, school bell, chirping of birds, barking of dogs, sounds of
musical instruments, etc. Among them, some sounds are pleasant while others are
not. All sounds produce a sensation of hearing in our ears. So, sound is a form of
energy which produces a sensation of hearing in our ears. Sound is produced due
to the vibration of bodies. Every vibrating body is a source of sound. It travels in all
directions and requires a medium for propagation. Sound cannot travel through a
vacuum. It can travel through solids, liquids and gases.

8.2 Sources of Sound

An object which
produces sound is
called a source of
sound. Radio, television,
horn of automobiles,
harmonium, guitar,
machines, human Guitar Radio Loudspeaker

beings, animals, etc. are Figure 8.1: Some sources of sound

some sources of sound. Every vibrating object is the source of sound and every
source of sound is a vibrating body. A rapid back and forth motion of a particle or a
body about its mean position is called vibration. A body can be vibrated by hitting,
plucking, blowing, stroking, etc.

In our surroundings, many living beings and non-living beings act as the sources
of sound. Different types of sounds can be heard in our home, school, road, market,
jungle, etc. Human beings, horns of vehicles, school bell, air, water, birds and other
animals are the sources of sound in aforementioned places. When the particles
of a body vibrate, sound is produced. The sound thus produced propagates in
our surroundings in the form of waves. Those waves reach our ears and vibrate
the eardrum. The vibration of the eardrum transmits sound impulses to the brain
through auditory nerve. As a result, we hear the sound.

Activity 1

• Press your throat with your fingers and sing a song or speak for 10-15
seconds. What do you feel on your fingers? You will feel the vibration in
your fingers.

• Now, stop singing or speaking for 5-10 seconds. Can you feel vibration?
When there is vibration, sound is produced and when the vibration stops,
the sound also stops. So we can produce sound by vibrating our vocal
cord.

impulse /ˈɪmpʌls/ - a force or movement of energy that causes sth else to react 87
PHYSICS Oasis School Science and Environment - 7

Activity 2

• Bring a rubber band of length about 15 cm.
• Tie one end of the rubber band in the nail of

a wall and hold another end in your hand.
Pull the middle part of the rubber band with
another hand and release it. What do you
observe ? What type of sound is produced
in this case?
You can observe that the rubber band
vibrates to and fro when we release it Fig. 8.2 Vibrations in a stretched rubber
after pulling. It produces sound as long
as it remains vibrating. You will find that when the rubber band stops
vibrating, it will also stop producing sound.

Activity 3

• Bring a rubber pad and a tuning fork.
• Tie a table tennis ball with a thread and suspend it to a stand as shown in

the given figure.
• Hit the tuning fork hard against the rubber pad.

Thread Thread

Vibrating Rubber pad Tuning fork

tuning fork
Tennis ball

Fig. 8.3 Tuning fork produces sound while vibrating

• The tuning fork vibrates and produces sound. Look at the prongs of the
tuning fork after striking the rubber pad. The prongs look hazy because
they are vibrating.

• Now, bring the prongs of the vibrating tuning fork near the table tennis
ball. The ball oscillates to and fro. It proves that the prongs of the tuning
fork are vibrating.

• When the prongs of the tuning fork remain vibrating, sound is produced.
When the prongs stop vibrating, they will also stop producing sound.

88 Oasis School Science and Environment - 7 PHYSICS

Activity 4

• Bring a plastic whistle and observe it carefully. You can see a ball inside it.
• Now, blow the whistle. You will hear the sound produced by the whistle.

This sound is produced due to the vibration of the ball of the whistle.
When the ball of the whistle stops vibrating, the sound also stops.

Activity 5

• Bring a ‘madal’ and keep it in the vertical position so that its skin surface
remains horizontal.

• Now, place some powder over the surface of the skin and bang lightly on
the skin. What do you observe?
You will observe that the powder vibrates due to the vibration of the skin
and the vibration of the skin produces sound.

8.3 Wave Motion

A material medium is required for propagation of sound. Generally, sound
propagates through air. However, it propagates through liquids and gases as well.
Sound propagates in the form of waves. Wave motion is a periodic disturbance travelling
through a medium which is produced by a vibrating body. The disturbance in a medium
is called wave.

A wave motion travels in all directions through Fig. 8.4: Ripples produced in water when a
the medium in a constant speed but the particles stone is dropped into a pond
of the medium do not move from one place to
another. They only vibrate passing on energy
they possess from one particle to another. During
a wave motion, the medium does not move as a whole
but only the disturbance travels through the medium.

Wave Piece of paper Wave If we drop a piece of stone in
a pond, ripples (i.e. circular
Piece of stone waves of water) spread out in
Fig. 8.5: Wave motion does not carry matter with it all directions on the surface of
water. If we place a piece of
paper on the water surface, the
paper moves up and down at the
same place but does not move
away along with the waves.

This activity shows that the disturbance moves from one place to another but water

wave /weɪv/ - the disturbance in a medium

PHYSICS Oasis School Science and Environment - 7 89

is not carried along with it. Water molecules simply move up and down. When a
stone is dropped into a pond, the energy carried by the stone disturbs the water
molecules close to it. By gaining energy from the stone, water molecules near the
stone vibrate. Those vibrating molecules transfer some energy to the nearby water
molecules. As s result, water waves (ripples) are formed.
Waves are of two types, viz. transverse wave and longitudinal wave. Sound travels in the
form of longitudinal waves. Let us perform following activity to develop the concept
of waves.

Activity 6

• Visit a nearby pond. Drop a piece of stone at the centre of the pond. What
happens to the surface of water? Do ripples form on the surface of water?
Now, throw a leaf on the surface of the waves in water. Does the leaf move
with the waves of water?
Write down the conclusion of this activity.

8.4 Transverse Wave

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. Ripples produced on the
water surface in a pond, vibration of stretched wave in musical instruments, waves
formed in a slinky when moved up and down, etc. are some examples of transverse
waves.
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.
A transverse wave can propagate only in solids and at the surface of liquids. A
transverse wave is shown in the given figure.

Crests

AC

X Zero Y

disturbance B D Vibrations
(up and down)
Troughs

Figure 8. 6: Transverse wave in a slinky showing crests and troughs

90 Oasis School Science and Environment - 7 PHYSICS

8.5 Longitudinal Wave

The wave in which the particles of the medium vibrate to – and – fro (back and forth)
in the direction of the wave is called a longitudinal wave. Sound wave, waves formed
in a slinky when pulled and pushed, etc. are some examples of longitudinal waves.

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 a rarefaction is the part of the wave in which the
particles are farther apart. A longitudinal wave is shown in the given figure.

Rarefaction Rarefaction Pull

Compression Compression Push of a slinky

Fig. 8. 7: Longitudinal wave in a slinky showing compressions and rarefactions

Differences between Transverse Wave and Longitudinal Wave

S.N. Transverse Wave S.N. Longitudinal Wave
1.
The wave in which the particles 1. The wave in which the particles of
2. of the medium vibrate up and the medium vibrate to-and-fro in
3. down perpendicular to the the direction of the wave is called
direction of the wave is called a a longitudinal wave.
transverse wave.

It consists of crests and troughs. 2. It consists of compressions and
rarefactions.

It can be produced only in solids 3. It can be produced in solid, liquid
and surface of liquids. and gas.

8.6 Propagation of Sound Wave

The transmission of the sound from one place to another is called propagation of
sound. Sound is produced by a vibrating body and it travels in all directions from
the source. Sound requires a medium for its propagation. The sound waves coming
from a vibrating body propagate through air and reach our ears. As a result, we
hear the sound. Sound can propagate through solids, liquids and gases. But it cannot
propagate through vacuum.
Sound can propagate through air. Air is the mixture of various gases like nitrogen,
oxygen, carbon dioxide, etc. The speed of sound in air is about 332m/s. Sound
waves cannot propagate in the absence of air. There is no air on the surface of the
moon. So sound cannot propagate on the surface of the moon as that happens on
the surface of the earth.
Sound also propagates through liquids. The sound produced by dolphin, whale,
etc. propagates through water. The speed of sound in water medium is about

PHYSICS Oasis School Science and Environment - 7 91

1498m/s. Similarly, sound propagates through solid medium. The speed of sound
in steel medium is about 5200m/s. The sound waves travel faster in solids than in
liquids or gases. The speed of sound is least in gases, lesser in liquids and the most
in solid medium.
Sound travels in the form of longitudinal wave. When a sound wave passes through
air, the particles of air vibrate back and forth parallel to the direction of propagation
of sound wave. It forms regions of compressions and rarefactions.

Compression Rarefaction Compression Rarefaction

Figure 8.9 Propagation of sound wave in air

Activity 7

Objective: To demonstrate the
propagation of sound through a solid

Materials required: Two ice-cream cups,
a long thread, needle, scissors

Procedure: Take an ice-cream cup and Figure 8.10
make a small hole at its bottom by using
a needle.

Take a long string and insert one end of the string through the hole and tie a knot so
that it does not slip back through the hole.

Repeat the same process with another cup also.

Now, ask your friend to take one end of the string with the cup and you hold the
other end.

Move as far from one another as possible so that the string is held tight. But do not
let the string slacken.

Now, hold the cup to your ear and ask your friend to speak some words softly. Can
you hear the sound produced? Repeat this activity for 3-4 times.

Now, ask another friend to cut the string in its middle.

Ask your friend to speak some words as in the previous activity. Can you hear the
sound produced? What can you conclude from this experiment?

Conclusion: From this activity, it can be concluded that sound propagates through
a solid.

92 Oasis School Science and Environment - 7 PHYSICS