Blooming Science-7 (2077) final for press

Activity

To demonstrate atmospheric pressure
Materials required: a glass tumbler, water, a postcard
Method:

Postcard

Water

Glass tumber

Fig: Atmospheric pressure

1. Take a tumbler with water.
2. Cover it with a postcard.
3. Invert the tumbler by supporting the postcard with your palm.
4. Remove the support from the postcard. Does the postcard fall?
5. Bring the tumbler as in the figure, what happens?
The postcard does not fall down because of the atmospheric pressure on it. The
activity proves the presence of the atmospheric pressure.

Measurement of Pressure
Pressure can be measured with the help of the instruments. They are presssure gauge,
manometer, barometer, etc.

Pressure Guaze
This is an instrument used to measure the pressure of tubes of the
tyres of vehicles. Tyre may brust out if maximum air is filled into
it and it does not move properly if it has less air. So, optimum air
pressure is to be maintained in the tubes for safety and efffciency.

Manomter
It is an instrument used to measure the pressure of the lungs. It consists of
U-shaped tube containing liquid. Both arms are open to the atmosphere.
The liquid inside the arms has equal depth.
By blowing the air from one arm of the U tube, the increase in the liquid
level in another arm is noticed.

Generally, mercury is used inside the manometer to measure the lungs pressure.

Blooming Science & Environment Book 7 51

Barometer

Barometer is an instrument used to measure the atmospheric pressure. It
consists of a one meter long glass tube closed at one end and completely
filled with mercury. After filling a tube, it is put in an inverted position
in a trough containing mercury. Care should be taken not to let air
inside it. The tube is supported with the help of stand and graduated
scale is fitted parallel to it. When air pressure decreases, the mercury level in the tube
starts falling down. When the air pressure increases, mercury level in the tube rises up.

Main Points to Remember F
A
1. Pressure is defined as the force per unit area. P=
2. The SI unit of pressure is N/m2 or Pascal.

3. Liquid exerts pressure due to its weight.

4. The force due to the vertical columns of air on a unit area is called atmospheric
pressure.

5. The liquid transmits the pressure equally in all the directions.

6. Manometer is an instrument that helps us to find out the pressure exerted by
lungs.

7. Pressure gauze is an instrument that helps us to find the pressure exerted by air
in tubes, tyres, etc.

8. Barometer is used to measure the atmospheric pressure.

10. The characteristics of a substance by virtue of which it is recognized are termed

as its properties.

PRO J ECTWORK

Manage a pair of high heeled shoes and a pair of flat shoes. Wear both of these turn
by turn and try to walk on muddy road or in sand. Feel the difference and share your
experience with your friends and teacher in class

Exercises

1. Write 'True' or 'False' for the following statements.
a. Pressure is force per unit area.
b. The unit of density is pascal.
c. It is comfortable to wolk with flat shoes.
d. The pressure increases with increase in force.
e. The pressure increases with increase in area.

52 Blooming Science & Environment Book 7

2. Answer the following questions.
a. What is pressure? Write its unit.
b. Define 1 pascal.
c. What is the reaction of pressure with force and area
d. Describe an activity which proves atmospheric pressure.
e. Define atmospheric pressure. What is the value of atmospheric pressure at
sea level.
f. Write down any two points of difference between force and pressure.
3. Give reasons for the following.

a) It is easier to cut the fruits by using a sharp knife than a blunt one

b) There are ten types kept in a truck which is used to carry heavy loads.

c) The foundation of a building is made under than its wall.

d) The feet of camel are broad

e) The rear wheels of a tractor are wider.
4. Numerical Problems.

a) The weight of a girl is 400N. The area of each shoe with which she is standing
is 40cm2. Calculate the pressure exerted by her both feet

b) A rectangular wooden block of dimensions 3m × 2m ×1m has 600N weight,
Find the pressure exerted by it .

c) The pressure exerted by a man with a foot is 800N/m2 and his body weight is
500N. Find the area of the ground covered by his one foot.

d) Calculate the weight of a boy when he is standing on ice skate with area
12.5cm2 and exerting the pressure of 392000 Pa by his skate board.

Glossary

Blunt : without a sharp edge or point

Foundation : a layer of bricks, concrete, etc. that forms the solid
underground base of a building


Immiscible liquids : those liquids which don’t dissolve together at any
proportion.

Blooming Science & Environment Book 7 53

5Chapter Energy, Work and Power

Learning Outcomes

On the completion of this unit, students will be able to: Estimated Periods: 3

• define work, energy and power
• describe various types of energy.
• define and explain transformation of energy.
• tell different types of work done.
• solve some simple numerals related to work, energy and power.

Energy

We eat food to get energy. Different types of fuels are used in vehicles to run there.
The fuels provide them energy. By using energy, different type’s of work are done,
means energy is the capacity or ability to do work. Therefore, energy is defined as the
capacity to do work.

The SI unit of energy is Joule [J]. One joule energy is that which can lift or slide a body
having weight of 1 N along to 1 m.

Forms of Energy

There are different forms of energy around us. Many types of them are used in a daily
life. Some of them are explained in short below.

a. Mechanical Energy

Potential and kinetic energy are combinely called as
mechanical energy because in most of the machines
they are in use. Potential energy is possessed due to
position of body. The water in a reservoir, the energy in
stretched spring are some examples of it.

Kinetic energy is defined as that type of energy which is
possessed due to motion of a body. This energy is found in flowing water, wind, rolling
stone, etc.

54 Blooming Science & Environment Book 7

Differences between Potential and Kinetic Energy.

Potential Energy Kinetic Energy

1. The energy possessed by a body by 1. The energy of a body by virtue

virtue of its position or configuration of its motion is called the kinetic

is called potential energy. energy.

2. It may be due to the position of body 2. It is only due to the motion of the

at certain height above the ground. bodies.

It is denoted by PE. 3. It is denoted by K.E. 4 mv2 12
3. It is calculated by PE = mgh It is calculated by KE = 3

b. Chemical Energy

It is defined as that stored energy which is liberated only after chemical
reaction. Food, fuel and electric cells contain this type of energy.

c. Heat Energy

It is defined as that type of energy which is possessed due motion of
molecules. It is also defined as total kinetic energy of molecules contained
in a body. Its main source is the sun but it is also obtained by electricity
and fuels.

d. Light Energy

The form of energy which is possessed usually due to more heat and makes
the bodies visible is called light energy. The sun, moon, burning candle,
glowing lamp, etc. are some examples of it.

e. Sound Energy

The energy possessed due to the vibration of bodies and can be felt
through ears is called sound energy. A ringing bell, playing radio,
guitar, etc. are some examples of sound energy.

f. Magnetic Energy

The energy which affects the magnetic bodies is called
magnetic energy. It is found in magnets. Magnet is used
for generating hydroelectricity in radio, tv, telephone,
microphone and in lifting heavy loads of magnetic materials.

g. Electrical Energy

The energy found in household wiring and in a wire connected
roperly with a cell or dynamo is called electrical energy. It is
occurred due to the flow of electrons which is called current electricity. It also may

occur by the change in number in electrons in bodies which is called static electricity.

Current electricity is used for various purposes in our daily life.

Blooming Science & Environment Book 7 55

h. Nuclear Energy
The energy possessed due to nuclear fission and nuclear fusion is called nuclear
energy. Nuclear fission is a process in which heavy nuclei of elements are broken into
light nuclei. Nuclear fusion is a process in which light nuclei are fused into heavy
nuclei. Nuclear fusion occurs in the sun by combining hydrogen into helium. Electrical
energy can also be generated by using nuclear fission reaction.
Transformation of Energy
When a candle burns, it changes chemical energy into light and heat energy. Similarly
when an electric bulb glows, it changes electrical energy into light and heat energy.
These examples show that one type of energy can be changed into another type.
The process in which one type of energy is changed into another type is called
transformation of energy. Some more examples of transformation of energy are as
given below.

a. In a battery operated radio

Chemical energy Electrical energy (when Sound energy (when
(in battery) the radio is switched on) the radio plays)

b. In our body Heat and muscular Sound energy
energy (by respiration) (when we speak)
Chemical energy
(in the eaten food)

c. In an electric heater

Electrical energy Heat and light energy
(in wire) (when the heater glows)

Work

You are familiar with the term ‘work’ in daily life. Reading, writing, walking holding
something on hand, cooking, cleaning, bathing, brushing, etc. are the examples of work
that we do in our daily life. The meaning of the word ‘work’ is somehow different in
physics.

Work is said to be done when the force is applied on an object and the object moves
due to the application of the force. When we try to push a heavy stone and are unable
to move that, no work is done scientifically. When a boy pushes hard again a wall but
is unable to move it, no work is done by him.

Similarly, a gatekeeper performing his duty standing beside the gate is not doing any

56 Blooming Science & Environment Book 7

work. A man standing with a heavy load on his head or back is not doing any work
though he feels tired. In both these examples, no distance is travelled by the body.
Therefore, in science, work is said to be done when.
(i) a force is applied on an object, and
(ii) object moves in the direction of force.
Kicking a ball, lifting a load, cycling on the road, pushing a wheelbarrow, running
upstairs, etc. are the activities in which the work is done.
Measurement of Work
The work done by a force is defined as the product of force applied on the object and
distance moved by the object in the direction of force.

Work done = force × distance moved in the direction of force.
Or, W = F × D
The SI unit of work done is Joule and denoted by J.
If a force of 1N acting on a body makes it move through a distance of 1 m in the
direction of force, then the work done is said to be one joule.
1J = 1N × 1m = 1Nm
The CGS unit of work done is calorie.

Solved Numerical Problem

A force of 500 N moves a body through a distance of 4 m in the direction of the
force. Calculate the work done.
Solution:
Here,
Force applied (F) = 500 N
distance covered (d) = 4m

Blooming Science & Environment Book 7 57

Work done (W) =?

We know that,

W = F × d

= 500 × 4

= 2000 J

Work done = 2000J

Differences between Force and Work.

Force Work

1. The force is the product of mass 1. When certain force is applied through

and acceleration due to gravity. a certain distance, work is done.

2. It can be calculated by F = m × g. 2. It can be calculated by W = F × d

3. It’s SI unit is Newton (N). 3. Its SI unit is Joule (J).

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

If the force is applied in opposite direction of friction, while doing work, it is called
work against friction, e.g. sliding a box, rolling a drum, etc.

If the force is applied in opposite direction of gravity, while doing work, it is called
work against gravity, e.g. lifting an object, throwing a body vertically upward,
etc.

Power

Different persons may have different rates of doing work. Different machines also may
have different rates of doing work. The machines and persons which have capacity
of doing work in short period have more power than those who do the same work by
taking long time.

58 Blooming Science & Environment Book 7

Therefore, power is defined as the rate of doing work. It is also defined as the rate of
transformation of energy.

It is calculated by using the formula;

Power = work done
time taken

P = W
t
F×d
Or,P = t

Or, P = mgh [: F= m×g and d=h]
t

The SI unit of power is watt and represented as watt and represented as W. It is also

measured in unit horse power(hp).

P = W
t

1W = 1J
1S

= 1J/S

One watt power is defined as that power in which one joule work is done in one second
Power is also defined as the rate of conversion of energy in unit time. For example

1W = 1J
1s

Power of a bulb is 60 watt means that the lamp can convert 60 J electrical energy
into light and heat energy in one second. Power is alos measured in horse power (hp).
The bigger units of watt are kilowatt and megawatt.

1hp =750 w (approx,) [infact it is 746 w]

1000 W = 1 KW (Kilowatt)

1000 KW = 1 MW (Megawatt)

Differences between Work and Power.

Work Power

1. Work is the product of force and 1. Power is the rate of doing work.

the distance moved by the body in (P = W )
the direction of force. (W = F × d) t

2. Its SI unit is Watt (W) and its CGS

2. Its SI unit is Joule (J), calorie is CGS unit is ergs.

unit. 3. Its value depends in time.

3. Its value does not depend with time.

Blooming Science & Environment Book 7 59

Activity Scan for practical experiment

To calculate your power while climbing stairs,

Materisals required : Weighting machine, measuring
tape,stopwatch etc

Method : using a visit: csp.codes/c07e09

a) Measure your body weight by
weighing machine and note down it .

b) Measure the height of stairs that you want
to climb.

c) Climb the stairs and carefully measure the
time taken you need to climb.

Observation:

You can find your power to climb stairs by using the formula.

Solved Numerical Problems

1. Hari carries a load of 120 kg upto 60 metres high. What work is done by him? If
he does this work in 2 minutes, also calculate his power.

Solution:

We have,

Mass (m) = 120kg = 120 ×10 = 1200N [F = mg & g = 10 m/s2]

Height (h) = 60m

Time taken (t) = 2min = 2 × 60s = 120s [1 min = 60s]

Work done (W) = ?

Now, W = F × d

= 120 × 60
= 72000J

Again, P = W
t

= 72000
120

= 600W

Hence, work done by Hari is 72000 J and his power is 600 W.

60 Blooming Science & Environment Book 7

2. Power of a crane is 25000 watt. Calculate the time required by it to lift a load
of 600kg upto 20 metres.

Solution:

We have,

Power [P] = 25000 W

Mass [m] = 6000 kg = 6000 ×10 N = 60,000N

Height [h] = 20m

Time [t] = ?

Now, P = W
t

= W×d
t

25000 = 60000×20
t

or, t = 60000×20
25000
∴ t = 48 s

Hence, the required time for the crane to lift the load is 48s

Main Points to Remember

1. The capacity of doing work is called energy. Its unit is also Joule [J].

2. There are different forms of energy. They are mechanical energy, heat energy, light
energy, etc.

3. One type of energy can be changed into other type. It is called transformation of
energy.

4. Energy can neither be created nor can be destroyed but can be changed from to
one form to another, which is called principle of conservatoin of energy.

5. Potential energy is the energy stored by virtue of position of a body.

6. When a body covers any distance by applying any force, it is said that work is
done by that body. W = F × d

7. SI unit of work is joule [J]. Its CGS unit is calorie.

8. The energy of a moving body is kinetic energy. W
t
9. Power is defined as the rate of doing work. P = .

10. SI unit of power is watt (W). Its CGS unit is erg.

Blooming Science & Environment Book 7 61

PRO J ECTWORK

Fill the table given below on the basis of the conversion of energy, that occurs in
your home or in school.

S.N Energy transformer Transformation of energy
1. Burning candle Chemical energy to Light energy
2.
3.
4.
5.
6.

Exercises

1. Fill in the blanks.

a. The capacity of doing work is called..............................

b. The formula which is used to calculate work is..............................
c. To lift a body is a type of work against..............................
d. Combined form of potential and kinetic energy is called.................energy.
e. The stored energy which is liberated by chemical reaction is called........
energy.
f. Rate of transformation of energy is..............................

2. Answer the following questions.
a. Define energy. Relate energy with work
b. Point out the different forms of energy. Mention one object possessing each
form of energy.
c. Define work. Write its unit.
d. What are the types of work? Write about them in brief.
e. Differentiate between kinetic and potential energy.
f. Define power. Write its unit.
g. Define 1J and 1W.
h. Stat law of conservation of energy.
i. What is meant by 100W marked in an electric bulb?

62 Blooming Science & Environment Book 7

j. Write the energy transformation that takes place in the following cases:
i. Burning a kerosene lamp
ii. Rotating a fan
iii. Making an electromagnet.
iv. Projecting a stone from a catapult

3. Define.

a. Light energy b. Power c. Work

d. 1 J work e. Potential energy f. Transformation of energy

4. What do you mean by?

a. 10 J work b. 60 W written in an electric bulb

5. What factors affect the work? Write the relation of the factors with work.

6. Write short notes on:

a. Heat energy b. Nuclear energy c. Kinetic energy d. Work

7. What type of transformation is done by the following things?

a. electrical lamp b. electric fan c. battery operated radio

d. kerosene lamp e. human body f. car

[Show by flow chart as given in the example.] e.g. A torch light

Chemical energy Electrical energy (when Light energy
(in battery) switched on) (when the lamp glows)

8. Numerical Problems.

a. Hari drags a load of 60 kg along to 12m what amount of work does he do?

Also mention the type of work. (Ans: 7200J)

b. Rama has her weight 40 kg. She is carrying a load of 20 kg upto 20 meter
high. What work does she do? Also mention the type of work. (Ans: 12000J)

c. Power of a crane is 25000 watt. What work will be done by it in 4 seconds?
(Ans: 6250J)

d. Sarita has 480 watt power. Calculate the amount of mass lifted by her upto 10

meter in 12 seconds. (Ans: 57.6 kg)

5. Rahul is carrying a load of 80 kg along to 200 meters in 5 minutes. But the same
load is carried by Reena for the same distance in 4 minutes. Who has more
power? Also show by calculation.

(Ans: Rahuls power is 533.33 W and Reena’s power is 666.66W.)

Blooming Science & Environment Book 7 63

Glossary

Work : product of force and distance.
Energy : the capacity to do work.
Kinetic energy : energy in a body due to its motion.
Potential energy : energy stored in a body because of its condition or position.
Heat energy : energy produced due to heat.
Chemical energy : energy stored in coal, oil and gas which is released in the

Light energy form of heat and light by chemical reaction.
Sound energy : energy obtained by light.
Magnetic energy : form of energy which is obtained due to vibration.
Electrical energy : energy which is obtained by magnet.
Nuclear energy : energy obtained due to electricity.
Joule : energy due to nuclear reaction.
Watt : unit of work, energy.
Horse power : unit of power
: unit of power equal to 750 w.

Acceleration due to gravity : the acceleration produced by the force of gravity

64 Blooming Science & Environment Book 7

6Chapter Heat

Learning Outcomes

On the completion of this unit, students will be able to: Estimated Periods: 3+1

• define heat and explain effcts of heat on a body.
• describe and demonstrate various methods of transmission of heat.
• define tempereture and mention its unit.
• measure temperature by using thermometer.

Introduction

Heat is a form of energy, which gives us sensation of warmth. Sensation of hotness or
coldness are very familiar to us. While sitting near an electric heater you feel hot, but
while touching a piece of ice you feel cold.

Heat can be obtained from the sources like fuel, Sun, combustion of food etc. In
cooking, in drying the cloth, to run the vehicles, to run the industry and factories and
to make our body warm, we need heat. The quantity of heat depends upon the mass
and the temperature of the body. That’s why, more heat is required to heat the body of
large mass and less heat is sufficient for the body of small mass. Similarly, the quantity
of heat is more in the hotter body than the colder body.

Change of State

You often see that water in the pond dries quickly in the summer day. Water from the
pond changes into the vapour and forms the clouds. The water changes into the vapour

not only from the pond but also from the sea, river and even from the soil.

Heating Heating
Cooling Cooling

Ice Water Steam

When the candle burns, the wax at the top changes into liquid and if the candle blown
off, the liquid wax changes back into the solid wax. These all changes we discussed
above, are due to the heat.

There are three states of matter: solid, liquid and gas. Nowadays another state of matter

Blooming Science & Environment Book 7 65

at very high temperature is also discovered, which is called the plasma. Plasma is also
known as the fourth state of matter. Under heating one state of matter changes into the
another which is known as change of state (phase change).

Change of Solid into Liquid beaker
ice cubes
When a solid is heated it changes into liquid. For
example ice changes into water on heating. The change burner
of solid into liquid on heating is called fusion or melting.
The temperature of the substance remains constant
throughout the process of melting. This temperature is
called the melting point. The ice melts at 0oC. When you
heat the ice it begins to melt but the temperature does not
increase until the whole ice melts.

On the other hand, the process by which liquid changes

into solid on cooling is called solidification. The temperature of the substance remains

constant during the process of solidification and this temperature is called the freezing

point. Water freezes at 0oC. Scan for practical experiment

In some crystalline substances, the melting and freezing points

are equal and in some non-crystalline solid the melting and

freezing point are not equal. For example, butter melts at or

above 30oC, while freezes at 22oC. Normally, volume of the

solid increases on melting but solid like ice, wax, etc. contracts

on melting.

Change of Liquid into Vapour visit: csp.codes/c07e12

The change of the substance from liquid to its vapour state is called vaporization. For
example, water changes into steam under heating. Vaporization takes place by two
ways: evaporation and boiling.

Evaporation Dish

It is the slow conversion of liquid into the gaseous state at Water
all temperature; the change of state takes place only from evaporated water
the surface of the liquid. Liquid can evaporate even at the
low temperature above the melting point.

Boiling

It is the rapid conversion of liquid into the vapour state
at the fixed temperature. The change of state takes place
throughout the liquid. The water changes into steam at
100oC, which is called boiling point of water.

On the other hand, the change of state of a substance from Boiling water

the vapour to liquid on cooling is called the liquifaction or condensation. The steam of

water changes into the water on cooling.

66 Blooming Science & Environment Book 7

Transmission of Heat

Heat flows from one point to another. If an iron spoon is kept half-immersed in a hot
water bath for a short while, its other end also becomes hot. Similarly, if you stand near
a heater, you will feel hot.

These examples show that heat is transferred from a hot body to a cold body.

The process of transfer of heat from one part to another is called transmission of heat.

There are three ways in which transmission of heat takes place. Scan for practical experiment

(i) Conduction (ii) Convection (iii) radiation

Conduction

Heat transmits through solids by the process of conduction.

Activity visit: csp.codes/c07e11

Take an iron or steel rod of 15 cm length. Fix four pins with the help of wax on
the rod. Place the rod on a wooden block as shown in the fig and hold it in its
position by a brick. Now heat the other end of the rod by means of a lamp or a
burner.

What do you observe?

You will find that the nails begin to fall one by one starting from the hot end of the
rod to the other end.

This shows that heat energy travels from the hot end of the ruler towards its cold
end. Let us suppose in a class, students are seated in their benches as shown in
figure. If a teacher wishes to pass the book to the far students of the bench, how
does the students get the book?

Good Conductors and Bad Conductors Steel rod

When one end of a spoon is put in the flame, the other end
becomes hot after a short while. But if we place one end of
a pencil in the flame, it starts burning and other end does not
get heated up.

This clearly shows that all substances do not conduct heat
equally.

Blooming Science & Environment Book 7 67

Those substances, which allow heat to flow through them easily are called good
conductors of heat. All metals are good conductors of heat. It has been found that silver
is the best conductor of heat.

Those substances which do not allow heat to flow through them easily are called bad
conductors of heat. Wood, plastics, glass, stone, rubber, cotton, wool are bad conductors
of heat. Due to this wooden handle or plastic handles are used in most of the kettles,
spoons used in cooking because a metal would soon get hot from the other end.

The process of transmission of heat in solids without the actual movement of particles
from their positions is called conduction.

Convection

Heat transmits through liquids and gases by the process of convection.

Activity

Take a beaker and fill three-fourth of it with water. Drop some portion of
potassium permanganate or some particles of ink. Keep the beaker on a tripod
stand and heat it with the help of a lamp or burner. Watch the movement of the
coloured solution.

What do you observe?

The coloured steam of water from the bottom of Water
the beaker rises to the top at the center and then fall beaker
downwards along the sides.

Why does this happen? Stand

When we heat the beaker, the water at the bottom gets Source of heat

heated. On being heated, it expands and becomes less

dense or lighter. The lighter hot water moves upward. The water at the top is

cooler and comparatively heavier so it comes down to take the place of hot water.

The process continues until the whole water becomes hot.

While moving hot molecules up and cold molecules down a current is formed called
convection current.
In this activity, we have noted the actual movement of water molecules that causes the
transfer of heat. Thus, convection can be defined as follows:
The process of transmission of heat in liquids and gasses due to actual movement
of their particles is called convection.
Radiation:
If you stand in the sun on a sunny day, you will feel warm.

68 Blooming Science & Environment Book 7

How do you feel the warmth?

Earth

Radiation

As we know that the sun is at a large distance from our earth and there is no air
beyond earth’s atmosphere. This means there is no material medium between the sun
and outside of the earth’s atmosphere. Hence, heat is not transmitted from the sun to
the earth by conduction or convection.

The process by which heat is transmitted from one part to another without the help of
any material medium is called radiation.

When we stand in front of a fire or heater, we feel warmth. The heat from the fire
transmits in all directions due to the process of radiation.

Radiation is a very quick process in which heat travels with the velocity of light.

Temperature

The hotness or coldness of different bodies can be compared by introducing a term
called temperature. Temperature measures the degree of the hotness or coldness of the
bodies.

The hot body is called a body of high temperature and the cold body is called the
body of low temperature. Heat flows from the body of higher temperature to the body
of lower temperature untill the temperature of both bodies becomes equal. When the
temperature of the two bodies is equal then these body are said to be in the state of the
thermal equilibrium.

The SI unit of temperature is Kelvin (K), The other common units are degree Celsius
(oC), degree Fahrenheit (oF) and degree Reaumur (oR).

Differences between Heat and Temperature

Heat Temperature

1. Heat is a form of energy, which gives 1. Temperature is the degree of hotness

the sensation of warmth. or coldness of the body.

2. It is measured in the unit Joule and 2. It is measured in the unit of Kelvin,
Calorie.
3. Caloriemeter is its measuring 3. Centigrade and Fahrenheit. of
4. It is measured by thermometer.
instrument. Temperature measures the flow
4. Heat flows from the body of heat.

high temperature to the body of low

temperature. 5. Temperature is the effect of heat.

5. Heat is the cause of temperature.

Blooming Science & Environment Book 7 69

Thermometer
Thermometer is an instrument which is used to measure the temperature of body. All
the thermometers are constructed on the principle that the substances expand on heating
and contract on cooling. On the basis of the material filled inside the thermometer,
there are two types of thermometer-alcohol thermometer and mercury thermometer.

The ordinary mercury thermometer consists of a glass tube having a narrow fine bore
and a bulb. The bore extends from one end to another end of the thermometer. One end
of the bore is scaled and the other end is connected to the bulb. The bulb and the small
portion of the bore is filled by dry, pure and clean mercury.

Thermometric Substance
The liquid kept inside the thermometer is called the thermometric liquid. The commonly
used thermometric liquids are alcohol and mercury.

Mercury
It is a good thermometric liquid because of the following reasons:
1. It is opaque and bright so it can easily be seen in the glass.
2. It is a good conductor of heat.
3. It does not wet the walls of glass. Hence, the rise and fall of mercury in the tube
is clean, smooth and regular.
4. It has uniform rate of expansion and contraction. It can expand or contract even
due to very small change in temperature.
5. If freezes at -39oC, and boils at 357oC. So, it can measured the wide range of
temperature between -39oC to 357oC.

Disadvantages

1. It cannot be used to measure the temperature of very cold regions. Mercury
freezes at -39oC so it cannot measure temperature below it.

2. It cannot measure the temperature more than 357oC as it turns into vapour at this
temperature.

3. Mercury is a poisonous substance.

Alcohol

Alcohol is a thermometric liquid because of following reasons:

1. It has a good expansion and contraction rate. Its expansion rate is about seven
times more than of mercury.
2. It freezes at -117oC and boils at 78oC. Thus, alcohol thermometer can measure
very low temperature (as low as -117oC).

70 Blooming Science & Environment Book 7

Disadvantages:
1. It is a colourless liquid so it should be coloured before use.
2. It has low boiling point. It cannot measure the temperature of even the boiling
water.
3. Its rate of expansion and contraction is not uniform as that of mercury.

Clinical Thermometer

A clinical thermometer is a mercury thermometer which is used to measure the
temperature of human body.

A clinical thermometer consists of a long capillary tube with a fine bore. The capillary
is connected to the thin-walled cylindrical bulb at one end at the other end is sealed.
The bulb and small portion of the capillary is filled with pure and clean mercury. The
capillary tube along with the bulb is covered by a thick glass tube. The stem of the tube
is graduated in Celsius or Fahrenheit scale. In Celsius scale it is graduated between
35oC to 43oC whereas in Fahrenheit scale it is graduated between 90oF to 110oF.

The capillary tube of clinical thermometer consists of a small constriction or kink
near the bulb. When the temperature rises, the mercury easily moves up through the
constriction and reaches upto a certain scale. But, when the thermometer is removed
from the body, the mercury column of the capillary does not fall down immediately
due to the constriction. Thus, the constriction helps to take the correct reading. After
the use, the thermometer is given a jerk to make the mercury column of the capillary
fall down. Generally, the shape of the clinical thermometer tube is prismatic.

Laboratory Thermometer

A laboratory thermometer is a simple mercury
thermometer, which is used to measure the
temperature of lab reagents, chemicals and
solvents. It consists of a thick walled glass
capillary tube with a fine bore. The capillary tube
is connected to the bulb at one end and the other
end is sealed. The bulb is filled with mercury
but the capillary does not have any constriction.
The stem of the glass tube is graduated in degree
from -10oC to 110oC.

Blooming Science & Environment Book 7 71

When the bulb of the thermometer is placed in contact with a hot body, the mercury
level rises in the capillary and shows a certain temperature. We should take reading of
the temperature of the body by keeping thermometer in contact because the mercury
level cannot remain in the same point due to the lack of constriction.

Calibration of Thermometer

The graduation of thermometer by marking the number on a chosen scale is called
calibration of thermometer. In order to graduate thermometer, first we find the lower

and upper fixed points of the scale.

Determination of Lower Fixed Point

To determine the lower fixed point, the thermometer is placed

vertically in the funnel containing melting ice at the place of
the standard atmospheric pressure (760mmHg). The level of Ice point
the mercury inside the tube falls gradually and after some time Melting ice
it attains the constant value fixing in a point which is called Funnel

the lower fixed point. The lower fixed point is marked as 0oC

or 32oF.

Determination of the Upper Fixed Point

Round-bottomed flask filled with water at the atmospheric Stem point

pressure 760mmHg is fitted with a cork. Two small holes

are made on the cork for a thermometer tube and glass tube.

The glass tube helps the steam to escape. The thermometer Flask
Steam
tube is kept in such a way that, the bulb of the thermometer Boiling water

lies just above the water surface as shown in fig. When the

flask is heated, the temperature of the water increases. The

mercury inside the thermometer expands on heating and

gets the steady value at a fixed point. This fixed point is called the upper fixed point of

the thermometer. This point is marked as 100oC or 212oF. Length between the upper

fixed point and the lower fixed point is divided into 100 equal parts. The spacing

between two consecutive parts is called one degree. This is called the calibration of

the thermometer.

Scales of Temperature

The commonly used temperature scales are given below:

Centigrade (Celsius) Scale

In this scale, the upper fixed point is the boiling point of the water i.e., 100oC and the
lower fixed point is the freezing point of the water i.e., 0oC. Between the 100oC and

72 Blooming Science & Environment Book 7

0oC, there are 100 equal parts; each parts is called degree centigrade (oC). The normal
body temperature is 37oC in this scale.

Fahrenheit Scale

In this scale, upper fixed point is the boiling point of water i.e. 212oF and lower fixed
point is the freezing point of water i.e., 32oF. There are 180 equal parts between 32oF
and 212oF, each part is called a degree Fahrenheit (oF). The normal temperature of our
body is equal to 98.6oF in this scale.

Kelvin Scale

In this scale, water boils at 373 K and freezes at 273 K. There are 100 equal parts
between 273 K and 373 K. Each part is called one-Kelvin. The relation between

centigrade, Fahrenheit and Kelvin can be written as:

C−0 = F−32 = F−273
100 180 100

or, C = F−32 = F−273
5 9 5

Main Points to Remember

1. Heat is a form of energy.
2. Different effects of heat on objects are:
Change of temperature
Change of state
Expansion
3. The process of transfer of heat from a hot portion to a cold one is called
transmission of heat.
4. The methods of transmission of heat are:
Conduction that is without the movement of particles in solids.
Convection that is due to the movement of particles in liquids and gases.
Radiation that is without the need of a medium
5. Temperature is the degree of hotness or coldness of a body which is measured by
a thermometer.
6. A thermometer is based on the principle that a substance expands on heaing and
contracts on cooling.

Blooming Science & Environment Book 7 73

7. The F.P. of mercury is -39oC and B.P. is 357oC.
8. The F.P. of alcohol is -117oC and B.P. is 78oC.
9. We get heat from the sun by radiation.
10. Constriction of clinical thermometer helps to note accurate reading.
11. The normal body temperature is 37oC or 98.6oF.
12. The lower fixed point of thermomter is 0oC (32oF) and upper fixed point is
100oC (212oF).

PRO J ECTWORK

Measure the temperature of your body in the beginning of the school and at
the end of the school and also measure the temperature of your friend and
notice whether or not any difference in temperature.

Exercises

1. Fill in the blanks.
a. Heat is a form of...................................
b. Nuclear reaction takes place in...................................
c. Burning fuel gives...................................
d. Temperature of our body is always...................................
e. Temperature measures..................................or.............................of the body.
f. Combustion of food gives...................................
g. Normal temperature of our body is...................................
h. Alcohol and mercury are..................................Liquids.
2. Write down True or False and correct the false statements.
a. Thermometer measures the quantity of mass of the body.
b. Fuel is the ultimate source of energy.
c. Sun is the major source of energy.
d. Centigrade is the instrument to measure the temperature.
e. Our body temperature is different in summer and winter.

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f. Mercury is commonly used in thermometers.

g. Temperature is the qualitative measurement of heat.

3. Match the following.

Column A Column B

Heat Burns

Temperature source of heat

Fuel Thermometer

Thermometer Clinical thermometer

Sun - 39oC

Mercury instrument to measure temperature

Constriction energy

4. Write difference between:

a. Heat and temperature b. Celsius and Fahrenheit scale

c. Conduction and convection d. Conductor and insulator.

5. Answer these questions.

a. What is heat? Write its four uses.

b. What is freezing point? Is it possible for all substances to have the same
freezing and melting points?

c. Define conductors and insulators with example.

d. What are solidification and liquification?

e. What is temperature?

f. What are the scales of temperature? Give their relation.

g. Write four advantages of mercury thermometer than alcohol thermometer.

h. What is the main use of thermometer?

i. What is calibration of thermometer? Write about the upper and lower fixed
point.

j. What is conduction? It is possible in liquid and gases?

k. Define radiation and convection.

Blooming Science & Environment Book 7 75

l. How can you say that heat convection is very useful to us?
m. Write about some natural and artificial convection current.
n. Conductivity of the different material is different. Justify.
p. Constriction is kept in clinical thermometer, why?

Glossary

Heat : form of energy

Temperature : degree of hotness or coldness of a body

Expansion : increase in volume due to heat

Conduction : method of transmission of heat in solids

Convection : heat transmission by movement of molecules

Radiation : process of transmission of heat without medium

Conductor : matter which allows heat and electricity to pass through it

Insulator : material which does not allow heat and electricity to pass
through it

Thermometer : device to measure temperature of body

76 Blooming Science & Environment Book 7

7Chapter Light

Learning Outcomes

On the completion of this unit, students will be able to: Estimated Periods- 3+1

• identify luminous and non-luminous bodies.

• define reflection of light and explain type of reflection.

• explain and demonstrate the laws of reflection of light

• prepare some simple instruments like prescope and kaleidoscope and

describe their working principle.

Light is a form of energy that produces the sensation of vision. It helps us to identify
the shape and the colour of objects. All objects can be seen only in presence of light.
For example, when we are in a dark room, we cannot see any things. But we can see
many objects in the presence of light in a room. Actually, we can see any object only
when the light coming from the object enters in our eyes.

The branch of science that deals with the study of light and its activities is called
Optics.

There are two theories about light. One of them explains that light is composed of
tiny particles and another explains that light is a wave. Actually, both theories are
equivalent as light can be transmitted from one place to another place, reflected or
refracted.

Luminous and Non-luminous Bodies

Burning candles, glowing electric bulb, the sun, the stars, fireflies, etc. emit light
themselves. Therefore, these bodies are called luminous bodies. Books, trees, chairs
etc., do not have their own light, but they are seen in the presence of light. Thus, these
bodies are called non-luminous bodies. What about the moon? Is it luminous or non-
luminous?

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How is light a form of energy?

A glowing electric bulb transforms the supplied electrical energy into light and heat.
A burning candle transforms its chemical energy into light and heat. When rays of the
sun are focused on a paper by a hand lens, it may cause it to burn. Here, solar energy
of the sun is changed into heat. Solar heaters, solar cookers, etc. use solar energy and
convert into heat energy.

These examples show that light is a form of energy. This is because light can be
obtained by the transformation of other forms of energy such as electrical energy,
chemical energy, etc.

In the similar way, light can also be converted into other forms of energy such as heat
energy, electrical energy, chemical energy, etc.

Light travels with the velocity of 3,00,000 kilometers per second in air or vacuum. It
takes about 8 minutes 20 seconds for the light of the sun to reach the surface of the
earth. Can you calculate the distance between the earth and the sun?

Reflection of Light

When light falls on the surface of a body, some of it is
absorbed, some of it may be transmitted and most of
it is reflected. The phenomenon of returning of light
in the same medium after striking a surface is called
the refection of light. A polished smooth surface is a
good reflector of light.

Plane Mirror Reflecting surface

A plane mirror is usually a flat sheet of glass with

a thin layer of a shiny metal such as silver or Polished surface

aluminium behind the glass. The surface of the mirror is very smooth. The quality

of the image formed by a plane mirror depends on the smoothness of its reflecting

surface. To represent a plane mirror in a diagram, a straight line is drawn with some

shaded lines on one of its sides. The shaded lines represent the polished surface of the

mirror.

78 Blooming Science & Environment Book 7

Some related terms to the Reflection of Light

Let a ray of light AO be incident on A NB
Normal
a plane mirror PQ as shown in the Incident ray
Reflected ray
Angle of
Angle of reflection
figure. By the definition of reflection, incidence OQ

the ray. AO is sent back in the medium P
along OB. The perpendicular drawn at

O on PQ is ON.

Incident Ray

The ray falling on the surface of an object from the source of light is called an incident

ray. In the figure, the ray AO falls on the surface PQ. So, the ray AO is an incident

ray.

Point of Incidence Scan for practical experiment

The point on the surface at which an incident ray falls is called
the point of incidence. In the figure, O is the point of incidence.

Normal

The perpendicular drawn at the point of incidence is called a
normal. In the figure, ON is a normal.

Reflected Ray visit: csp.codes/c07e13

The incident ray returns in the same medium after striking the
surface, such returned ray is called a reflected ray, in the figure OB is a reflected ray.

Angle of Incidence

The angle formed between the incident ray and the normal is called the angle of
incidence. In the figure, AON is the angle of incidence, It is generally denoted by i.

Angle of Reflection

The angle formed between the reflected ray and the normal is called the angle of
reflection(reflected angle). In the figure BON is the angle of reflection. It is generally
denoted by r.

When a ray of light is incident on a plane surface, it is reflected by following the
certain rules. These rules are called laws of reflection.

Laws of Reflection

a. The incident ray, the reflected ray and the normal at the point of incidence all lie
in the same plane.

b. The angle of incidence (i) is always equal to the angle of reflection (r). (i = r)
c. If a ray falls normally, it retraces its path.

Blooming Science & Environment Book 7 79

Activity

To verify the laws of reflection.
Materials required : A drawing board, a sheet of white paper, thumb pins and
pins, a plane mirror with a stand, a geometrical box, a pencil
Method:

a. Fix a sheet of white paper on a drawing board with a help of thumb
pins.
b. Draw a straight line PQ in the middle of the paper and a slanted line BO on it
as shown in the figure (i).
c. Place a plane mirror in upright position along the straight line PQ.
d. Fix two pins a and b vertically in front of the mirror at a certain distance. You
will see the images a’ and b’ of the pins a and b, respectively in the plane.
e. Observe the image of the pins from the other side and fix two pins c and d
in such a way that it should overlap the images of the pins a and b, this ensures
the images a’, b’ and the pin c and d lie in a straight line as shown in the figure
(ii).
f. Mark the positions of the pins a, b, c and d with a pencil. Remove these pins
and the plane mirror from the drawing board.
g. Draw a line joining the points c and d which meet the mirror again at
O.
h. Draw a normal ON at O. Measure BON and CON.
Conclusion :
a. It is found that
∠BON = ∠CON
i.e., i = r
Thus, the angle of incidence (i) is equal to the angle of reflection (r).
b. It is noticed that the incident ray (BO), the reflected ray (OC) and the normal
(ON) at the point of incidence (O) all lie in the same plane.
From this activity, the laws of reflection are verified.

80 Blooming Science & Environment Book 7

Types of Reflection

Wall Incident beam
Spot
Incident rays Reflected rays

Plane mirror
Reflected beam

Regular reflection Reflection of the light of the sun from a plane mirror

Hold a plane mirror facing the sunlight and allow the reflected light to fall on a wall.
You will get a bright spot on the wall. As the position of the plane mirror is changed,
the position of the bright spot on the wall also changes. This activity shows that the
light reflected from the mirror travels in one direction only. Such types of reflection is
called regular reflection.

When a parallel beam of light falls on a smooth surface such as a plane mirror, the
reflected beam is also parallel. This is called regular reflection.

Now, repeat the above activity with a flat piece of paper. This time, you will not get any
bright spot on the wall. This activity shows that the light reflected from the wood or
paper does not travel in one direction only, i.e. it is scattered in different directions.
Such type of reflection is called irregular reflection.

When a parallel beam of light falls on a rough Incident rays Reflected rays
surface such as wood, paper, etc. the individual
rays are reflected in different directions from the
surface. This is called irregular reflection.

Can you answer that a steel plate shines but a
wooden vessel does not, why?

During a sunny day, the light falls on the objects

of the surroundings. These objects reflect the light Irregular reflection
irregularly. Such irregularly reflected light or

diffuse light enters into the classroom and the classroom becomes bright. Thus, our

classroom becomes bright due to the irregular reflection or diffusion of light.

The counterpart of an object formed by the reflection of light is called image of that
object. When the reflected rays from an object intersect or appear to intersect, then an
image of that object is formed. A real image is formed by the actual intersection of the
reflected rays while a virtual image is formed by the virtual intersection of the reflected
rays. A real image can be obtained on a screen but a virtual image cannot.

Can an image be formed by both regular and irregular reflections?

Take a trough with water. Keep it undisturbed for a while. When the water in the
trough becomes still, you can see your image in it. The reflection from the surface of
the still water is regular.

Blooming Science & Environment Book 7 81

Now, disturb the water in the trough. This time, you will not be able to see your image.
In this case, the reflection from the disturbed water surface is irregular. Thus, an image
is formed only due to regular reflection. Because of this, we can see our image in a
plane mirror, but not in a plane sheet of paper, wood, concrete wall or on any rough

surface. The quality of image formed by a surface depends on its smoothness.

Periscope

It is an optical instrument used to see objects which Plane mirror

are not in the same level of the observer. It is based on Plane
mirror
the reflection of light by a plane mirror. It consists of
two plane mirrors strips fixed at 45o to the ends of a

long narrow tube. These narrrow tubes are parallel to
each other. Each mirror turns the light rays by 90o. The

observer looking from below can see the image of the

objects in the lower mirror.

Periscope is an instrument for observation from a concealed position. In its simplest
form it consists of a tube with at each end set parallel to each other at a 45-degree
angle. This form of periscope, with the addition of two simple lenses, served for
observation purposes in the trenches. Military personnel also use periscopes in the war
to view their enemies..

Method of Construction

a) Take a circular or rectangular
paper pipe ( If not found it can
be made by using carton also )

b) Find two plane mirrors of
appropriate size and fix these
two mirrors at the two corners
of the tube parallel to each
inclined at 45o in each circular
hole of opposite sides of the
tube.

c) Make the holes on the tube by
cutting their ends.

d) Join those tubes in Z-shape
and make as shown in the figure.

On viewing from the lower mirror of periscope, We can see the object on the height it
is used to see the object on height from the lower places.

Kaleidoscope

A Kaleidoscope is a circle of mirrors containing loose, coloured objects such as beads
or pebbles and bits of glass. As the viewer looks into one end, light entering the other
end create a colourful pattern, due to the reflection of light by mirrors. In 1817 a

82 Blooming Science & Environment Book 7

Scottist inventor Sir David Brewster coined the word “Kaleidoscope” and is derived
from Ancient Greek “Kalos” (kalóx) meaning beautiful or beauty and “eidos” meaning
“that which is seen: from shape” and “skopeó” meaning “to look, to examine” hence
“observer of beautiful form S”
Kaleidoscope operates on the principle of multiple reflection, where several mirrors
are attached together. Typically there are three rectangular mirrors put side by side
touching each other lengthwise. Setting the mirrors at 45o angle creates eight duplicate
images.

Fig: Kaleidoscope

Method of Construction
a) Take three equal rectangular mirror strips and make their edges blunt by rubbing
on stone or cemented wall.
b) Fix those strips to an equilateral
triangular shape with their shiny face
inside with the help of tape. Wrap the
mirrors with your favourite coloured
paper.
c) Cover its one end with white sheet and
another end with opaque sheet of paper.
d) Keep some coloured pieces of broken
barleys or litters inside the tube .
e) Make a small hole at the centre of opaque sheet and cover the hole with transparent
sheet .
f) Now the kaleidoscope is prepared shake the equipment to get different patterns
inside and observe from the small hole.

Blooming Science & Environment Book 7 83

Main Points to Remember

1. Light is a form of energy, which gives the sensation of vision.
2. The body, visible due to its own light is called the luminous body and the body
which is appeared due to the light of other, is called the non-luminous body.
3. Light can behave as wave as well as particle.
4. Shadow is the region in the screen made due to the blockage the incident ray by
the opaque object.
5. Image is a figure on the screen made by reflection or refraction of light.
6. There are two types of image real and virtual; the real image can be taken on the
screen and virtual image cannot be taken on the screen.
7. The phenomenon in which the light coming from one medium strikes on the
object and returns in the same medium is called reflection of light.
8. The laws of reflection are:
a. The angle of incidence is equal to the angle of reflection.
b. The incident ray, reflected ray and the normal all lie in the same plane or the same
point.
9. There are two types of reflection: regular reflection and irregular reflection.
10. Perscope and Kaleidoscope are application of reflection of light.

PRO J ECTWORK

Make a model of a periscope or a Kaleidoscope by using the local materials
available in your locality and demonstrate in your class.

Exercises

1. Fill in the blanks.
a. The image formed by plane mirror is...................................
b. Real image can be taken in the...................................
c. In the regular reflection, the angle of incident is......................to angle of
reflection.
d. The length of the shadow increases with..................................the distance of
screen from the object.
e. In the mirror, the right hand appears as...................................
f. Inversion is the process of..................................

84 Blooming Science & Environment Book 7

2. Write down True or False and correct the false statements.

a. Regular reflection occurs in rough surface.

b. Mirror reflects the complete light that falls on the mirror.

c. Stopping the light by transparent material forms shadow.

d. In the regular reflection, the angle of incident is not equal to angle of reflection.

3. Match the following.

Column A Column B

Regular reflection perpendicular to the plane

Irregular reflection opaque

Shadow rough surface

Image smooth surface

Normal Figure

4. Choose the correct answer.

a. Regular reflection occurs in

i. Wood ii. Water iii. Charcoal iv. Book

b. Reflection occurs in

i. Single medium ii. Two mediums

iii. Without mediums iv. Interface between two mediums

c. Angle made by incident ray with normal

i. Incident angle ii. Refracting angle

iii. reflected angle iv. Critical angle

d. Diffuse reflection helps

i. To make real image in the screen

ii. To make small brightness in the room

iii. To make the dark in the room

iv. To destroy the opaque object.

5. Differentiate between.

a. Regular and irregular reflection.

b. Real image and Virtual image

c. Luminous and non-luminous objects

Blooming Science & Environment Book 7 85

6. Answer these questions.
a. What is light? Give the two theories that explain the nature of the light.
b. What is shadow? How does it form?
c. Why don’t we see the image of the body in the rough surface?
d. What is reflection of light? Explain with the help of diagram.
e. Write down the two laws of reflection.
f. How does the image form in the plane mirror?
g. What is lateral inversion? Give one example.
h. The length of the shadow is different in evening and in the day. Justify.
i. Define real and virtual image.
j. Write the use of Periscope and Kaleidoscope.

k. Draw a periscope and write its working principle.

Glossary

Reflection : process of returning back of light in the same medium
when it trikes on the surface of body.


Concave mirror : a spherical mirror in which the inner surface reflects
light.

Convex mirror : spherical mirror in which the outer surface reflects

light.

Real image : the image producible on the screen.

Virtual image : the image not producible on the screen.

Magnified image : bigger image

Diminished image : smaller image

Refraction : the bending of light it goes from one medium to
another.

Lateral inversion : the sideways inversion of objects in its image.

86 Blooming Science & Environment Book 7

8Chapter Sound

Learning Outcomes

On the completion of this unit, students will be able to: Estimated Periods: 3+1

• define sound and mention the different sources of sound.
• define longitudinal wave.
• describe the origin and propagation of sound wave.
• describe the factors affecting transmission of sound.
• demonstrate sound propagates with different speed in different medium.

We hear different types of sound every day. We hear songs from radio or television,
the chirping of birds, the humming of bees, the sound of a madal or a guitar, the
barking of dogs and the horns of vehicles. Some sounds are pleasant and others are
unpleasant. An unpleasant sound is called noise. We should not listen to loud sound
as it damages our hearing. Sometimes, even a pleasant sound can be unpleasant if it is
too loud. We can hear strong sound when the source of sound is close to us. If it is far
away, we hear weak sound. Sound is a form of energy that produces the sensation of
hearing in our ears.

Sources of Sound

Objects that produce sounds are called sources
of sound. A guitar, a madal and an electric bell
are some sources of sound.

Hold your finger against your throat. Read
out some sentences from your book, or talk to
your friend. What do you feel? You will feel
the vibrations of your vocal cords. Sound is
produced by the vibration of an object. The up

and down, or side to side, movement to an object

is called vibration. A tuning fork is a source of

sound that is used in a laboratory. It is U-shaped metal instrument with a stem. The

U-shaped part has two arms called prongs. If one of the prongs is struck, it vibrates

and a sound is produced.

Blooming Science & Environment Book 7 87

Activity

Take a rubber band.

Hold one end of it in your mouth and the other end in
one hand and stretch it. Pull the middle of the rubber
band with your other hand and release it.

What happens to the rubber band?

You will notice that it starts vibrating and it produces
sound.

Activity for Practice

• Take a drum and beat it with a drumstick.
• You will hear sound, but you cannot see the vibration.
• Now place few grains of rice on the surface of drum
and beat it again.
• What will you see?
• You will see the grains of rice jumping up and down,
showing that the surface of the drum is vibrating.

Try to find out what produces sound in the following cases.

i. Singing a song ii. Plucking a guitar

iii. The humming of mosquito iv. The beating of a madal

v. The blowing of a whistle

Wave

Activity

Take some water in a trough. Throw a small stone into
the middle. The disturbance of water surface in all
direction is seen. It produces a disturbance in which
the water molecules vibrate up and down and form
waves which move continuously towards the trough.
During vibration on any object, the waves are formed.

88 Blooming Science & Environment Book 7

A wave is a kind of disturbance that travels through medium matters and space. Wave
motions transfer energy from one place to another. There are two types of waves that
transmit in matter. They are

1) Transverse wave

2) Longitudinal wave

a) Transverse wave

The waves that from on water in above example are transverse waves. The wave in
which the particles of medium move up and down perpendicular to the direction of
propagation of wave is called transverse wave.

A transverse wave propagates in the form of crest and trough. The maximum upward

displacement of molecules is called crest and maximum downward displacement of

molecules is called trough.

Crest Crest

Trough Trough Trough

b) Longitudinal wave

The wave in which the particles' movement is parallel to the direction of the wave
propagation is called longitudinal wave. Sound wave is an an example of longitudinal.
It can be observed in a helical spring

Activity
Take a coiled spring of about 1m long and hold its ending by two friends. Let

one end be free and observe it carefully. You can observe to and fro vibrations of

spring. Some of the adjacent coils come closer to one another while some move

father from one another. compressions

rarefaction

A longitudinal wave propagates in the form of compression and rarefaction.
A compression is a region in a longitudinal wave where the particles are closest
together similarly a rarefaction is a region in a longitudinal wave where the particles
are farthest apart.

Blooming Science & Environment Book 7 89

Nature of Sound

Sound is a form of energy which is produced Wavelength
by the vibrating objects. It is propagated in the Amplitude

form of waves. Sound is propagated in the form

of longitudinal wave. As light has its properties

like reflection, refraction, diffraction and sound Compression Rarefaction Compression

also have similar properties. Therefore, sound is a form of energy. Sound also travels

in the form of waves. Sound waves are longitudinal waves. Longitudinal waves are

those in which the vibration of particles of the medium move to and fro in the direction

of wave. The particles of the medium vibrate in such a way that they come close and go

apart and make alternate compression and rarefaction. Compressions are those where

the particles come close and the rarefactions are those where the particles go apart.

The distance from one compression to the consecutive compression is called
wavelength.

The compression and rarefaction can be seen on a helical coil holding in two hands.
The number of waves produced in one unit time is called frequency. The frequency
is measured in Hertz (Hz). You have studied about these terms in preceding sections.

If an object vibrates at any frequency between 20 and 20,000 vibrations per second,
its vibrations will affect the average human ear so that sound is heard. A person can’t
move his arms fast enough to produce sound, but the bees can move their wings so fast
that a buzzing sound is heard when they are flying.

When vibrations occur too rapidly for the human ear to hear, they the called ultrasonic
vibrations (ultra = beyond, sonic = sound) i.e. more than 20 KHz. Dogs can hear sound
caused by frequencies up to 40,000 vibrations per second. Vibrations of frequency
less than 20 vibrations per second are infrasonic vibrations. They have been used
successfully in drilling deep oil wells, because this low-frequency vibrations break
up hard rock more easily than ordinary drill bits do. We can hear the sound wave of
frequency ranges 20 Hz to 20 KHz called audible sound.

Transmission (Propagation) of Sound

When a body vibrates, it makes the air molecules around it Scan for practical experiment
vibrate. These molecules transfer their vibrations to other
molecules nearby, and so on. When these vibrations reach our visit: csp.codes/c07e14
ears, we hear sound. The phenomenon of sound travelling from
one place to another is called the transmission of sound. A
material medium is required for the transmission of sound. The
material medium may be solid, liquid or gaseous. Sound travels
fastest in a solid medium and slowest in the gaseous medium.
Sound cannot travel through a vacuum.

Take an electric bell and place it inside a glass bell jar that does not contain air. Now,
switch on the electric bell. Can you hear the sound? Why?

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Sound can transmit through material medium. Here are some experiments to determine
the transmission of sound through material medium.

Transmission of Sound in solid

Activity

Take two tin cans. Fix a paper at one end of the tin
can tie a thread to the bottom of the can and pass
the thread to the other end of another tin can. Let
the next tin can be held by your friend at a certain
distance, straining the thread straight. As you speak
in the tin can and make your friend listen, he can hear the sound at that distance.
Similarly, let your friend put his ear on the desk and you tap on the desk. He can
hear the sound. Sound travels very fast in solid medium.

Transmission of Sound in Liquid

Activity

Take a large bucket and full it with water. Place a bell in the water and strike with
a hammer. The bell should not touch the walls of the bucket. You may hear very
weak sound.

Water Metal disc Hammer
Bucket Water

Metal disc not touching the Metal disc touching the

bucket bucket

Now touch the bell with the walls of the bucket and ring the bell. You can hear

louder sound.

The depth of ocean can be measured by using sound. Ships send sound waves towards
the bottom of the sea. The sound gets reflected to the surface after striking the bottom
of the sea. The time taken by the sound is divided by 2. Thus, the depth of the sea can
be measured.

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Dolphin can produce high pitch sound. Recognizing the reflected sound, the dolphin
can measure the distance of the solid surface.

Transmission of Sound in Gases

The sound travels in gases. The sound we hear is transmitted through the air. Air is a
material medium. The velocity of sound in air is very low compared to the liquid and
solid.

Activity

To demonstrate that sound travels in the air:
Materials required: a plastic bottle, a
plastic bag, a rubber band, scissors and a
small candle.
Method:
• Cut the base off the plastic bottle.
• Then, cut a piece from the plastic bag
to cover the end of the bottle.
• Stretch the piece of plastic tightly over the end of the bottle.
• Secure it with the rubber band. Light the candle.
• Hold the bottle with its neck about 2.5 cm (1 inch) away from the
candle.
• Now tap the piece of plastic sharply with your fingertips. What happens to the
flame?
Observation:
• When you tap the piece of plastic, you make the tiny particles of air next to the
plastic vibrate.
• These vibrating particles make the particles beside them vibrate too.
• The vibration travels through the bottle and disturbs the flame.

Speed of Sound in Gas

There are different factors which affect the speed of sound.

a. Temperature: Greater the temperature of the gas greater wil be the speed of sound.
It occurs due to the less density of the gas at higher temperature.

b. Density: Speed of sound directly varies with the density of the gas i.e. greater the
density greater will be the speed of sound.

c. Humidity: The amount of water vapour in the atmosphere is called humidity. It
has actually the negligible affect on the speed of sound. i.e. speed of sound is
slihtly more in humid air than in dry air at the same presure.

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Medium is required for the Transmission of Sound

Activity Wire connected to
battery
Material medium is required for the
transmission of sound. Rubber string

Method: Electrical bell
Bell jar
Take a jar and an electric bell as shown in fig. Make Glass sheet
the electric circuit connection as shown. Ring the
bell. You can hear the sound. After connecting the
vacuum compressor, remove the air from the jar.
Ring the bell. You can see that the hammer hits the
bell but you cannot hear he sounds. This is due to
the reason that there is no air and without medium
no sound can be transmitted.

The velocity of sound in different media (object) at NTP is given below:

Medium Velocity
Air (at NTP) 332 m/s
Water (at 0ºC) 1493 m/s
Sea water 1540 m/s
Steel 5000 m/s
Brass 3650 m/s

NTP = Normal Temperature and Pressure

Main Points to Remember

1. Sound is a form of energy. It is produced by a vibrating body.
2. The bodies that produce sound are called sources of sound.
3. Wave is a kind of disturbance that travels through matter or space.
4. Sound is produced by the vibration of molecules and propagated in the form of
wave.
5. Sound wave is a longitudinal wave.
6. Sound travels in any medium like solid, liquid and gas but does not travel in space.
7. The sound wave with frequency less than 20 Hz is called infrasound.
8. The sound wave with frequency from 20 Hz to 20 KHz is called audible
sound.
9‑. The sound wave with frequency more than 20 KHz is called ultrasound.

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PRO J ECTWORK

Take plastic cups and a roll of thread. Make a device to show propagation of sound
in solid medium and observe this in pair in playground of your school.

Exercises

1. Fill in the blanks.
a. ..................................is produced by a vibrating body.
b. Sound travels in the form of........................
c. The speed of sound in air is...................................
d. Sound needs.......................to propagate.
2. Use a tick () for the correct statements and a cross (×) for the incorrect
ones.
a. Even a pleasant sound turns unpleasant if it is too loud.
b. If a body stops vibrating , sound is heard.
c. Our vocal cords vibrate to produce sound.
d. A tuning fork is a convenient source of sound in the laboratory.
e. An unpleasant sound is called music.
3. Answer the following questions.
a. What is a source of sound? Write any five sources of sound.
b. How do you show that a tuning fork vibrates?
c. How is sound produced?
d. What is longitudinal wave?

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e. What are the factors affecting velocity of sound in air?

f. Draw diagrams of any two sources of sound.

g. Sprinkle sand on the horizontal surface of a drum and then tap it gently. What
do you notice?

h. Describe an activity to show sound propagates in liquid.

i. How can you prove sound can not travel through vacuum?
j. Define compression and rarefaction with their sketch.
k. Why does sound travel faster in solid than in liquid?

Glossary

Murmuring : to make a quite continuous sound

Soothe : to make somebody who is anxious upset, etc, feel calmer

Prong : each of the two or more long pointed parts of a fork

Cleft : a natural opening or crack, for example in the ground or in a rock

Blooming Science & Environment Book 7 95

9Chapter Magnetism

Learning Outcomes

On the completion of this unit, students will be able to: Estimated Periods: 3

• differentiate natural and artificial magnet

• tell the utilities of different types of magnet.

• describe properties of magnet.

• describe and demonstrate methods of preparing artificial magnets.

Introduction

The substances, which can attract small pieces of iron and always rests in the north -
south direction when suspended freely are called magnets.

A magnet attracts other magnetic materials towards it. The materials made by iron
cobalt and nicked are magnetic in nature and these are attracted by magnet. Magnets
are used in radio, transistors, microphone, telephone, etc. A magnet is also used to
generate electricity. Magnets are found in nature as well as can be made artificially.
There are two types of magnets; natural magnet and artificial magnet.

Natural Magnet

The magnet which is obtained from nature is called natural magnet. Long ago, people
of Magnesia, a place in Asia Minor (Greece) found out a particular stone that could
attract small iron objects. This wonderful stone was called magnet after Magnesia - the
place of discovery.

The people of those days (the Chinese) knew that when this stone was suspended
freely with a fine thread, it always points to the north and south direction. Navigators
used this property to find their way in the sea. Thus, they named it ‘leading stone’. The
word gradually changed to ‘lodestone’. Lodestones today are called natural magnets.
These are found on the earth and have a property of attracting small pieces of iron.

Artificial Magnet

The magnets which are made by human beings are called artificial magnets.
Nowadays, magnets are widely used in toys, magnetic compass, loudspeakers, electric
motors, bicycle dynamo etc. So, the humans made different types of magnets like bar
magnets horse shoe shaped magnet, etc. There are two types of manmade magnets.
They are; permanent magnet and temporary magnet.

Permanent Magnet

The artificial magnet which retains its magnetic properties for longer period is called
permanent magnet. Generally permanent magnets are made by using iron, cobalt and
nickel. Such magnets are used in radio, television, microphone, etc.

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Bar Magnet Horse shoe U-shaped
shaped
Compass

Temporary Magnet on off cell

The artificial magnet which does not retain its magnetic properties long
period is called temporary magnet. Such magnets are made by soft iron

core, iron or steel and generally used in electric bell, electric fans, etc.

Differneces between Natural Magnet and Artificial Magnet Electric bell

Natural Magnet Artificial Magnet
1. It is found in the nature. 1. Man made magnet is called artificial magnet.

2. It has less magnetic power 2. Magnetic power is more.

3. Lodestone is an example of 3. Bar magnet, horse shoe magnet, U-shaped

natural magnet. magnet, compass needle etc.

Magnetic Materials

Faraday showed that all substances are affected by magnet. Substances
like iron, nickle and cobalt are strongly attracted by a magnet and
are known as ferromagnetic substances. Substances like manganese,
platinum and oxygen show feeble attraction and are known as
paramagnetic substances. Substances like bismuth, antimony,
phosphorous, zinc, mercury, lead, tin, water and copper are repelled by
magnets and are known as diamagnetic substances.

Properties of Magnet

1. Magnets can attract only some metals: Magnets cannot attract all objects.
Magnets attract only some metals like iron, steel, nickel and cobalt. Magnets do
not attract other metals. Magnets also do not attract other materials like wood,
books, copper, brass, plastics, rubber etc.

Substances which are attracted by magnets are called magnetic substances. Iron,
steel, cobalt, nickel are magnetic substances.

Substances which are not attracted by magnets are called non-magnetic
substances. Wood, plastic, paper, glass, copper, brass, rubber are non-magnetic
substances.

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2. Poles of a magnet are located near the ends: The attracting power of a magnet is
maximum at the two ends and minimum at the middle. These two ends of a
magnet are called poles of the magnet.

Activity

Spread out some iron filings or common pins on
a piece of paper. Now roll a bar magnet on the
pins.

What do you observe?

You will find that most of the iron filings cling near the ends of the magnet while
there are very few iron filings or pins at the middle.

Why are more iron filings or pins at the ends?

The two ends of the magnet have the greatest attracting power. So they attract
more iron filings.

3. Freely suspended magnet always points in north- Scan for practical experiment
south direction: When a bar magnet is suspended
freely and horizontally from a thin thread, the magnet visit: csp.codes/c07e15
will come to rest pointing to the earth’s north and south
directions.

The end of magnet which, points towards the north is
called the North Pole and the other end which points south is
called the South Pole. These north and south poles are
marked by letter N and S respectively.

Activity

Take a bar magnet and suspend it freely as shown in
the figure. The magnet comes to rest after some time.

Observe the direction pointed by the two ends of the
magnet.

Again, disturb the magnet a little from its position of rest and allow it to come to
rest.

What do you observe again?

You will find that the magnet always points to the same north-south direction.

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4. Like poles repel and unlike poles attract: When the north pole of a magnet is
brought close to the north pole of another suspended magnet, the two magnets
repel each other. But when the south pole of a magnet is brought close to the
north pole of another suspended magnet, they attract each other. This is also

called law of magnet.

Activity

Take two bar magnets. Suspend one magnet freely by a piece of thread. After a
while, the magnet will come to rest in the north-south direction. Bring the north

pole of the other bar magnet near the south pole of the suspended magnet.

Like pole repel each other Unlike pole repel each other

Do you observe the attraction between the two poles?

Now, bring the south pole of the other magnet near the south pole of the
suspended magnet.

Do you observe the repulsion between the poles?

5. Each magnet has its own magnetic field: The effect of a magnet is felt in a certain
region around a magnet.

If you place a compass needle swings and then rests in a
definite direction. When the compass needle is taken
away from the magnet, the influence on the needle slowly
decreases. The effect on the needle completely disappears
after a certain distance away from the magnet.

This clearly shows that effect of a magnet can be felt in a certain region around a
magnet.

The region around a magnet in which its influence can be felt is called “magnetic
field”. The magnetic field is stronger near the magnet and decreases with the in

crease in distance from the magnet.

Blooming Science & Environment Book 7 99

Activity

Place a sheet of white paper over a magnet and
sprinkle some iron filings on the sheet. Tap the surface of the
sheet gently.
What do you see?
The iron filings arrange themselves with a definite patterns.
Why does it happen? Explain.

6. Magnetic poles always exist in pairs: In a magnet north and south poles exist in
pairs. They cannot be isolated. If a magnet is broken into two pieces at the mid
dle, each piece will become a complete magnet containing a north and a south
pole. If the two pieces are further broken, each smaller piece still acts as a full
magnet containing the both poles. On breaking the pieces of magnet further, pro
duces a large number of magnets with both the poles at the ends.

Every piece of magnet is a complete magnet

This clearly shows that poles of a magnet cannot be separated and always exist in pairs.
Methods of Making Magnet
Magnetic substances like iron, nickel, cobalt, steel etc. can be made magnets, whereas
a non-magnetic substances cannot be made magnets. There are various methods of
making magnets. Some of the simple method of making magnets are as follows:

1. Stroking Method (Rubbing Method)
In this method, we magnetize the substances like blade, clip, nail etc. There are two
types of stroking method in which a magnet is made by using one or two permanent
magnet.
i. Single touch method
ii. Double touch method
Single touch method
In this method a magnetic material is magnetised by
rubbing with one magnet.

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