School Science Health & Physical Education 6

energy, light energy, sound energy, magnetic energy, electric energy, nuclear
energy, chemical energy, etc.

burning fire: heat torch light : light speaker: sound magnet: magneting energy

Activity

What kind of energy is present in a battery? What energy is there in an apple,
paracetamol, water in a dam, a river, a compressed spring, a rolling rock and a
burning candle? Guess your answers and discuss them with your teacher.

Mechanical energy

There are various objects around us. Some of them are in motion and others are
at rest. The objects in motion gain energy due to their mass and velocity. Objects
at rest gain energy due to their position at a height from the ground or their
deformation. Thus, the energy of an object due to its motion or position is called
mechanical energy. Broadly, the mechanical energy is divided into two groups:
kinetic energy and potential energy.

Reason to remember

The unit of energy is the same as that of work, i.e. joule (J). Why?
Energy is the capacity to do work. It is equal to the amount of work done by the
body. For example, 1J of energy is the capacity to do 1J work.

Potential energy

When an object is lifted from the ground, some work has been done. That work
done is stored in the object in the form of energy called potential energy. Similarly,
when you stretch a spring, its shape changes. That change in shape is due to the
deformation of the object. This also stores potential energy. Therefore, we can
conclude that the energy present in a body due to its position or deformation is
called potential energy.

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water reservoir stretched spring leg raised to kick football stretched catapult

Mathematically, potential energy depends on the mass of the body, height of the
body from the ground and acceleration due to gravity.

It is expressed as:

Potential energy (P.E.) = mgh

Here, ‘m’ is the mass of an object, ‘g’ is the acceleration due to gravity and ‘h’ is
the height of where the object is placed from the ground.

Fact file

1. The potential energy stored in a body travelling vertically upward is called
gravitational potential energy.

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

Reason to remember

Why does a cat at the top of a pole possess potential energy?

A cat at the top of an electric pole has some potential energy. The amount of
work done by the cat against gravity while it was climbing the pole is stored in
it in the form of potential energy.

Solved numerical: 1

A boy of mass 40 kg climbed a vertical height of 3 meters. Calculate the potential
energy stored in his body at that height.
Solution:
Mass of the boy (m) = 40 kg
Height from the ground (h) = 3 m
Acceleration due to gravity (g) = 9.8 m/s2
Potential energy (P.E.) = ?

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Now, potential energy (P.E.) = mgh

= 40 × 3 × 9.8

= 1176 J

Therefore, the potential energy of the body is 1176 J.

Activity

When we sit on a sofa, we feel we bounced a bit. How does the sofa get the
energy to push us up? Discuss this in a group.

Factors affecting potential energy

Let’s suppose you have to carry two stones, Fig.: water has potential energy
one of 5 kg and another of 10 kg, to the upper
floor. Which one would be easier to carry?
Obviously the 5 kg stone, right? But why?
Because 5 kg is a lesser mass than 10 kg. If the
mass is less, the amount of work to be done to
carry it to the upper floor is less because less
amount of potential energy is stored in it.
From this example, we can say that the
potential energy is directly proportional to
the mass of the object.

Similarly, we need to do more work to carry the stone to a greater height. Hence
the potential energy is directly proportional to the height travelled by the object.

At the same time, the potential energy is directly proportional to the acceleration
due to gravity at the place as well.

∴ PE = mgh

Kinetic energy

An object in motion can do work. For example, a flying bullet can bore a hole on
a steel plate. A moving hammer can break a rock. A swinging pendulum runs a
clock. Therefore, an object in motion has energy, which is the kinetic energy. The
energy present in an object due to its motion is called kinetic energy. For example,
flowing rivers, blowing air, falling raindrops, etc.

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flying bird flowing river rotating fan

Mathematically the kinetic energy is expressed as KE = 1 mv2. Where, KE is the
2
kinetic energy, ‘m’ is the mass of the object in motion and ‘v’ is the velocity of that

object.

Reason to remember

Kinetic energy increases if the mass of a moving object is increased. Why?

Kinetic energy increases if the mass of a moving object is increased because
kinetic energy is directly proportional to the mass of the object.

Solved numerical: 2

A cricket ball having mass 163 gram is shot with a velocity of 45m/s, calculate
the kinetic energy of the cricket ball?

Solution:

Mass of the cricket ball (m) = 163 gram = 0.163 kg

Velocity (v) = 45 m/s

Kinetic energy (K.E.) =?

Using formula,

K.E. = 1 mv2
2

or, K.E. = 1 × 0.163 × (45)2
2

= 1 × 0.163 × 2025
2

= 1 × 330.075
2

= 165.0375 joule

Therefore, the kinetic energy of the bullet is 165.037 joules.

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Reason to remember

Potential energy and kinetic energy are types of mechanical energy. Why?

Kinetic energy and potential energy are types of mechanical energy because
they are both there in the object. Kinetic energy is present in a body due to its
motion and potential energy is present in a body due to its position of rest at a
height.

Factors affecting kinetic energy

When you are hit by a cricket ball and then by a table tennis ball with the same
velocity, which one will hurt you more? Obviously, the cricket ball. What is
different between these balls? Their mass, right? The cricket ball has more mass
than the table tennis ball. From this example, we can conclude that even though
both balls have the same velocity, a cricket ball hurts us more because it has more
kinetic energy due to its higher mass.

hitting earthen pot by a cricket ball hitting earthen pot by a table tennis
(pot broken) ball (pot not broken)

From this observation, we can say that the kinetic energy is directly proportional
to the mass of the object in motion.

i.e., K.E. ∝ m (when ‘v’ or its velocity remains constant)

What will happen if you throw a cricket ball at an earthen pot twice, first, at a
very low velocity and then at a very high velocity? Hopefully, the low velocity
throw will not break the earthen pot. But when the cricket ball has a very high
velocity while hitting the pot, chances are great that the pot breaks.

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hitting earthen pot by high velocity hitting earthen pot by low velocity
cricket ball (pot broken) cricket ball (pot not broken)

Kinetic energy is directly proportional to the square of the velocity of an object.
i.e., K.E. ∝ v2 (when ‘m’ remains constant)

∴ KE = 1/2 mv2

Activity

What happens to the kinetic energy of an object if its velocity is doubled keeping
the mass constant? Discuss this in groups. Share your calculations.

Transformation of energy

Water stored in a dam has some potential energy. It changes into kinetic energy
when it flows down the tunnel. This kinetic energy runs the turbine and produces
electrical energy. Electric bulb, tube light, etc. change the electrical energy into
light energy. Thus, the process of change of energy from one form to another is
called energy transformation.

Dam water Running water Running turbine Electric bulb
(Potential energy) (Kinetic energy) (Electric energy) (Light energy)

Fact file

An electric bulb changes 10% of electricity into light energy and the remaining
90% into heat energy.

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Examples of energy transformation

a) Green plants perform photosynthesis. They convert solar energy into
chemical energy.

Solar Energy Chemical Energy

b) When a torchlight is switched on, the chemical energy of the dry cells is
converted into electrical energy. Now, the bulb converts electrical energy
into light energy.

Chemical Energy Electrical Energy Light Energy

c) In an automobile engine, fuel is burnt to change chemical energy into heat
energy and finally into mechanical energy.

Chemical Energy Heat Energy Mechanical Energy

Steam engine

Steam engines run on burning coal. Here, the chemical energy is converted into
heat energy. Heat boils the water and changes it into a jet of steam. The steam
produces kinetic energy and the train runs.

Coal On burning Steam engine
Chemical energy → →

Heat energy Kinetic energy

Solar panel

The solar panel converts light energy into electricity and stores it in a battery.

Light energy → Electricity

Fact file

Photosynthesis is an example of natural transformation of energy.

Law of conservation of energy

One form of energy can be converted into another form with the help of suitable
devices. But there can be no loss or gain in the total energy. For example, when
firewood is burning, the chemical energy changes into heat energy and light
energy. Here the amount of chemical energy must be equal to the sum of heat
energy and light energy. This is how energy is conserved even as its form is

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changing. According to the law of conservation of energy, “Energy can neither be
created nor be destroyed, but it can be transformed from one form to another.”

SN Transformation of energy SN Conservation of energy

The process in which one form According to the law of conservation
of energy, “energy can neither be
1. of energy gets transformed into 1. created nor be destroyed but it can
another form is called the be transformed from one form to
another.”
transformation of energy.

2. This suggests that one form of This also suggests that the total
energy can be converted into 2. amount of energy remains constant.
another.

Reason to remember

If energy cannot be created, how is an atom bomb able to produce a massive
amount of energy that can destroy life and property?

Energy cannot be created, but an atom bomb produces a tremendous amount of
energy because it blows up the mass to change it into energy.

Activity

Transformation of energy is very useful for us. Most machines operate on the
principle that one form of energy can be transformed into another in the process.
Imagine the world without the transformation of energy being possible. How
hard would life be? Discuss this with your friends as a group activity.

Summary

1. Energy is the ability to do work. The SI unit of energy is joule (J) and its CGS
unit is erg.

2. The energy of an object due to its motion or position is called mechanical
energy.

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3. The energy present in a body due to its position or deformation is called
potential energy.

4. The energy present in an object due to its motion is called kinetic energy.

5. The process of change of energy from one form to another is called energy
transformation.

6. According to the law of conservation of energy, “Energy can neither be
created nor be destroyed, but it can be transformed from one form to another.”

Exercise

1. Fill in the blanks with appropriate words.
a. There is no work without ……….
b. The ……….. energy depends on mass and velocity.
c. Mathematically, potential energy is expressed as………….
d. Potential energy depends on ……………,…………….and………..
e. An electric heater converts electrical energy into ……..

2. Write True for the correct and False for the incorrect statements.
a. Joule is the CGS unit of energy.

b. Objects at rest also have potential energy.

c. Water stored in a dam has kinetic energy, which changes into potential
energy when it starts to move.

d. Kinetic energy is directly proportional to the square of the velocity.

e. Energy can be created and destroyed under suitable conditions.

3. Select the best answer from the given alternatives.

a. Which is not the form of energy?

i. heat energy ii. light energy

iii. sound energy iv. gravity

b. Which does not have potential energy?

i. stone at the top of a house ii. stretched rubber

iii. water in a dam iv. flowing river

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c. Which does not have kinetic energy?

i. water in a dam ii. rainfall

iii. wind iv. moving train

d. How does a microphone transform energy?

i. electricity to sound ii. sound to magnetic energy

iii. sound to electricity iv. electricity to magnetic energy

e. Which is the mathematical expression for the potential energy?

i. mgh ii. dgh iii. mv2 iv. hgd

4. Match the following:

CGS unit of energy i. potential energy

Stretched spring ii. Kinetic energy

Blowing wind iii. Chemical energy

Cell iv. Energy

Capacity of doing work v. erg

5. Write your answer to the following questions in a word each.

a. What type of energy is present in vegetables, drugs, petrol, firewood
and dry cell?

b. What is the major type of energy produced by burning of a matchstick,
fire and volcano?

c. What is the form of energy present in a stretched rubber band, stretched
catapult, an object at a certain height and water in the dam?

d. Write the mathematical expression for the kinetic energy.

e. “Energy can neither be created nor be destroyed, but it can be transformed
from one form to another.” Which law is related to the given statement?

6. Write any two differences between:
a. Potential energy and kinetic energy

b. Transformation of energy and conservation of energy

7. Give reasons:
a. A raised stone possesses potential energy.

b. A falling raindrop has kinetic energy.

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c. A moving cricket ball hurts more than a table tennis ball moving with
the same velocity.

8. Observe the given diagram and identify the type of energy each possesses.

9. Answer the following questions.
a. Define energy and write its SI unit.
b. List different forms of energy.
c. What is mechanical energy? Write down its types.
d. How is energy transformed by the torchlight?
e. How can electricity be the transformation of solar energy? Explain.
f. If energy can neither be created nor destroyed, how does a nuclear
bomb produce a tremendous amount of energy?

10. Numerical problems
a. A donkey of mass 55 kg, carrying a 50 kg rice sack, climbed a vertical
2-metre height. If the value of ‘g’ is 9.8 m/s2, calculate the potential
energy.
b. A rabbit of mass 5 kg is at a height of 7 metre on the earth where ‘g’ is
9.8 m/s2. Calculate the potential energy.
c. If a kid of 25 kg mass is riding a 10 kg bicycle and travelling north at a
velocity of 10 m/s, calculate the kinetic energy.

11. Project Work
a. Take a football. Measure its mass. Measure the length of a slide. Put the
ball at top of the slide and let it roll. Use a stopwatch to measure the
time it takes for the ball to travel the length of the slide. Then calculate
the kinetic energy.

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7.2 Heat

Topics in Their Order according to CDC Curriculum

1. Definition of heat energy
2. Factors affecting absorption of heat (surface area and colour)
3. Application of absorption of heat

Learning outcomes

At the end of this unit, students will be able to:
1. Define heat energy.
2. Show that an object absorbs heat depending on its surface area and this has

applications in our life.
3. Show that an object absorbs heat depending on its colour and this has

applications in our life.

Key terms of the unit

Heat: The form of energy that gives us sensation of warmth is called heat.
Heat energy: The sum of kinetic energy present in the molecules of the matter
is called heat energy.
Temperature: Temperature is the degree of hotness and coldness of a body.

Sources of heat: The objects from which heat energy can be obtained are called
sources of heat.
Transmission of heat: The process in which heat energy transfers from one
object to another is called transmission of heat.
Absorption of heat: The transfer of heat into a body is called absorption of heat.

In winter, we prefer wearing woolen clothes and basking in the sun. Woolen
clothes and sunlight give us a warm sensation. This sensation is heat. Thus, the
form of energy that gives us a sensation of warmth is called heat. All matters are
made up of molecules. These molecules, in motion, have the kinetic energy called
the heat energy. Therefore, the sum of the kinetic energy present in the molecules
of the matter is called heat energy. The more the molecules of the matter vibrate,
the greater the amount of kinetic energy they will have. Similarly, if molecules of
the matter vibrate less, they will have less kinetic energy.

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Unit of heat

The SI unit of heat is joule and its CGS unit is calorie. One calorie equals 4.2
joules. Heat can be measured by a device called a calorimeter. It is found that
objects with a greater mass have more amount of heat and objects with smaller
mass have lesser amount of heat.

Thermometer Ignition wires
Stirrer

Water Oxygen
atmosphere

Sample
in cup

Fig.: calorimeter

Reason to remember

Heat is a form of energy. Why?

Heat is a form of energy because it can do work and can be changed into other
forms of energy.

Temperature

Temperature is the degree of hotness and coldness of a body. It
is also called the average kinetic energy present in the molecules
of the matter. Its SI unit is Kelvin. It is measured by using a
thermometer.

Source of heat

The sun, burning of firewood, burning of fossil fuel, electric
heater, induction rod, electric iron, electric kettle, etc. produce
heat. They are called sources of heat. Thus, the objects from
which heat energy can be obtained are called sources of heat.
The sun is the ultimate source of heat energy on the earth.

Fig.: thermometer

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Reason to remember

The sun is the ultimate source of heat energy on the earth, why?

The sun is the ultimate source of heat energy on the earth because the solar heat
helps life in the germination of seeds, continuing the water cycle, creating wind
and keeping the earth warm.

sun burning fire electric room heater electric heater for cooking food

Transfer of heat

We feel warm while staying in the sunshine. This is because our body absorbs the
solar heat. Similarly, we feel cold while touching metal pots in winter because
heat flows out of our body to the pot. Thus, the process in which heat energy
transfers from one object to another is called the transmission of heat.

Activity

Pour water into two pots. Put the first pot in a refrigerator and the second pot
near the fire. Put your hand in the first pot. What do you feel? What might be
the reason? Now put your hand in the second pot. What do you feel? Discuss
this with friends.

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Heat always flows from hotter objects to colder objects until both of them have an
equal temperature.

Fact file

There are three methods of transmission of heat. They are conduction, convection
and radiation.

Reason to remember

Why does heat flow from hotter objects to colder objects?

Heat flows from hotter objects to colder objects because molecules of the hotter
objects have more average kinetic energy than the molecules of the colder
objects.

Effects of heat energy

Heat is a form of energy. When two objects come into contact, heat flows from the
hotter object to the colder one. As a result, physical or chemical changes occur in
the object. Effects of heat are discussed below:
a) Heat causes a chemical reaction
When heat is supplied, molecules of matter get excited and a chemical reaction

takes place. For example, when you boil an egg, it changes into a white solid
substance.
b) Heat changes the shape and size of matter
Have you observed that plastic becomes soft on heating? The reason is
molecules of the substance gain kinetic energy on heating. As a result, the
shape and size of the object change. The process in which substances expand
on heating and contract on cooling is called thermal expansion.
c) Heat changes the state of matter
Heat changes the state of matter. For example, heat changes solid substances
to liquid state and liquids to gases.

Activity

Put a bowl of water in the deep freeze of a refrigerator. Why does the water
change into ice? What happens when it is heated and why? Discuss this in
groups.

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d) Heat changes the temperature of an object
Molecules of an object vibrate more vigorously with heat. Their kinetic

energy, therefore, keeps rising. As a result, the temperature rises.

Reason to remember

An iron nail glows red on continuous hammering. Why?

An iron nail glows red on continuous hammering because its molecules vibrate
to raise the average kinetic energy significantly. On excessive rise in the
temperature, the iron emits light.

Absorption of heat

Heat energy can be absorbed and released. When we cook food, we burn fire that
releases heat energy. The metallic pot above the fire absorbs it. Absorption and
release of heat depend on various factors such as the nature, colour, surface area
and mass of the objects.

Thus, the transfer of heat into a body is called absorption of heat. An object having
more mass absorbs more amount of heat energy than an object with less mass. It
is also found that an object at a lower temperature absorbs heat from the object at

a higher temperature.

SN Absorption of heat SN Transmission of heat

1 The transfer of heat into a 1 The process in which heat energy goes
body is called absorption of from one object to another is called
heat. transmission of heat.

2 A black body usually 2 Heat flows from the object at a higher
absorbs more heat. temperature to the object at a lower
temperature.

Factors affecting the absorption and release of heat

a. Colour of the object

A light colour has less pigment in it than a dark colour. As a result, when colours
are exposed to sunlight, the light colour absorbs less amount of solar heat than

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the dark colour. This is a reason why clothes with light colour such as white,
yellow, light green, etc. are preferred in the summer season. Likewise, dark
clothes such as black, blue, red, etc. are preferred in the winter season.

Reason to remember

1. Black utensils with a wider base are preferred in kitchens. Why?
Black kitchen utensils with a wider base are preferred because they absorb
a large amount of heat energy and food is cooked faster. It also helps save
the fuel.

2. The surface of the solar heater has a black coating. Why?
We know a black colour absorbs heat faster than other colours do. Therefore,
to heat water fast, the surface of the solar heater is made black in colour.

3. We prefer wearing black dresses in winter but white dresses in summer.
Why?
We know the black colour absorbs more heat than the white colour does.
Therefore, in winter, black dresses are preferred to keep our body warm.
Similarly, in summer, white dresses are preferred to keep our body cool.

Fact file

Aeroplanes are painted white so that they can reflect the maximum amount of
sunlight.

b. The surface area of an object

You might have seen dogs and cats curl up during the winter. A fist feels warmer
than a spread palm. A soup in a bowl remains warmer for a longer time than the
soup in a plate. What might be the reasons behind these phenomena? What
happens when the animals curl up in balls or when the soup is in a bowl instead
of a plate? In all these instances, the surface area of the bodies has decreased. The
smaller surface area causes less amount of heat loss, hence, the body stays warm
and for longer.

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a curled dog soup in a bowl wider base of the cooking pot

Thus, amount of heat lost or absorbed is directly proportional to the surface area
of the object.

Reason to remember

1. Spreading the cloth in the sunshine dries it faster. Why?
Spreading the cloth to dry in the sun increases its surface area, where it
absobs more heat and, therefore, dries faster.

2. Chopped radish dries faster. Why?

Chopped radish has more of its surface area exposed, so it absorbs more heat
and dries faster.

Activity

Take a bowl and a plate with the same quantity of water in each. Keep them in
the sunshine for the same amount of time. Which water warms up faster and
why? Explain.



drying clothes drying grains

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Summary

1. The form of energy that gives us a sensation of warmth is called heat.
2. The sum of kinetic energy present in the molecules of the matter is called heat

energy.
3. The SI unit of heat is joule and its CGS unit is calorie. One calorie equals 4.2

joules.
4. Temperature is the degree of hotness and coldness of a body.
5. The objects from which heat energy can be obtained are called sources of

heat.
6. The process in which heat energy transfers from one object to another is called

transmission of heat.
7. The transfer of heat into a body is called absorption of heat. It depends on the

nature, colour and surface area of the object.

Exercise

1. Fill in the blanks with appropriate words.
a. Calorie is the CGS unit of………………..
b. Heat can be measured by a device called ……..
c. Electric heaters, electric rods, rice cookers, etc. are …………….heat.
d. The process in which substances expand on heating and contract on
cooling is called …….
e. ……..can change the state of matter.

2. Write True for the correct and False for the incorrect statements.
a. Temperature is measured by using a thermometer.
b. Electricity is the ultimate source of heat energy for the earth.
c. Heat always flows from hotter objects to colder objects.
d. Dark colours absorb less amount of solar heat than light colours.
e. Amount of heat lost or absorbed is directly proportional to the surface
area of an object.

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3. Select the best answer from the given alternatives.
a. Which of the following energy gives us a sense of warmth?

i. light energy ii. heat energy

iii. sound energy iv. electricity

b. How many joules make a calorie?

i. 4.2 ii. 4.02 iii. 2.4 v. 4

c. Which of the following is SI unit of heat?

i. joule ii. calorie

iii. kelvin iv. all of the above

d. Which of the following colour absorbs the maximum amount of heat?

i. black ii. white iii. pink iv. green

4. Match the following: i. heat
CGS unit of heat

Average kinetic energy ii. hotter to the colder object

Total kinetic energy iii. thermal expansion

Flow of heat iv. temperature

Principle of thermometer v. calorie

5. Write your answer to the following questions in a word each.

a. Which device is used to measure the heat energy present in the
substance?

b. Write the name of the device that measures the temperature of a
substance?

c. Which colour reflects the maximum amount of solar energy?

d. What is the ultimate source of energy on the earth?

e. Why does the plant need heat energy?

6. Write any two differences between:
a. Heat and temperature

b. Absorption and transmission of heat

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7. Give reasons:
a. Ice melts on heating.
b. Making a fist feels warmer than spreading the palm.
c. Dogs curl up in winter nights.
d. We feel cold on touching a metal bar.
e. It feels warm when we dip our feet in warm water?

8. Answer the following questions.
a. What is heat? Write its SI and CGS units.
b. What are the sources of heat energy? Give examples.
c. Define the transmission of heat.
d. Heat causes a chemical reaction. Explain with an example.
e. List the effects of heat energy.
f. What is absorption of heat energy? On which factors does it depend?

9. Project Work
Take a long metal spoon. Burn a candle. Drop the melted wax on the spoon

and quickly attach five pins to it in a row. Then put the handle of that spoon
on fire. As time passes, the pins stuck on the spoon start dropping turn by
turn. Observe what is happening carefully and explain the physics behind
this.
10. Glossary
a. calorimeter: a device to measure the amount of heat
b. thermometer: an instrument to measure temperature
c. fossil fuel: coal and petroleum materials

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7.3 Light

Topics in Their Order according to CDC Curriculum

1. Demonstration of rectilinear propagation of light
2. Definition of ray and beam of light with examples
3. Preparation and demonstration of pinhole camera
4. Formation of shadow
5. Introduction to sundial

Learning outcomes

At the end of this unit, students will be able to:
1. Show they are familiar with rectilinear propagation of light.
2. Define ray and beam of light with diagrams.
3. Make and demonstrate pinhole camera.
4. Explain and demonstrate formation of shadow.
5. Show the change in the length of shadow in the morning, day and evening.
6. Describe the concept of sundial and how to make it.

Key terms of the unit

Light: Light is a form of energy which causes the sensation of vision.
Sources of light: The objects that give out light are called sources of light.
Luminous objects: The objects having their own source of light for shining are
called luminous objects.
Non-luminous objects: The objects which do not have their own source of light
for shining are called non-luminous objects.
Transparent objects: The objects that allow light to pass through them are
called transparent objects.
Translucent objects: The objects which allow only a part of light to pass through
them are called translucent objects.

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Opaque objects: The objects that do not allow the light to pass through them
are called opaque objects.

Propagation of light: The transmission of light when it comes out from the
source is called propagation of light.

Rectilinear propagation: Light always travels in a straight path as long as
it is travelling in the same medium. This phenomenon is called rectilinear
propagation of light.

Ray of light: The narrow path through which light can travel is called a ray of
light.

Beam of light: The collection of rays of light is called a beam of light.

Pinhole camera: An optical device that collects light through a tiny hole into a
lightproof box to make a real image is called a pinhole camera.

Shadow: The dark spot formed behind an opaque object when it is kept in the
path of light is called the shadow.

Umbra: The dark shadow is called an umbra shadow.

Penumbra: The faint shadow is called the penumbra shadow.

Sundial: Sundial is a circular disc with a gnomon. It tells the time of a day
depending upon shadow made by sunlight.

Observe your surroundings. You see a variety of things during the day. Similarly,
observe the same place at night. You cannot see things even though you keep
your eyes open. This is because in the day time, there is light but at night there is
no light. This activity proves that light helps us see the things present in our
surroundings. So, light is a form of energy which causes the sensation of vision.
Light is an essential form of energy on the earth. We depend on light for food,
oxygen and fuel. Green plants need sunlight to prepare their food.

Reason to remember

Light is a form of energy. How?

Light is a form of energy because it can do work and can be changed into
another form of energy.

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Sources of light

The objects that give out light are called sources of light. For example, the sun,
lantern, glowing bulb, burning candle, etc. These sources emit light and make
objects visible. The sun is a natural source of light. It gives a tremendous amount
of light and heat to the earth. It is the ultimate source of energy on the earth.

sun lantern glowing bulb burning candle

Luminous and non-luminous objects

We see thousands of objects around us all the while. Among them, only some
objects emit light. Depending on their ability to emit light, objects are broadly
classified into two groups: luminous and non-luminous.

Luminous objects

Does a burning candle emit light? Yes, it does. Does a desk emit light? No, it does
not. Objects that can emit light are called luminous objects. The sun, star, burning
candle, compact fluorescent lamp (CFL), LED light, firefly, etc. emit light. They
shine due to their own light. Therefore, they are luminous objects. Objects having
their own source of light for shining are called luminous objects. All luminous
objects are sources of light.

Reason to remember

Why is the sun called a luminous object?
The sun is a luminous object because it has its own source of light for shining.

sun CFL stars burning candle

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Non-luminous objects

Objects such as a chair, table, wood, glass, stone, ball, moon, earth, book and
brick do not emit light by themselves. These objects are called non-luminous
objects. Thus, objects which do not have their own source of light for shining are
called non-luminous objects. Normally, they do not shine. When they do, they
are only reflecting the light coming to them from elsewhere. Non-luminous
objects are mainly of three types: transparent, translucent and opaque.

Reason to remember

Why is the moon a non-luminous object?
The moon does not have its own source of light. So, it is called a non-luminous
object. The light from the sun falls on the moon and the moon reflects the
sunlight for us to see it.

Activity

Make a list of luminous and non-luminous objects present in your house.

Luminous bodies Non-luminous bodies

Transparent objects

Some objects such as water, air, glass or the cornea of our eye allow a maximum
amount of light to pass through them. These objects are called transparent objects.
The objects that allow light to pass through them are called transparent objects.
Transparent objects cannot cast shadows because they do not block the light
falling on them.

Translucent objects

Some objects such as a tracing paper, ground glass and white paper immersed in
oil allow only some part of light to pass through them. These objects are called
translucent objects. Thus, objects which allow only some part of light to pass
through them are called translucent objects. We cannot see things clearly while
looking through the translucent objects. These objects form faint images.

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Opaque objects

Some objects such as a chair, brick, book, table, wood, the soil and stone do not
allow the light to pass through them. They are called opaque objects. Thus, objects
that do not allow the light to pass through them are called opaque objects. Opaque
objects form dark images as light does not pass through them.

Activity

Collect various objects available in your surroundings. For example, look for a
white paper, a piece of plywood, a tracing paper, a piece of plastic, a piece of
glass and a brick. Keep them in the sunlight turn by turn. Observe different
types of shadows they cast. On the basis of your observation, group the objects
into transparent, translucent and opaque categories.

Propagation of light

The sun emits light that travels to the earth. The moon does not have its own
light. It reflects the sunlight. This reflection of sunlight travels from the moon and
reaches the earth. Similarly, a torchlight can travel upto a certain distance. Thus,
the transmission of light when it comes out of the source is called propagation of
light. Light does not need a medium to travel because it is an electromagnetic
wave.

Reason to remember

We cannot see through a stone wall. Why?
We cannot see through a stone wall because the wall blocks 100% of the light
falling straight on it.

Rectilinear propagation of light

Observe the beam of a car headlight at a misty night. Similarly, observe the light
entering through a narrow hole. We will see how the light travels in a straight
line. Thus, light always travels in a straight path as long as it is travelling in the
same medium. This phenomenon is called rectilinear propagation of light.

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Activity

Objective: To demonstrate the rectilinear propagation of light
Materials required: Cardboard papers, scale, scissors, iron nail or a driller,
candle, match box, knitting needle, thread, marker, pencil, suitable stands
Procedure: Cut the cardboard into three identical pieces. Make a hole in each
cardboard at the same place. Arrange a source of light and three cardboards in
a row in a straight line as shown in the diagram. Observe the source of light
from the hole of the third cardboard. It is visible. Now slightly displace any one
of the cardboards and observe the source of light again through the hole of the
third cardboard. You cannot see the source of light.
Observation: We saw that the source of light was visible when holes of the
cardboard were in a straight line. The source of light was not visible when the
holes were not in a straight line.

Conclusion: From the above experiment, we can conclude that light travels in
a straight line.

Ray and beam of light

Light coming out of the source travels in a straight line. It can pass through the
narrowest of an opening. Thus, the narrow path through which light can travel is
called a ray of light. It is the path represented by a straight line with an arrowhead
in it. The collection of rays of light is called a beam of light. A beam of light may
be parallel, convergent or divergent.

ray beam

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The rays of light coming from a long distance are mostly parallel beams of light.
For example, light beams from the moon, sun, stars, etc. The beam of light that
disperses away from the source is called divergent beam of light. For example,
light from the torchlight, bulb and candle. Similarly, if many rays of light are
collected at a single point it is called a convergent beam of light. For example,
light reflected by a concave mirror, light passing through the convex lens and
light passing through hand lens.

parallel beam divergent beam convergent beam

Fact file

A ray of light can get reflected, refracted and dispersed.

Differences between ray and beam of light

SN Ray of light SN Beam of light

1 The narrowest path through which 1 The collection of rays of light is
light can travel is called a ray of light. called a beam of light.

2 A ray of light is a hypothetical 2 A beam of light exists and can
concept. It cannot be observed. be observed easily.

When we see a divergent or convergent beam of light, it appears that the light is
travelling in a curved line. For example, light coming from a bulb and torchlight.
But that is not the case. This is just an illusion.

Reason to remember

Why does light bend while passing from one medium to another?

Light bends while passing from one medium to another because its velocity
changes in the media with different densities.

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Pinhole camera

We have seen devices that work with the help of light. They are called optical
devices. For example, camera, telescope, microscope, binocular, hand lens etc.
The pinhole camera is also an optical device. It collects light and makes a real
image. It works on the principle of rectilinear propagation of light. It has no lens.
An optical device that collects light through a tiny hole into a lightproof box to
make a real image is called a pinhole camera.

Fig.: pinhole camera

Fact file

Light splits when it passes through the raindrop and forms a rainbow.

Structure of a pinhole camera

A pin-hole camera contains a lightproof box. A tiny pore is made at the centre of
one of the surfaces of the box. The film is kept on the wall opposite to the pin
hole. The pin hole is covered with an opaque object. To take a picture, the cover
is removed for a short duration.

Activity

Objective: To construct a pin-hole camera and demonstrate real image
formation
Materials required: A black cardboard paper, scissors, gluestick, cellophane
tape, tracing paper, candle, matchbox, common pin

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Procedure: Take a black cardboard paper and cut it to make a cylinder. Now,
the cylinder has two holes. Cover one hole of the cylinder with a black cardboard
paper and another with a tracing paper (i.e. screen). Pierce a small hole at the
centre of the cardboard paper. Put a burning candle on a table and observe the
image of the candle through the pinhole camera.

Observation: The image of the candle is seen on the tracing paper (i.e. screen).
The image is real and inverted.
Conclusion: Pinhole camera forms a real, inverted and diminished image.

Fact file

1. A real image means it can be cast on the screen.
2. Inverted image has its look upside down.

Shadow

Light always travels in a straight line and makes
objects visible. When an opaque object comes
in the path of light, it leaves a dark spot
behind it. This dark spot is called a shadow.
Thus, the dark spot formed behind an
opaque object when it is kept in the path of
light is called the shadow.

The size of the shadow varies depending on the

distance between the object and source of light.

It also varies with the distance between the Fig.: shadows
object and the screen. If the source of light is

closer to the object, the shadow becomes bigger.

Similarly, if the object moves away from the source, the shadow becomes smaller.

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Fig.: making different size of shadow by a standing man

Types of shadow

Depending on the darkness, there are two types of shadow. They are:

(i) Umbra (ii) Penumbra

The dark shadow is called an umbra. It is formed when light is completely blocked
by an opaque object. It is also formed when multiple shadows overlap with one
another. If we view the source of light from the umbra region, we just see the
dark area on the source of the light. The faint shadow is called the penumbra. It
is formed when light is partially blocked by the opaque object. If we view the
source of light from the penumbra region, we see a light dark area on the source
of light.

Fig.: umbra and penumbra shadow

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Fact file

The shadow of one heavenly object falling on another heavenly object is called
an eclipse.

Sundial

Have you ever seen a sundial? It is a traditional as well as a local technology.
Ancient people used the sundial to tell the time of the day. Sundial is a circular
disc with a gnomon, a stick, put upright on it. It tells the time of a day depending
on the shadow the sunlight casts over the stick. Sundial is fixed on a disc. Hours
of the day are marked on the disc. When sunlight falls on the sundial, gnomon
blocks the light and casts a shadow on the disc. Time can be estimated by
observing the size, position and direction of the shadow.

Fig.: sundial

Activity

Take a wooden or plastic or metallic rod, about half a metre in length, and a
cardboard. Early on a sunny day, erect the rod on the cardboard and put it on
the ground. Record the length and position of the shadow every hour. What
difference did you observe in the length, position and direction of the shadow?
Discuss this in the class.

Time 8 AM 9 AM 10 AM 11 AM 12 AM 1 PM 2 PM 3 PM 4 PM 5 PM
Length

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Summary

1. Light is a form of energy which causes a sensation of vision. It is an essential
form of energy on the earth.

2. Objects that emit light are called sources of light. For example, the sun,
lantern, glowing bulb, burning candle, etc.

3. Objects having their own source of light for shining are called luminous
objects. For example, the sun, star, burning candle, firefly, etc.

4. Objects which do not have their own source of light for shining are called
non-luminous objects. For example, chair, table, wood, glass, stone, ball,
moon, earth, book, brick, etc.

5. Objects that allow light to pass through them are called transparent objects.
For example, water, air, glass, cornea of eye, etc.

6. Objects which allow only some part of light to pass through them are called
translucent objects. Examples are a tracing paper, ground glass and white
paper dipped in oil.

7. Objects that do not allow the light to pass through them are called opaque
objects. For example, a chair, brick, book, table, wood, soil, stone, etc.

8. The transmission of light when it comes out from the source is called
propagation of light.

9. Light always travels in a straight path as long as it is travelling in the same
medium. This phenomenon is called rectilinear propagation of light.

10. The narrowest path through which light can travel is called a ray of light.
12. The collection of rays of light is called a beam of light.
13. An optical device that collects light through a tiny hole into a lightproof box

to make a real image is called a pinhole camera.
14. The dark spot formed behind an opaque object when it is kept in the path of

light is called the shadow.
15. Sundial is a circular disc with a gnomon. It tells the time of a day depending

upon the shadow made by sunlight.

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Exercise

1. Fill in the blanks with appropriate words.
a. The moon shines brightly at night by reflecting the ……….

b. When ………. beam of light enters into our eyes, we see that object.

c. …………. cannot cast shadow because these objects do not block the
light.

d. Objects which do not allow light to pass through them are called
………..objects.

e. When an opaque object blocks light, it casts ………….

2. Write True for the correct and False for the incorrect statements.
a. The sun is a natural source of light.

b. Stone, soil, water, air, etc. cannot emit light.
c. The collection of rays of light is called a beam of light.

d. Light also can travel in a curved path.

e. Pinhole camera works on the principle of rectilinear propagation of
light

3. Select the best answer from the given alternatives.
a. Light is a form of energy which causes the ____________.

(i) sensation of sight (ii) sensation of heat

(iii) sensation of sound (iv) sensation of cold

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

(i) moon (ii) burning candle

(iii) firefly (iv) sun

c. The object that allows light to pass through it is called .........................

(i) luminous object (ii) translucent object

(iii) opaque object (iv) transparent object

d. Which one of the following is the luminous body?

(i) sun (ii) wood (iii) moon (iv) brick

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e. Pinhole camera contains…………………

(i) convex lens (iii) concave lens

(iii) concave mirror (iv) none of them

4. Match the following:

Glass i. translucent object

Oiled paper ii. opaque object

Stone iii. very narrow path of light

Ray iv. no lens is used

Pinhole camera v. transparent object

5. Write your answer to the following questions in a word each.

a. Write the name of the traditional tool which is used to measure time.

b. What is the dark shadow called?

c. Write the principle of pinhole camera.

d. What is the collection of rays called?

6. Write any two differences between:
a. Luminous objects and non-luminous objects

b. Transparent object and opaque object

c. Umbra and penumbra

7. Give reasons:
a. The sun is a luminous object but the moon is a non-luminous object.
b. Clear water is a transparent but a brick is opaque.
c. Air does not cast shadow.

8. Observe the diagram of the solar eclipse and answer the following
questions.

a. Write the name of the shadow at region A and the shadow at region B.
b. Why is region B faint?

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9. Answer the following questions.
a. What is light? What are its sources?
b. What are luminous and non-luminous objects? Write with examples.
c. Define transparent, translucent and opaque objects with any two
examples of each.
d. What is rectilinear propagation of light?
e. What are rays and beam of light?
f. Define shadow. How is it formed?
g. What is sundial? How does it work?
h. Describe the structure of the pinhole camera.

10. Project Work
a) Construct a sundial.

7.4 Sound

Topics in Their Order according to CDC Curriculum

1. Origin of sound waves
2. Propagation of sound waves in solid, liquid and gas mediums
3. Application of propagation of sound waves in solid, liquid and gas mediums

Learning outcomes

At the end of this unit, students will be able to:
1. Show that sound is produced due to vibration of the body.
2. Demonstrate that sound requires material medium for its propagation.
3. Describe the application of sound wave propagation in solid, liquid and gas.

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Key terms of the unit

Sound: Sound is a form of energy which produces a sensation of hearing in the
ears.

Source of sound: The vibrating body which produces sound is called a source
of sound.

Musical sound: The sound which is pleasant to hear is called musical sound.

Noise: The sound which is unpleasant to hear is called noise.

Wave: A wave is a disturbance in a medium that transfers energy from one
point to another.

Transverse wave: The wave in which the medium particles vibrate perpendicular
to the direction of wave motion is called a transverse wave.

Longitudinal wave: The wave in which the particles of the medium vibrate
parallel to the direction of the wave motion is called a longitudinal wave.

Propagation of sound: The disturbance in a medium that transfers sound
energy away from the source is called propagation of sound.

We get to listen to a variety of sound every day. For example, musical sound, sounds of
birds, insects, animals, etc. Sound is produced due to vibrations of solid, liquid and gas
mediums. On reaching us, the vibration gives us a sense of hearing. It is perceived by our
ears. Thus, sound is a form of energy which produces a sensation of hearing in the
ears. We do not like all sounds. Sounds from old machines, old vehicles and crowds are
unpleasant. But, the sounds from birds, flutes and guitars are pleasant. We produce sounds
from the vocal cord and words are formed by the twisting of the tongue.

sound from the speaker people talking birds singing

Fact file

The loudest natural sound on the earth is the sound from the eruption of a
volcano.

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The SI unit of intensity of sound is decibel.

Source of sound

Have you ever played a guitar? Or, popped a balloon? They make sounds.
Flowing river, blowing wind, moving bus, playing kids, etc. make sounds. The
vibrating body which produces sound is called the source of sound. Sounds
produced by different objects are different. For example, the sound produced by
a madal is different from the sound produced by the drum. Sound produced by
an adult is different from that by a kid. Usually a big source of sound produces
louder sound as compared to the small source of sound. A thin and small source
produces a sharp sound because it vibrates faster. A loudspeaker, bell, electric
bell, siren, horn etc. are some examples of sources of sound.

loudspeaker guitar pressure horn a barking dog

Reason to remember

The barking of a dog is louder than the sound of a mouse. Why?

The barking of a dog is louder than the sound of a mouse because the dog has
a big vocal cord and the mouse has a smaller one.

Activity

Observe in your neighborhood the sounds produced by various sources and
classify them as pleasant and unpleasant.

Pleasant sound Unpleasant sound

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Types of sound

Depending on the frequency and intensity of audible sounds, we can classify
them as musical and noisy.

Musical sound

Some sounds such as a singer singing, a baby laughing, flute tunes and rustling
leaves are pleasant to hear. They are called musical sounds. Thus, the sound
which is pleasant to hear is called a musical sound. Listening to the musical sound
feels nice. It helps relieve our stress, calm us and improve our mood and memory.

a girl playing with flute a girl singing a girl listening to music with earphone

Noise

Some sounds such as those of a crying baby, snoring, the ringing of the landline
phone, and rattles of old and heavy machinery are unpleasant to hear. They are
called noise. Noise is irritating. The sound which is unpleasant to hear is called
noise. Noise is bad for our health. It increases stress, decreases concentration,
causes heart diseases and insomnia.

crying baby sound from old and heavy machine quarreling people

Wave

Touch your throat with a finger as you speak. Do you feel any vibration? Similarly,
play the madal and put your hand over the sounding surface. Do you feel any
vibration? Yes, you do, right? Vibrations are waves produced due to disturbances
in the medium. Thus, a wave is a disturbance in a medium that transfers energy
from one point to another. For example, during an earthquake, the tectonic

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disturbance produces waves. These waves travel to the surface of the earth
carrying energy. As a result, the ground trembles.

Fig.: wave

Reason to remember

Sound is a mechanical wave. Why?
Sound is a mechanical wave because it needs a material medium to travel
through.

Activity

Throw a stone in a pond. It creates a disturbance. This disturbance creates
ripples. These ripples travel away from the source. Here, the water does not
travel as such but its ripples do. This means when a disturbance is created in a
medium energy travels away from the source.
The transfer of energy from one point to another without the actual transfer of
matter is called a wave motion.

Fig.: ripples in the water

Types of waves

Depending on the direction of vibration, waves are divided into two types. They
are transverse waves and longitudinal waves.
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Transverse waves

Throw a stone in still water. The disturbance causes water molecules to vibrate
up and down along the direction of the wave motion. These waves are called
transverse waves. The waves in which the medium particles vibrate perpendicular
to the direction of the wave motion is called a transverse wave. For example,
ripples in water, waves in string, light waves, etc.

displacement + crest crest

– trough trough
distance

Fig.: representation of transverse wave showing crest and trough

Longitudinal waves

Compress a spring and release it. The coil of the spring vibrates to and fro along
the direction of the spring. The waves created this way are called longitudinal
waves. Thus, the wave in which the particles of the medium vibrate parallel to
the direction of the wave motion is called a longitudinal wave. Sound wave is a
longitudinal wave.

Fig.: longitudinal wave showing compression and rarefaction

Sound wave and its propagation

Vibration of a body produces sound. Sound
propagates in solid, liquid and gaseous mediums
through the wave motion. But it cannot travel
through a vacuum. Thus, the disturbance in a
medium that transfers sound energy away from the
source is called the propagation of sound. The sound
waves coming from a vibrating body propagate
through air and reach our ears. As a result, we hear the sound.

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Reason to remember

Astronauts on the moon need electrical devices to communicate among them.
Why?
Astronauts on the moon need electrical devices to communicate among them
because the moon has no air to propagate sound. An electrical device produces
electromagnetic waves to transfer the sound.

Propagation of sound in solid medium

Solids have closely packed molecules. If there is a disturbance in the solid
medium, it produces sound waves. These sound waves transfer sound energy
very fast. It has been proved that the speed of sound is the highest in a solid
medium. The maximum speed of sound in a solid medium is 6000 m/s.

Activity

Objective: To demonstrate the propagation of sound through a solid
Materials required: Two ice-cream cups, a long thread, needle, scissors
Procedure: Take two plastic cups and make a small hole on the bottom of each.
Insert ends of a long string through the holes and tie knots to keep the string
attached to each cup. Now, ask your friend to take one cup away to a distance
as you hold the other cup. Hold the cup to your ear and ask your friend to
speak some words softly enough that you won't hear them without the cup on
your ear. Did you hear the sound because of the string?

Hi ! Hello !

Thread

Conclusion: The above activity proves that sound can travel through a solid
medium.

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Activity

Objective: To demonstrate that sound propagates through a solid medium
better than through a gaseous medium

Take a watch that produces ticking. Keep it on
one end of a table. Stand at another end of the
table and try to hear the ticking sound of the
watch. Can you hear the sound? Now, place
your ear on the table and listen to the sound of
the watch. Is the sound louder than before?
What can you say from this experiment?

Fact file

The fear of music is called melophobia.

Propagation of sound in liquid medium

The molecules of a liquid are closer together than the molecules of a gas are. So,
the propagation of sound waves in a liquid is faster than in a gaseous medium.
Similarly, the molecules of liquid are more loosely packed than the molecules of
solid. Therefore, the speed of sound in a liquid is slower than in a solid medium.
The maximum speed of sound in water is about 1500m/s.

Activity

Objective: To demonstrate that sound propagates through a liquid medium

Materials required: A bucket, water, a bell, a hammer

Procedure: Take a bucket full of water. Immerse a bell into the water. Hit the
bell with a hammer. Listen to the sound.

Observation: Sound can be heard clearly when Bell Bucket
produced by the bell inside the water. Hammer
Water
Conclusion: From this activity, it can be
concluded that sound propagates through a
liquid.

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Propagation of sound in gas

The molecules of gas are very far from one another.
So, the sound waves propagate very slowly in a
gaseous medium. The average speed of sound in air is
around 332m/s.

Propagation of sound in vacuum Fig.: a barking dog

We cannot hear the sound produced in the sun and stars. Similarly, astronauts
cannot talk with one another on the surface of the moon. Do you know why? The
reason is there is no air in the space. As a result, we cannot hear the sound
produced in the sun and stars. As sound is a mechanical wave, it needs a material
medium to travel. Astronauts must use electronic devices for their communication.

Reason to remember

We do not hear the ringing of an electric bell inside a vacuum chamber.
Why?

We do not hear the ringing of an electric bell inside a vacuum chamber because
sound needs a medium such as air for it to travel.

Fact file

Cows that like to listen to music give more milk than those which don't.

Activity

Objective: To demonstrate that sound does not travel in a vacuum
Materials required: a vacuum chamber and an electric bell
Procedure: Put an electric bell inside a vacuum
chamber. Ring the bell.
Observation: Sound cannot be heard outside the
chamber.
Conclusion: This activity proves that sound
cannot travel in a vacuum.

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Summary

1. Sound is a form of energy which produces a sensation of hearing in the ears.
2. The vibrating body which produces sound is called the source of sound. For

example, a loudspeaker, bell, electric bell, siren, horn etc.
3. The sound which is pleasant to hear is called a musical sound.
4. The sound which is unpleasant to hear is called a noise.
5. A wave is a disturbance in a medium that transfers energy from one point to

another.
6. The wave in which the medium particles vibrate perpendicular to the direction

of the wave motion is called a transverse wave. For example, ripples in water,
waves in string, light waves, etc.
7. The wave in which the particles of the medium vibrate parallel to the direction
of the wave motion is called a longitudinal wave.
8. The disturbance in a medium that transfers sound energy away from the
source is called propagation of sound.
9. The speed of sound is the highest in solid and the lowest in gas as a medium.

Exercise

1. Fill in the blanks with appropriate words.
a. Sound is perceived by our ……….
b. The SI unit of intensity of sound is ……………
c. Any object that produces sound is called ………….
d. The sound which is unpleasant to hear is called ……..
e. A ……….is a disturbance in a medium that transfers energy from one
point to another.

2. Write True for the correct and False for the incorrect statements.
a. A big source of sound usually produces louder sound than the smaller
one.

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b. A thin and small source of sound produces sharp sound.
c. Listening to music helps to relieve stress.
d. The speed of sound is the highest in gas medium.
e. Sound cannot travel through solid.

3. Select the best answer from the given alternatives.

a. Which of the following gives a sense of hearing?

i. sound energy ii. light energy

iii. heat energy iv. electricity

b. Which of the following is not a source of sound?

i. bulb ii. loud speaker

iii. horn of vehicle iv. guitar

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

i. 332m/s ii. 232 m/s

iii. 223m/s iv. 322m/s

d. Which one of the following medium cannot transfer sound?

i. solid ii. liquid

iii. gas iv. vacuum

e. What does a wave do?

i. transfer matter ii. transfer energy

iii. both i and ii iv. none

4. Match the following:

The maximum speed of sound i. wave

Harmonious sound ii. noise

Irritating sound iii. longitudinal wave

Disturbance in a medium iv. music

Sound wave v. solid

5. Answer each of the following questions in one word.

a. What is the type of wave if the particles vibrate perpendicular to the
direction of the wave?

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b. What is the type of wave if the particles vibrate parallel to the direction
of the wave?

c. What is transferred through the wave?

d. What is the term for regular and harmonious sound?

e. In which medium is the speed of sound the fastest?

6. Write any two differences between:
a. Noise and music

b. Longitudinal wave and transverse wave

7. Give reasons:
a. Sound is called a mechanical wave.

b. Sound travels faster in solid and slower in gas.

c. Astronauts cannot hear one another on the moon without devices.

d. We do not hear the ringing of an electric bell inside a vacuum chamber.

8. Observe the given diagrams and identify the types of waves.

displacement + crest crest

– trough trough

distance

9. Answer the following questions:
a. What is sound? How is it produced?

b. What is meant by a source of sound? Name any three sources of sound.

c. What is a wave motion? Name the types of waves.

d. Describe the speed of sound in solid, liquid and gas?

e. What is propagation of sound?

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8Unit Electricity and Estimated Teaching Periods:
Magnetism
Theory Practical
11 4

8.1 Magnetism 8.2 Electricity

8.1 Magnetism

Topics in Their Order according to CDC Curriculum

1. Magnetic field of a bar magnet
2. Drawing magnetic field lines of a bar magnet
3. Making magnets using rubbing method and electricity
4. Method to increase magnetism of an electromagnet
5. Application of an electromagnet

Learning outcomes

At the end of this unit, students will be able to:
1. Show they understand magnetic fields.
2. Identify the magnetic field of a bar magnet with the help of iron dust.
3. Draw the magnetic field lines of a bar magnet.
4. Make magnets using rubbing method and electricity.
5. Demonstrate the method to increase magnetism of an electromagnet.
6. Discuss the application of an electromagnet.

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Key terms of the unit

Magnet: The substances which attract iron, cobalt, nickel, etc. and always rest
in the north-south direction when suspended freely are called magnet.

Magnetic substances: The substances that are attracted by a magnet are called
magnetic substances.

Non-magnetic substances: The substances that are not attracted by the magnet
are called non-magnetic substances.

Natural magnet: The naturally occurring black rock of magnetite that has
magnetic properties is called a natural magnet.

Artificial magnets: The magnets which are formed by human beings are called
artificial magnets.

Magnetism:The properties shown by the magnet is called magnetism.

Pole: The region of a magnet where maximum amount of magnetic force is
concentrated is called pole.

Magneticlength: The total distance between two ends of the magnet is called
the magnetic length.

Magnetic length: The distance between two magnetic poles is called effective
magnetic length.

Magnetic field: The space around a magnet where the magnetic effect can be
felt is called magnetic field.

Magnetization: The process in which a magnetic substance changes into
permanent or temporary magnet is called magnetization.

Single touch: The process of magnetization in which particular pole of a
permanent magnet is rubbed against an iron bar in a single direction is called a
single touch method.

Double touch method: The process of magnetization in which the opposite
poles from two different permanent magnets are rubbed against an iron bar in
opposite direction is called double touch method.

Electromagnet: The temporary magnet formed by passing current in an
insulated coil around a soft iron core is called an electromagnet.

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In our surroundings, we may have noticed some objects pushing or pulling other
objects nearby. Some substances attrace iron, cobalt and nickel. Suspended freely,
these substances always rest in the north-south direction. They are magnets.
Thus, the substances which attract iron, cobalt, nickel, etc. and always rest in the
north-south direction when suspended freely are called magnets. The property
of a magnet due to which it attracts magnetic substances is called magnetism. A
magnet is a very useful substance. The naturally occurring magnet is called a
lodestone. The Chinese sailors used the lodestone as a magnetic compass for
navigation. Similarly, a magnet is used to produce electricity by using a dynamo
or generator. A magnet is also used to remove eye splinters as it attracts magnetic
substances.

magnetic compass electric bell magnetic lines of force around the earth

Fact file

The magnetic property of the earth is called terrestrial magnetism.

Magnetic and non-magnetic substances

The magnet attracts only some metallic substances such as iron, nickel and cobalt.
It does not attract, say, copper, gold or aluminium. Depending on this property,
substances are divided into two groups: magnetic and non-magnetic. The
substances that are attracted by a magnet are called magnetic substances.
Similarly, the substances that are not attracted by a magnet are called non-
magnetic substances.

Reason to remember

A rubber band cannot be pulled by a magnet. Why?

A rubber band cannot be pulled by a magnet because it is a non-magnetic
substance.

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