Science 10

surface. The atmospheric pressure is due to the weight of air column
acting on a specified area at any place.

The pressure of air that acts upon the unit area on the surface of the
earth is called atmospheric pressure.

The atmospheric pressure at the sea level is 105 N/m2 or 760 mm of
Hg or 1 atmosphere. This pressure is known as standard atmospheric
pressure. There are different other units for the measurement of
pressure.

1 atm = 760 mm mercury

1 atm = 760 torr

1 atm = 101325 Pascal

1 atm = 1.01 x 105 N/m2

1 atm = 1.01325 bar

The pressure is 1,000 times greater at the depth of 10,000 m from the
surface.

Fact &Reason

Why is it impossible to suck lemonade with soda straw on the surface of the
moon?
When suction is caused at the end of the soda straw, partial vacuum is created
inside the straw. The lemonade goes up inside the straw to fill the vacuum due
to the pressure exerted by the atmosphere on the earth surface. But there is
no atmospheric pressure on the surface of the moon, so it is impossible to suck
lemonade with soda straw.

The atmospheric pressure decreases at high altitude because the air
column high above the earth surface is rare. We cannot live in the place
with very high or very low atmospheric pressure. The aeroplanes fly
at the high altitude but we do not feel difficulty in respiration and feel
normal. It is because the air pressure inside the aeroplane is controlled
so that the comfortable air pressure for our body is maintained.

The atmospheric pressure affects each and every thing on the surface
of the earth. But we do not feel the atmospheric pressure because our
blood pressure is slightly greater than the atmospheric pressure. Our
body also exerts equal pressure against the atmospheric pressure.
Hence, we do not feel atmospheric pressure.

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Activity 2 .7 To demonstrate that air exerts pressure.

Materials required:
A cylindrical tin can,
water, burner or stove,
water tap, etc.

Procedure:
1. Take a cylindrical tin
can and pour some
water into it.
2. Heat for sometime taking

it on the stove or using a burner.
3. When the water inside the tin box vapourizes rapidly, close the
mouth of the cylinder with a lid, such that no air can enter into it.
4. Then put cylinder under a tap of running water and observe

what happens there?

Observation:
The tin will collapse, i.e. the sides of the can are pushed inward.

Explanation:
Before heating, the pressure inside the tin was balanced by the
pressure outside the tin. When the tin containing some water is
heated, the water vapour pushes air from tin and the total space
inside the cylinder is occupied by water vapour. If the cylinder
is put under the tap water, it becomes cool and water vapour
inside the cylinder condenses to form water. As the mouth of the
tin is closed tightly, the air from outside cannot enter into the
cylinder.
This creates a partial vacuum inside the cylinder. Because of
the low pressure inside the cylinder, the large atmospheric
pressure gives force on the surface of the cylinder and it gets
collapsed.

Conclusion:
This experiment shows that air exerts pressure. The use of different
instruments is possible due to the atmospheric pressure. Filling ink
in a pen, filling medicine in a syringe, working of water pump,
operating of a bicycle pump, etc are all possible due to atmospheric
pressure.

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Mercury barometer

The barometer is an instrument which Pressure due to Torricellian
is used to measure the atmospheric Atmosphere vacuum

pressure. It uses mercury as the

barometric substance. A mercury

barometer is constructed as follows: Pressure due to the

A strong glass tube closed at one end Mercury column

is filled with clean and dry mercury. Trough
Then the open end of the tube is

closed with palm and is inverted. Mercury

The open end of the inverted tube is

carefully dipped in mercury taken in Barometer
a trough. While inserting the tube

into the mercury of the trough, the atmospheric air should not get

into the tube. Then the level of mercury in the tube falls to a certain

height and remains stationary. This creates a vacuum inside the tube.

This vacuum is called Torricellian Vacuum in honour of the Italian

scientist E. Torricelli, who has invented the mercury barometer. The

tube is kept erect using a stand and graduated scale is made in the

tube. Since mercury is poisonous in skin contact also, we should wear

gloves while handling mercury.

The mercury level is 760 mm in the tube above the trough at the sea
level. It means that the atmospheric pressure is 760 mm of mercury
at the sea level. The level of mercury falls down if the atmospheric
pressure is less and it rises if the atmospheric pressure is more. The
atmospheric pressure decreases as we travel to higher altitude from
the sea level.

Syringe Needle Piston

A syringe is a medical instrument

which is used to send medicine

into the body through blood. It is

also used to draw blood from the

body of a patient. It consists of Storage cylinder

three main parts – needle, storage cylinder and piston.

The piston is pushed up to the mouth of the storage cylinder and the
needle is inserted into a bottle of liquid medicine. When the piston
is pulled backward, a partial vacuum is created inside the storage
cylinder. Since the atmospheric pressure is higher than that inside

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the cylinder, it pushes the medicine into the syringe. When piston is
pushed towards mouth, medicine goes inside blood. A liquid always
flows from the region of high pressure to the region of low pressure.

Air pump

An air pump is a device which is used
to pump air into bicycle tube, football,
volleyball, etc. It consists of three main
parts – piston, cylinder and nozzle. The
piston is moved up and down while filling
air into the bicycle tube. When the piston
is pulled up, a partial vacuum is created
inside the cylinder so the air from the
atmosphere moves into the cylinder and
fills it. If the piston is pushed down, the air
of the cylinder is pushed into the tube of the
bicycle. The tube of bicycle contains a valve to regulate the entry of air into it.

Water pump

A water pump is used to Piston Valve (V1)
draw water from a well. It Foot valve (V2)
works on the principle of
atmospheric pressure.

A water pump has two
main parts – barrel and
piston. Both of them consist
of valves. The piston valve
(V1) moves up and down
along with the piston. The
foot valve (V2) is connected
at the junction of barrel and
pipe. The pipe is long which
is sent down to draw water
from the well. Two strokes,
i.e. upstroke and down
stroke are involved while
drawing water from the well.



When the handle of the pump is pushed downward, the piston moves
up and creates a partial vacuum between the valves V1 and V2. In such

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case, the valve V1 gets closed whereas the valve V2 opens and water
rises into the barrel. This process is called upstroke. The upward
movement of water is due to the effect of atmospheric pressure upon
the water of well.

When the handle is pulled upward, the piston moves down. Then the
distance between the two valves decreases. In such case, the foot valve
experiences more pressure and gets closed but the piston valve opens
due to water pressure. Thus, water comes out through the spout. This
process is called down stroke. When the handle is pushed downward,
the piston moves upward and the water raised above piston valve
comes outside through spout.

This process gets repeated and water comes out continuously.

Solved Numerical Problem 2.4

Mass of a brick is 2kg and its density is 2.5 g/cc. How much water is
displaced by it when it is kept in water?

Solution:

Given,

Mass of the brick (m1) = 2 kg = 2000g

Density of brick (d1) = 2.5g/cc

We have, m1
d1
Volume of the brick (V1) = = 2000 = 800cc
2.5

Since the density of brick (2.5g/cc) is more than that of water (1g/cc),

it sinks in water.

For sinking body

Volume of displaced water = volume of sinking body

(V2) = 800cc

Density of water (d2) = 1g/cc

Mass of displaced water (m2) = ?

Again, m2 = d2 × v2

= 1 × 800

= 800g.

Thus, the brick displaces 800cc or 800g of water.

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Solved Numerical Problem 2.5

Weights of a piece of stone when weighed in three different media: air,
water and salt solutions are given in the table.

Medium Weight
A 10N
B 15N
C 12N

a. Which one is air, salt solution and water? Write with
reasons.

b. What is the upthrust given by the water and salt solution?

Solutions:

a. Salt solution has the highest density and it exerts highest
upthrust. Therefore, the weight of stone in the salt solution is the
least. Thus, medium ‘A’ is salt solution. Similarly, density of the
water is less than that of the salt solution but more than that of
air. Thus, it exerts more upthrust than the air but less than the
salt solution. Hence, the weight of the stone in water is more than
in salt solution but less than in air. From the table, medium ‘C’ is
water. The remaining medium ‘B’ is air.

b. Weight of the stone in air = 15 N

Weight of the stone in water = 12N

Weight of the stone in salt solution =10N

We have,

Upthrust given by water = Weight of the stone in air –

Weight of the stone in water

= 15 – 12 = 3 N.

∴ Upthrust given by water = 3N

Similarly, upthrust given by salt solution

Weight of stone in air-weight of the stone in salt solution

= 15 – 10 = 5N.

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Learn and Write

1. An egg sinks in pure water but floats on salt solution. Why?

Density of salt solution is more than that of pure water. Due to
this the salt solution gives more upthrust to the egg to make it
float but the upthrust given by the pure water is not sufficient to
make the egg float.

2. A piece of iron sinks in water but a ship made up of iron floats on
water. Why?

Density of iron is more than that of water. Therefore, the amount
of water displaced by a piece of iron is less than its weight and
the upthrust applied by the water is not sufficient to make the
iron piece float. But, a ship made up of iron is hollow and contains
air. So, its average density is less than that of water. Therefore,
it displaces water equal to its weight making the ship float.

3. It is easier to pull a bucket of water when it is inside water. Why?

When a bucket of water is inside the water, it experiences more
upthrust given by water. But, when it is pulled out of water,
the upthrust given by the air is less. Thus, a bucket of water is
lighter when it is inside the water.

Main points to remember

1. Pressure is defined as force acting per unit area.
2. Atmospheric pressure is defined as the pressure exerted by the

atmosphere.
3. Pascal’s law state that the pressure applied at a place on

a liquid contained in a closed container transmits equally
perpendicularly in all directions.
4. Hydraulic garage lift is a device which is used to lift light
vehicles in the garage during their maintenance or service.
5. Density is defined as the mass present in a unit of volume of a
substance.
6. Archimedes' principle states that when a body is wholly or
partially immersed in a liquid, the upthrust experienced by it is
equal to the weight of the displaced liquid.
7. Law of flotation states that a floating object displaces liquid
equal to its weight.
8. Hydrometer is a device which is used to measure density of a
liquid.

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Exercise

A. Very short answer questions (1 mark)

1. What is one Pascal pressure ?

2. Name the factors on which the pressure depends on.

3. State Pascal's law of liquid pressure

4. What is upthrust? Write its SI unit.

5. Name any two equipment which are based on Pascal's law of

liquid pressure.

6. Write the formula based on Pascal's law.

7. What is thrust? Write down its SI unit.

8. State law of floatation. Name one instrument which is

constructed on this principle .

9. Weight of an object is decreased when immersed in water,

which law does this statement justify?

10. What is density ? Write down its SI unit.

11. State Archimedes principle.

12. The weight of any object decreases inside water. Why? How

much weight will an object lose in water?

13. What is atmospheric pressure?

14. Why are hydraulic brakes used?

15. What is hydraulic press?

16. Write down the use of hydraulic jack.

17. What is water pump?

18. How is it possible to fill ink in a fountain pen ?

19. Name two instruments that are based on the law of floatation.

20. What is mercury barometer?

21. What is a syringe?

22. Name three main parts of a syringe

23. What is air pump?

24. At what condition does a body float on liquid?

25. What change in atmospheric pressure is observed while

moving upwards from the earth's surface?

26. Write the value of atmospheric pressure at sea level.

27. Why does air give pressure towards ground ?

28. What is atmosphere?

29. Name any two equipment based on atmospheric pressure.

B. Short answer questions (2 marks)

1. Why is it easier to pull a bucket of water from the well until

it is inside the water but it is difficult when it is taken out of

water?

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2. The bucket is filled slower in the tap of top storey than that of
down storey, Why?

3. Differentiate between force and pressure.
4. An object weights 20 N in air and 16 N in liquid then answer

the following questions.
i) What is the upthrust of the liquid on the object?
ii) What is the weight of the displaced liquid? Write with
reason.
5. Differentiate between Archimedes principle and law of flotation.
6. What is the difference between the floating of an empty ship
and a cargo loaded ship?
7. Write two differences between density and relative density.
8. By which two properties of liquid it is used in hydraulic brake?
9. Hydraulic press is called force multiplier. Why?
10. If a hydrometer is immersed in sugar solution and pure
water, in which it immerses more? Why?
11. The weight of stone inside water is 9N and water displaced by
stone is 2N. What is the weight of the stone in air?
12. One feels difficult to breathe in the deep well while cleaning
it, why?
13. What is the relationship between the density of a liquid and
its upthrust? Explain.
14. Write two conditions for a substance to float on liquid.
15. An iron nail sinks in water but floats on mercury, why?
16. A ship coming from sea enters the river, where will its hull
sink more? Give reason.
17. Write any two importance of atmospheric pressure.
18. What happens to balloon filled with air, when it goes very
high altitude from surface of the earth? Why?
19. An iron ball sinks in water but a ship made up of iron floats, why?
20. A balloon filled with hydrogen gas rises up but the balloon
filled with air falls down. Give reason.
21. How much water should a girl, weighing 600 N, displace in
order to float on water?
30. How is it possible to fill medicine in a syringe? Explain

C. Long answer questions (3 marks)

1. How is mercury barometer constructed? Explain.

2. An effort of 100N can raise a load of 2000N in a hydraulic

press. Calculate the cross-sectional area of a small piston in

it. The cross-sectional area of a large piston is 4m². Write any

one application of hydraulic press.

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?

3. What is the condition for an object to sink in water?
s 4. It is easier to swim in ocean water, Why?

5. Differentiate between:
i) Force and pressure

y? ii) Thrust and pressure
iii) Archimedes principle and law of floatation
iv) Density and relative density
v) Thrust and upthrust
vi) Liquid pressure and atmospheric pressure

D. Very Long answer questions (4 marks)

1. Describe the structure of a water pump with neat and labeled
diagram. Also explain the working mechanism.

2. Describe the working mechanism of air pump with neat and
labeled diagram.

3. Weights of a piece of stone when weighed in three different
media: air, water and salt solutions are given in the table.

Medium Weight
A 20N
B 25N
C 22N

i) Which one is air, salt solution and water?

ii) In which medium is the weight of the stone minimum? Why?

iii) In which medium is the weight maximum? Why?

Numerical Problems

d 1. The weight of a piece of stone when fully immersed in water is
18N. It displaces 4N of water. What is the weight of the stone

in air?

2. In a hydraulic press, area of the large piston is 20cm2 whereas

the area of the small piston is 5cm2. What weight can be

balanced in the large piston if 20N effort is applied on small

piston? B 375N

It 3. Pistons A,B and C of the apparatus
given are frictionless. What is the area 250N

of the piston B? What force is exerted A 20cm2 C 10cm2
on the piston C?

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4. Weights of a stone in three different mediums air, water and
salt solutions are as follows.

Medium Weight
A 10N
B 15N
C 12N

i. Which medium is air and water? Why?

ii. What is the upthrust given by water and salt solution to
the stone?

Answers Numerical Problems

1. 22N 2. 80N 3. 30 cm2, 125N 4. (ii) 3N,5N

Project Work

Take a bucket and half fill it with water. Place an air-filled ball on water.
The ball floats. What will happen if you try to dip the ball in water by
pressing it with your hands? Write your observation with proper
justification.

Glossary

• Submerge : sink

• Magnitude : amount or quantity

• Barometer : an instrument that measures the
atmospheric pressure

• Pressure gauze : an instrument that measures the air
pressure inside the tubes of tyres

• Graduated : divided into equal parts in a scale

• Fluid : liquid or gas, a substance that can
flow freely

• Thrust : force

• Floatation : act of floating on the surface of a liquid

• Perpendicular : at the angle of 90º

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Chapter

3 Energy

Albert Einstein

Albert Einstein was a German-born theoretical physicist.
He developed the general theory of relativity, one of the
two pillars of modern physics.

Estimated Periods: 5 (5T+0P)

ObAjetctthieveesnd of the lesson, students will be able to:

• define energy and explain sources and uses of energy;
• explain nuclear reaction as the source of solar energy;
• identify the main factors of energy crisis;
• describe the measures of energy conservation and explain alternative sources

of energy.

All living things need energy. We get energy from food. We feel hungry
and tired after doing work. It is because the energy contained in our
body is used and we need other foods. In the same way, machines also
need energy to operate. Fuels or electricity provide necessary energy
to the machines. Fuels burn in the air to provide energy. Petrol, diesel,
kerosene, LP gas, coal, biomass, etc are fuels.

Energy present in an object develops capacity of doing work. If a
body has energy, it can do work. A vehicle having no fuel cannot
run because it has no energy. Fans cannot run without electricity. A
hungry porter cannot carry heavy load. Thus, energy can be defined as
the capacity of doing work.

Sources of energy

Sources of energy are classified as primary and secondary sources.
The sources of energy which can be used in the same form in which
they are present in the nature are called primary sources. Firewood,
coal, cattle dung, solar energy, wind energy, etc are primary sources.
The sources of energy which are obtained from primary sources are
called secondary sources. Biogas, LP gas, petrol, diesel, kerosene, etc
are examples of secondary sources.
Sources of energy are further divided into two types on the basis of
their replenishing periods. They are:

a. Renewable source of energy
b. Non-renewable source of energy

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Renewable Source of energy

The sources of energy which can be replenished within certain period
of time even their sources get finished are called renewable sources of
energy. Biomass, forest, soil, water, etc. are renewable sources of energy.

Non- renewable source of energy

The sources of energy which cannot be replenished easily when their
sources get finished are called non-renewable sources of energy. It
takes million of years to get replenished. Petrol, diesel, kerosene,
coal, natural gas, etc are non-renewable sources of energy.

Sometimes, we notice that some sources of energy neither decreases
nor increases although we use them continuously. Such sources are
called perpetual sources of energy.

The sources of energy which do not get exhausted despite of their
continuous use are called perpetual source of energy. Solar energy,
wind energy, geo-thermal energy, energy of flowing water, etc are
perpetual sources of energy.

Fossil fuel

The dead remains of animals and plants which were burried under
the soil millions of years ago are called fossils. These remains can
also be used as fuels called fossil fuels. Coal, petrol, diesel, natural
gas, etc are examples of fossil fuels.

When plants and animals of millions of years ago got burried under
the swamp soil, the dead bodies of plants and animals were covered
by soil. It prevented the supply of oxygen to them due to which no
bacteria could survive. In the absence of bacteria, the dead bodies
were prevented from decaying.

Due to high pressure and temperature, chemical reaction took place
in the dead bodies. As a result, coal and petroleum were produced
from them.

Fact &Reason

Why is fossil fuel called non renewable sources of energy?
Fossil fuel cannot be replenished in short time. It takes millions of years to
produce fossil fuel. So, fossil fuel is called non renewable sources of energy.

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Coal

Coal is a black or brown coloured solid form of fossil fuel. It is said that
coals were formed from the trunks of plants of millions of years ago. It
is found in deep mines under the surface of the earth. Coal is of four
types. They are lignite, sub-bituminous, bituminous and anthracite.
They differ from each other in their carbon content, volatile matter
and moisture content.

In Nepal, only low quality of coal mines (lignite) are present in Dang
district. High quality of coals (anthracite) have not been found yet.
Coals are used in the following ways:

1. It is used as fuel in different industries and factories.
2. It is also used as fuels for cooking, heating and in steam engines
3. It is essential for the manufacture of drugs, dyes, fertilizers,

plastics, etc.
4. It is used as fuel in trains.

Mineral oil

Petrol, diesel, kerosene, paraffin, lubricating oil, etc are mineral oils.
Crude oil is the main source of these oils which is trapped between
rocks under the surface of the earth. Natural gas is present on the
surface of the crude oil. Crude oil is drilled out of the underground
mine and refined. During refining process, petrol, diesel, kerosene,
etc are obtained along with the residual oil. The residual oil is further
refined to get asphalt, paraffin and lubricating oil. The ultimate
residual matter is coal tar which is used to make tarred roads. The
uses of mineral oil are as follows.

a. Petrol and diesel are used to run various types of vehicles like
bus, car, truck, motor-cycle, etc.

b. Kerosene is used as fuel for cooking food, for heating, etc.
c. It is used to run aeroplanes.
d. Lubricating oil is used as lubricant in engines and machines.
e. It is used to generate electricity from diesel plant.
f. Paraffin is used in the manufacture of wax, candle, boot polish,

etc.

Advantages of fossil fuel

1. It is cheaper and easily available than other fuels.
2. It can be transported from place to place easily.
3. It can be stored.
4. It is a multi purpose fuel. It can be used to run vehicles, run

engines, generate electricity, etc.

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Hydro-power

The energy of flowing water can be used as an important source of
energy. It is called hydropower. People have been using hydropower
for running water mills in hilly region. Such water mills (Panighatta)
are still in use.

The energy of flowing water can be used to generate electricity. The
electricity generated from water is called hydroelectricity. Water is
stored by constructing dams. Such collected water is allowed to fall
on the blades of the turbine. The turbine rotates the generator which
produces electricity. Nepal is rich in water resources. It is estimated
that 83,000 MW electricity can be produced from the fast flowing rivers
of our country. Although the cost of generation of hydroelectricity is
high in the beginning, it becomes cheaper in long run.

Advantages of hydroelectricity

1. Although the production cost of hydroelectricity is high in
the beginning, it becomes
cheaper in long run.

2. The hydroelectricity is
pollution free.

3. It is renewable source
of energy and is never
depleted.

4. It can easily be ransformed
to other forms of energy.

5. It is the only energy which
can be used to operate
modern devices such as
computer,television, fridge,
etc.

6. Modern lighting devices operate only with electricity.

Energy crisis

According to the present status of energy use in the world, 80 percent
of the necessary energy has been fulfilled from mineral oil, natural
gas, coal, etc. The reserve of such fuels is limited. But, the demand
of these fuels is increasing due to rapid population growth and use of
more number of vehicles. The consumption of coal is also increasing

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in industrialized countries. In this way, the yearly demand of fuel
energy has been increasing in the world by 2.3 percent.

According to the survey conducted in 1984A.D., it was estimated that
these fuels will be used up within 25-30 years. New oil storage three
times bigger than the existing one has been considered to be present
in Mid-Eastern Asia which can fulfil the demand of the fuel energy
for 50 years more. Thus, the reserve of fuels can fulfil the demand of
people for 2037 AD only if the demand increases by the same pace.
Thus, the world is facing the problem of scarcity of energy in near
future.

Thus, the shortage or scarcity of energy sources on the earth due to
overuse of the non-renewable source of energy is called energy crisis.

The problem of energy crisis can be solved by the following ways:

a. Using the fuels wisely and economically.
b. Conserving the existing fuels.
c. Developing alternative sources of energy.
d. Controlling the rapid population growth.
e. Making the people aware about energy crisis.
f. Developing advanced and modern technology to use alternative

sources. For example: developing vehicles which can run with
electricity or solar energy.

Alternative sources of energy

The sources of energy which can be used instead of conventional non-
renewable source of energy are called alternative source of energy.

Renewable and perpetual sources of energy can be used as alternative
source of energy. Nuclear energy, a non-renewable source of energy can
also be used as alternative source of energy. Some of the alternative
sources of energy are as follows:

1. Bio-mass fuel Bio-gas plant

Biomass fuel is any organic matter which
can be used as energy source. Wood,
cattle dung, straw, bio-gas, etc are bio-
mass fuel. They are used in villages
for domestic purpose. Certain plants
and algae, including sugarcane and sea
weed can be grown especially for use as
biomass. Biomass fuel provides energy by

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burning. In village areas, the cattle dung is dried to get dry mass
called ‘Guintho’ which is used to cook food.

Firewood is widely and commonly used source of fuel in Nepal.
Wood is converted to charcoal by burning it in insufficient amount of
oxygen. Similarly, briquette cake is made from ‘Banmara jhar’ a type
of useless weed. Charcoal and briquette coal can be used as energy
source in place of coal.

Bio-gas which is obtained by the fermentation of bio-mass is also an
emerging energy source in remote areas of our country. Bio-gas is
produced from bio-gas plant which is constructed by using modern
technology. In a bio-gas plant, cattle dung, human excreta, even
poultry droppings are decomposed to obtain bio-gas. The bio-gas
mainly contains methane and carbondioxide. Nepal government is
also providing loan and subsidy to the farmers for the construction of
bio-gas plant.

Fact &Reason

Farmers are suggested to build biogas plant, why?
Biogas is cheap and convenient source of energy. The raw materials required for
biogas plant are also easily available as our country is an agricultural country.
Moreover, the byproduct of the biogas also can be used as manure by the farmers.
Hence, farmers are suggested to build biogas plant.

Advantages of bio-gas

1. It does not produce any smoke.
2. The raw material for the bio-gas are easily available in village areas.
3. It can be used for cooking as well as lighting.
4. It is renewable source of energy.
5. The waste from the bio-gas plant is further used as fertilizer

in crop fields.

2. Nuclear fuel

The elements which produce large amount of energy as a result of
nuclear reactions are nuclear fuels. Radioactive elements like
uranium, plutonium, etc are nuclear fuels. Nuclear reactions are of
two types: nuclear fission and nuclear fusion.

Nuclear fission

When a heavier nucleus of radioactive element is bombarded by slow
neutrons, it splits into smaller nuclei with the release of huge amount

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of energy. It is called nuclear fission. For example, uranium nucleus
splits into krypton and barium nuclei when bombarded by neutrons.

92U235+ 0n1 → 56Ba141 + 36Kr92 + 30n1 + Energy.

The amount of energy released is calculated by Einstein’s mass - energy
relation theory. According to this theory, amount of energy released E
is calculated by E = mc2 where m is mass converted and c is speed of light.
The huge amount of energy released during nuclear fission is used to
produce steam from water which is used to rotate blades of generator
to produce electricity.

Nuclear fusion

In this reaction, two or more light nuclei combine to form a heavier
nucleus. Tremendous amount of heat and light energy is released
during this reaction. For example, four hydrogen nuclei fuse together
to form a helium nucleus.

21H1+ 21H1 → 2He4+ 1e°+ Energy.
Hydrogen bomb is based on the nuclear fusion reaction. This reaction
requires high temperature and pressure and cannot be controlled.

Advantages of nuclear fuel

a. A small amount of nuclear fuel produces huge amount of energy.
b. Once the nuclear fuel is fed into the nuclear reactor, it produces

energy for 2-3 years.
c. It is used by very less countries of the world. Therefore, it can

be used as alternative source of energy.
Disadvantages

a. The pollution created by nuclear fuel is more dangerous than
other sources.

b. Production cost is too high. So, poor countries cannot afford for it.
c. Harmful wastes produced by nuclear reaction cannot be

disposed or dumped inside the earth, in river or in the seas or
in open fields as other wastes.
d. Its byproduct causes long term genetic problem.

3. Tidal energy

The surge and ebb of water level from oceans due to gravitational pull
of the moon and the sun is called tide. The water raised from oceans
or seas are collected in a dam made at the sea-shore. Pipes are kept

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to return the water from the dam to the seas. This water is allowed
to fall on the turbines which drives generator to produce electricity.

4. Wind energy

In most parts of our country, wind blows with
high speed. The wind contains energy called wind
energy. Wind energy has been used to drive wind
mills, to propel sailboats, to separate husks from
food grains, etc. Nowadays, wind energy is used to
generate electricity. Wind rotates the fans which
in turn rotate the generators connected to them to
produce electricity.

Advantages

1. It is a cost free perpetual source of energy.
2. It can be used to operate water- lifting pumps, flour mills, etc.
3. It can be used to generate electricity.
4. It provides energy without depleting environment.

Disadvantages

1. It cannot produce large amount of energy.
2. It is not as reliable as hydropower.
3. It cannot provide the continuous supply of energy.

5. Geo-thermal energy

The interior part of the earth is extremely hot. The heat energy
contained in the interior part of the earth is called geothermal energy.
The temperature of the interior part of the earth rises by 30°C per
kilometer depth in common areas but by 80°C per kilometer depth
in volcanic and earthquake regions.
In order to use geo-thermal energy, two parallel holes are drilled.
Water is sent to the inside of the earth through one of the holes.
The water readily changes into steam due to very high temperature.
Thus generated steam comes out from another hole forcibly which is
used to rotate turbine. The turbine rotates the generator to produce
electricity.

The geo-thermal energy is also obtained from the natural phenomena
such as volcanic eruption, hot water spring, etc.

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Advantages

a. It supplies energy without pollution
b. It is a perpetual source of energy.
c. It can generate electricity.

6. Solar energy

The energy obtained from the sun is called solar energy. It is the main
source of energy of the earth. Solar energy has been used for many
purposes from the ancient times.

Mass: 2x 1030 kg ( about 3.33x105 times bigger than the earth)

Main constituents: Hydrogen (70%), helium (28%), other elements (2%).

Diameter: 1.4 × 106 km (109 times that of the earth)

Part by mass in solar system: combined mass of all planets & satellites of solar
system form only 0.0015th part of the sun.

Surface temperature: 5700°C

Temperature at the core: 15 million degree Celsius

Energy radiation: 4 × 1026 Joule of energy per second

Solar energy received by earth: 1.4 KW per sq. metre per second.

Some of the traditional uses of solar energy are as follows:

a. For drying clothes, food grains, obtaining salts from sea water, etc.
b. For preserving fruits, vegetables, fish, etc by drying them in the sun.
c. For providing heat and light to the earth.
d. For the preparation of foods by the plants.

Besides the traditional uses, the solar energy can be used by converting
it into other forms of energy. Many modern devices are constructed to
use solar energy. Solar cooker, solar heater, solar cells, solar furnaces,
etc are such devices. Solar cookers, and solar heaters are painted
black on the outer surface. Black colour absorbs much solar heat and
provides it to the food and water kept in them.

Similarly, solar cells convert solar energy into electrical energy. A
group of cells are connected in solar panel. It produces huge amount
of electricity. It can be used to operate lighting devices, television,
computer, etc in remote areas. It can also be used in artificial satellite,

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in communication devices, etc.

Fact &Reason

How much energy is emitted by the sun in one second?
The energy emitted by the sun in one second is 4× 1026 Joules..

Fact &Reason

What amount of average solar energy does the earth surface receive per square
meter?
The earth receives 1.4 KW solar energy per square meter.

Advantages

1. It is pollution free source of energy.
2. It is perpetual source of energy.
3. It is multi purpose source of energy.

Limitations

1. It is not available at all the time and at all the places.
2. It reaches the earth surface in diffused form. So, it cannot be

used directly.
3. Costly equipments are required to convert solar energy to

other forms.

The sun: The ultimate source of energy

The sun is extremely hot mass of gases. It is estimated that the
surface temperature of the sun is 5700°C and the core temperature
is 13 million degree Celsius. Since even iron is vapourised at about
2700°C, everything is vapourised near the sun at that temperature.

The sun is the main and ultimate source of energy for the earth.
Directly or indirectly, most of the sources of energy of the earth are
the outcome of solar energy.

Wind energy is the outcome of solar energy

The heat of the sun causes the air of one place to be heated. The hot
air being lighter rises up and a vacant place is created. The cooler
air from other places moves towards such vacant place with greater
speed. Thus, wind is formed. Hence, wind energy is the outcome of
solar energy.

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Bio-fuel and fossil fuel are outcomes of solar energy

Plants convert solar energy into chemical energy and store it in their
body. Animals eat plants and form their body. The bio-mass of plants
and animals are used as bio-fuel. When plants and animals of million
years ago died and got burried under the soil, they were converted into
fossils. These fossils can be used as fuels. Thus, petroleum, natural
gas, coal, etc are outcomes of solar energy.

Hydroelectricity is the outcome of solar energy

The water of seas, oceans, rivers, etc evaporates due to heat of the
sun. The water vapour rises upwards due to the solar heat. The water
vapour gets converted to the water or snow due to cooling in the
atmosphere. The water falls back to the earth surface in the form
of rain. Similarly, the snow melts forming rivers due to the solar
heat. The flowing water is used to produce electricity. Thus, the
hydroelectricity is the outcome of solar energy.

Present status of the energy use

Nowadays, mineral oil, natural gas, coal, and bio-fuel are the major
sources of energy in the world. With the increase in development and
industrialization, the use of mineral oils is increasing. The present
status of consumption of energy in the world is as shown below in the
table.

S.N. Source of energy Percentage

1. Mineral oil 35

2. Coal 27

3. Natural gas 17

4. Bio-fuel 13

5. Hydroelectricity 5

6. Nuclear fuel 3

The yearly demand of energy source has been increasing in the world
by 2.3 percent.

Fact &Reason

How can energy crisis be pushed forward?
Energy crisis can be pushed forward by conserving the available sources of energy,
developing alternative sources of energy and also by developing the technology
for using the alternative sources of energy.

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Conservation of energy

The sources of energy can be conserved in the following ways:
1. Alternative sources of energy should be developed and used
wisely.
2. The conventional sources of energy should be used wisely and
economically.
3. Modern technology should be developed to use alternative
energy resources, e.g. developing vehicles which can be run
on solar or electrical energy.
4. Electricity should be generated from water power, wind
power, tidal energy etc rather than from fossil fuel.
5. The use of solar energy should be encouraged. Solar cookers,
furnaces, water heaters, dryers, solar cells etc work well using
solar radiation.
6. Rapid population growth should be controlled and people
should be made aware about energy crisis.

Fact &Reason

Why should we conserve the non-renewable sources of energy?
The non-renewable sources of energy are available in limited amount and it
would take very long period of time to regenerate once exhausted. Hence, we
should conserve the non-renewable sources of energy.

Learn and Write

1. We will face energy crisis in near future. Why?

At present, 80% of the demand of energy is fulfilled by fossil
fuels. The reserve of the fossil fuel is limited and it cannot
be replenished easily. But, the yearly demand of such fuel is
increasing by 2.3 percent. Thus, the world will face energy crisis
in near future.

2. Coal is a fossil fuel. Why?

Coal is formed from dead bodies of animals and plants of millions
of years ago. Therefore, coal is fossil fuel.

3. Scientists are busy to design solar equipments. Why?

Solar energy is the perpetual source of energy. It is cost free and
pollution free source of energy. Thus, it is one of the alternative
sources of energy. Therefore, scientists are busy to design solar
equipments.

4. Differentiate between nuclear fusion and nuclear fission

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Nuclear fusion Nuclear fission

1. It is a nuclear reaction in 1. It is a nuclear reaction in

which two or more light nuclei which a heavier nucleus

fuse together to form a larger splits to form two or more

nucleus. light nuclei.

2. It is uncontrollable reaction. 2. It is controllable reaction.

Therefore, energy cannot be Therefore, energy can be

converted into useful form. converted into useful form.

3. It produces comparatively more 3. It produces comparatively

amount of energy. less amount of energy.

4. Hydrogen bomb is based on this 4. Atom bomb is based on this

principle. principle.

5. What chemical is mixed with liquefied petroleum gas? Why?

The natural gases present in liquefied petroleum gas are
colourless and odourless. Hence, a strong smelling chemical
named ethyl mercaptan is mixed with LPG to detect any leakage
of the gas during its usage.

Main points to remember

1. Capacity of doing work is called energy.
2. The sources of energy which can be used in the same form in

which they are present in the nature are called primary sources
of energy.
3. The sources of energy which are obtained from primary sources
are called secondary sources.
4. The sources of energy which can be replenished within short
period of time are called renewable source of energy.
5. The sources of energy which cannot be replenished easily when
their sources get finished are called non-renewable source of
energy.
6. The sources of energy which do not get exhausted are called
perpetual sources of energy.
7. The sources of energy which are obtained from the dead remains of
plants and animals of millions of years ago are called fossil fuels.
8. The energy due to flowing water is called hydropower.
9. The shortage or scarcity of energy sources on the earth due to
overuse of the non-renewable source of energy is called energy crisis.
10. The sources of energy which can be used instead of conventional non-
renewable source of energy are called alternative source of energy.

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11. Bio-mass is any organic matter which can be used as a source
of energy.

12. The elements which produce large amount of energy as a result
of nuclear reactions are called nuclear fuels.

13. When a heavier nucleus splits into smaller nuclei due to
bombardment of neutrons, it is called nuclear fission.

14. The nuclear reaction in which two or more nuclei fuse to form a
heavier nucleus is called nuclear fusion.

15. The energy contained in the wind of high speed is called wind
energy.

16. The heat energy contained in the interior part of the earth is
called geo-thermal energy.

17. The energy obtained from the sun is called solar energy.
18. The sun is the ultimate source of energy of the earth.

Exercise

A. Very short answer questions ( 1 mark )

1. What is primary source of energy?

2. What type of energy is coal ?

3. What is renewable source of energy?

4. Define nuclear reaction that takes place in the sun.

5. Write two examples of biomass.

6. What is fossil fuel?

7. What is alternative source of energy?

8. What is bio-fuel?

9. Write any two natural phenomena from which geothermal

energy can be obtained.

10. What is energy crisis?

11. Write the name of high quality coal.

12. What is hydroelectricity?

13. What is the estimated hydroelectricity generation capacity of

our country?

14. Write a major disadvantage of fossil fuel.

15. Name the coal of highest quality.

16. What is geothermal energy?

17. What is non-renewable source of energy?

18. For what two natural process is the solar energy being used?

.

19. What is energy ? Write down the SI unit of energy.

20. Write the type of energy sources based on time of

replacement.

21. Write the name of any two radio active elements.

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22. What is nuclear fuel?
23. What is the main source of solar energy?

B. Short answer questions (2 marks)

1. Sun is also called atomic power plant, why?

2. Justify with reasons that the hydroelectricity is the best

alternative source of energy in the context of Nepal.

3. Government is giving high priority for solar energy. Why?

4. Suggest any two alternative ways to prevent the energy crisis

in the context of Nepal.

5. Why is the use of wood as a fuel not advised although forests

can be replenished?

6. Energy contained in fossil fuels is the outcome of solar energy,

why?

7. Fossil fuel is known as non-renewable source of energy, why?

8. The sun is considered as the main source of energy". Justify

the statement.

9. "Hydroelectricity is an indirect source of solar energy".

Clarify this statement.

10. What are the main reasons of energy crisis? Write any four

reasons.

11. Energy crisis cannot be solved immediately but it can be

pushed up to the next generation. Give reason.

12. Large amount of energy is produced during the fission of

uranium atom. Give reason.

13. The use of solar energy should be increased in the context of

Nepal". Give any two reasons to justify this statement.

14. Why should energy be conserved? Mention any two steps you

would take at home to conserve energy.

15. Mineral oil is a fossil fuel. How do you explain it?

16. Wind energy is the outcome of solar energy. Explain.

C. Long answer questions (3 marks)

1. Write down any three ways of solving energy crisis.

2. Write any three advantages of mineral oil. How is biogas

produced ? Describe in brief.

3. Biogas is considered as a good domestic fuel. Justify the

statement with three reasons.

4. Differentiate between renewable and non-renewable sources

of energy.

5. Differentiate between nuclear fusion and nuclear fission.

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D. Very long answer questions (4 marks)

1. What is wind energy? How is electrical energy generated

from the blowing wind?

2. How is enormous energy produced due to decomposition of

uranium?

3. The sun is considered as an ultimate source of energy.

Justify the statement.

Project Work

Organize a field visit or an excursion to a hydroelectricity
project that lies near your school or the district. Observe
the different aspects of production of electricity by using the
running water of the river and prepare a brief report. Also
include the advantages and disadvantages of hydroelectricity
production.

Glossary : everlasting, unending

• Perpetual : energy emitted in rays or waves
• Radiation
• Swamp : an area of land that is full of water and
mud
• Ultimate
• Briquette : decisive, crucial

• Fusion : a small hard block made from a mixture
• Fission of clay, coal, saw dust or the dust of
• Fossil plant parts

• Crisis : complete combination
• Alternative
: break down

: dead remains of the organisms of millions
of years ago that are buried inside the
earth's crust

: shortage, problem

: something different that performs the
same job, substitute for

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Chapter

4 Heat Anders Celsius

Anders Celsius was a Swedish astronomer, physicist and
mathematician. He founded the Uppsala Astronomical
Observatory in 1741, and in 1742 proposed the Celsius
temperature scale which bears his name.

Estimated Periods: 6 (5T+1P)

Objectives

At the end of the lesson, students will be able to:
• explain heat as a form of energy;
• differentiate between heat and temperature;
• describe specific heat capacity and derive heat equation;
• solve simple numerical problems related to the heat.

Introduction

When we touch a cup of hot tea or hot iron, we feel hot. Here, energy
transfers from those hot bodies to our body and we feel hot. This
energy is heat energy.

Heat is defined as a form of energy which brings sensation of warmth.

Its SI unit is Joule. It is also measured in calorie. Heat always flows
from hotter objects to colder objects.

Every substance is made up of molecules. These molecules are in
vibrating condition. Due to the vibration of the molecules, every molecule
contains kinetic energy. The total sum of kinetic energy of molecules
of a body is called heat energy. The kinetic energy of each molecule
depends upon its mass and speed of vibration. (KE = ½mv2). Thus, total
heat energy contained in a substance depends upon the total mass of
the molecules and average speed of vibration of molecules.

The more the speed of vibration of the molecules, the more the heat
energy contained in the object. The more the total mass of molecules,
the more the heat energy contained in the object.

One caloric heat: The amount of energy required to raise the
temperature of 1g pure water by 1oC is called one calorie heat.
i.e. 1 calorie = 4.2 Joule

Fact &Reason

What is the direction of the flow of heat?
Heat always flows from higher temperature to lower temperature.

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Temperature

When we touch warm water, we feel hot. But we feel hotter when
we touch boiling water. The warm water and boiling water are not
equally hot. We need numerical quantity to express degree of hotness
or coldness. This quantity is called temperature. Temperature is
defined as a quantity which measures degree of hotness or coldness of
a body. The hotter objects have more temperature than colder objects.

Molecular theory states that each object is made up of molecules
and these molecules are in vibrating state. When a body is heated,
the vibration of the molecules contained in it increases then the
temperature, too. The increase in vibration of molecules causes the
increase in temperature. Thus, temperature measures the average
speed of vibration of molecules. In other words, temperature is defined
as the average kinetic energy of the molecules of a body.

The SI unit of temperature is Kelvin (K). It is also measured in degree
centigrade (°C) and degree Fahrenheit (°F).

Heat and temperature

When we heat an object, the molecules contained in the object vibrate
faster and it will gain more kinetic energy. The quantity of heat
contained in an object depends upon following two factors:

i) Number of molecules

ii) Average kinetic energy of the molecules

Thus, the heat contained in an object is directly linked to the
kinetic energy of its molecules. Heat contained in an object is the
sum of average kinetic energy of all the molecules contained in
it. The average kinetic energy in the molecules of hotter object is
more than that in colder objects.

When an object is heated, its temperature also increases. It is
due to increase in the average kinetic energy of the molecules
contained in that object. An object becomes colder or hotter due to
difference in temperature. Hotter objects have more temperature
than colder ones. The molecules contained in hotter bodies vibrate
with higher speed than that in colder objects. Thus, heat and
temperature are very much related. On heating an object, the heat

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energy is absorbed by the molecules and amount of heat as well as
the temperature of the body increases. Similarly, on cooling, the
temperature of the object decreases.

Differences between heat and temperature

Heat Temperature

1. It is a form of energy which 1. It is the degree of hotness or
produces sensation of coldness of an object.
warmth.

2. Heat contained in an object 2. Temperature of a substance

depends upon the mass of depends upon the speed

total molecules and the speed of vibration of molecules

of vibration of the molecules. but not on the mass of the

molecules.

3. Its SI unit is Joule (J). 3. Its SI unit is Kelvin (K).

4. It is measured by a 4. It is measured by a
calorimeter. thermometer.

Thermometer

When we touch lukewarm water, we can estimate temperature
of the water. But, it will not be accurate. For measuring accurate
temperature, we need a special device named thermometer. Thus,
thermometer is defined as a device which measures temperature of a
substance.

It works on the principle that “the liquid expands on heating and
contracts on cooling”.

The bulb of a thermometer is filled with a liquid called thermometric
liquid. Mercury or alcohol is used as thermometric liquid in a
thermometer.

Fact &Reason

On which principle does thermometer work?

Thermometer is based on the principle which is “ liquid expands on heating and

contracts on cooling”.

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Characteristics of thermometric substance

a. Mercury
1. It is a silver coloured shiny liquid. Therefore, it can be seen
clearly in the glass.
2. It is good a conductor of heat. Therefore, it easily passes heat
from the object.
3. It does not wet the wall of glass tube. Thus, rise and fall of
mercury in the tube is clear.
4. It freezes at −39°C and boils at 357°C. Therefore, it
measures the temperature from −39°C to 357°C. Hence,
mercury thermometer cannot be used to measure very low
temperature.
5. It has uniform rate of expansion and contraction.

b. Alcohol
1. It is a transparent colourless liquid. Therefore, it is to be
coloured before use.
2. Its expansion rate is six times more than that of mercury.
3. It freezes at −115°C and boils at 78°C. Therefore, alcohol
thermometer is used to measure very low temperature. But,
it cannot measure the temperature higher than 78°C.

Fact &Reason

Alcohol thermometer is used to measure the temperature of very cold reason
instead of mercury thermometer, why?
The freezing point of alcohol is very low i.e. -115 degree centigrade in comparison
to the freezing point of mercury i.e. -39 degree centigrade. Therefore, alcohol
thermometer is used to measure the temperature of very cold reason instead of
mercury thermometer

Types of thermometer

a. Clinical thermometer

A clinical thermometer is an instrument which is used to measure the

temperature of human body. It contains bulb filled with mercury as

the thermometric liquid. It has very short temperature range of 35°C

to 42°C (or 94 °F to 108°F). This thermometer is widely used by the

doctors and nurses to measure the temperature of patients.

Alcohol capillary tube Glass tube

Bulb constriction temperature scale
Clinical thermometer

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A clinical thermometer consists of a fine uniform bore and a little
constriction in the bore of the mercury tube just above the bulb.
This constriction helps us to measure the temperature of the patient
correctly. When the thermometer is placed under the tongue or armpit
of the patient, the expansion of mercury takes place due to the increase
in temperature. This causes the rise of mercury in the tube passing
through the constriction. As the thermometer is removed from the
body, the mercury thread breaks at the constriction. The mercury in
the bore above the constriction continues to stay there and thus helps
to record the temperature of the patient.

Fact &Reason

Why is kink present near the bulb of the clinical thermometer?
Kink is present near the bulb of the clinical thermometer to avoid the backward
flow of mercury when it is removed from the body. So, we can get a accurate
reading of the temperature even after sometime.

b. Digital thermometer

An electronic (digital) thermometer is in common use to measure the
temperature of the human body in present days. This thermometer
does not use any thermometric liquid and shows the temperature in
digits.



Digital thermometer

Digital thermometer is an electronic thermometer. It consists of a
temperature sensitive resistor as a thermometric substance. It also
comprises a memory function and beep alarm. A cap is provided on
the top of the thermometer which protects the battery fitted inside it.
The working of a digital thermometer is based on the principle that the
resistance of a resistor changes with the change in temperature.

Non-contact electronic thermometers are also used to measure the
temperature of human body. They can measure temperature of a
body from a distance, i.e. they need not touch the body to measure its
temperature. They consist of thermopiles to record temperature from a
distance by detecting an object’s infrared energy.

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c. Laboratory thermometer

A thermometer which is used to measure the temperature of different
chemicals, solvents and reagents in the laboratory is called laboratory
thermometer. It consists of a thick walled glass tube having uniform
and fine bore called capillary tube. The capillary tube is connected to
bulb containing mercury at one end. The other end of the capillary is
sealed. The capillary tube and the bulb are covered by a glass cover.
The capillary tube is straight and it does not have any constriction.

Mercury capillary tube Glass tube

Bulb Laboratory thermometer temperature scale

The stem of the laboratory thermometer is graduated in degree
between –10°C to 110°C. When the bulb of the thermometer is placed
in contact with a hot body, the mercury of the bulb begins to rise
and reaches a constant value. The temperature of the body can be
measured by keeping the thermometer in contact with the body.

Differences between clinical and laboratory thermometer

Clinical thermometer Laboratory thermometer

1. It measures the temperature 1. It measures the temperature of

of human body. different objects.

2. It is calibrated between 2. It is calibrated between –10°C to
35°C to 42°C (94°F to 108°F). 110°C.

3. It has a constriction or kink 3. It does not have any kink.
near the bulb.

d. Minimum and maximum thermometer

Maximum and minimum thermometer is used to record the maximum
temperature during day and the minimum temperature at night. It
consists of a U-shaped tube having two limbs A and B. These limbs
are partially filled with mercury. These limbs are connected to two
bulbs X and Y respectively.

Limb A is connected to the bulb X containing alcohol. The change in
volume of alcohol with temperature is responsible for registering the
minimum and maximum temperature. Limb B is connected to the

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X Y
minimum maximum

bulb 'Y' partially filled with alcohol. There vacuum

is an empty space in the bulb 'Y' to allow A B

the further expansion. When the alcohol °C °C
-25 alcohol 40
of the bulb X expands due to increasing -20 D metal indices C 35
30
temperature, it pushes the mercury column -15 mercury 25
20
of limb B sending the steel index above the -10 15
-5 10
level C. This index enables to record the 0 5
0
maximum day temperature that can be read 5 -5
10 -10
on the scale marked along the limb. 15 -15
-20
The temperature of a place falls down at 20 -25
25
night. When the temperature falls, the 30
alcohol of the bulb at X contracts and the 35

mercury is drawn back pushing steel index 40

above the level D. The index, therefore gives

the minimum temperature reached at night. Minimum and maximum
thermometer
Specific heat capacity

When heat is supplied to an object, the rise in temperature of the
object depends upon the amount of heat supplied, mass of the object
and the nature of the object.

Activity 4 .1 To show that the rise in temperature of a substance depends

upon the nature of the substance

Materials required:
Two beakers of same size, two thermometers, oil, water, spirit lamp,

stand, stop watch, etc.

Procedure:
a. Take two beakers of same size.
b. Put 200 ml of soyabean oil in one of the beakers and 200 ml of

water in another beaker.

c. Fix a thermometer in each beaker with the help of stand as
shown in the figure and measure the temperature of both oil
and water.

d. Heat them with two lamps of identical size and burning capacity.

Thermometer

Beaker

Water Oil

Tripod
stand

Burner

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e. Exchange the spirit lamp for every 1 minute.
f. Measure the temperature of both the liquids in every two

minutes.

Observation:
The temperature of the oil rises faster than the water even though
equal amount of heat is supplied to equal amount of both oil and
water.

Conclusion:

The rise in temperature of a substance does not depend only upon
the amount of heat supplied and the mass of the substance but also
upon the nature of the substance.

From the above activity, we can say that the increase in temperature
is different eventhough the equal quantity of heat is supplied to equal
amount of different substances. In this way, the heat capacity or heat
retention capacity of different substances is different.

The amount of heat required to increase the temperature of an object
of 1 kg mass by 1°C is called the specific heat capacity of the object.
It is measured in Joules per kilogram per degree Celsius (J/kg°C).
Different substances have different specific heat capacities. The
bodies having higher specific heat capacity change their temperature
slowly whereas those having lower specific heat capacity change
their temperature faster. Water is the substance having the highest
specific heat capacity. The specific heat capacities of some objects are
as follows:

S.N. Materials Specific heat capacities in J/kg°C

1. Lead 126
2. Mercury 138
3. 234
4. Silver 380
5. Brass 390
6. Copper 447
7. Steel 470
8. Iron 900
9. Aluminium 670
10. Glass 800
Sand

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11. Kerosene 2200
12. Ice 2100
13. Water 4200
14. Petrol 1670
15. Vegetable oil 2000
16. Alcohol 2400

Fact &Reason

The specific heat capacity of different materials is different, why?
The molecular composition of different material is different and they absorb the
heat energy in different ways. The vibration of the molecules and the intensity
of the molecules is also different even when same amount of heat is applied. So,
different materials have different specific heat capacity.

Heat capacity

If the mass of an object is not specified, the amount of heat required
to raise temperature of an object through 1°C is called heat capacity.
The SI unit of heat is Joule per degree Celsius (J/°C).

Consequences of the highest specific heat capacity of water

1. Water is used to cool the engines of vehicles.

Water is the substance having the highest specific heat capacity,
i.e. 4200J/kg°C. It can absorb large amount of heat from hot engines
without raising its temperature by significant value. Thus, it makes
the engine cool.

2. Water is used in hot water bag.

Water has the highest specific heat capacity, i.e. 4200 J/kg°C. When
water is heated to boiling point, it absorbs huge amount of heat. The
same heat is given out to the surrounding if it is placed in hot water
bag. Thus, the large amount of heat is made available for longer time.

3. The night in desert is very cold but day is very hot.

Desert is full of sand. The sand has less specific heat capacity, i.e.
only 800 J/kg°C. During day time, sand gets heated faster due to
absorption of heat from the sun. At night, the sand loses heat rapidly
and gets cooled faster. Thus, the nights in desert are colder but days
are hotter.

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Heat equation

When a body is heated, its temperature increases. On the other hand,
when a body is cooled, its temperature decreases. Suppose, a body of
mass m having initial temperature t1 is heated by supplying heat Q
then its temperature rises to t2. Now, the change in temperature is:
dt = t2– t1.
It has been proved that the heat gained or lost (Q) by a body is directly
proportional to (i) mass of the body (m) and (ii) change in temperature.

Mathematically,

Q α m ……… (i)

Q α dt ……….. (ii)

Combining equations (i) & (ii), we get:

Q α mdt

Or, Q = smdt

Where, ‘s’ is a constant called specific heat capacity of the body.

∴ Q = msdt

Thus, amount of heat lost or gained by an object is equal to the product
of mass (m), specific heat capacity (s) and the change in temperature
(dt). The above equation is called heat equation. It is used to calculate
the value of any one variable when other remaining values are given.

Solved Numerical Problem 4. 1

How much heat is required to raise temperature of 5 kg water from
10°C to 30°C? (Given Specific heat capacity of water is 4200 J/kg°C.)

Solution:

Given,

Mass (m) = 5kg

Initial temperature (t1) = 10°C
Final temperature (t2) = 30°C
Change in temperature (dt) = t2–t1 = 30–10 = 20°C
Specific heat capacity (S) = 4200J/kg°C

Amount of heat gained (Q) = ?

We have,

Q = m s dt = 5×4200×20 = 420000J.

Therefore, the amount of heat required is 420000 Joule.

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Principle of calorimetry

Calorimetry is a branch of physics which deals with the measurement
of quantity of heat lost or gained. According to the principle of
calorimetry, when a cold body comes in contact with a hot body, the
amount of heat lost by the hot body is equal to the amount of heat
gained by the cold body if there is no heat loss to the surrounding
environment, i.e.

Heat lost by a hot body = Heat gained by a cold body

i.e. Q1 =Q2 or, m1s1dt1 = m2s2dt2 or, m1s1(t1-t) = m2s2(t-t2)

Solved Numerical Problem 4. 2

An iron ball of mass 2 kg with temperature of 90°C is kept in water of
mass 3kg with temperature of 10°C. What will be the final temperature
if the specific heat capacity of iron is 470 J/kg°C and that of water is
4200 J/kg°C?

Solution:

Given,

Mass of iron (m1) = 2 kg
Initial temperature of iron (t1) = 90°C
Specific heat capacity (s1) = 470 J/kg°C
Mass of water (m2) = 3kg
Initial temperature of water (t2) = 10°C
Specific heat of water (s2) = 4200 J/kg°C
Let, the final temperature be ‘t’ after keeping the iron ball in the
water. According to the principle of calorimetry,

Heat lost by iron = Heat gained by water

m1s1(t1-t) = m2s2 (t - t2)
Or, 2×470 (90-t) = 3×4200 (t - 10)

Or, 940 (90 - t) = 12600 (t - 10)

84600 - 940t = 12600t - 126000

13540t = 210600 Or, t = 15.55°C

Therefore, the final Temperature is 15.55°C

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Solved Numerical Problem 4. 3

Calculate the final temperature when 5 kg iron of 40°C is heated by
supplying heat of 2400 Joules of heat. (The specific heat capacity of
iron = 470 J/kg°C)

Solution:

Given,

Mass (m) = 5 kg

Initial temperature (t1) = 40°C
Specific heat capacity (s) = 470 J/kg°C

Amount of heat supplied (Q) = 2400J

Final temperature (t2) = ?
We have,

Q = msdt Or, Q = ms(t2– t1)

Or, 2400 = 5 × 470 (t2 – 40)

Or, 2400 = 2350t2 – 94000

Or, 2350t2 = 96400 Or, t2 = 41.02°C

Therefore, the final temperature of the iron is 41.02°C

Learn and Write

1. The water in the earthern pots becomes colder than that in metal pots. Why?

In the earthern pots, there are tiny pores. Water oozes out
through the tiny pores of the earthern pots. Evaporation of oozed
water takes heat from the water inside the pot making it colder.
Such oozing does not take place in metal pots. Thus, the water
in the earthen pots becomes colder.

2. Mercury is used as a thermometric liquid. Why?

Mercury is a good conductor of heat having less specific heat
capacity. Even it absorbs less amount of heat from a body, it has
significant rise in temperature making the thermometer more
sensitive. Therefore, it is used as a thermometric liquid.

3. Which one is heated faster: one kilogram iron or one kilogram
mercury when equal amount of heat is supplied ?

One kilogram mercury gets heated faster than one kilogram iron
because mercury has less specific heat capacity (138°J/kg) than the iron
(470 J/kg°C).

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4. Sea breeze occurs at day while land breeze occurs at night.

The specific heat capacity of water is much more compared to
that of sand or soil. During day time, the land is heated faster
than water. Therefore, the air just above the land gets heated
and rises up creating a partial vacuum near the land. But air
above the sea is still cold and it blows toward the land to occupy
the partial vacant space. This cold wind that blows from sea to
land is called sea breeze.

During the night, the land (soil and sand) cools down faster. But
the sea water is still hot as it cools down slowly. As a result the air
above sea water becomes less dense and rises upward. Thus, the air
from land moves towards the seas and land breeze occurs.

5. The specific heat capacity of mercury is 140 J/kg °C. What does
it mean?

It means that 140 Joules of heat energy is required to raise the
temperature of 1 kg of mercury by 1°C. In other words, if you have
1 kg of mercury at the temperature of 25°C, you have to supply 140
Joules of heat energy to it to increase its temperature up to 26°C.

6. Water starts freezing from the top whereas ghee starts freezing
from the bottom.

When the temperature of water falls below 4°C, it becomes
lighter and rises up to the surface. Thus, the colder water of 0°C
remains at the top but the hotter water remains at the bottom.
Hence, water starts freezing from the top. But, ghee becomes
heavier when it becomes colder, and remains at the bottom.
Thus, the ghee starts freezing from the bottom.

Main points to remember

1. Heat is a form of energy which is produced due to vibration of molecules.
2. SI unit of heat is Joule.
3. Temperature is the degree of hotness or coldness of a substance.
4. A thermometer which is used to measure temperature of human

body is called clinical thermometer.
5. A thermometer which is used to measure temperature of any

substance is called laboratory thermometer.
6. The amount of heat required to increase the temperature of an object

of mass 1kg by 1°C is called the specific heat capacity of the object.
7. The amount of heat lost or gained by an object is equal to the product

of mass of the object, specific capacity and change in temperature.
8. Amount of heat required to raise temperature of 1 gram of

water through 1oc is called 1 calorie heat.

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Exercise

A. Very short answer questions (1 mark)

1. Define specific heat capacity.

2. What are the factors on which heat depends on?

3. Define one calorie heat.

4. Write the mathematical relation between calorie and joule.

5. Write the relation between heat gained or lost by a body with

its surface area.

6. What is the specific heat capacity of water?

7. What is heat equation ?

8. Define temperature in terms of kinetic theory.

9. Write the relation between heat gained or lost by an object

with its mass.

10. Define heat according to kinetic molecular theory.

11. Write down the principle of calorimetry.

12. What is thermometer ? On which principle is it constructed?

13. Write down the freezing and boiling point of mercury and

alcohol.

14. Name any two types of thermometer. Write the name of any

two thermometric liquids.

15. What is the normal body temperature of a healthy human in

Fahrenheit scale?

16. Name the instrument which is made on the basis of expansion

of heat.

17. Write the relation between heat gained or lost by a body with

the change in temperature.

B. Short answer questions (2 marks)

1. Write any two differences between heat and temperature.

2. Why is water used to cool the engine of vehicle?

3. During the winter, animals sit into curl, why?

4. If equal quantity of vegetable oil and water in two different

beakers are heated with equal amount of heat . Which will be

heated faster, why?

5. Why does specific heat capacity of objects differ?

6. What is the meaning of the statement “ the specific heat

capacity of water is 4200 J/kg°C “?

7. A substance having low specific heat capacity gets heated faster

than another one having high specific heat capacity, why?

8. When hot iron nail is kept in cold water, the temperature of

iron is reduced, why?

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9. Object A has more specific heat capacity than object B. If both
are given equal amount heat, which of the object has more
temperature and why?

10. In winter morning as soon as we wake up, the air inside the
room is cold but if we go out and return to the same room, we
feel warm, why?

11. If two equal sized beakers one having water and another
having paraffin oil are heated equally, which beaker gains
more temperature? Why?

12. 10 kg of water at 80°C cools faster than 15 kg of water at the
same temperature kept in the identical vessels. Why?

13. The temperature of coastal regions remains fairly constant.
Give reason.

14. In desert, it is very hot during day and very cold during night.
Why?

15. A wet handkerchief is kept on the forehead of the patient
suffering from fever, why?

16. Write two reasons for using mercury as a thermometric liquid.
17. Write any two differences between clinical thermometer and

simple thermometer.
18. A narrow constriction is kept near the bulb of the clinical

thermometer. Why?
19. Mercury thermometer is used to measure higher temperature but

alcohol thermometer is used to measure lower temperature, why?
20. Alcohol thermometer can measure the temperature of an ice

but cannot measure the boiling point of water. Why?
21. It is given jerk to clinical thermometer before using it for

measuring temperature of human body, why?
22. Nowadays clinical thermometer is replaced by digital

thermometer. Why?
23. The shape of clinical thermometer is made prismatic. Why?

C. Long answer questions (3 marks)

1. If three liquids A, B and C having equal masses are taken in

three same size of beakers and exposed in sunlight. After 10

minutes the increased temperature in each is given in the table:

Liquid Temperature in °C

A 60

B 70

C 50

i) Which liquid has highest specific heat capacity? Why?

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ii) If all the three liquids are heated to same temperature and
left to cool down, which one cools down faster? Why?

2. Prove that: Q=msdt
3. Write any three applications of high specific heat capacity of

water.
4. Study the given table and answer the following questions:

Name of metals Specific heat capacity
A 210 J/kg°C

B 460 J/kg°C

C 630 J/kg°C

i) What do you mean by the statement that the specific heat
capacity of iron is 460 J/kg°C ?

ii) If equal amount of heat is given to equal mass of these
substances, which one will have the highest temperature?
Why?

iii) If equal mass of these substances at 100°C temperature are
kept on wax slab, which one will make the deepest hole in
wax slab? Why?

D. Very long answer questions (4 marks)

1. Draw a neat and labelled figure showing maximum-minimum

thermometer. Describe the structure of the maximum-minimum

thermometer.

2. What is laboratory thermometer? Describe the structure of

laboratory thermometer with a labelled diagram.

Numerical problems
1. How much heat is required to increase temperature of 5kg of
water from 20°C to 70°C? (Given specific heat capacity of water
= 4200J/kg°C.)
2. Calculate the final temperature of a pressure cooker of mass 1.5kg
and temperature 30°C, when it is heated with 7.5×104 Joule. (The
specific heat capacity of aluminium is 1000J/kg°C.)
3. The temperature of 20kg water in the radiator of an engine of a
car is 30°C. If the temperature of the water increases to 100°C
after the engine is heated. What quantity of heat is absorbed by
the water?
4. What will be the quantity of heat required to raise the temperature
of 2 kg paraffin by 10°C if 44000 Joules of heat energy is required
to raise the temperature of 1 kg paraffin by 20°C?
5. Hot water of 100°C is added to 300g of water at 0°C and the final

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temperature of the mixture reached 40°C. Find the mass of hot water
added. (Given, the specific heat capacity of water = 4200J/kg°C).
6. Temperature of water is 5°C in winter season. If 20 litres of
water is to be heated to 35°C for taking a bath, calculate the
amount of heat required for it. (Specific heat capacity of water
= 4200J/kg°C and 1 litre water = 1 kg)
7. For taking a bath, water at 40°C is required. Calculate the
mass of water at 15°C to be added to 80kg water at 80°C for
making it suitable for bathing.

Answers Numerical problems

1. 1050000 Joule 2. 80°C 3. 5.88×106 Joule

4. 2.2×104 Joule 5. 0.2kg 6. 2.52×106 Joule 7. 128 kg

Project Work

Put equal amount of water, mustard oil, kerosene in three
different glasses. Keep them in sunlight for one hour which
gets heated more. What conclusion do you get from above
observation ?

Glossary

• Vibration : rapid to and fro motion of a body

• Transformation : change

• Kinetic : related to motion

• Digital thermometer : a thermometer that displays
temperature in digits, an electronic
thermometer

• Frozen : covered by or made into ice, solidified
into ice

• Breeze : gentle and cool wind

• Ooze : to exude a liquid substance slowly
and in small quantities

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Chapter

5 Light Willebord Snell

He is known for the Snell's Law.

Estimated Periods: 7 (5T+2P)

Objectives

At the end of the lesson, students will be able to:

• demonstrate the refraction of light through lens;
• explain the uses of lenses in practical life with ray diagrams;
• describe human eye and causes and remedies of defect of vision;

We can see the objects only in the presence of light. In dark, we
cannot see the objects. Thus, light can be defined as a form of energy
which brings sensation of vision. Light can be produced from natural
or artificial substances. The sun is the natural substance which
produces light. Electric bulb, candle, fire, fluorescent lamps, etc are
some artificial substances which produce light.

Lens

You must have heard about the use of lens in camera, spectacles,
microscope, telescope, etc. It is very useful device. It is present even in
human eye. The lenses used in optical instruments help for the clear
visibility of objects. The lens of the human eye refracts the light rays
coming from the objects and forms images on the retina.

A lens is defined as an optical device which is made up of a transparent
medium bounded by spherical surfaces. It is usually made up of glass
or plastic. It is of two types. They are:

i. Convex lens ii. Concave lens

Convex lens Thin

The lens which is thick in the Thick
middle and thin at the edge is F
called convex lens. When parallel
rays of light fall on a convex lens,
they get converged at a point after

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refraction. Therefore, a convex lens is called converging lens.

Concave lens

The lens which is thin in the middle and thick

at the edge is called concave lens. O

When parallel rays of light fall on a concave F

lens, they get diverged after refraction. They

appear to diverge from a point as shown in

the figure. Therefore, a concave lens is called

diverging lens.

Activity 5 .1 To show that a convex lens converges parallel rays to a point.

Materials required:
A convex lens, a paper, etc.

Procedure:
1. Take a convex lens and a paper.
2. Focus the sunlight with the help of the

convex lens to the paper for about 5 minutes.

Observation:

The paper starts burning after sometime.

Conclusion:

The convex lens being a converging lens focuses the beam of sunlight
to the paper.

Repeat the above activity with a concave lens. What happens? The

concave lens cannot converge the parallel rays of light to the paper.

So, it does not burn.

Lens as a group of prisms

A lens is assumed to be made by the

combination of many prisms in correct

order. A convex lens is supposed to consist F
of prisms in upper half with their bases

downwards and prisms in the lower half (a)
with their bases upwards. The parts of the i. Convex lens

lens are like those shown in the figure.

The rays of light always deviate towards O
the base of lens after refracting through F
it. Thus, the prism in the upper part of the
(b)
ii. Concave lens

lens bends the incident light downward,

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whereas the prism in the lower part of the lens bends the incident
light upwards. The central part of the lens acts just like a rectangular
glass slab, and hence it does not bend the incident light. The incident
light travels straight through the middle part of the lens. It is evident
from the above discussion how a convex lens converges the rays of
light after refraction.
A concave lens can also be considered as composed of the combination
of prisms. However, the bases of prisms lie just opposite to those of
convex lens. The prisms in the upper half contain their bases upwards
and the prisms in the lower half have their bases downwards. The
middle part of the lens acts as a glass slab. Since the parallel rays
of light deviate toward the base after refraction, the upper part of
the concave lens bends the rays upwards and the lower part bends
the rays downwards. The light rays travel straight (without bending)
through the central part as it acts as a rectangular glass slab. In this
way, a concave lens diverges the parallel rays of light after refraction.

Terminology X F F Y
Different terms are used while C1 O C2
discussing the lens. Some of them are as
follows:

Centre of curvature FF Y
Surface of a lens is bounded by two C1 O C2
spherical surfaces. Thus, surface of a
lens is a small portion of a sphere. The Sphere I Sphere II
centre of the sphere of which the lens X
is a part is called centre of curvature. A R1 R2 B
lens has two centres of curvature. They C1 O C2
are denoted by C1 and C2.

Radius of curvature
The radius of a sphere whose surface forms
a part of a lens is called radius of curvature. A
A lens has two radii of curvature.

Principal axis R1 R2 Principal
C1 O C2 axis
An imaginary line which joins two centers A
of curvatures (C1 and C2) is called principal B
axis. In the figure, AB is principal axis.
Sphere I Sphere II

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Optical Centre

It is the centre of a lens which is equidistant from all points of its
surface. It lies at principal axis. It is denoted by O. When a ray of
light passes through it, it does not deviate.

Principal focus

It is a point on the principal axis where the rays of light parallel to the

principal axis converge after refraction

through a convex lens or appear to diverge

after refraction through a concave lens. It F

is denoted by F.

In the figure (a), the parallel rays of light (a) f

after refraction through convex lens meet

at a point ‘F’. It is principal focus. The O
F
principal focus of a convex lens is real.

In the figure (b), a parallel beam of light (b)
after refraction through a concave lens

diverge. The diverged rays when produced meet at a point ‘F’ on

the principal axis. It is the principal focus. The principal focus of a

concave lens is virtual.

Focal length

The distance between optical centre (O) and principal focus (F) of a
lens is called focal length. It is denoted by f . It is positive (+) for a
convex lens and negative (–) for a concave lens.

Activity 5 .2 To determine focal length of a convex lens.

Materials required:

A convex lens, a white sheet of paper, etc.

Procedure
a. Hold a convex lens about 20cm away from a white sheet of paper.
b. Make the rays from distant object (say house or tree) fall on the lens.
c. Try to get clear image of the object by moving the lens closer or

away from the paper.
d. Measure the distance between the image and the lens when the

image becomes clear and distinct.

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This distance is focal length of the lens. A thicker lens has shorter
focal length and a thinner lens has longer focal length. The process
of adjusting distance between the lens and the screen in order to get
the clear and distant image is called focusing. Focusing is done to
get clear image in microscope, telescope, camera and even in the
eyes.

Rules of refraction through a lens

F FF F F F

(a) (b) (c)

a. A ray parallel to the principal axis passes or appears to pass

through the principal focus after refraction through a lens.

b. A ray passing through optical centre does not bend after refraction.

c. If a ray passes through the principal focus, the refracted ray

becomes parallel to the principal axis.

Rules for drawing ray diagrams in a convex lens

Following rules are to be followed while drawing ray diagrams:

Draw a convex lens with the help of a protractor. Draw a dotted line

from upper tip to lower tip of the lens to represent optical axis of the

lens. M

2F F Ov
u F 2F

N

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Draw a line perpendicular to the optical axis passing through the
centre of the lens. It is principal axis. The point of intersection of
the optical axis and the principal axis is optical centre ‘O’. Mark two
points on the principal axis equidistant from the optical centre ‘O’ at
both sides of the lens as ‘F’. It is principal focus. Mark ‘2F’ at a point
on the principal axis which is exactly double distance of ‘F’ from the
optical centre.
Draw an upright arrow from a point on the principal axis behind ‘F’ as
an object. Draw a straight line parallel to the principal axis from the
tip of the arrow. The line touches the optical axis at point M. Then,
bend it and pass through ‘F’ at another side. It shows that the ray
parallel to the principal axis passes through the principal focus after
refraction. Draw a straight line from the tip of the arrow through the
optical centre. Extend it such that it meets the line passing through
the principal focus at point ‘N’. Draw a perpendicular on the principal
axis from the point ‘N’. This line represents image. The image is real
and inverted. The real image can be obtained on the screen.
All the distances in a lens are measured from the optical centre (O).
The distance between optical centre and object is called object distance
(u). The distance between the optical centre and image is called image
distance (v).

Images formed by a convex lens

A convex lens forms images of different sizes on the basis of positions
of objects.

1. When an object is at infinity

When an object is at infinity, the

parallel rays coming from the

object meet at the principal focus F

after refraction through the lens.

Thus, the image is formed at the

principal focus.

The image is real, inverted and very much smaller than the object, i.e.

highly diminished.

2. When an object is beyond 2F

When an object is kept beyond 2F, B F A'
image is formed between ‘F’ and A O 2F
‘2F’ at another side. The image is 2F
real, inverted and smaller than the B'
object (diminished).

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