Science and Environment 10

Activity 8

• Take a glass. Fill it with water up to the brim so Water
that there is no space for air. Cover the glass with Glass
a postcard. Invert the glass as shown in the figure.
Now, remove the hand from the postcard gently. Postcard
What do you observe?

Fig. 2.25

It is found that water does not fall from the glass immediately. The atmospheric
pressure helps to hold water in the glass by pressing the postcard upward.

Activity 9 Fig. 2.26

• Take water in a glass. Suck the water slowly with a straw.
How is it possible to draw water from the glass? Think!

When air in the straw is sucked, it creates a partial vacuum
inside the pipe. So, it helps to reduce the pressure in the
straw, and the atmospheric pressure helps to raise the water
level upward.

A similar process takes place while filling ink in a fountain
pen.

Importance of atmospheric pressure
Atmospheric pressure is very important for our day to day activities. We are able to use various
equipment due to the presence of atmospheric pressure. It is important to:

i. fill ink in a fountain pen.
ii. fill medicine in a syringe.
iii. fill air in a bicycle tube or tube of vehicles.
iv. lift water by using a water pump.
v. draw soft drinks through a straw.
vi. use rubber suction pads on the walls to hang clothes, calenders, etc.

Fact File - 4

• Atmosphere exists upto 1000 km vertically upwards from the earth's surface.
• Atmospheric pressure decreases while moving upwards from the earth's surface.

PHYSICS Oasis School Science - 10 43

2.18 Barometer

A barometer is a scientific instrument used to measure atmospheric
pressure. It was invented by Evangelista Torricelli in 1643 AD. There are
different types of barometers like mercury barometer, aneroid barometer,
water-based barometer, vaccum-pump oil barometer, etc. Among them,
the mercury barometer and aneroid barometer are commonly used.

Mercury barometer Fig. 2.27

A mercury barometer consists of a graduated glass tube of one metre height closed at one
end with an open mercury-filled reservoir at the base. The weight of the mercury crates a
vacuum at the top of the glass tube, which is called Torricellian vacuum. Mercury in the tube
adjusts until the weight of the mercury column balances the atmospheric force exerted on the
reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury to
move higher in the column. The height of the mercury in the glass tube shows the atmospheric
pressure of a place. On the other hand, low pressure allows the mercury to drop to a lower
level in the column by lowering the force placed on a reservoir. The average atmospheric
pressure at sea level is 760 mm Hg. Atmospheric pressure decreases with the increase in
altitude of a place.

Fig. 2.28

Aneroid Baromter

An aneroid barometer is an instrument which is used for measuring Fig. 2.29
pressure without using a liquid. It was invented by a French scientist,
Lucien Vidi, in 1844 AD. In this barometer, a small flexible metal box
called an aneroid cell is used. So, it is named the aneroid barometer.
The aneroid cell is made from an alloy of beryllium and copper. The
evacuated capsule is prevented from collapsing by a strong spring.

Small changes in the external air the pressure causes the aneroid cell to expand or contract.
This expansion or contraction drives mechanical levers such that the tiny movements of the
capsule are amplified and displayed on the face of the aneroid barometer, with a pointer. This
type of barometer is commonly used in homes, vehicle service centers for measuring pressure
in the tubes of vehicles, in boats and aircraft. It is also used in meteorological stations or
weather stations and scuba diving.

44 Oasis School Science - 10 PHYSICS

2.19 Syringe

A syringe is a simple pump consisting of a plunger that fits tightly in a tube. The plunger or
piston can be pulled and pushed along inside a cylindrical tube called the barrel, allowing the
syringe to take in and expel a liquid or gas through an orifice at the open end of the barrel.

Fig. 2.30

The open end of the syringe may be fitted with a hypodermic needle, a nozzle or tubing to
help direct the flow of liquid or gas into and out of the barrel. Syringes are generally used to
administer injections, insert intravenous drugs into the bloodstream, apply glue or lubricant,
and measure liquids.
The word "syringe" is derived from the Greek word "syrinx" meaning "tube". Hypodermic
syringes are used with hypodermic needles to inject liquid, or gas, into body tissues or to
remove it from the body.
The barrel of a syringe is made of plastic or glass. It usually has graduated marks that indicate
the volume of fluid in the syringe. Most medical syringes are made of plastic with a rubber
piston. Medical syringes are sometimes used without a needle for orally administering liquid
medicines to young children or animals, or milk to young animals.

When the piston of the syringe is pulled out by inserting the nozzle into a medicine or any
liquid, partial vacuum is created inside the barrel or storage cylinder. As a result, medicine
or liquid flows into the storage cylinder. When the piston is pushed inside, it creates high
pressure in the storage cylinder and expels the medicine or liquid.

2.20 Hand Pump or Water Pump

A hand pump is a manually operated pump which uses human power and mechanical
advantage to lift water. Hand pumps are widely used in rural plain areas to lift water vertically
upward from the ground. There are different types of hand pumps. Among them, the sunction
and lift hand pumps are widely used.

Handle

Barrel

Fig. 2.31 (a) Fig. 2.31 (b)

PHYSICS Oasis School Science - 10 45

Sunction and lift are important considerations while pumping fluids. Sunction is the vertical
distance between the fluid to be pumped and the center of the pump. Similarly, lift is the
vertical distance between the pump and delivery point. The depth from which a hand pump
will suck is limited by atmospheric pressure to an operating depth of less than 7 meters.

A water pump consists of two valves V1 and V2. The valve V1 moves up and down along with
the piston, and valve V2 is connected to the base of the barrel. Similarly, the barrel is connected
to a pipe, and the pipe is inserted to the ground. When the handle is pushed down, the piston
moves upward, which creates a vaccum between the valves V1 and V2. In this state, valve V1
closes and water enters in the barrel due to atmospheric pressure.

The process by which underground water enters the barrel of the hand pump is called
upstroke. When the handle is pulled upward, the distance between V1 and V2 decreases which
causes increase in the pressure in value V2 and gets closed. As a result, valve V1 opens due to
water pressure, and water comes out.

2.21 Air pump or Bicycle pump

A bicycle pump or air pump is a type of positive displacement pump
specifically designed for inflating bicycle tyres. It consists of three
main parts. They are the piston, cylinder and nozzle.

A bicycle pump functions via a hand operated piston. During the
upstroke, this piston draws air through a one-way valve into the
pump from the outside. During the down-stroke, the piston then
displaces the air from the pump into the bicycle tyre.

The bicycle pump compresses air. When the cylinder is compressed, Fig. 2.32
air is pushed down the tube of the pump and then into the tube via

the valve, which is forced open by the pressure of the air. When the handle is pulled up again,

the valve shuts off automatically so that the air cannot escape from the tyre, and new air is

forced back into the cylinder. So, the process can be repeated. Once the tyre is at the correct

pressure, the pump valve can be removed from the tyre, and the cap can be replaced on the

tyre.

Fig. 2.33
46 Oasis School Science - 10 PHYSICS

SUMMARY

• The thrust per unit area of a surface is called pressure. Its SI unit is pascal
(Pa) or N/m2.

• The total perpendicular force exerted by a body on the surface in contact is
called thrust. Its SI unit is N.

• The pressure is said to be 1 pascal when 1 newton force is acting on 1m² surface
area.

• The thrust exerted by a liquid per unit area of the surface is called liquid
pressure.

• Pascal's law states, "Pressure is transmitted equally in all directions when
pressure is applied at a place on a liquid kept in a closed container."

• The resultant thrust exerted by a liquid is called upthrust.
• Archimedes' principle states, "When a body is partially or wholly immersed in

a liquid, it experiences an upthrust which is equal to the weight of the liquid
displaced by it."
• Law of flotation states that, "A body floats on a liquid if it can displace the liquid
equal to its own weight."
• Mass per unit volume of a substance is called density. Its SI unit is kg/m3.
• The ratio of the density of a substance to the density of water at 40C is called
relative density or specific gravity.
• A barometer is a scientific instrument which is used to measure atmospheric
pressure.
• A syringe is a simple pump consisting of a plunger that fits tightly in a tube.
• A hand pump is a manually operated pump which uses human power and
mechanical advantage to lift water.
• A bicycle pump, or air pump, is a type of positive displacement pump
specifically designed for inflating bicycle tyres.

PHYSICS Oasis School Science - 10 47

Exercise

Group-A
1. What is pressure?
2. Write down formula and SI unit of pressure.
3. What is thrust? Write down its SI unit.
4. Name the two factors on which the pressure exerted by a body depends?
5. What is hydraulic pressure?
6. On which factors does hydraulic pressure depend? Write.
7. State Pascal’s law of liquid pressure and write any one equipment based on this law.
8. On which law is the given diagram based? State the law.

B = 375N

250N C = 10 cm2

A=20 cm2

9. What is hydraulic press?

10. Write down the principle of hydraulic press.

11. What is hydraulic brake?

12. Write down the use of hydraulic lift.

13. Name the equipment shown in the given diagram. Also, write down the principle on
which this equipment works.

20N F2

A B
320cm2
8 cm2

14. Why is hydraulic jack used? Write.
15. State Archemedes' principle.
16. Name any two instruments based on Archimedes’ principle.
17. What is atmospheric pressure?
18. What is barometer?
19. What is syringe?
20. What is upstroke?
21. What is down stroke?

48 Oasis School Science - 10 PHYSICS

Group-B
1. Write two differences between force and pressure.
2. Write two differences between pressure and upthrust.
3. Write two differences between Pascal’s law and law of flotation.
4. Write two differences between Pascal’s law and Archimedes’ principle.
5. Write two differences between density and relative density.
6. An iron nail sinks in water but a ship made up of iron floats. Why?
7. An egg floats on saturated salt solution but sinks in fresh water. Why?
8. An iron nail sinks in water but floats on mercury. Why?
9. On which law the given experiment is based? Write with reason.

Spring balance

20N

Stone Ureka
Water can

Beaker
5N

9. It is easier to swim on the sea than on the pond, why?
10. What is the difference between an empty ship and loaded ship moving on the sea water?
11. An egg sinks in pure water but floats on solution of salt with water, why?
12. Why is it easier to lift a heavy stone under water than in air?
13. In the given figure, which is sea water and which is river water, why?

Egg

Water

Fig-1 Fig-2

14. What will be the effect in the weight of displaced water if load is added into the ship
floating in the ocean? Write with reason.

15. Although a ship is made up of iron, it can float on water, why?

PHYSICS Oasis School Science - 10 49

Group-C
1. On which principle does hydraulic brake base? Prove that the hydraulic press is a force

multiplier.
2. Why does a ball try to come up when it is pressed in water?
3. Answer the following questions after observing the given diagram of hydraulic press.
i) On what principle does this machine work? State the principle.
ii) What role is played by water to multiply the force in this device?

Load

Piston

Water

Hydraulic Press

4. Describe in brief the working mechanism of a syringe.
5. Describe the working mechanism of a water pump with figure.

Group-D
1. Study the given figure and answer the following questions.

Spring balance

20N

Stone Ureka
can

Water

Beaker
5N

i) On which principle is the experiment based? State the principle.

ii) What is the weight of displaced water? (Ans: 5N)

iii) What is the reason that the weight of the object is less in water than in air? A
B

2. The ice made of water floats on water, why? Which instrument

is shown in figure? If cross-sectional area of piston A is 40cm2

and that of piston B is 4m2, what load is necessary on the piston

‘B’ to balance 600N force kept on piston A?

(Ans: 6 × 105N)

Liquid

50 Oasis School Science - 10 PHYSICS

3. Three objects with different densities A, B and C are in water A
as shown in the figure. Now, answer the following questions:
B
i) Which object has higher density than the water? Why? C

ii) If the mass of object A is 1 kg. How much kg of water is
displaced by the object?

iii) Which law is applicable for object B? State the law.

4. A ship coming from sea enters the river, will its hull sink more or less in river water?
Give reason. The weight of an object is 20N in air, the weight of that object in the water
is seen 12N only then:

i) What is the value of upthrust on the object by water? (Ans: 8N)

ii) What is the weight of the displaced water by it? (Ans: 8N)

5. Study the given table and answer the following questions:

Substances Density (in g/cm3)

W 0.8

X 13.6

Y1

Z 0.9

i) If volume of all substances is equal, which substance has the greatest mass?

ii) Among the substances in the table, which substance sinks in water, why?

iii) Among the given substances, name the substance that gives the least upthrust. Give
reason also.

PHYSICS Oasis School Science - 10 51

3UNIT Estimated teaching periods

Theory 5

Practical 1

ENERGY
Objectives
Albert Einstein
After completing the study of this unit, students will be able to:
(1896–1955 AD)

• define energy and describe the sources of energy with their uses.

• describe the sun as the main source of energy.

• identity the causes of the energy crisis and state the methods of solving it.

• describe the methods of conservation of energy with examples.

3.1 Introduction

We have many sources of energy, such as the sun, wind, flowing water, coal, gasoline, diesel,
natural gas, biogas, etc. We utilize energy to perform a wide range of activities, e.g., industrial,
commercial and household, etc. Energy is defined as the capacity to do work. It is measured
in joule, calorie, etc. Energy is required to pull, push and lift objects while doing various
works. Energy produces force, and force performs work. We need energy to operate different
machines. Energy governs all the activities on this planet. It has a vast range of applications
like biological, physical, agricultural, industrial, etc. According to the law of conservation of
energy, "Energy can neither be created nor destroyed, but it can be transformed from one form
to another."

3.2 Types of Energy Sources Reasonable Fact-1

The natural sources of energy that are used in Fossil fuel is known as non-renewable
the same form as they are available are called source of energy, why?
primary sources of energy. These sources
of energy can be broadly classified into two Ans: Fossil fuel is called non-renewable
categories as follows: source of energy because it gets depleted
and cannot be produced again and again
1. Non-renewable sources of energy as it has a limited stock in nature.

2. Renewable sources of energy

1. Non-renewable sources of energy: Energy sources like coal, petroleum and natural gas
are natural sources which have accumulated over a period of millions of years. Coal,
petroleum and natural gas are also known as fossil fuel, as they were formed from the
dead remains of plants and animals that existed on the earth millions of years ago. The

fossil /ˈfɒsl/ - the dead remains of an ancient plant or animal
deplete /dɪˈpliːt/ - to reduce something by a large amount
exhaust /ɪɡˈzɔːst/ - to use all of something so that there is none left

52 Oasis School Science - 10 PHYSICS

conversion of dead organic matter into fossil fuels occurs over a long period of time.
However, the rate at which we are using these fuels is high, when compared to the rate at
which they are formed in nature. So, their reserves available to us are depleting quickly.
Non-renewable sources of energy are formed in nature after millions of years but deplete
with use over time. Their replacement is not possible at least over a human life span.
The type of energy sources which get depleted and cannot be replaced quickly when
exhausted are called non-renewable sources of energy. For example, coal, petrol, diesel,
etc. Such energy sources have limited stock in nature. They are also called conventional
sources of energy.

2. Renewable sources of energy: Energy sources such as the sun, flowing water, wind,
tides, biogas, etc. are inexhaustible. Some of these sources of energy are abundant in
nature, and they will be available for billions of years, e.g. solar energy . Other sources
such as wind and tides are phenomena which regularly occur in nature. The energy
sources that are being produced continuously in nature and are inexhaustible are called
renewable sources of energy. For example, solar energy, wind energy, biomass energy,
tidal energy, etc. These sources of energy will never get exhausted and can be used again
and again.

Differences between Renewable and Non-renewable sources of energy

S.N. Renewable sources of energy S.N. Non-renewable sources of energy

1. Those energy sources which are 1. Those energy sources which are not

obtained continuously are called obtained continuously are called non-

renewable sources of energy. renewable sources of energy.

2. These sources of energy are 2. These sources of energy cannot be formed

formed in a short period of time. in a short period of time and take millions

of years.

3. They never get exhausted from 3. They get exhausted from nature after their

nature. use.

3.3 The Sun: An Ultimate Source of Energy Fig. 3.1

The sun is the main source of energy. It is an extremely large mass
of hot gases. It provides energy for all living beings on the earth
directly or indirectly. The sun provides both heat and light energy
for the living organisms. The sun is a store house of energy. The
energy produced from the sun is called solar energy. Solar energy
is produced due to the nuclear fusion reaction going on inside it all
the time. All of other energy sources are the outcome of the solar
energy. Let us see how the solar energy helps to form other sources
of energy.

Fact File - 1

It has been estimated that the surface temperature of the sun is about 57000C, and the temperature in
the core of the sun is about 13 million degrees Celsius.

ultimate /ˈʌltɪmət/ - from which something originally comes

PHYSICS Oasis School Science - 10 53

Fact File - 2

The sun radiates about 2.7×1014kW of energy per second, and about one part of the 2.2
trillion solar energy reaches the earth. One square meter area of the earth receives 1.4kW
of energy per second.

a. Wind energy is the outcome of solar energy
The heat of the sun helps to change the pressure of air at different places. This change in

pressure causes the air to move. In this way, solar energy helps to form wind energy.

b. Fossil fuel is the outcome of solar energy
The plants and animals buried under the earth's crust long ago have been converted into

fossil fuels like coal, petroleum oil and natural gas. The energy from the fossil fuel can be
used to produce heat energy, electrical energy and mechanical energy. Thus, the energy
stored in fossil fuels is also an outcome of solar energy.

c. Hydroelectricity is the outcome of solar energy

The water from the sea, river, etc. evaporates due to the heat of the sun and lifts the water
high into the sky in the form of clouds. When this water falls back to the earth in the form
of rain (or snow), it is stored in high dams. This water is used to rotate the turbine for
producing hydroelectricity. Thus, solar energy helps to produce hydroelectricity.

Solar energy is produced due to thermonuclear fusion reaction. This is the nuclear reaction
in which light nuclei combine to form a single heavy nucleus at very high temperature
and pressure. Solar energy consists of electromagnetic radiations (such as x-rays, infra-
red rays, UV-rays, visible light, etc.). The sun emits energy in all direction in space.
However, only a small fraction of this energy reaches the earth and other planets. The
ultraviolet (UV) rays are prevented from entering the earth's atmosphere by the ozone
layer. Thus, solar radiations are rich in infra-red rays and visible rays when they spread
over our planet. Infra-red rays are responsible for the heating up of objects. Technological
progress has made humans competent enough to harness this invaluable gift of nature,
i.e., solar energy, by inventing different devices, such as the solar cooker, solar water
heater, solar cell, solar water pump, solar toys, solar batteries, etc. The solar energy is not
equally received in all parts of the country. The Terai region of Nepal receives more solar
energy per square meter than that received by the Kathmandu Valley. Similarly, more
heat is felt in the Terai region than the Himalayan region of Nepal.

3.4 How is Enormous Energy Produced by the Sun?

The sun provides all other forms of energy due to a reaction called thermonuclear fusion
reaction. In this reaction, two or more light nuclei combine to form a single heavy nucleus
at very high temperature and pressure, e.g., formation of the helium nucleus from hydrogen
nuclei. The following is the most possible reaction in the sun among many nuclear reactions.



54 Oasis School Science - 10 PHYSICS

1 H1 + 1H1 → 1H2 + 1eo

1 H2 + 1H1 → 2He3 + γ − radiation

2 He3 + 2He3 → 2He4 + 1H1 +1 H1 + energy
41 H1 → 2He4 + 21 e0 + γ − radiation + energy

Simply,

41 H1 →2 He4 + energy

In the 1st step of the above reaction, two protons (1H1) combine to form a deuteron (1H2)
and positron (1eo). Then the deuteron thus formed combines with one more proton (1H1) and
produces a light helium atom (2He3) and gamma radiation. Finally, two light helium atoms
combine to form a stable form of helium (2He4) along with two protons and energy. During
this process, a vast amount of energy is produced. All this energy is released in the form of
heat and light. This energy can be calculated by Einstein's mass energy relation, i.e., E = mc2.

Particles formed during the thermonuclear fusion reaction

Isotopes

Isotopes are the atoms of an element having the same atomic number but different
mass number. Isotopes can also be defined as the atoms of the same element
containing the same number of protons but different numbers of neutrons in their
nuclei.

Isotopes of hydrogen

Hydrogen has 3 isotopes having the same atomic number (1) but different mass
number (1, 2 and 3) which are as follows:

1p+ 1p+ 1p+
0 n0 1 n0 2 n0

Protium (1H1) Deuterium (1H2) Tritium (1H3)

These isotopes give the respective nuclei:

1p+ 1p+ 1p+
0 n0 1 n0 2 n0

Proton Deuteron Triton

Mass number = No. of protons + No. of neutrons
Atomic number = No. of protons

Positron (1eo)

After the combination of proton (1H1) in the 1st step, a particle called positron (1eo) is
formed which can also be called positive electron. A positron is a positively charged

particle having 1 unit positive charge but mass equal to that of an electron.

PHYSICS Oasis School Science - 10 55

According to German physicist Albert Einstein, mass and energy are equivalent. During the
nuclear reaction some mass is lost. The lost mass is converted into energy. This energy can be
calculated by the famous equation called mass-energy relation which is given below.

E = mc2

Where, E = energy produced
m = lost mass (also called mass defect)
c = speed of light, i.e., 3 × 108 m/s

Worked out Numerical 1

Calculate the amount of energy produced when 1 g mass is lost during a nuclear reaction.

Solution: Mass (m) = 1g = 1 kg
Given, 1000

Speed of light (c) = 3 × 108 m/s

Energy produced (E) = ?

According to Einstein’s mass-energy relation,

E = mc2

= 1 × (3 × 108 )2
1000

= 3 × 1013 J
∴ The amount of energy produced is 3×1013 J.

Conditions required for nuclear fusion reaction in the sun
i. In the sun, there is sufficient amount of hydrogen gas.
ii. There is very high temperature in the sun, which is essential for the formation of free

protons.
iii. There is necessary pressure for the combination of free protons.

3.5 Fossil Fuel

The remains of animals and plants which were buried millions

of years ago are called fossils. The fuel formed from the fossils of

plants and animals buried under the earth's crust for a long time

is called fossil fuel. It includes coal, mineral oil and natural gas.

All these fuels are formed from the remains of plants and animals

that lived and died millions of years ago. So they are called fossil Fig. 3.2
fuels. In the past, the earth was covered with swamps. As the

dead plants and animals died, they slowly sank to the bottom of these swamps. During the

geological changes, the dead remains of plants and animals were compressed and squeezed

over millions of years. They finally turned into coal, petroleum and natural gas.

56 Oasis School Science - 10 PHYSICS

Coal

Coal is a solid form of fossil fuel. It is a hard black mineral found deep in the earth's surface.
It is one of the important fuels for the iron and steel industry, brick industry, cement industry,
etc. Coal is burnt to produce heat. Coal takes millions of years to form in nature and its
deposit, decreases when used continuously. Therefore, coal is considered as non-renewable
source of energy. There are different types of coal with varying carbon content as given in
the following table.

Types of coal Carbon percentage (%)
Peat 60
Lignite 70
Bituminous 80
Anthracite 90

Peat has the lowest percentage of carbon, and it is an inferior form of coal. Bituminous is the
most common form of coal. It is also known as the household coal. It produces more heat than
peat. Anthracite is a purest form of coal with the highest carbon content. It is a coal of superior
quality. In Nepal, a mine of high quality coal, i.e., anthracite has not been discovered so far.
However, mines of low quality coal have been discovered in Dang district.

Mineral oil

The liquid form of fossil fuel, such as petrol, diesel, Fact File - 3
kerosene, mobil, etc. is called mineral oil. It is used
in vehicles, industries, factories, etc. Mineral oil is The biggest oil fields are located in
obtained from rock traps beneath the earth by a Saudi Arabia and Kuwait. Mineral
drilling process. The oil thus obtained is crude and oil takes millions of years to form in
is purified to get various forms of oil like petrol, nature. Therefore, it is also called non-
diesel, kerosene, LP gas, etc. The residue of the renewable source of energy.

mineral oil is called tar, which is used to black top

roads. It is to be noted that raw materials for the detergents, plastic, synthetic chemicals, etc.

are also obtained from mineral oil. These days, the economy of any nation largely depends on

its mineral oil deposits. Hence, it is also known as black gold.

Advantages of fossil fuel (coal and mineral oil)
1. It is cheaper for long term production.
2. It can easily be transported from one place to another.
3. It is used for various purposes to run automobiles, generators, factories, etc.
4. It can be used to generate electricity.

5. It is more abundant in comparison to other fuels.

Disadvantages of fossil fuel
1. It produces smoke and toxic gases on burning, which pollute the environment.
2. It is a non-renewable source of energy. So, it gets depleted after use.

PHYSICS Oasis School Science - 10 57

3.6 Hydroelectricity

Flowing water consists of kinetic energy. Such Fig. 3.3 Hydropower plant
energy of water has been successfully used by
humans for rotating turbines to generate Fact File - 4
electricity. The electricity generated by rotating
turbines using water is called hydroelectricity. It Hydropower is the most potential
is one of the main sources of energy. It is a source of energy in Nepal. Our country
pollution free and renewable source of energy. is the second richest country after Brazil
Energy trapped in flowing water is used to in hydroelectric power potential due to
generate electricity on a large scale in hydroelectric its flowing water in many rivers. The
power stations. Dams are constructed to utilize estimated hydroelectricity generation
the kinetic energy of flowing water. Those dams capacity of Nepal is about 83000 MW.
obstruct the free flow of water in river and collect
in the form of an artificial lake. Consequently, the
kinetic energy of water is transformed into
potential energy. The water is made to fall from a
height through pipes and made to run over the
blades of huge turbines at the bottom. This sets the
turbines into motion which produces electricity.
The figure shows the schematic representation of a
hydropower plant.

Advantages of hydropower
1. It is a renewable source of energy. So, it will never get depleted.

2. It is a pollution free source of energy.

3. It is cheaper in the long term although the initial cost is high.

4. It is widely used in electric and electronic devices, like computers, television sets,
bulbs, CFL, etc.

Reasonable Fact-2

Write two reasons why higher priority is given to the production of hydroelectricity
in Nepal.

Ans: Two reasons for giving higher priority in Nepal for the production of
hydroelectricity are:

(i) Hydropower is the most potential source of energy in Nepal. Nepal is the second
richest country after Brazil in hydroelectric power potential due to its many rivers.

(ii) Hydroelectricity is cheaper in the long term. It is a renewable and pollution free source
of energy.

abundunt /əˈbʌndənt/ - existing in large quantities, more than enough

turbine /ˈtɜːbaɪn/ - a machine or an engine that receives its power form a wheel that is turned by the pressure of water

58 Oasis School Science - 10 PHYSICS

Reasonable Fact-3

Give two reasons to justify that the use of hydroelectricity should be increased more than
that of fossil fuel energy.

Ans: Two reasons to justify that the use of hydroelectricity should be increased more than
that of fossil fuel energy:

(i) Hydroelectricity is a pollution-free source of energy, while fossil fuel causes air
pollution when used.

(ii) It is a renewable source of energy.

3.7 Alternative Sources of Energy

Those sources of energy which are used to reduce the load of non-renewable sources of
energy are called alternative sources of energy. Such sources of energy are used to replace the
conventional sources of energy. There are various types of alternative sources of energy, but
here we discuss only the following five types:

1. Biofuel 2. Nuclear energy

3. Tidal energy 4. Wind energy 5. Geothermal energy

1. Biofuel

The organic source of energy that can be obtained from biomass, wood, hay, straw, chaff,
etc. is called biofuel. The decay of garbage, cow dung, sewage and other plant residue in the
absence of air produces biogas. Biogas is a mixture of methane, carbon dioxide, hydrogen and
hydrogen sulphide. It is an excellent fuel in our villages where people raise cattle. People also
use the dry form of cattle dung, called 'guintha', as fuel. It creates less pollution than firewood.
After using the plants as biofuel, afforestation should be conducted.

Nepal is an agricultural country. About 80% of the people of Nepal are involved in
farming. Cattle dung, straw and other plant products are easily available in our country.
These materials can be used to produce biogas. Similarly, electricity can also be generated
from biogas. The by-product of biogas is a good manure to increase the fertility of soil.
The energy obtained from biomass is cheap and pollutionless. Therefore, biofuel, or
biomass energy, is the best source of alternative energy in the context of Nepal.

The Government of Nepal is also providing help and subsidy for the construction of biogass
plants. The use of biogas is more effective in the rural areas of Nepal. In our country, the
sources of fuel, such as wood, straw, hay, chaff, etc. are used for domestic purposes. In the
rural areas, the dung of cattle is dried in the sun to make dung patties, or guintha, and is
used for cooking food.

Preparation of biogas

Biogas is produced by the anaerobic decomposition of animal waste (like animal dung)

or plant waste in the presence of water. Anaerobic bacteria decompose these waste

materials in the presence of water and convert them into methane and some other gases.

In this way, biogas is produced in biogas plants. The construction of biogas plants is shown

below:

chaff /tʃɑːf/ - the outer covering of seeds of grain, straw, etc.

PHYSICS Oasis School Science - 10 59

Outlet of biogas Outlet Slurry of dung Outlet of
Gas tank and water biogas
Slurry
Overflow
Inlet pipe tank

Outlet pipe

Slurry Digester Partition wall

a) Fixed dome type b) Floating gas-holder type
Fig. 3.4 Biogas plants

Major uses of biogas
1. Biogas is used as a fuel for cooking purpose.

2. It is used as a fuel to run engines.

3. It is used for generating electricity or getting light.

Fact File - 5

Why does the Government of Nepal encourage farmers to establish biogas plants? Write
any two reasons.

Ans: The Government of Nepal encourages farmers to establish biogas plants for the
following reasons:

i. Biogas is a renewable and cheap source of energy and can be generated easily.
ii. The by-product of biogas is a good manure, which increases the fertility of the soil.

Advantages of biogas

1. Biogas burns completely without smoke, and hence it does not cause air pollution.
So, it is also called a clean fuel.

2. It produces more heat while burning.

3. It is cheaper and can be produced easily.

4. It can be used to generate electricity.

2. Nuclear Energy

The nucleus of an atom is a store house of energy. The nucleus is the central portion of
an atom, where most of its mass is concentrated. Electrons, the small pockets of energy,
revolve round the nucleus in shells. The nuclear components are two types of particles,
i.e., protons and neutrons, each of which are nearly 2,000 times heavier than an electron.
The energy present in the nucleus of an atom is called nuclear energy. Nuclear energy is
produced during two reactions: nuclear fusion and nuclear fission.

fission /ˈfɪʃn/ - the process of splitting the nucleus of an atom

60 Oasis School Science - 10 PHYSICS

(i) Nuclear fusion

Nuclear fusion is the nuclear reaction in which the lighter nuclei fuse or combine to form
a heavier nucleus. This reaction is accompanied by the release of tremendous amount
of energy. The most evident example of nuclear fusion is the release of solar energy. A
nuclear fusion reaction cannot be controlled and requires a very high temperature and
pressure. Hydrogen bombs are made on the basis of nuclear fusion reaction.

(ii) Nuclear fission

The nuclear reaction in which an unstable nucleus of a heavy atom (like uranium-235)
splits into two or more lighter nuclei with the liberation of a large amount of energy is
called nuclear fission. Such a splitting may be spontaneous or may occur when an atom
is struck by a particle, such as a neutron. This reaction is always followed by the release
of a tremendous amount of energy in the form of heat. Nuclear fission was first observed
when a uranium atom was seen to split into two smaller nuclei as it was hit by a neutron.
Upon splitting, along with two smaller atoms, it releases three fast-moving neutrons.
These neutrons in turn cause three more uranium atoms to split. The process continues,
and if uncontrolled, it results in a series of reactions involving continuous splitting of
uranium atoms (called a chain reaction), causing a gigantic explosion and producing
unimaginable heat.

U235 + 0n1 Fission 56Ba139 + 36 Kr94 + 30n1 + Energy

92

(Uranium) (neutron) (Barium) (Krypton) (neutrons)

nU Smaller atoms or nuclei called n Nuclear fission
Neutron 'fission fragments' Un U
n U
Uranium atom n 3 fast-moving neutrons n U
n

n n U
n Un Un U

n n U
U
n U
Un U

n

Fig. 3.5 A chain reaction showing the splitting of uranium atoms

An atomic bomb also works on the principle of chain reaction. In 1945, the USA dropped
two such bombs on Hiroshima and Nagasaki in Japan on the 6th and 9th of August
respectively. It led to massive destruction of both lives and property. The radiation
released during those blasts not only killed people but also produced physical and
genetic deformities in subsequent generations.

Nuclear fission, if carried out in a controlled manner, is a source of energy that can be
utilized for a constructive purpose like the generation of power or electricity. Such a
controlled reaction is brought about in a nuclear reactor. The released energy in nuclear
power plants heats up water to produce steam. The steam produced sets the turbine into

PHYSICS Oasis School Science - 10 61

motion, and electricity is generated. Those substances which are used to produce nuclear
energy are called nuclear fuel, e.g., uranium, plutonium, etc.

Differences between Nuclear fission and Nuclear fusion reaction

S.N. Nuclear fission reaction S.N. Nuclear fusion reaction

1. The reaction in which a single heavy 1. The reaction in which two or more

nucleus splits into two or more light nuclei combine to form a single

light nuclei is called nuclear fission heavy nucleus is called nuclear fusion

reaction. reaction.

2. It can be controlled. 2. It cannot be controlled.

3. An atom bomb is made on the basis 3. A nuclear bomb is made on the basis of

of this reaction. this reaction.

4. It occurs in the form of a chain 4. It does not occur in the form of a chain

reaction. reaction.

5. In this reaction, comparatively less 5. In this reaction, comparatively more

energy is produced. energy is produced.

3. Tidal Energy

The energy generated in oceans or seas when the tides rise and fall is called tidal
energy. It is a renewable source of energy. Tides are caused due to a combination of the
gravitational forces between the earth, the moon and the sun. These waves of water, i.e.,
high and low tides are rich in kinetic energy, which can be utilized to rotate turbines
located in dams to generate electricity. Dams are built across a narrow opening of the sea
to harness tidal energy for the production of electricity.

Fig. 3.6 Electricity generated from tides of the sea

4. Wind Energy

Moving air is called wind. It has kinetic energy. This energy is utilized for rotating wind
mills to generate electricity. The energy produced by the fast-moving wind is called wind
energy. The sun is responsible for the generation of this form of energy as it governs the
movement of air in the atmosphere. Wind energy is used to rotate turbines to generate
electricity.

62 Oasis School Science - 10 PHYSICS

Fact File - 6

Nowadays, wind mills are operated by
wind energy to generate electrical and
mechanical energy. Investigations have
revealed that a minimum velocity of 15
km/h is required to keep a wind mill
operational.

Fig. 3.7 Wind mill

Human beings have been using wind as a source of energy for ages. In the olden days,
it was used for sailing, pumping water, removing husk from the grains and for grinding
grains or milling.

5. Geothermal Energy

The heat energy obtained from the hot rocks present Fig. 3.8
inside the earth is called geothermal energy. This
energy is obtained by trapping the heat of the earth
itself, usually from kilometers deep in the earth's
crust. It is expensive to build a power station, but the
operation costs are low, resulting in low energy costs
for suitable sites. Ultimately, the energy is obtained
from heat in the earth's core.

Geothermal energy from the core of the earth is closer to the surface in some areas than in
others. In places where earthquakes and volcanoes are active, the temperature increases
by 800C per kilometer depth in the earth's crust whereas the temperature increases by
300C per kilometer depth in normal regions. The hot underground steam or water can be
trapped and brought to the surface and is used to generate electricity. Such geothermal
power sources exist in certain geologically unstable parts of the world, such as Iceland,
New Zealand, United States, Philippines and Italy. Geothermal energy can also be obtained
from hot dry rocks. Holes at least 3 km deep are drilled into the earth. Some of these holes
pump water into the earth, while other holes pump hot water vapour out. This hot water
vapour is used to run steam engines for rotating turbines to generate electricity.

6. Solar Energy

Energy which can be obtained from the sun is called

solar energy. Solar energy is taken as the most

important alternative source of energy as it is available

in all parts of the earth. The light of the sun is converted

into electrical energy with the help of a solar panel.

Solar energy can be used to get light, to heat water, to

lift water, to cook food, etc. Solar energy is a very good Fig. 3.9

alternative source of energy to push the energy crisis

further. Therefore, we should spread the use of solar energy in Nepal.

PHYSICS Oasis School Science - 10 63

3.8 Present Status of Use of Energy Sources

In the present day world, about 35% of the world's energy demand is being fulfilled by mineral
oil and 27% by coal. The demand for energy sources is increasing at a rate of 2.3 % every
year. Similarly, biomass fulfills about 13% of the world's demand for fuel. Hydropower fulfills
about 5% and nuclear reaction supplies about 3% of the demand for fuel. If energy sources are

not properly managed, there will be an energy crisis in the world in the near future.

3.9 Energy Crisis

An energy crisis is a situation in which there is an Fact File - 7
acute scarcity of energy sources. The major source
(about 80%) of energy for the growing population If the demand of the energy grows by
of the world is fossil fuel. Thus it is very difficult 2.3% every year, the remaining mineral
to meet the increasing demand for energy only by oil lasts upto 2037 AD only. So, the
fossil fuels. So, the acute scarcity of energy sources alternative sources of energy must be
due to overuse by the growing population is called developed for use in the future.
energy crisis.

The main causes of energy crisis are:

(i) High demand for energy sources as a result of population growth
(ii) Over use of non-renewable sources of energy
(iii) Lack of developing and using alternative and renewable sources of energy

(iv) Advancement and technological inventions

In the very near future, the world will have to face an acute scarcity of energy sources. The
demand and use of energy sources are increasing day by day with the pace of development.
So, scientists are busy developing alternative sources of energy. Some of the methods for
solving energy crisis are:

(i) Developing and using alternative energy sources to replace the conventional
energy sources

(ii) Developing and using devices which can be operated by renewable sources of
energy

(iii) Using existing sources of energy wisely and economically
(iv) Conserving available sources of energy and avoiding unnecessary use of fossil fuels

Reasonable Fact - 4

The development of hydropower helps to push the energy crisis further. Justify.

Ans: Hydropower or hydroelectricity is the only major source of energy for various
electronic devices, industries, machines, etc. Modern light and heat producing appliances
are operated by electricity. Hydropwer is a renewable source of energy which is never
depleted. Therefore, we can say that the development of hydropower helps to push the
energy crisis further.

64 Oasis School Science - 10 PHYSICS

SUMMARY

• Energy is defined as the capacity to do work. It can neither be created nor
destroyed, but it can be transformed from one form to another.

• The type of energy sources which get depleted and cannot be replaced
quickly when exhausted are called non-renewable sources of energy.

• Energy sources that are being produced continuously in nature and
inexhaustible are called renewable sources of energy.

• Solar energy consists of electromagnetic radiations, such as x-rays, infra-red
rays, UV-rays, visible light, etc.

• The fuel formed from the fossils of plants and animals is called fossil fuel.
• Coal is burnt to produce heat. Coal takes millions of years to form in nature

and it decreases when used continuously. Therefore, coal is considered as a
non-renewable source of energy.
• The liquid form of fossil fuel, such as petrol, diesel, kerosene, mobil, etc. is
called mineral oil.
• Those sources of energy which are used to reduce the load of non-renewable
sources of energy are called alternative sources of energy.
• The energy present in the nucleus of an atom is called nuclear energy. Nuclear
energy is produced during two reactions: nuclear fusion and nuclear fission.
• Nuclear fusion is the nuclear reaction in which the lighter nuclei fuse or
combine to form a heavier nucleus.
• The nuclear reaction in which an unstable nucleus of a heavy atom (like
Uranium-235) splits into two or more lighter nuclei with the liberation of a
large amount of energy is called nuclear fission.
• Nuclear fission, if carried out in a controlled manner, is a source of energy
that can be utilized for a constructive purpose like the generation of power or
electricity.
• The energy produced by the fast-moving wind is called wind energy. It is
used to rotate turbines to generate electricity.
• The heat energy obtained from the hot rocks present inside the earth is called
geothermal energy.
• The acute scarcity of energy sources due to overuse by the growing population
is called energy crisis.

PHYSICS Oasis School Science - 10 65

Exercise

Group-A
1. What is energy? Write down its unit in SI system.
2. What are various types of energy sources? Name them.
3. What is renewable source of energy?
4. Give any two examples of renewable source of energy.
5. What is non-renewable source of energy?
6. How much part of the solar energy reaches the earth?
7. What is nuclear reaction? Give one example of nuclear reaction.
8. What are two conditions required for nuclear fusion reaction that takes place in the sun?
9. What is fossil fuel energy? Give example.
10. What is mineral oil?
11. What is hydroelectricity? What is the estimated hydroelectric potentiality of Nepal?
12. Name any two alternative sources of energy.
13. What is biomass?
14. Define biomass energy.
15. What is biofuel?
16. What is biogas?
17. What is nuclear fission reaction?
18. What is tidal energy?
19. What is wind energy?
20. What is geothermal energy?
21. What is energy crisis?
22. List any two causes of energy crisis.
23. Write any two natural phenomena from which geothermal energy can be obtained.
24. Name any two main sources of energy used in the world.

Group-B

1. The sun is considered as an ultimate source of energy. Justify this statement.

2. What is thermonuclear fusion reaction? Write with chemical equation.

3. The use of hydroelectricity should be increased rather than that of mineral oil. Justify
this statement.

4. What do you mean by alternative sources of energy? Write with examples.
5. Write two differences between renewable sources of energy and non-renewable sources

of energy.
6. Write two differences between nuclear fusion reaction and nuclear fission reaction.
7. Write two differences between fossil fuel energy and biofuel.
8. The demand of fossil fuel is increasing day by day in the present day world. Why?
9. State any two causes of energy crisis.

10. The higher priority has been given for the use of biogas energy in Nepal. Give two
reasons.

66 Oasis School Science - 10 PHYSICS

11. Justify with two reasons that the hydroelectricity is the best alternative source of energy
in the context of Nepal.

12. In the present world, the use of hydroelectricity is increasing. Write any two reasons.

13. The use of alternative source of energy helps to control the energy crisis, how?

14. “Nepal has high potentiality of producing hydroelectricity”. Give any two reasons to
justify this statement.

15. Write two causes that Nepal Government has given higher priority for the production of
hydroelectricity.

Group-C

1. The generation and use of hydroelectricity is spreading throughout the world. Justify
this statement by giving any three reasons.

2. The development and use of alternative energy sources is very necessary in present day
world. Explain this statement.

3. Biomass energy is the best source of alternative energy in the context of Nepal. Explain
this statement with reason.

4. How is nuclear energy produced? Describe in brief.
5. Write any three ways to solve the situation of energy crisis in the context of our country.

6. Why should energy be conserved? Mention any three steps you would take at home to
conserve energy?

7. In future, the hydroelectricity will be main alternative source of energy for Nepal. Justify
it with three reasons.

8. How is the enormous amount of energy produced in the sun? Describe it with chemical
equation.

Group-D

1. How is energy produced from nuclear fission reaction? Describe in brief.
2. Explain in brief about present status of energy use in the world and its conservation.

3. All the sources of energy present on the earth are the outcomes of solar energy. Justify
this statement with examples.

4. Government of Nepal encourages the farmers to establish bio-gas plant. Write any two
scientific reasons of it. In the context of Nepal, it is better to use energy like solar power.
Justify your answer with two reasons.

5. In the context of Nepal, both bio-fuel and solar energy are useful. Out of these two
sources of energy, which one will be more useful? Justify your answer with two reasons.

6. Write down two causes of extreme use of petrol, although it is a non-renewable source of
energy. Give the equation of process of formation of helium atom from hydrogen atoms
in the sun.

7. Justify giving four reasons that the use of hydropower should be increased than that of
coal and mineral oil for energy.

8. Why is the development and use of alternative energy sources necessary in today’s
world? Explain.

9. Explain the method of generating electricity utilizing the nuclear fission reaction.

10. How can we generate electricity from geothermal energy? Explain with figure.

PHYSICS Oasis School Science - 10 67

4UNIT Estimated teaching periods : 6

H E AT

Objectives James Prescott Joule
(1818-1889 AD)
After completing the study of this unit, students will be able to:

• define heat and differentiate between heat and temperature.
• describe specific heat capacity and derive the heat equation.
• explain the structure of thermometers and use them.
• solve simple problems related to heat and temperature.

4.1 Introduction

Every living organism on this earth requires heat. We cook our food by using heat. We dry our
clothes by using the heat of the sun. Heat is a form of energy which produces the sensation of
warmth. It makes substances hot. Heat is the cause of change in temperature. The temperature
of a body increases due to the application of heat. When heat is given to a substance, the kinetic
energy of the molecules increases, and they move with greater speed at higher temperature
and vice versa. The heat of a body is considered as molecular motion. Heat flows from a hot
body to a cold body. In the SI system, heat is measured in joule (J) and in CGS system, it is
measured in calorie (cal.). Heat energy is measured by using calorimeter.

Insulated lid Stirring stick

Inner vessel Insulated ring
Air space Outer vessel
Fig 4.1 Calorimeter

68 Oasis School Science - 10 PHYSICS

Definition of heat in terms of kinetic energy
Heat energy of a body is the total sum of kinetic energy contained by the molecules of the
body. The quantity of heat contained by a body depends on:

i. Number of atoms (or mass of the body)
ii. Average kinetic energy of molecules of the body
The quantity of heat contained by a body is directly proportional to the average kinetic energy
(KE) of the molecules and mass of the body, i.e.,

Quantity of heat ∝ average KE of molecules × mass of the body

The properties of the molecules of different matters are different. So, when an equal amount
of heat is given to an equal mass of different substances, the average kinetic energy of the
molecules is also different. Thus, they do not have the same temperature.

Heat is required to keep our body warm. Living organisms require heat energy to survive. Heat
is used to cook our food, run various means of transportation, such as vehicles, aeroplanes,
rockets, etc.

One calorie heat

The amount of heat required to increase the temperature of 1 gram of pure water by 1°C or 1
K is called 1 calorie heat. Similarly, 4200 J heat is required to change the temperature of 1 kg
mass of pure water by 1°C or 1 K.
From this relation, it becomes clear that 4.2 joules of heat energy is required to raise the
temperature of one gram of pure water by 10C or 1K. Thus, 1 calorie = 4.2 joules .

4.2 Effects of Heat

Heat is produced due to the vibration of molecules of a body. Heat can show the following
effects on different substances.

i. Heat changes the volume of a body.
ii. Heat changes the physical state of a body.
iii. Heat changes the solubility of a substance.
iv. Heat brings about a chemical change.
v. Heat changes the temperature of a body.

4. 3 Temperature

The degree of hotness or coldness of a body is called temperature. The normal temperature of
the human body is 98.6°F or 37°C. Temperature is measured in kelvin (K), degree Celsius (°C)
and degree Fahrenheit (°F). The temperature of a body is measured by using a thermometer.

anomalous / ə ˈ n ɒ m ə l ə s / - different from what is normal or expected

PHYSICS Oasis School Science - 10 69

Fig. 4.2 Thermometer

Fact File - 1

If an equal amount of heat is given to unequal masses of the same body, the temperature of
the body with less mass will be higher and vice-versa.

When heat is supplied to a body, it becomes hot and as the heat is removed, it becomes cold.
Obviously, a hot body has more temperature than a cold one.

Definition of temperature in terms of kinetic energy

The average kinetic energy of the molecules of a body is called the temperature of that body.
The temperature of a body is directly proportional to the molecular vibration of the body.

Activity 1
• Take two beakers of equal size and put different amounts of water in those beakers.
• Heat the beakers with the same sized lamp.
• Observe the reading of the thermometer dipped into these liquids after a certain
time interval. What do you observe ?

Thermometer

Stand Water Stand

Bunsen burner

Fig. 4.3
• The beaker having less amount of water has a higher temperature if an equal amount

of heat is supplied for an equal interval of time.

70 Oasis School Science - 10 PHYSICS

Activity 2

Take two beakers of the same size. Put 150 ml of mustard oil in one beaker and 150 ml of
water in another. Measure the initial temperature. Heat those liquids using an equal-sized
lamp for five minutes. Measure the temperature of each liquid every one minute. Which
liquid shows faster change in the temperature?

The rate of rise in temperature depends on the nature of the body.

The temperature of oil will increase faster than that of water. This is due to the nature of
the molecules of oil and water. The heat retention capacity of the molecules of these liq-
uids is different. So, they have different capacity to absorb and lose heat.

It is found from the experiment that 1 kg of water requires 4200 J of heat energy to raise
its temperature by 10 C. But 1 kg of oil requires less amount of heat (about 2000J) than that
of water to increase its temperature by 1º C.

Differences between Heat and Temperature

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

3. Heat is measured by a calorimeter. 2. Temperature is measured by a
4. thermometer.
The SI unit of heat is joule. 3.
5. The SI unit of temperature is kelvin.
Heat flows from a hot body to a cold 4.
body. Temperature gives the direction of flow
of heat.
Heat is the cause of change in 5.
temperature. Temperature is the effect of heat.

4.4 Measurement of Temperature

The device used for measuring the temperature of a body is called the thermometer. The
working principle of the thermometer is, "Liquid expands on heating and contracts on cooling."
Mercury and colored alcohol are generally used as thermometric liquids. Those liquids are
used in the bulb of a thermometer.

Now, let us observe the boiling and freezing points Fact File - 2
of alcohol and mercury. Which of the following
thermometric liquid is the right liquid to measure Alcohol is a cheap and easily
100°C? Why? available thermometric liquid.
Its volume increases six times
Substances Freezing point Boiling point more than that of mercury. It is a
1. Mercury – 39°C 357°C colorless liquid. So, it is colored to
2. Alcohol – 115°C 78° C make it visible.

PHYSICS Oasis School Science - 10 71

A thermometer consists of a glass tube in which a bulb is attached with a capillary tube. The
bulb of the thermometer is filled with a thermometric liquid. When heat is supplied to the
bulb, the liquid in the bulb expands and rises up in the capillary tube. After sometime, it
shows a constant reading, and the reading shows the temperature of the body. The structure
of a laboratory thermometer is shown below:

Scale Capillary tube
Upper fixed point
Bulb containing
thermometric liquid

Lower fixed point Glass stem

Fig. 4.4 Laboratory thermometer

Reasonable Fact-1 Reasonable Fact-2

The boiling temperature of water cannot Write two reasons for using mercury as a
be measured with the help of an alcohol thermometric liquid.
thermometer, why?
Ans: Mercury is used as a thermometric liquid
Ans: The boiling point of alcohol is 780C whereas because:
the boiling point of water is 1000C. Since the boiling i. It is a good conductor of heat.
point of water is more than that of alcohol, the
boiling temperature of water cannot be measured ii. It has low specific heat capacity, i.e., 140 J/
with the help of an alcohol thermometer. kg0C.

4.5 Temperature Scales

There are three temperature scales:

i. Celsius scale ii. Fahrenheit scale iii. Kelvin scale

Boiling point 373.15 100 212 Relation among °C, °F and K
of water Scale – Lower fixed point
100 K 100 0C 180 0F
(steam point) Upper fixed point – Lower fixed point

Melting point 273.15 0 32 C–0 = F – 32 K – 273
of ice =
100 – 0 212 – 32 373 – 273
(ice point)

C–0 = F – 32 K – 273
=
Absolute 0 -273.15 -459.7 100 180 100
Zero kelvin Celsius Fahrenheit

4.6 Types of Thermometers

1. Laboratory thermometer

A laboratory thermometer consists of a thick walled capillary tube made up of glass in
which one end is closed and its other end has a cylindrical bulb. The cylindrical bulb
and a small portion of the capillary tube are filled with pure mercury. The air from the

72 Oasis School Science - 10 PHYSICS

capillary tube is taken out completely before closing the upper end of the capillary tube.
In this thermometer scale ranges from – 10°C to 110°C. The vacuum made above the
mercury column makes it easy to expand the thermometric liquid.

Fig. 4.5 Laboratory thermometer

The basic differences between a laboratory thermometer and a clinical thermometer are:
(i) The clinical thermometer has the graduations from 35°C to 42°C or 95°F to 107.6°F be-

cause the temperature of the human body varies only in this range. But the laboratory
thermometer has a range from -10°C to 110°C.
(ii) The clinical thermometer has a constriction near the bulb but not in the laboratory
thermometer.

2. Clinical thermometer
The clinical thermometer is constructed for measuring the temperature of the human

body. That is why it is also called the doctor's thermometer. A clinical thermometer is
shown in the following figure.

Fig. 4.6 Clinical thermometer

The clinical thermometer has a constriction near the bulb. It allows the mercury to rise
up in the capillary tube when the thermometer is in contact with the body. But it does
not allow the mercury to fall back into the bulb after the removal of the thermometer
from the body. Due to the constriction in the clinical thermometer, the temperature of the
human body can be taken easily and accurately.

To measure the temperature of the human body, the bulb of the clinical thermometer
is kept under the tongue or in the armpit of the person at least for two minutes. The
mercury of the bulb in the thermometer absorbs the heat energy of the human body and
expands. As a result, the level of the mercury rises in the capillary tube showing the body
temperature of the person.

When the thermometer is taken out of the mouth or armpit of the person, the sudden
cooling and contraction of the mercury in the bulb breaks the thread of mercury at the
constriction. Hence, the mercury stays at the original level and the temperature can be
noted easily without any difficulty. The mercury in the stem of the thermometer must
be returned to the bulb by shaking it before its reuse. The thermometer must be washed
with water before using it.

3. Digital thermometer
A digital thermometer is a modern thermometer which shows the temperature in digits.

It does not have any calibration on it. It has an elongated body with a bulb filled with
mercury. When the bulb of the thermometer comes in contact with a body while it is on,
it shows the temperature on the monitor.

PHYSICS Oasis School Science - 10 73

Fig. 4.7 Digital thermometer

A digital thermometer has a built-in sensor. It determines the amount of indicators and
displays them on the monitor (display).

The principle of operation of the thermometer is recording the change of electrical
resistance of a conductor and converting it into a numeric value.

The digital thermometer operates through the contact method, as the mercury
thermometer. To obtain accurate results, it is necessary to ensure close contact of the
thermometer with the human body.

A digital thermometer monitors temperature fluctuations. When the figures srabilizes,
the thermometer beeps and a digital display shows the results.

4. Maximum and minimum thermometer

The thermometer that is used to measure the maximum and minimum atmospheric

temperature of twenty four hours of a min. max.
particular place is called the maximum 0C 0C

and minimum thermometer. - 20 – – + 50
-10 – –
It has a U-tube which is partially filled – +40
with mercury, and the remaining – –

U– tube is filled with alcohol, keeping 0– – +30 Y
a small vacuum for expansion. The –
limb X is completely filled, but limb Y X– – +20
is partially filled for the vacuum. When –
the temperature of the surroundings +10 – – +10
increases, the alcohol in limb X expands. –
+20 – –0

+30 –

This expansion pushes the mercury, +40 – – -10
and the mercury pushes the index in +50 – –
the limb. When the temperature falls, mercury – -20
the alcohol in the limb contracts. As

a result, the mercury flows towards magnet
the alcohol of limb X. This movement

helps to displace the index in limb X. Fig. 4.8 Maximum-minimum thermometer
The index of Y gives the maximum

temperature and the index of X gives

the minimum temperature. We need to note the reading of the lower point of the index.

Once the reading is taken, the indices are drawn down to the mercury surface by using
a C magnet because the index is made up of magnetic substance.

74 Oasis School Science - 10 PHYSICS

4.7 Heat Equation

When a hot body is cooled, it gives out heat. As a result, the temperature of the body falls. But
when a cold body is heated, it absorbs heat and its temperature rises. It has been proved that
heat gained or lost (Q) by a body is directly proportional to:

i. Mass of the body (m), i.e.,

Q ∝ m ________ (i)

ii. Change in temperature (dt), i. e.,

Q ∝ dt ________ (ii)

Combining (i) and (ii), we get

Q ∝ mdt

or, Q = smdt (where 's' is a constant called specific heat capacity of the body)
∴ Q = msdt _______ (iii)

The amount (quantity) of heat lost or gained by a body (Q) is equal to the product of mass (m),
the specific heat capacity (s) and change in the temperature (dt) of that body.

The above equation (iii) is called the heat equation. The equation that shows the relation
among the amount of heat lost or gained by a body, the mass of the body, specific heat capacity
and change in the temperature is called the heat equation. It is Q = msdt.

The amount (quantity) of heat lost or gained by a body depends on:
i. Mass of the body (m).

ii. Specific heat capacity of the body (s).

iii. Change in temperature (dt).

4.8 Specific Heat Capacity

The specific heat capacity of a substance is defined as the amount of heat required to raise the
temperature of 1kg mass of that substance by 1°C. It is also called specific heat. The SI unit of
specific heat capacity is J/kg°C or J/kg K.

Values of specific heat capacities of different substances

S.N. Substances Specific heat capacity S.N. Substances Specific heat capacity
900 J/kg°C
1. Lead 130 J/kg°C 10. Aluminium

2. Mercury 140 J/kg°C 11. Petrol 1670 J/kg°C

radiator /ˈreɪdieɪtər/ - a device for cooling the engine of a vehicle or aircraft

PHYSICS Oasis School Science - 10 75

3. Silver 234 J/kg°C 12. Wood 1755 J/kg°C
4. Brass 380 J/kg°C
5. Copper 400 J/kg°C 13. Vegetable oil 2000 J/kg°C
6. Steel 447 J/kg°C
7. Iron 460 J/kg°C 14. Kerosene 2200 J/kg°C
8. Glass 670 J/kg°C
9. Sand 800 J/kg°C 15. Ice 2100 J/kg°C

16. Alcohol 2400 J/kg°C

17. Water 4200 J/kg°C

From the above table, we see that the specific heat capacity of water is 4200 J/kg0C. It means that
4200 J heat is required to raise the temperature of 1 kg mass of water by 1°C. It should be noted
that the specific heat capacity of water is higher than that of all other common substances.

Fact File - 3

Due to the high specific heat capacity (4200J/kg0C), water is used for heating purposes (as
in hot water bags) and for cooling purposes (as in the radiator of vehicles).

Mercury has low specific heat capacity, i.e., 140 J/kg0C, and it is a good conductor of heat. So,
it is used as a thermometric liquid. It was found that when different substances having equal
masses are given equal amounts of heat, the rise in temperature will be different. This is due
to the different specific heat capacity. Thus, on supplying equal amounts of heat, the rise in
temperature will be more in the substances having low specific heat capacity whereas the
rise in temperature will be less in the substances having high specific heat capacity. This fact
becomes clear from the following mathematical relation:

Q = msdt

or, dt = Q
ms

If 'Q' and 'm' are constants, then

∴ ∴ ddtt ∝α 1
s

The above relation shows that the change in temperature is inversely proportional to the
specific heat capacity of a substance.

76 Oasis School Science - 10 PHYSICS

Reasonable Fact-3

Water is used as a coolant in the radiator (engine) or vehicles. Or, why is only water used
to cool the engine of a vehicle?

Ans: Water absorbs a large amount of heat from the engine as it has very high specific heat
capacity (i.e., 4200 J/kg0C). Due to this, the radiator (engine) does not get heated up when
filled with water. Therefore, water is used in the radiator of vehicles to prevent the engine
from excess heating.

Reasonable Fact-4

A wet handkerchief is kept on the forehead of a patient suffering from fever, why?
Ans: When a person is suffering from fever, a wet handkerchief is kept on his forehead to
lower his body temperature. As the wet handkerchief contains water, it can absorb a large
amount of heat due to its high specific heat capacity. As a result, the temperature (i.e., high
fever) of the patient decreases. Therefore, a wet handkerchief is kept on the forehead of a
patient suffering from fever.

Reasonable Fact-5

Well water feels warmer in the morning during the winter season, why?
Ans: The specific heat capacity of soil (land surface) is less compared to that of water. So,
during the winter night, the temperature of the soil decreases faster than that of water. As
a result, heat flows from well water to our body when we touch well water. Therefore, well
water feels warmer in the morning during the winter season.

4.9 Calorimetry and Its Principle

Calorimetry is the measurement of heat lost or gained by an object. The principle of calorimetry
states, "When a hot body is mixed with a cold body, the heat lost by the hot body is equal to the
heat gained by the cold body, provided that no heat escapes to the surroundings", i.e.,

Heat lost = Heat gained

or, m1s1 (t1 – t) = m2s2(t–t2)

where, m1 = Mass of the hot body
s1 = Specific heat capacity of the hot body
t1 = Temperature of the hot body
m2 = Mass of the cold body
s2 = Specific heat capacity of the cold body
t2 = Temperature of the cold body
t = Final temperature of the mixture

PHYSICS Oasis School Science - 10 77

Worked out Numerical 1

Calculate the amount of heat required to change the temperature of 1.5 kg of iron from 20°C
to 100°C. The specific heat capacity of iron is 460 J/kg °C.

Solution:
Given,

Mass of iron (m) = 1.5 kg

Specific heat capacity (s) = 460 J/kg°C

Change in temperature (dt) = 100 ° C – 20 °C = 80 °C

Amount of heat (Q) = ?

We have,

Q = msdt

= 1.5 × 460 × 80

= 55200 J.

∴ The required heat is 52200 J.

Worked out Numerical 2

The initial temperature of a pressure cooker of mass 2 kg is 30°C. The specific heat capacity
of the alloy of the pressure cooker is 1000 J/kg°C. Calculate the final temperature of the
pressure cooker. The amount of heat supplied is 7.5 ×104 joule.

Solution:
Given,

Mass of pressure cooker (m) = 2 kg

Specific heat capacity (s) = 1000 J/kg 0C

Amount of heat (Q) = 7.5 × 104J

Initial temperature (t1) = 30 0C
Change in temperature (dt) = ?

We have,

Q = msdt
Or,
dt = Q = 7.5 × 104 = 37.5º C
Now, m×s 2 ×1000

Final temperature (t2) = t1 + dt

= 300C + 37.50C

= 67.50C
∴ The final temperature of the pressure cooker is 67.50C.

78 Oasis School Science - 10 PHYSICS

Worked out Numerical 3

A bucket contains 10 kg of hot water at 700C. The water is cooled for bathing by mixing
20 kg of water at 100C. Calculate the final temperature of the mixture. [Neglect the heat
absorbed by the bucket].

Solution:
Given, Mass of hot water (m1) = 10 kg

Temperature of hot water (t1) = 700 C

Specific heat capacity of water = s (both hot and cold)

Mass of cold water (m2) = 20 kg
Temperature of cold water = (t2) = 100 C
Let, the final temperature of the mixture be t 0C.

Now, According to the principle of calorimetry,

Heat lost = Heat gained

or, m1 × s × (70–t) = m2 × s (t–10)
or, 10 × s × (70–t) = 20 × s × (t–10)

or, 10 × (70–t) = 20 (t-10)

or, 700 – 10t = 20t – 200

or, 700 + 200 = 20t + 10t

or, 900 = 30t

or, t = 900 = 30
30

or, t = 300C

∴ The final temperature of the mixture is 300C.

Reasonable Fact-6

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

Ans: The surface of the desert is made up of sand and the specific heat capacity of sand is
low (i.e. 800 J/kg0C). Due to this, the sand becomes very hot during the day as it gets heat
of the sun whereas the hot sand loses its heat very soon during the night. As a result, it is
very hot during the day and very cold during the night in the desert.

Reasonable Fact-7

The difference in temperature is low near the sea. Give reason.
Ans: A sea consists of a large amount of water, and the specific heat capacity of water is
high, i.e., 4200 J/kg0C. It gets heated very slowly during the day and cools very slowly
during the night. Therefore, the difference in the temperature is very low near the sea
because of the land breeze and sea breeze.

PHYSICS Oasis School Science - 10 79

SUMMARY

• The heat energy of a body is the total sum of the kinetic energy contained by
the molecules of that body. The SI unit of heat is joule (J).

• Heat flows from a hot body to a cold body.
• The quantity of heat contained by a body is directly proportional to the

average kinetic energy (KE) of molecules and mass of the body.
• The amount of heat required to increase the temperature of 1 gram of pure

water by 10C, or 1 K, is called 1 calorie heat.
• The average kinetic energy of the molecules of a body is called the temperature

of that body. The SI unit of temperature is kelvin (K).
• The working principle of the thermometer is, "Liquid expands on heating

and contracts on cooling."
• The specific heat capacity of a substance is defined as the amount of heat

required to raise the temperature of 1 kg mass of that substance by 1°C.
• The equation that shows the relation among the amount of heat lost or

gained by a body, the mass of the body, specific heat capacity and change in
temperature is called the heat equation. It is Q = msdt.
• The principle of calorimetry states, "When a hot body is mixed with a cold
body, the heat lost by the hot body is equal to the heat gained by the cold
body, provided that no heat escapes to the surroundings".
• Mercury has low specific heat capacity, i.e., 140 J/kg0 C, and it is a good
conductor of heat. So, it is used as a thermometric liquid.
• The clinical thermometer has graduations from 35°C to 42°C or 95°F to
107.6°F because the temperature of the human body varies only in this range.
But the laboratory thermometer has a range from -10°C to 110°C.
• The clinical thermometer has a constriction near the bulb but not in the
laboratory thermometer.
• A digital thermometer is a modern thermometer which shows the
temperature in digits.

80 Oasis School Science - 10 PHYSICS

Exercise

Group-A
1. Define heat. Write its SI unit.
2. State the factors on which the amount of heat present in a body depends.
3. Define heat energy on the basis of molecular kinetic theory.
4. Define one calorie of heat energy.
5. Define one joule of heat energy.
6. Define temperature and write down its SI unit.
7. Write down formula and SI unit of specific heat capacity.
8. Which principle is the construction of thermometer based on?
9. Name any two liquids which are used in the thermometer.
10. Write down the boiling and melting points of mercury in normal pressure.
11. What is clinical thermometer?
12. Why is clinical thermometer used?
13. What is digital thermometer?
14. What is maximum-minimum thermometer?
15. Name the instrument which is made on the basis of expansion of a liquid on heating.
16. What is the body temperature of a healthy human?
17. Write two factors that affect heat energy.
18. What is the boiling point of water in Kelvin unit?
19. Write down the formula of heat equation.
20. What is meant by 1 calorie heat?
21. What is the value of specific heat capacity of water?
22. What is the relation between the specific heat capacity and the rate of increase or decrease

in its temperature?

Group-B
1. What is the relation among the amount of heat energy contained by a body, its mass and

kinetic energy of the molecules?
2. Write any two differences between heat and temperature.
3. What do you mean by the specific heat capacity of a substance?
4. What is the meaning of the statement that the specific capacity of water is 4200 J/kg 0C?
5. What is used to cool the engine of vehicles? Why?
6. What is used in hot water bags? Why?
7. A new quilt is felt warmer than an old one. Why?
8. A wet handkerchief is kept on the forehead of the patient suffering from fever. Why?

PHYSICS Oasis School Science - 10 81

9. We wear woolen clothes during winter. Why?
10. A solid melts on heating. Why?
11. Well water is felt warmer in winter. Why?
12. Write any two differences between mercury and alcohol.
13. Write any two differences between clinical thermometer and laboratory thermometer.
14. If chilled coke and hot tea are kept together, tea cools down but coke gets warm, why?
15. When hot iron nail is kept in cold water, the temperature of iron decreases, whereas

temperature of water increases, why?
16. The temperature of mercury is more when equal amount of heat is given to one kilogram

of water and mercury each, why?
17. If equal amount of heat is given to the equal mass of water and alcohol substance, which

warms up faster and why?
18. Why do specific heat capacity of objects differ?
19. During high fever, a wet clean cloth is kept on the forehead of the patient, why?
20. The difference in temperature is low near sea. Give reason.
21. State the meaning of specific heat capacity of copper is 380 J/kg0C.

22. Write down the cause that the night in desert is very cold and day of that place is very hot.

Group-C
1. How is body temperature measured by using clinical thermometer? Explain.
2. Write any three advantages of high specific heat capacity of water.

3. Draw a neat and labeled figure showing maximum-minimum thermometer.

4. Describe an activity to show that heat always flows from the body at high temperature
to the body at low temperature.

5. Write any three applications of high specific heat capacity of water.

Group-D
1. Study the given table and answer the following questions.

Metal Specific heat capacity

A 140 J/kg 0C

B 460 J/kg 0C

C 380 J/kg 0C

i) The specific heat capacity of metal ‘A’ is 140 J/kg 0C. What does it mean?

ii) If the same amount of heat energy is given to the same mass of all three metals at
the same temperature, which one will gain the lowest temperature, why?

iii) Which metal will penetrate into the greatest depth if each with equal mass is put
on a wax slab after heating to 75 0C each? Why?

82 Oasis School Science - 10 PHYSICS

2. Specific heat capacities of three different substances are given below:

Substances Specific heat capacity
A 910 J/kg 0C
B 380 J/kg 0C
C 470 J/Kg 0C

i) What do you mean by specific heat capacity of A is 910 J/kg 0C?

ii) Which above given substance will gain least temperature while heating equal mass
of all the three substances supplied with equal amount of heat? Give reason.

iii) Which one will go down least in depth while keeping equal mass at the three
substances heated for the same temperature of 100 0C and kept on a wax slab, why?

3. Write any two advantages of high specific heat capacity of water. Calculate specific

heat capacity of the alloy of pressure cooker of mass 1.5 kg is made, if quantity of heat

necessary to raise its temperature by 60 0C is 81 kJ. (Ans: 900 J/kg 0C)

4. Why does frost bite occur in mountain climber? What will be the final temperature of a
mixture of 10kg of water at 70 0C and 20 kg of water at 10 0C. Neglect the heat absorbed
by the container. (Ans: 30 0C)

5. Three liquids ‘A’, ‘B’ and ‘C’ of equal mass are kept in the same type of container and
placed in the sun for 30 minutes. The increase in temperature is given in the table.

Liquid Increase in temperature
A 170C
B 300C

C 230C

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

ii) If equal mass of all three liquids at the same temperature are cooled, which one
will cool down faster? Why?

6. What will happen if oil is used in hot water bag instead of water? Explain with reason.

Hot water of mass 10 kg at 900C is cooled for taking bath by mixing 20 kg of water

at 20 0C, what is the final temperature of the water? (Specific heat capacity of water

is 420 J/kg0C) (Ans: 43.330C)

7. Write the relationship between heat gained or lost by an object with its mass. An iron ball

of mass 5 kg at 100 0C temperature is dipped in water of mass 10 kg at 200C. Find the final

temperature of water. (The specific heat capacity of water and iron are 4200 J/kg0C and

470 Jkg0C respectively.) (Ans: 24.330C)

8. Study the given figure and answer the following 10 kg of water at 600C
questions.
5 kg of water at 800C

(i) What is the direction of heat and why? Stopper

(ii) What will be the equillibrium temperature if
stopper is opened, assuming there is no heat gain

or loss in the system. (Ans: 66.660C) A B

PHYSICS Oasis School Science - 10 83

5UNIT Estimated teaching periods

Theory 5

Practical 2

LIGHT

Objectives Galileo Galilee

After completing the study of this unit, students will be able to: (1564-1642 AD)

• define two types of lenses (convex and concave) with their uses.
• demonstrate refraction of light through lenses.
• draw ray diagrams showing the image formed by lenses.

• Introduce optical instruments and state their utility.

• describe the human eye and defects of vision with their remedy.

5.1 Introduction

Light is a form of energy which produces the sensation of vision. Light travels in a straight
path when it travels in the same medium of the same density. But if it travels from one medium
to another, it bends due to the change of direction in contact between any two media. So, the
rays of light refract as they pass from air medium to glass medium. The process of bending of
rays of light when they travel from one medium to another is called refraction of light. In our
day to day life, we use various instruments having one or more lenses, e.g., a camera, microscope,
telescope, etc. Instruments having one or more lenses are called optical instruments. These
instruments are used to produce images with the help of one or more lenses.

5.2 Lens

A watchmaker uses a hand lens to view extremely Fact File - 1
small parts of a watch clearly. Spectacles are used
to make things visible. In fact, a lens plays an The speed of light is very high. It
important role in our every day life. A mirror works travels at the speed of 300000km in one
on the basis of reflection of light whereas a lens second through a vacuum.

works on the basis of refraction of light. A lens is a

piece of transparent glass bounded by two refracting surfaces, which are usually spherical.

5.3 Types of Lenses

On the basis of nature, there are two types of lenses.

They are:

i. Convex lens and Convex surface Convex surface

ii. Concave lens

i. Convex lens: A lens which is thick in the Fig. 5.1 Convex lens
middle and thin at the edges is called a

84 Oasis School Science - 10 PHYSICS

convex lens. A convex lens is also known as converging lens because it converges
the parallel rays of light at a point after refraction. Since the rays of light actually
pass through the focus of the convex lens, it has a real focus.

Fact File - 2

Convex lenses are of
three types . They are:
(i) Biconvex lens
(ii) Planoconvex lens
(iii) Concavoconvex lens

Fig. 5.2 Converging action of a convex lens

Activity 1

Take a convex lens (hand lens) and a piece of paper.
Focus the sunlight on the paper with a fine beam.
What happens to the paper after 3 – 4 minutes?

The paper starts burning after some time since the
convex lens converges the parallel beams of light at
a point after refraction.

Fig. 5.3 Burning a piece of
paper by using a hand lens

Repeat the activity with a concave lens.
What happens?
The paper does not burn since the concave lens diverges the parallel beams of light.

Fact File - 3
ii. Concave lens: A lens which is thin in the middle
but thick at the edges is called a concave lens. Concave lenses are of three
A concave lens is also known as diverging lens types. They are:

because it diverges the parallel rays of light after (i) Biconcave lens
refraction. Since the rays of light do not actually
pass through the focus of a concave lens, it has a (ii) Planoconcave lens

virtual focus. (iii) Convexoconcave lens

virtual / ˈ v ɜ ː t f u ə l / - almost or very nearly the thing described, not real

PHYSICS Oasis School Science - 10 85

Concave Concave Parallel rays Concave lens
or diverging lens
(Focus)
Principal axis F OF
(Focus)

f
(Focal length)

Fig. 5.4 Concave lens Fig. 5.5 Diverging action of a concave lens

5.4 Lens As a Group of Prisms

A convex lens is made of a set of prisms in such a way that the prisms in the upper half
have their bases downwards and the prisms in the lower half have their bases upwards.
The central part of the lens acts as a glass slab. The prism in the upper part of the lens
bends the incident ray downwards, while the prism in the lower part of the lens bends
the incident ray upwards. The central part of the lens, which is just like a rectangular
glass block, allows the incident ray to pass undeviated.

Convex lens Concave lens

Fig. 5.6

Similarly, a concave lens is made of a set of prisms in such a way that the prisms in the
upper half have their bases upwards and the prisms in the lower half have their bases
downwards.

The central part of the lens is just like a glass slab. The prism in the upper part of the lens
bends the incident ray upwards, while the prism in the lower part of the lens bends the
incident ray downwards. The central part passes the incident ray undeviated.

5.5 Terminology Related to Lens

i. Center of curvature (2F or C): The center of the sphere from which the lens has been cut

is called the center of curvature (2F or C). A lens has two such points.

Convex lens Concave lens
A
Center of A Center of Center of Center of
curvature curvature curvature curvature

XO YX O Y

2F F F 2F Principal 2F F F 2F
axis R R
RR

B B

Fig. 5.7

aperture /ˈæpətʃər/ - the maximum portion of the lens through which refraction takes place
curvature /ˈkɜːvətʃər/ - the state of being curved

86 Oasis School Science - 10 PHYSICS

ii. Aperture: The aperture of a lens is the maximum portion of the spherical surface through
which refraction takes place. In the above figure, AB is the aperture of the lens.

iii. Optical center (O): The geometrical center of a lens is known as its optical center. In the
given figure, the point 'O' is the optical center. A ray of light passing through it doesn’t
suffer any deviation and goes straight ahead.

iv. Principal axis: The line joining both the centers of curvature of a lens is called the principal
axis. In the given figure, XY is the principal axis.

v. Principal focus or focus (F): Principal focus is the point on the principal axis where the
rays of light parallel to the principal axis converge after refraction through a convex lens
or appears to diverge from a point on the axis through a concave lens. In the figure, the
point 'F' is the focus.

vi. Focal length (f): The distance between the principal focus and the optical center of a lens
is called the focal length.

vii. Radius of curvature (2f): The radius of the sphere from which the lens has been cut is
called the radius of curvature. It is the distance between the optical center and center of
curvature. It is denoted by 'R' or 2f.

Convex lens Concave lens
or converging lens or diverging lens

Parallel rays Parallel rays of light
of light

Principal axis O F Focus O F
F' (Focus) Principal axis F (Focus)

(Focus)

(a) (b)
f f

(Focal length) (Focal length)

Fig. 5.8

5. 6 Process of Finding Focal Length of a Convex Lens

When the object is at infinity, the distance of the image from the lens will be equal to the focal
length of the lens. This is used to find the focal length of a convex lens. Take a convex lens
whose focal length is to be determined. Keep the lens in front of a distant object and put a
card board as a screen behind the lens. Change the distance of the screen from the convex lens
until a clear inverted image of the distant object is formed on the screen. Measure the distance
between the screen and the lens. The distance will be the rough focal length of that lens.

Activity 2

Take a convex lens. Try to get an image on the screen. Measure the distance between the
screen and the lens.

This is the rough value of the focal length of the lens.

PHYSICS Oasis School Science - 10 87

Focusing: The process of adjusting the distance between a lens and the screen so that the
image becomes clear and distinct is called focusing. It is done to produce a sharp and clear
image in a camera, microscope, telescope, etc.

5. 7 Rules for Drawing Ray Diagrams

Rule 1. The ray of light which is parallel to the principal axis of a convex lens always passes
through the focus after refraction. In the case of a concave lens, the ray appears to be
diverging from the principal focus.

O FO
F

Convex lens Fig. 5.9 Concave lens

Rule 2. An incident ray passing through the principal focus of a convex lens becomes parallel
to the principal axis after refraction. In the case of a concave lens, the incident ray
appearing to meet the focus is refracted parallel to the principal axis.

O FO
F

Convex lens Fig. 5.10 Concave lens

Rule 3. The ray of light that passes through the optical center of a convex lens or a concave
lens goes straight without deviation after refraction.

OO

Convex lens Fig. 5.11 Concave lens

5. 8 Image and Its Types

When an object is placed in front of a lens, the rays of light from the object fall on it and get
refracted. The refracted rays produce a picture of the object, which is called the image. An
image is a physical likeness or representation of an object produced by a lens after refraction.

deviation /diːvɪˈeɪʃn/ - the act of moving away from what is normal

88 Oasis School Science - 10 PHYSICS

Lenses form two types of images. They are:

i) Real image: The image which can be obtained on the screen is called a real image. It
is always inverted. A convex lens generally forms a real image. This type of image is
formed when the refracted rays actually meet at a point.

ii) Virtual image: An image which cannot be obtained on the screen is called a virtual
image. It is always erect. A concave lens always forms a virtual image. This type of image
is formed when the refracted rays appear to meet at a point.

5. 9 Images Formed by a Convex Lens

The type of image formed by a convex lens depends on the position of the object in front of
the lens. The object can be placed at different positions to get different types of images. The
object can be placed:

(i) at infinity (ii) beyond 2 F

(iii) at 2 F or C (iv) between F and 2F

(v) at F (vi) between F and O

A brief description of the images formed by a convex lens is given below:
1. When an object is at infinity

Characteristics of the Image

pPoainrat lBoAlef laradyisstafrnotmobtojepct Position : At F
F'
D Size : Highly diminished

F Nature : Real and inverted
O B'
Utility : This type of image
ImaAg'e formation represents the
(Real, inverted) action of the objective
lens of an astronomical
Fig. 5.12 telescope and a camera
lens focused at infinity.

2. When object is beyond 2F

Convex lens Characteristics of the Image

A Position : Between F and 2 F
B 2F'
B' Size : Diminished
F 2F
F' O Nature : Real and inverted
A'
2f Image Utility : This type of image
(Real, inverted) formation is used in a
Fig. 5.13 photographic camera.

PHYSICS Oasis School Science - 10 89

3. When object is at 2F

A Characteristics of the Image
Position : 2 F

Object F 2F Size : Same size as the object
B B'
F O Nature : Real and inverted
2F A'
Image
2f (Real, inverted) Utility : This type of image formation
is used in a terrestrial
FFigig. 5. .51.314 telescope.

4. When object is between 2F and F

A D F B' Characteristics of the Image
C 2F Position : Beyond 2 F
B Size : Enlarged (magnified)
2F Object F' Nature : Real and inverted
Utility : This type of image
Fig. 5.15 A'
Image (Real, formation is used in film and
inverted and slide projectors to produce
an enlarged image on the
magnified) screen.

5. When object is at F Characteristics of the Image

Convex lens Position : At infinity
A

B F Size : Highly enlarged (magnified)
F
O Iimnfaingitey at X Nature : Real and inverted
Object

Y Utility : This type of image formation
is used in making search
f Rays lights and spot lights in
become theaters.
Fig. 5.16 parallel

6. When object is between F and O Characteristics of the Image
B'

Image Convex lens Position : Beyond the object

B Size : Enlarged (magnified)

Object Nature : Virtual and erect
FA
Image (virtual, A' O F Utility : This type of image
erect and formation is used in a
enlarged) simple microscope or a
hand lens.
Fig. 5.17

90 Oasis School Science - 10 PHYSICS

Summary of the images formed by a convex lens

Position of the object Position of image Size of image Nature of image

1. At infinity At F (focus) Highly diminished Real and inverted
2. Beyond 2F Between F and 2F
3. At 2 F At 2 F Diminished Real and inverted

Same size as the Real and inverted
object

4. Between F and 2F Beyond 2F Magnified (enlarged) Real and inverted
5. At F
6. Between F and O At infinity Highly magnified Real and inverted

On the same side of the Magnified Virtual and erect
object (beyond object)

5. 10 Image Formed by a Concave Lens

When an object is placed anywhere between the optical center and infinity, the image formed
by a concave lens lies in between the principal focus and optical center on the same side of the
lens. The image is always diminished, virtual and erect.

1. When the object is kept anywhere between infinity and the optical center

A Concave lens Characteristics of the Image
Object X Position : Between F and O

A' Size : Diminished

BF B' C Y Eye Nature : Virtual and erect
Image
Utility : This type of image forma-
(Virtual, erect and diminished) tion is used in spectacles for
correcting myopia or short
Center line of the lens sightedness.

Fig. 5.18

5.11 Power of a Lens

A convex lens converges the light rays falling on it whereas Remember

a concave lens diverges. The ability of a lens to converge or Physical Quantity SI unit

diverge the rays of light falling on it is called the power of D (dioptre)
a lens. The power of a lens is defined as the reciprocal of its Power

focal length in meters. 1 Focal length m (meter)
Focal length (f)
Thus, Power of a lens (P) = ∴P = 1
f
∴P 1
= f



The SI unit of power of a lens is dioptre (D). The power of a lens is inversely proportional to its

focal length. Thus, a lens of short focal length has more power and vice-versa.

One dioptre (1 D): The power of a lens is 1D if its focal length is 1 metre.

PHYSICS Oasis School Science - 10 91

Fact File - 4

The instrument used to measure the power of a lens is called the dioptremeter. It is used by
an optician to find the power of the lens of spectacles.

Fact File - 5

A concave lens has negative focal length so it has negative power. A convex lens has positive
focal length so it has positive power.

Worked out Numerical 1

Calculate the power of a lens having focal length 10 cm. Also, write down the type of
the lens.

Solution:
10 1
Here, Focal length (f) = 10 cm = 100 m = 10 m

Power of the lens (P) ==1f =11 10 D
10

Since the focal length is positive, the lens is a convex lens.

Worked out Numerical 2

The power of a spectacle lens is –2.5 D. Calculate the focal length of the lens. Also, name
the type of the lens.

Solution:

Here, Power (P) = –2.5 D

Focal length (f) = ?

We have, P ==1f =11 10 D
or, – 2.5 ==1f =1110 10 D
∴ 1 1=0
f = −2.5 – 0.4 m

Since the focal length is negative, it is a concave lens.

5.12 Magnification

The magnification produced by a lens is defined as the ratio of the height of the image to the

height of the object, i.e.

Magnification (M) = Height of the image(I) ∴oMr =MOI = I
Height of the object (O) O

92 Oasis School Science - 10 PHYSICS