Modern Graded Science 10

On the basis of atmospheric pressure, many devices like syringes, air pumps, water
pumps, etc. are made. These devices are used for different purposes in our daily life.

Measuring atmospheric pressure

Atmospheric pressure is measured by using barometers. They are of mainly two
types; mercury barometer and aneroid barometer. Here, mercury barometer is described.

Mercury barometer glass tube vacuum
atmospheric
Mercury barometer was invented by an Italian physicist pressure 760mm
named Evangelista Torricelli. It is also known as Torricellian (29.92in)
barometer. The barometer is made of a one meter long glass
tube containing mercury. The upper end of the tube is closed mercury
and has vacuum above the mercury. It is also calibrated with a
scale at its side.

When atmospheric pressure increases, the mercury level rises
up in the tube and when the atmospheric pressure decreases, the
mercury level falls down. In this way, the instrument works.

Some instruments based on atmosphere pressure Fig: 2.15 mercury barometer

Many types of instruments are invented based on atmospheric pressure. Some of them

are syringe, air pump, water pump etc. Now, study their structure and working methods.

A. Syringe piston

A syringe is used in the medical field to inject medicine nozzle
in patients' body. It consists of plastic or glass. It has mainly needle
two parts: cylinder and piston. The cylinder is a hollow
pipe-like structure connected with a nozzle at its one end cylinder
and other end is opened. The nozzle is fitted with a needle Fig: 2.16 syringe structure
and inside the cylinder, the piston is adjusted. The piston
can be moved to and fro inside the cylinder.

Working method A

The instrument works by the repetition of two processes. They a. pulling b. pushing
are the pulling piston and the pushing piston. piston Fig: 2.17 piston

When the piston is pulled by keeping the needle inside a handle
liquid, the volume of part A of the cylinder increases and pressure
decreases there. In this condition the atmospheric pressure pushes
the liquid inside A.

When the piston is pushed, the volume of part A decreases
and the pressure increases here. It pushes the liquid out of the
syringe.

B. Air Pump

An air pump is a device used to pump air in to wheel piston with
stoves, balls, toys, balloons, bicycle's wheels, etc. It with valve washer
may be hand-operated or machine-operated. In the cylinder
diagram, a hand operated bicycle pump is shown.
The structure shows its two parts mainly. They are a Fig: 2.18 air pump structure

PRESSURE CLASS - 10 MODERN GRADED SCIENCE 47

cylinder and a piston. The cylinder is a pipe having connected with a nozzle with rubber
tube. The other end is opened through which a piston is passed inside it. The piston
contains a handle, a rod and a washer. The washer is made soft by applying oil/grease on
it and it works as the valve for the pump. The grease also reduces friction while the piston
slides in the cylinder.

Working method

An air pump also works by the repetition of pulling and BB
pushing activities of the piston. For it the rubber tube of the
pump is connected with bicycle's wheel. At the joint there v1
is a valve in the bicycle's wheel. When the piston is pulled, v1
the volume of part A increases and it reduces pressure in this
part. The air of part B at the atmospheric pressure compresses AA
the washer and passes into A. At this time, the valve in the v2 v2
bicycle's wheel is closed which blocks the air of the bicycle
wheel from coming out. a. pulling piston b. pushing piston
Fig: 2.19

When the piston is pushed, the volume of part A decreases and it increases pressure
in this part. The increased pressure spreads the washer to stop the passage of air from A
to B. The air of part 'A' enters the bicycle's wheel by opening the valve.

C. Water pump

Water pump is also based on atmospheric pressure. It rod
is used to pull underground water. Its structure shows that spout
it consists of four parts. They are cylinder, piston, pipe and
handle. handle

The cylinder is a hollow pipe having a spout at its one piston with cylinder
side. Its one end is opened and the other end is connected valve B
with a pipe. There is a valve (valve A) at the joint of the
cylinder and the pipe. The pipe is taken to the source of water. valve A
Inside the cylinder, a piston is fitted. The piston contains a pipe
rod, a washer and a valve (valve B). The outer end of the rod Fig: 2.20 water pump structure
is connected with a handle. The handle is the first class lever
used to pull or push the piston.

Working method

A water pump pulls underground water by repetition of the process of downstroke
and upstroke.

Downstroke: When the handle of a water pump is lifted B B
up, the piston is pushed down. It is downstroke of the A A
piston. In this condition, the volume of part A decreases
and water pressure increases in this part. The increased
pressure shuts the valve 'A' and opens the valve 'B'. It
makes the water of part A pass into part B of the cylinder.

Upstroke: When the handle is pushed down, it lifts the a. downstroke Fig: 2.21 b. upstroke
piston up. It is upstroke. During upstroke, the volume of
part A increases and water pressure at that part decreases.

48 MODERN GRADED SCIENCE CLASS - 10 PRESSURE

It makes valve A opened and valve B closed. Thus, the water from the source enters into
part A of the cylinder. The water of part B is lifted up and it comes out from the spout.

S me Reasonable Facts

1. It is easier to lift our limbs in water than in air. This is because water exerts upthrust
on the limbs and thus it reduces the weight of the limbs apparently. Thus, force
required by a person to lift his/her limbs when immersed in water is smaller than the
force required for the same movement in air.

2. The blood pressure in human body is greater at the feet than at the brain. This is because
the depth of the feet from the heart is more than the depth of the brain. Since P = dgh
(where d = density, g = acceleration due to gravity and h = depth], it implies that pressure
is more in the feet where h is more and it is less in the brain where h is less.

3. A balloon filled with helium does not rise in the air indefinitely, but halts after a certain
height (neglecting wind). It is because the density of air goes on decreasing with the
height. Thus, the weight of air displaced (i. e. upthrust) decreases with height as the
balloon rises up in the air. At a certain height, the weight of the balloon and helium in
it equal to the upthrust due to air. In this condition, the net force acting on it is zero,
and the balloon stops rising further.

4. A piece of solid steel sinks, but a ship made of steel floats. The bodies which are less
dense than water float on it; those bodies which are denser than water sink in it. A
piece of steel sinks in water as the density of steel is more than the density of water.
On the other hand, a ship made of steel is hollow and contains air in its cavity, so its
average density is less than that of water. Because of this, it displaces water equal to
its weight, causing the ship to float on water.

5. An loaded ship sink more than empty ship in seawater. The weight of seawater
displaced by the immersed portion of each ship is equal to the total weight of each
ship. The empty ship is lighter than the loaded ship. Due to it, the loaded ship has to
displace more weight of water to be floated, so it sinks more. Thus, loaded ship sinks
more than an empty ship.

6. An egg sinks in pure water but floats on a saturated salt solution. Density of an egg
is higher than the density of pure water but it is less than the density of saturated salt
solution. The salt solution exerts more upthrust on the egg than the pure water due
to its more density. It makes the egg float.

7. The density of ice is less than that of water. Thus, ice is able to displace water equal to
its weight. Hence, ice floats on water.

8. The density of a steel ball is greater than that of water but less than that of mercury.
Thus, a steel ball cannot displace water equal to its weight and it sinks in water but
the density of mercury is greater than the density of steel and the ball can displace
mercury equal to its weight. Hence, a steel ball sinks in water but floats on mercury.

9. An air bubble becomes bigger and bigger as it moves towards the surface of a liquid
as liquid pressure is different at different depths of a liquid. The bubble is compressed
more at a greater depth due to more pressure there. Thus it is the smallest at the
deepest region.

PRESSURE CLASS - 10 MODERN GRADED SCIENCE 49

10. Washer of an air pump is used with lubricants. It is because the washer works as
a piston as well as a valve. In flexible condition only, it can expand and contract to
work properly. The smooth lubricant also reduces friction between the piston and the
cylinder.

11. A water pump cannot lift water higher than 10 metre from the earth's surface. It is
because a water pump is based on the principle of atmospheric pressure. Atmospheric
pressure cannot push up water more than 10 metre or even higher in the water pump.

Things To Know

1. The pressure exerted by a liquid of density 'd' at a depth 'h' from its free surface is dgh.
2. The pressure exerted by a liquid increases with depth and it maintains its level itself.
3. The pressure at any point in a liquid acts in all directions.
4. Pascal's law states that, "any pressure applied to an enclosed liquid is transmitted

equally in each part of the liquid."
5. Pascal's law is used in hydraulic machines such as hydraulic press, hydraulic brake,

hydraulic garage lift, hydraulic crane, etc.
6. When a body is immersed partially or completely in a liquid, the resultant force on

the body is called upthrust or buoyant force.
7. Archimedes' principles states that when a body is immersed completely or partially

in a liquid, it loses some of its weight apparently. This apparent loss in the weight of
the body i. e. upthrust is equal to the weight of the liquid displaced, i. e. upthrust =
wt. of displaced liquid.
8. Law of floatation is a special condition of Archimedes' principle which states that
the weight of a floating body is equal to the weight of the displaced liquid, i. e. wt. of
floating body = wt. of displaced fluid.
9. Density of a body is defined as the amount of mass of that body in unit volume. i. e.
d = m/v.
10. Relative density of a body is defined as the ratio of the density of a substance to the
density of pure water at 4° C.

Density of a body
Relative density = density of pure water at 4°C
12. The pressure given by the air of the atmosphere is called atmospheric pressure.

13. The standard pressure is 1 atmosphere i. e. 760 mm of Hg.

14. Atmospheric pressure is measured by using a barometer.

15. Syringe, air pump and water pump are some instruments based on the principle of
atmospheric pressure.

Things To Do

Prepare a set of apparatus for the verification of Archimedes' principle and law of
floatation using local and no cast materials (except a spring balance).

50 MODERN GRADED SCIENCE CLASS - 10 PRESSURE

Test Yourself

1. Multiple choice questions (MCQs).

a Any pressure applied on an enclosed liquid is transmitted equally in all parts.
The statement is:

A. Pascal's law B. Archimedes' principle

C. Principle of floatation D. Boyle's law

b. The value of standard atmospheric pressure is:

A. 7600 mm of Hg B. 760 cm of Hg C. 760 mm of Hg D. 76 mm of Hg

c. Mercury borometer was invented by:

A. Blaise Pascal B. Archimedes C. Aneroid D. Torricelli

d. In which instrument of the following the washer works as a valve?

A. Syringe B. Hydraulic garage lift

C. Air pump D. Water pump

e. A hydraulic garage lift is based on:

A. Pascal's law B. Law of floatation

C. Atmospheric pressure D. Archimedes' principle

f. The formula showing relation among a cross sectional area and efforts is given by:

A. A1 g=ivAFe2s2 the B. A1 × A2 = F1 × F2 C. AA12 = FF21 D. A1 × F1 = A2 × F2
U= WF12 conclusion of:
g.

A. Pascal's law B. Law of floatation

C. Atmospheric pressure D. Archimedes principle

h. The fraction of a floating solid inside liquid is given by:

A. V1 B. V1 × V2 C. VV12 D. V2 – V1
V2

i. Which of the following formula gives definition of relative density?

d d
A. d water at 4°C B. d water at 40°C

d d
C. d mercury at 4°C D. d alcohol at 4°C

2. Answer the following questions.

a. Prove that p = dgh.

b. Mention two factors which affect pressure due to liquid contained in a vessel.

c. Two drums of the same size and height are taken.

i. What will be the difference in liquid pressure on their bases if A of them is
completely filled and B is filled half and kept at the same place.

ii. What will be the difference in liquid pressure on their bases if both A and
B are filled with water completely but one of them is kept in Lumbini and
another in Pokhara? Why?

PRESSURE CLASS - 10 MODERN GRADED SCIENCE 51

iii. What will be the difference in liquid pressure on their base if A is filled with
water and B is filled with salty water and kept at Biratnagar in the same
position? Why?

d. When a body is fully or partially immersed in a liquid, name the forces acting on
the body.

e. State Pascal's law. Name any two instruments based on Pascal's law.

f. State law of floatation and Archimedes' principle. Also write the difference
between them.

g. While dipping a solid object in a liquid, upthrust on the object due to the liquid
and the weight of the displaced liquid are shown to be equal in an experiment.
Which principle does the experiment prove?

h. If a freely floating ship has a weight of 5 × 105N, what will be the weight of water
displaced by the ship?

i. Define atmospheric pressure. What instrument is used to measure it?

j. Describe the structure and working method of a barometer in short.

k. How can you prove that atmospheric pressure occurs?

l. How does an air pump work?

m. Describe downstroke and upstroke of a water pump.

3. Differentiate between:

a. Law of floatation and Archimedes' principle
b. Density and relative density
c. Air pump and water pump
d. Downstroke and upstroke of a water pump
e. Thrust and upthrust

4. Give reasons.

a. It is easier to pull a bucket of water from a well until it is inside the water but
difficult when it is out of water.

b. A bucket of water is filled faster in the downstair's tap than in the upstair's tap.
c. The blood pressure in human body is greater at the feet than at the brain.
d. Deep-sea divers wear diving suits.
e. An egg floats on salty water.
f. An iron nail sinks in water but a ship made of the same material floats on it.
g. It is easier to swim in an ocean than in a river.
h. Water cannot be used in a barometer.
i. The washer of an air pump is used with grease/oil.
j. A water pump cannot lift water more than 10 metre high from the earth's surface.
k. Water cannot be used instead of mercury in a barometer.

5. Diagrammatic questions A B h1 h2
D C
a. In the diagram, a body, ABCD is placed in water. Study
the diagram and answer following questions.

i. On what surface of the body, will pressure be the
maximum and why?

ii. On what level, will thrust be the maximum?
Explain why?

52 MODERN GRADED SCIENCE CLASS - 10 PRESSURE

b. Answer the following questions on the basis of the diagram given below.

i. Is there any difference in upthrust of water at A wall
B and C ? B
C
ii. Why is the wall made thicker at the bottom?

c. Densities of some of the substances are given in the table. Answer the following

questions on the basis of them. Substance Density (g/cm3)

i. If equal masses of all are taken, which one x 11
will have the largest volume?
y 8
ii. If all have equal volume, which one will
z 0.9
have least mass?

iii. Among the substances, which one will float on water?

d. In the diagram, the weight of a stone in air and in water 10N 7N
are shown, where the weight of the beaker is 1N. Study
the diagram and answer the following questions.

i. Find upthrust due to water and weight of the liquid 4
displaced.

ii. Write the conclusion of the experiment.

iii. State the principle can be verified by the experiment.

e. Study the diagram and answer the following questions. C D
B E
i. What is shown in the diagram?
ii. Label the parts A and E. A
iii. Is it downstroke or upstroke? How do you identify the

condition?
iv. Which part is the spout?
v. Which part blocks the water of the pump from going back

into the source of water?

F. Numericals

a. Calculate the pressure exerted by a mercury column of 76 cm high at its bottom.
Given that the density of mercury is 13600 kg/m3 and g = 9.8 m/s2.

b. The depth of water in a rectangular tank is 6 m. Find pressure exerted by water
at the bottom of it (g = 10 m/s2, density of water = 1000 kg m–3).

c. The depth of a circular well is 5 m. Water level is below 2 m from the upper face
of the wall. Find the pressure at its base? (g = 9.8 m/s2)

[Hint: P = hdg and d = (5 m–2 m) = 3 m)].

d. The weight of a piece of stone when fully immersed in water is 18 N and it
displaces 4 N of water. What is the weight of the stone in air?

e. Calculate the pressure exerted by a mercury column of height 75 cm at its bottom.
Given that the density of mercury is 13600 kg/m3 and g = 10 m/s2.

f. A cube of wood of volume of 0.2 m3 and density 600 kg/m3 is placed in a liquid of
density 800 kg/m3. What fraction of the volume of the wood be immersed in the
liquid?

PRESSURE CLASS - 10 MODERN GRADED SCIENCE 53

g. A block of wood of mass 24 kg floats on water. The volume of the wood is 0.032

m3. Find the volume of the block below the surface of the water and the density

of the wood. (density of water = 1000 kg/m3).

h. A 15 cm long cube made of oak floats in water with 10.5 cm of its depth below

the surface and with its sides vertical. What is the density of the cube? (density of

water = 1000 kg m–3)

i. Calculate the mass of displaced water when a piece of 30 cm thick iceberg with

surface area 1000 cm2 floats on water (density of ice = 0.9 g/cm3 and density of

water = 1 gm/cm3).

j. A rectangular body is dipped into water

as shown in the figure. The upper or lower 2m

surface area of it is 2 m2. Find out the upthrust 6m Rectangular
acted on it by water (density of water is 1000 body

kg/m3. Water

k. A piece of stone with its volume 400 cm3

and density 7.8×103 kg/m3 is immersed totally in water of density 1000 kg/m3.

Calculate the weight of the stone in air and the upthrust of water.

l. A solid weighing 277.5 g in air and 212.5 g is totally immersed in the liquid of

density 0.9 g/cm3. Calculate the density of the solid.

m. In a hydraulic press, the cross sectional area of the big cylinder is 0.5 m2 and a

load of 6000 N can be lifted by it using 500 N effort. Calculate the cross sectional

area of the small cylinder. A = 20 cm2
F = 100 N

n. Calculate X and Y in the given diagram. A

o. The ratio of the cross sectional area of F = 60 N B
the small cylinder to the big cylinder Y=? A = 60 cm2
is 1:6, what weight can be lifted on the
big piston by applying an effort of 200 C X=?
N on the small piston?

a. 1.01 × 105 Pascal b. 6 × 104 Pascal c. 2.94 × 104 Pa d. 22N
e. 1.02 × 105 Pa
i. 27 kg f. 3/4 g. 0.024 m3, 750 kg/m3 h. 700kg/m3
m. – 0.041 m2
j. 78400 N k. 31.2 N, 4 N l. 3.84g/cm3

n. X is 300 N and Y is 12 cm2 o. 1200 N

Microscopic : too small to be seen without a microscope
Torr
Bar : unit of atmospheric pressure equivalent to 1mm Hg or 133.32 pa
: SI unit of atmospheric pressure equivalent to 106 dynes/cm2 or 105pa
Hydrostatics
(App.750mmHg)

: a branch of physics which deals with the properties of fluids

54 MODERN GRADED SCIENCE CLASS - 10 PRESSURE

Chapter ENERGY

3 Total estimated periods: 6 (T 5 + P 1)

explain energy/fuel with examples.
describe the sources of energy and their importance.
explain the sun as the ultimate source of energy.
identify the chief factors of energy crisis and ways to minimize energy crisis.
identify and describe the alternative sources of energy in our daily life.
describe the measures of energy conservation with examples.

Energy is defined as the ability or capacity to do work. It is measured by the amount of
work an object can do. For example, a fan uses electrical energy to move its blades. Energy
is found in various forms such as heat energy, light energy, electrical energy, chemical
energy, nuclear energy, kinetic energy, potential energy, etc. They are obtained from
different sources. In our everyday life, these forms of energy are required to do different
kinds of jobs. Different machines, vehicles like cars and buses, trains and aeroplanes use
energy sources to run their engines. Similarly, over population, urbanization, agricultural
and industrial growth with the modern civilization cause the extreme use or over
exploitation of energy sources. If the demand of the energy keeps on increasing in this
alarming rate, there will be no more energy sources left to use in near future because of
their limited sources. This causes the world to face the severe crisis of energy in the future.
Thus, wise, economical and scientific use of energy and its sources is a must to avoid
energy crisis.

The substance that reacts chemically with another substance to produce heat or that
produces heat by nuclear processes is called fuel. The term fuel is generally limited to
those substances, which burn in air emitting heat. The SI unit of the heating value of fuel
is Jkg–1. Coal, petroleum, solar, nuclear fuel, biomass, etc. are some examples of fuel. Fuel
is used to run different types of machines.

Classification of energy sources

Wood, coal, petroleum, natural gas, dung, bio-gas, etc. are some of the sources of
energy. They produce heat energy when they are burnt. Sources of energy are classified
as primary and secondary on the basis of the form they are used.

ENERGY CLASS - 10 MODERN GRADED SCIENCE 55

Those sources of energy which can be used in the same form in which they occur
in nature are called primary sources of energy. Wood, coal, crude oil, natural gas, dung,
solar energy etc. are some examples of such sources of energy.

Those sources of energy which are derived from primary source of energy are called
secondary sources of energy. Coal gas, bio-gas, kerosene, petrol, diesel, charcoal etc. are
some examples of secondary sources of energy.

On the basis of replacing period, sources of energy are classified into two types. They
are non-renewable sources of energy and renewable sources of energy.

1. Non-renewable sources of energy

Non-renewable sources of energy are found to be accumulated in nature over a very
long time and they cannot be quickly replaced when exhausted at their place of origin.
Coal, natural gas, petroleum or mineral oil, nuclear energy, etc. are some examples of it.

2. Renewable sources of energy

The sources of energy that can be replaced immediately if they are exhausted at
their place of origin are called renewable sources of energy. They are being produced
continuously in nature and are inexhaustible. Wood is a renewable source of energy.
This is because, if wood is cut from forests, it also can be grown there by plantation.
Hydropower, geothermal power, solar energy, wind energy, tidal energy, vegetable refuse
and bio-gas (gobar gas) are some other examples which can be replaced easily. Thus, these
are the renewable sources of energy.

Difference between renewable and non-renewable sources of energy

Renewable sources of energy Non-renewable sources of energy

1. It can be renewed or replaced shortly 1. It cannot be replaced shortly after it is

after it is exhausted at its place of origin. exhausted at its place of origin.

2. It can be used as alternative energy source. 2. It cannot be used as alternative energy

Examples: solar energy, bio-gas, etc. source. Examples: coal, petroleum, etc.

3. It cannot be stored for long time. 3. It can be stored.

Major sources of energy

There are different kinds of sources of energy such as fossil fuels, hydro-electric energy,
solar energy, nuclear energy, wind energy, tidal energy and geothermal energy. Out of
them, solar energy, fossil fuels and hydro-electric energy are the major sources of energy.

a. Solar energy

The sun is a big ball of fire. It is not a solid body. It is very large in comparison to the
earth. Its diameter is about 109 times more than the diameter of the earth. Due to this,
about 13 crore earthes can be adjusted in the sun. By mass, the sun is so huge that even the
total mass of the members of the solar system together becomes 0.0015th part of the sun.
The temperature of the surface of the sun is 5.7×103°C, while the temperature of its centre
is 1.5 × 107°C. In this high temperature, no substances can exist in solid and liquid states.
Hence, the sun is a mass of very hot gases.

56 MODERN GRADED SCIENCE CLASS - 10 ENERGY

The sun emits 2.7×1024 kW energy every second. The earth receives a small fraction
of the energy emitted by the sun due to a large distance between the earth and the sun
(1.5×108 km). The earth gets 2.6×1022 joule energy every second. The earth's surface gets
1.4 kW or 1.4×103 J/s energy per square metre in average. The energy from the sun is not
uniformly available; it keeps on changing every day and it differs from and place to place.
For example, the Terai belt of Nepal is hotter than the Kathmandu valley. This is because
the Terai belt gets more energy per square metre than the Kathmandu valley. The total
solar energy potential in our country is about to be 26.6 million MW.

The sun radiates a large amount of energy in the form of heat and light due to nuclear
fusion reaction. In this reaction, lighter nuclei fuse to form a slightly heavier nucleus with
the release of a large amount of energy. The sun has about 74% hydrogen, 24% helium and
1% of other elements. The sun is the most abundant and everlasting source of energy. This
energy given by the sun is called solar energy.

Limitations of solar energy

1. Solar energy reaches the earth in a much diffused form. This diffused solar energy is
too small for doing useful work. The energy released by the sun is radiated in all the
directions. About 47% of that solar energy is reflected back by the atmosphere. About
23% of that solar energy is received by every square metre of the earth per second
at day time. This solar energy is returned back to the atmosphere at night. The earth
receives only a fraction of this energy. Solar energy received by the earth per square
metre is estimated to be 1.4 kW. It is also called solar constant and it varies by o.2%
every 30 years. About 30% of the solar radiation is used by the continuous circulation
of water in water cycle.

2. It is not available uniformly all the time and at all places. The availability of solar
energy is more at some places on the earth and much less at other places. The
availability of solar energy also changes at a place every day and even keeps on
changing throughout the day.

3. It is not available during the night and cloudy days.

Traditional uses of solar energy

1. It has been used for drying clothes, obtaining salt from seawater, reducing the
moisture content in food grains after harvesting crops, etc.

2. By the process of sun drying, solar energy has been used for the preservation of fruits,

vegetables, fish, etc. 1. solar panel converts

Present uses of solar energy sunlight to DC current

Besides the traditional uses, nowadays, 4. extra electricity
solar energy is used either by converting it credited on grid
into heat or electricity. In a solar cooker or
solar water heater, solar energy is collected 3. takes electricity to
as much as possible. This is done by using your home requires
a black painted surface, a glass sheet cover 2. inverter converts
and a reflector. electricity DC to AC

Fig: 3.1 a solar plant

ENERGY CLASS - 10 MODERN GRADED SCIENCE 57

A solar cell is a device which converts solar energy directly into electricity. A group
of cells joined in a particular way is called a solar panel. It produces a large amount
of electrical energy. It is used in artificial satellites, operating communication devices
in remote areas, operating water pumps, etc. It is popular in houses as an alternative
and renewable source of energy. Evidently, use of solar energy also does not cause any
environmental pollution.

Sun as the ultimate source of the energy

Directly or indirectly, the sun supplies most of our energy that we use today. Wind
energy, hydroelectricity, fossil fuel, tidal energy and geothermal energy are all derived
from the sun.

The heat energy of the sun causes the air near the surface of the earth to be heated.
Being lighter, it rises up and at the same time, the cooler air from neighbouring regions
flows in to take the vacated regions in the form of wind. Thus, wind energy has been
derived from the energy of the sun.

During photosynthesis, the sun's energy is stored in plants in the form of organic
compounds. Animals eat plants and thus it is indirectly stored in animals. The plants and
animals which were buried under the earth millions of years ago, had sun's energy stored
in them. They were converted into fossil fuels. Hence, the energy of fossil fuels is also
derived from the sun's energy. Bio-fuel is also the stored energy of the sun.

Solar energy evaporates water from oceans and surface of the earth. The water vapour
thus formed rises up in the atmosphere to form clouds. When it gets cooled, it falls back
to the earth in the form of rain and snow. The water is used for running generators for
hydroelectricity. Thus, the real source of hydroelectricity also is solar energy.

Nuclear fusion as the source of solar energy

The sun is rich in hydrogen gas. At a very high temperature of the sun, a hydrogen
atom splits up into protons and electrons. Although there is repulsion between protons,
very high pressure of the sun binds two protons (1H1) to form a deuteron (1H2) and a
positron (1e0). A proton (1H1) and a deuteron or deuterium (1H2) unite to form the light
isotope of helium (2He3).

Note: Symbols used in nuclear energy

A sodium atom consists of 11 protons, 12 neutrons and 11 electrons. Hence

Atomic number (Z) = number of protons (or electrons)= 11

Mass number (A) = number of protons + number of neutrons

= 11 + 12 = 23

∴ Sodium nucleus is represented by 11Na23
Similarly, 2He4 is the symbol of helium nucleus, 1H1 is the symbol of a proton (or hydrogen
nucleus), 1H2 is the symbol of deuteron nucleus, -1e0 is the symbol for an electron and 1e0
is the symbol for a positron.

Positron (1e0) is a positively charged electron i. e. the mass and charge of a positron (1e0)
is equal to the mass and charge of an electron but the nature of charge of a positron is
opposite to that of an electron.

58 MODERN GRADED SCIENCE CLASS - 10 ENERGY

Then the two nuclei of light helium (2He3) unite to form an ordinary helium nucleus
(2He4) with some hydrogen nuclei. In each step of nuclear fusion [(i) to (iii) below], a vast
amount of energy is released.

i. 1H1 + 1H1 enormously high temperature and pressure 1H2 + 1e0 + energy

Deuterium

ii. 1H2 + 1H1 enormously high temperature and pressure 2He3 + energy

Light helium

iii. 2He3 + 2He3 enormously high temperature and pressure He2lHiume4 + 1H1 + 1H1 + energy

In short enormously high temperature and pressure 2He4 + 21e0 + energy

21H1 + 21H1

Conditions required for nuclear fusion in the sun

The following conditions cause nuclear fusion in the sun.

1. There is an abundance of hydrogen gas in the sun to participate in nuclear fusion.

2. The extremely high temperature of the sun (about 108K) causes hydrogen atoms to
split their nuclei.

3. The extremely high pressure of the sun causes the similar charged nuclei to fuse
together.

Evidence of nuclear fusion in the sun

1. The sun has huge amount of hydrogen which participates in nuclear fusion.

2. There is presence of abundance of helium gas on the sun, which is the product of
nuclear fusion.

b. Fossil fuels

Plants and animals, which died millions of years ago, got buried beneath the earth.
They got covered under clay and sand, which prevented oxygen of air from reaching the
remnants of dead plants and animals. Due to high temperature and pressure inside the
earth, those dead bodies got decomposed in the absence of oxygen. This resulted in the
formation of coal and petroleum. These coal and petroleum are called fossil fuels.

i) Coal: It is a non-renewable source of energy found in deep mines beneath the surface
of the earth. It is taken out by mining. Coal comes in four different varieties: lignite,
sub-bituminous, bituminous (soft coal) and anthracite (hard coal). They vary in their
carbon content, volatile matter and moisture content.

In Nepal, only low quality coal-mines (lignite) are found in Dang district. High quality
coal (anthracite) has not been found yet. It is extensively used in different industries,
factories and trains to run steam engine in the form of fuel. It is also used for cooking.
It has become a major source of energy fuel for metal industries, brick industries and
cement industries. It also can be converted into other sources of energy such as coal
gas, oil and electricity. It is essential for the manufacture of drugs, dyes, fertilizers,
plastics, synthetic fibres and explosives.

ENERGY CLASS - 10 MODERN GRADED SCIENCE 59

ii) Petroleum: Petrol (or gasoline), diesel, kerosene, asphalt, paraffin, lubricating
oil, petroleum gas, etc. are collectively called petroleum. Crude oil found trapped
between rocks beneath the earth usually mixed with salt, rock-particles and water is
the main source. Natural gas occurs above the surface of such crude oil. When a hole
is drilled in the earth's crust at the occurrence site of petroleum, natural gas comes
first and then crude oil begins to come out. When the crude oil is refined, we get
petrol, kerosene, diesel, etc. along with residual oil. On further refining the residual
oil, we get, paraffin, lubricating oil and asphalt.

Besides the purpose of heating, oil and gas are used for the production of carbon
black, needed in the rubber industry and for the manufacture of hydrogen in fertilizer
industries. Petrol is used to run small vehicles such as motorbikes, three-wheelers
and cars. Diesel is used to run heavy vehicles such as buses, trucks, trains, water
pumps and other diesel engines. Kerosene is used as fuel for domestic purposes. In
aeroplanes, pure kerosene is used as fuel. Asphalt is mainly used in road construction.
Paraffin is used for manufacture of candles, boot polish, wax and vaseline. Lubricating
oil is used as a lubricant in the moving parts of machines. Because of the following
reasons, petroleum is used more than other sources of energy in spite of its harmful
effects to the environment.

1. It is easy to transport.

2. It is cheaper and more easily available than other types of fuels.

3. It is used to run vehicles, many machines and engines in industries and factories.

4. It is used in factories and industries that require more heat and also used to
generate electricity.

c. Hydropower or hydroelectric energy
water
Traditionally, fast flowing water of intake

rivers has been used to run watermills power lines

(ghatta) in the remote hilly areas of our

country. When the wheel of the watermill

rotates due to the energy of fast moving reservoir dam generator
water, it drives various machines connected

to it. outflow
Nowadays, hydropower projects are in

use. In these projects, kinetic energy of fast

running water is converted into electrical turbine

energy. Water is collected by constructing a Fig: 3.2 a hydroelectric plant
high dam. Such collected water is allowed

to fall on the blades of a water turbine. The kinetic energy of the flowing water rotates the

water turbine rapidly, which rotates the armature of the generator that produces electricity.

It has been estimated that total hydropower capacity of Nepal is about 83,000 MW but, at

present, very less amount of electricity is produced through different hydropower projects.

Till the last of 2076 BS, the total electricity production in Nepal is about 1500 MW.

Advantages of using hydropower

1. Although the initial construction cost of hydropower projects is expensive, it seems
to be cheap in a long run.

60 MODERN GRADED SCIENCE CLASS - 10 ENERGY

2. Generating electricity from hydropower does not produce any environmental
pollution like smoke, radiation, dust, etc.

3. Hydropower is a renewable source of electrical

energy which will never get exhausted. About 5% of the world's power is now
produced by hydroelectric power stations.
4. It has multi-purpose use i. e., it is used to run
many types of instruments.

5. It operates many devices such as TV, computer, mobile, lighting and electrical devices.

Alternative source of energy

The renewable sources of energy that are used instead of non-renewable sources of
energy are called alternative sources of energy. In other words, the sources of energy,
which can be used to preserve the non-renewable sources of energy for the future are
called alternative sources of energy. All the perpetual and renewable sources of energy
fall in this category.

1. Bio-fuel

The source of energy which is obtained from plants and animals is called bio-fuel.
Biomass and bio-gas are some examples of bio-fuel. Biomass is any organic material that
can be converted into energy. It includes wood, dung, weeds, agricultural waste and
bagasse. Certain plants and algae, including sugarcane and seaweed, may be grown
especially for use as biomass. Biomass is easily available and renewable i. e. it can be
continually replenished. Biomass energy can be released from biomass by its burning.

Firewood is a common and widely used source of fuel in Nepal. Wood also can be
converted into charcoal. Charcoal is prepared by burning wood in an insufficient supply of air.

In villages, dried animal-dung (dung cake) is also used as fuel in kitchens.

To obtain bio-gas, biomass like dung or excreta of other animals is fermented or
treated with bacteria. Bio-gas is an excellent source of fuel for villages. The gas obtained
by the decay of garbage, cattle dung, sewage and other wastes in the absence of oxygen
is called bio-gas. Bio-gas is a mixture of hydrogen gas, carbon dioxide and methane. In
several thousands of houses in our country, toilet pipes connected to a bio-gas plant are
used for fuel. The government gives a loan and subsidies to farmers for the establishment
of bio-gas plants. Bio-gas can be used to glow lamps and cook food. Nowadays, husk, saw
dust, straw pieces are also pressed highly by machines to make bio-fuel like briquettes.
All the examples given above are bio-fuels.

Advantages of bio-gas
1. It is a renewable source of energy and produces more heat on burning.
2. It is cheaper and can easily be produced at the domestic level.
3. It is environment friendly as it does not cause air pollution.
4. It is used for cooking food, lighting and generating electricity.

2. Nuclear energy

When a heavier nucleus splits or light nuclei combine together, there is a tremendous
release of energy in the form of heat and light. It is called nuclear energy because it is the
product of nuclear reaction (nuclear fission and nuclear fusion).

ENERGY CLASS - 10 MODERN GRADED SCIENCE 61

Nuclear fission: When a heavier nucleus is bombarded with slow neutrons, it splits into
lighter nuclei with the release of a large amount of energy. This process is called nuclear
fission. For example, uranium nucleus (92U235) splits into barium and krypton nuclei with
the release of energy.

U235 + 0n1 56Ba141 + 36Kr92 + 3 0n1 + Q

92

Heavy Nucleus Lighter nuclei

Einstein's mass-energy relation is used for the calculation of nuclear energy. According
to it, mass 'm' can be converted into energy; E equivalent to mc2 [E = mc2], where c is the
velocity of light in vacuum (3 × 108 m/s). Mass-energy relation was founded by Albert
Einstein in 1905 AD. The enormous amount of heat energy released by nuclear fission is
used for producing steam that rotates the blades of generators. It then generates electricity.

Nuclear energy obtained from nuclear fission is non-renewable source of energy but it is
used by some countries only. Thus, it is also considered as alternative source of energy.

Nuclear fusion: When two or more light nuclei combine to form a heavy nucleus under
certain conditions, there is a tremendous amount of energy released in the form of heat
and light. This is called nuclear fusion. For example, hydrogen nuclei fuse together to
form a helium nucleus with the release of energy in the sun and other stars.

2 H + 2 H1 1 2He4 + 21e0 + Q
(De1uterium) (Deu1terium)
Lighter nuclei Heavier nucleus

The fusion reaction does not take place under ordinary conditions. It occurs only
under extremely high pressure and high temperature. Hydrogen bomb is based on
nuclear fusion. It is not possible to generate useful energy by fusion so far on the earth.

Advantages of using nuclear fission energy over fossil fuels

1. A small amount of the nuclear fuel (like uranium-235) can produce as much energy as
produced by huge amount of fossil fuel (like coal). For example, 1 g of uranium can
produce energy equal to that produced by three tons of coal.

2. Once the nuclear fuel is fed into a nuclear reactor, it produces energy for two or three
years. On the other hand, fossil fuels have to be fed at regular intervals in the thermal
power station.

3. As the source is used by very few countries of the world, it can be used as an alternative
source of energy.

Disadvantages of using nuclear energy over fossil fuels

1. The pollution caused by nuclear fuel is much more serious than that caused by fossil
fuel.

2. Harmful waste produced in nuclear fission cannot be stored or dumped in rivers,
seas or open fields as done for fossil wastes. Thus, there is a serious problem in the
disposal of nuclear wastes.

3. Tidal energy

The energy that can be obtained from the tides of seas and oceans is called tidal

62 MODERN GRADED SCIENCE CLASS - 10 ENERGY

energy. The tidal waves in the oceans build up and recede twice a day. The rise of ocean
turbine
level due to the attraction of the moon is called high tide generator

whereas the fall of ocean level is called low tide. The water

raised during the high tide is trapped in dams. This water air chamber

is then allowed to fall down slowly on the blades of water wave

turbines. The rotating turbine drives generators, which

ultimately produce electricity. In Nepal, there is no sea

and ocean, so the source of tidal energy cannot be used

here.

4. Wind energy Fig: 3.3

transformer

wind turbines substation electricity grid house

Fig: 3.4 use of wind energy

The energy obtained from strongly moving air i. e. wind is called wind energy. Wind
is the movement of large masses of air from one place to another. Wind possesses energy
due to its motion. Traditionally, wind energy has been used to separate husk from grains
and to drive windmills, which in turn operates water lifting pumps and flourmills. It is also
used to propel sailboats. Nowadays, wind energy is also used for generating electricity.
Wind energy does not produce any environmental pollution. There are so many high
wind-blowing regions in Nepal, especially in the Mahabharata region. We can reduce our
dependence on scarce petroleum products by installing windmills.

5. Geo-thermal energy

The energy that can be obtained from turbine generator
the heat present inside the earth is called

geothermal energy. The inner part of the steam

earth is extremely hot. It is obvious that at

the time of volcanic eruption or earthquake, hot water injection
the magma (lava) and other gases rush well

out explosively at high temperature. On

average, the temperature rises by 30°C per Fig: 3.5 geothermal power plant

kilometre but by 80°C per kilometre in the

volcanic and earthquake regions. By drilling a hole, if water is allowed to contact to such

hot regions, it instantly gets converted into steam. Such generated steam comes out from

the hole forcibly and rotates the turbines of generators to produce electricity. The energy

obtained in such a way is called geothermal energy.

Present status of energy/fuel

The main sources of energy being used by us are coal, biomass, nuclear energy,
hydro-power, mineral oil and natural gas.

World energy consumption in 2018.:

ENERGY CLASS - 10 MODERN GRADED SCIENCE 63

Sources of energy Percentage

Mineral oil 34%

Coal 27%

Natural gas 24%

Hydroelectric energy 7%

Nuclear energy 4%

Others 4%

Total 100%

The amount of energy consumed by a country depends on the living standard of
its citizens and the degree of its industrialization. For example, the population of the
USA and the previous Russia together is about 1/10 of the population of the entire world.
However, these two countries consume about half of the total energy sources in the world.
Although the population of developing and under-developed countries is high, they need
less energy sources. This is because the degree of industrialization and living standards of
people of these countries are low.

Energy/fuel crisis

Due to the rapid increase in population and industrialization, the demand for energy
is increasing at an alarming rate. Coal, mineral oils and natural gas jointly supply about
80% demand of energy in the world. The demand of energy is increasing at the rate of
2.3% per year. The non-renewable sources of energy will not last forever, because their
amount is limited. They have been formed in the earth over millions of years and they
cannot be replenished rapidly when exhausted.

According to a survey done in 1984 AD, it was estimated that the mineral oil reserves
will have been emptied within 25 to 30 years. New oil reserves in the countries of Middle
East Asia estimate that oil reserves will last for less than 50 years. If the mineral oil deposit
is used in the present ratio that will last up to 2076 AD. But, we know that the energy
demand is increasing by 2.3% per year. In this way, the mined oil will last up to 2037
AD. Thus, the world is likely to face a problem of energy scarcity in near future which
is called energy crisis. Thus, the future scarcity of sources of energy on the earth due to
overpopulation, urbanisation and industrialization is called energy crisis. It may lead the
existence of human beings to hazards.

Causes of energy crisis

1. Overuse of non-renewable sources of energy. From 2008 to 2030, world energy
2. Lack of reliable alternative sources of energy. consumption is expected to increase

3. Limited sources of non-renewable source of energy. more than 55%.

4. High demand of sources energy from overpopulation, industrial, agricultural and
urban development.

5. Lack of concrete plans about the conservation of present sources of energy.

6. Advanced lifestyle or improvement in the living standard increases the consumption
of energy.

64 MODERN GRADED SCIENCE CLASS - 10 ENERGY

7. Improper use or misuse of energy.
8. Consumption of more fuel by less efficient machines or devices.

Energy conservation/energy saving

It is a common saying that "energy saved is energy produced." That is, the solution
of energy crisis is energy saving. We should try to prevent the energy from being wasted
uselessly. Efforts on the individual and global levels must be made to alert about energy
crisis. Energy saving can be done in the following ways:
1. As far as possible renewable sources of energy should be used more.
2. As far as possible non-renewable sources of energy should be used less. In this way,

such sources of energy can be preserved for the future.
3. More alternative sources of energy should be developed and used.
4. Unnecessary use of energy should be avoided.
5. By controlling over population.
6. By conserving the existing sources of energy.
7. The use of bio-fuel for domestic purposes such as lighting and cooking should be

promoted.
In the context of Nepal, we can push energy crisis further by using the following
measures–
a. By using and producing more hydroelectricity
b. By using more bio-fuel
c. By using more solar energy
d. By using more wind and wave energy
e. By controlling over population

S me Reasonable Facts

1. Scientists are busy at designing solar equipment, it is because, solar energy is a
perpetual source of energy and by using it, non-renewable sources of energy can be
conserved for the future. It will push the energy crisis further.

2. Coal and mineral oil are fossil fuels as they are formed from those dead bodies of
plants and animals which were preserved in sedimentary rocks.

3. Nuclear energy can be considered as an alternative source of energy. It is because
nuclear energy is a non-renewable source of energy but very few countries of the
world are able to use it, as there is a big deposit of radioactive substances in the
world, which can be used for several years as the source of energy.

4. Hydropower is a pollution-free source of energy, as any fuel is not burnt to produce it.

5. Energy crisis is a burning issue at present. It is because energy crisis may bring hazards
to the existence of human beings, as in the absence of major sources of energy, they
cannot survive.

6. Crude oil is a primary source of energy. It is because crude oil is the raw oil obtained
from mines and in the same form as it is available in the nature.

ENERGY CLASS - 10 MODERN GRADED SCIENCE 65

7. Sun is the ultimate source of energy. It is because many sources of energy are products of
solar energy directly or indirectly. Fossil fuel is the stored solar energy. The energy was
stored during photosynthesis done by the plants, when they were alive. Hydroelectricity
is also product of solar energy as water cycle is conducted by the sun. Similarly, wind
energy, tidal energy and bio-fuel are also products of solar energy.

Things To Know

1. Substances used for producing heat energy by their burning or nuclear processes are
called fuels.

2. Fuels are obtained from different sources of energy.
3. Sources of energy can be classified into primary and secondary sources of energy on

the basis of the form of energy used.
4. Sources of energy are also classified into non-renewable and renewable sources of

energy on the basis of their replacement duration.
5. Sources of non-renewable energy cannot be replaced quickly when they are exhausted

at their place of origin. Fossil fuels and nuclear energy are its examples.
6. Sources of renewable energy are those which can be replaced quickly, when they are

exhausted at their place of origin. Solar, tidal, geothermal, wind and hydroelectric
energies are some examples of such type of energy.
7. The remnants of plants and animals were buried under the earth millions of years
ago; such preserved dead bodies are called fossils. These fossils are excellent fuels
called fossil fuels.
8. Moving air is called wind. It possesses kinetic energy. A windmill is a machine which
works with the energy of blowing air or wind.
9. The waste materials and dead bodies of plants and animals are called biomass.
10. When a heavier nucleus splits or light nuclei combine together, there is tremendous
release of energy in the form of heat and light. It is called nuclear energy.
11. The future problem of shortage of major sources of energy is called energy crisis.
12. The solution of energy crisis is called energy saving.
13. Alternative sources of energy are those which can be used instead of non-renewable
sources of energy to preserve them.

Things To Do

A. Make your own solar heater.

1. Take a big bucket with a tap.

2. Make a number of turns of a black polythene pipe and connect its two ends with
the bucket as shown in the diagram. (You can use a hot metallic pipe of the same
diameter of the polythene pipe to make holes in the bucket).

3. Keep the unit in the heat of the sun and pour water in the bucket. After some
hours, you can get hot water from its tap.

66 MODERN GRADED SCIENCE CLASS - 10 ENERGY

Test Yourself

1. Multiple choice questions (MCQs).

a. What is the main source of energy?

A. Hydroelectricity B. Sun

C. Mineral oil D. Natural gas

b. How much solar energy falls per square meter on the surface of the earth?

A. 1.5 kW B. 1.4 kW C. 140 kW D. 14 W

c. Which is the chief source of energy in Nepal?

A. Nuclear energy B. Tidal energy

C. Geothermal energy D. Hydroelectric energy

d. Which of the following is a non-renewable source of energy?

A. Coal B. Wood

C. Bio-gas D. Wind energy

e. Who propounded the formula E = mc2?

A. Newton B. Einstein C. Kepler D. Galileo

f. Nuclear energy is derived by …

A. Combustion of atoms of U235

B. Fission of atoms of U235

C. Fusion of atoms of U235

D. The breaking of U235 bonds

g. Solar energy stored in the materials such as wood, sugar, bio-gas is called…

A. Fossil fuels B. Biomass

C. Geothermal energy D. Tidal energy

h. Energy is produced on the sun by …

A. Nuclear fission B. Chemical reaction

C. Nuclear fusion D. Photoelectric effect

2. Answer the following questions.
a. What is fuel? What are primary and secondary sources of energy?
b. Define non-renewable sources of energy with examples.
c. What is fossil fuel? How is it formed? What kind of source of energy is it?
d. What is the cause that coal and petrol are called non-renewable sources of energy?
e. Write the limitation of solar energy.
f. What are renewable sources of energy? Also give examples.
g. Who proposed the relation E = mc2? What do the symbols E, m and c stand for?
h. How is geothermal energy trapped? State a drawback of this kind of energy?
i. What is wind? What type of energy is possessed by it? Give any three uses of this
energy.

ENERGY CLASS - 10 MODERN GRADED SCIENCE 67

j. Name three devices run by solar energy. Mention some traditional uses of solar
energy.

k. What are solar cell and solar panel? What are the conditions under which nuclear
fusion is expected to occur in the sun?

l. What is the advantage of the presence of hydrogen in large scale in the sun?
What are the advantages of solar energy over mineral energy?

m. Write two ways to be protected from the stage of energy crisis.

n. Give the main factor that causes energy crisis. How does nuclear reaction occur
in the sun? Explain with a chemical equation.

o. How can we conserve energy?

p. Which is the most useful alternative source of energy for Nepal? Give two reasons.

q. Define:

i. sources of energy ii. biomass energy iii. energy crisis

iv. nuclear energy v. tidal energy vi. alternative source of energy

vii. solar energy viii. nuclear fuel

r. Write a note on:

i. Nuclear energy ii. Limitations of solar energy

3. Write the difference between:
a. Renewable and non-renewable sources of energy.
b. Fossil fuel and bio fuel.
c. Nuclear fusion and nuclear fission.

4. Give reasons.
a. Energy of fossil fuels is also derived from solar energy.
b. The use of fossil fuels is more than other fuels at present.
c. Enormous energy is released in nuclear reaction.
d. Hydropower is an alternative source of energy.
e. Scientists are seen busy at designing solar equipment.
f. The problem of energy crisis can be solved by developing alternative sources of
energy.
g. The sun is the ultimate source of energy.
h. Hydropower is said to be an indirect source of solar energy.

Bagasse : dry part of sugarcane that is left after extraction of juice
Plasma state
Fossil fuel : a state of matter consisting of partially ionised gas

: the fuel formed by the preserved dead bodies of living beings in
sedimentary rocks

68 MODERN GRADED SCIENCE CLASS - 10 ENERGY

Chapter HEAT

4 Total estimated periods: 6 (T 5 + P 1)

describe the sources and importance of heat.
distinguish between heat and temperature.
explain specific heat capacity and derive the heat equation.
take the measurement of temperature (simple thermometer, clinical thermometer,

digital thermometer).
solve simple numerical problems related to the heat equation.

Heat is a very important source of energy. We use heat energy for different purposes
in our daily life. Heat is the total kinetic energy of molecules that provides us sensation
of warmth. Heat can flow from hotter to colder bodies. We use heat to cook our food. The
heat of the sun makes our body warm. Heat is also used for running vehicles and other
machines.

Heat as energy

Every substance is made up of a large number of tiny particles called molecules. These
molecules vibrate about their mean positions in a very small distance (≈ 10–10 m). Because
of this, the molecules in a motion possess kinetic energy. There is a mutual attraction
between the molecules which can be neglected here for simplicity. The sum of kinetic
energy for all molecules of a body is called its internal energy. Internal energy differs from
heat in the same way as rainwater differs from the water in a lake. Water in the form of
droplets in motion is called rain. After raining, rainwater mixes with lake water; it loses
its identity as rainwater and becomes lake water. Exactly, in the same way, heat differs
from internal energy. Heat and internal energy are the same thing, namely, energy, but
when internal energy is in flow, it is called heat. It is measured by using calorimeter and
the units used for this purpose are calorie and joule.

The total amount of heat possessed by a body i. e. internal energy depends on:
1. The number of molecules of the body i. e. mass of the body. It means larger the

number of molecules, higher is the value of internal energy of a body.
2. Average kinetic energy of the molecules (temperature of the body).

∴ Amount of heat ∝ No. of molecules of the body × Average kinetic energy of
molecules

i. e. Amount of heat ∝ of the body × temperature

HEAT CLASS - 10 MODERN GRADED SCIENCE 69

The nature of molecules is different for different bodies. If two different bodies having
the same mass are heated equally, their average kinetic energy will be different. That is, a
molecule of a body vibrates about its mean position at a different speed from the molecule
of another body at the same temperature.

When a body is heated, the magnitude of the vibration of the molecules of the body
increases. This consequently increases kinetic energy of the molecules and hence the
internal energy of the body rises. When the body is allowed to cool, the magnitude of the
vibration of the molecules begins to decrease and thus its internal energy decreases. That
is, a hot body has more internal energy than another identical cold body. If two bodies,
one hot and another cold, are kept in a thermal contact, the internal energy flows from the
hotter body to the colder body. This energy in flow is called heat. The moment it ceases to
flow, it cannot be called heat. In short, heat is a form of energy that flows from a hot body
to a cold body when they are placed in a thermal contact. In other words, heat is the total
kinetic energy of the molecules contained in a body.

Units of heat energy

Energy is measured in joule in the SI system. As heat is a form of energy, it is also
measured in joule. In the CGS system, it is measured in calorie. The amount of heat energy
required to raise the temperature of 1g of pure water by 1°C or 1K is one calorie heat. One
calorie heat is equal to 4.2 joules of heat. Calorimeter is used to measure heat.

Let's convert 4200 J into calorie i. e. from SI system to the CGS system.
∴ 4200 J = 4200/4.2 calorie ( 4.2 J = 1 calorie)
or, 4200 J = 1000 calorie
Thus, 4200 J and 1000 calorie are equivalent.
1000 calorie is also called one kilocalorie.
One kilocalorie (kcal) heat is defined as that heat which is required to raise the
temperature of 1 kg pure water by 1 °C or 1 K.
We get heat from different sources. The sun is the main source of heat. We burn
different types of fuels like petroleum, coal, wood and others to get heat. All of them are
sources of heat. Electricity is another source of heat as it has heat effect. When different
types of heaters are connected in a closed circuit, they change electrical energy into heat
energy. Heat is also possessed by friction. Thus, there are many sources of heat around us.
Thermo-nuclear reactions also possess heat in a large amount.

Importance of heat

Heat energy is one of the most important sources of energy. It is used for many
purposes in our daily life. Some of them are as the following–
1. It is used for cooking our food.
2. It makes our body warm which is necessary to run our different life processes

normally.
3. Heat for used for drying grains and our wet clothes.
4. Heat is used for running different machineries in industries.
5. Heat is also used for running vehicles.
6. Heat of the sun is used for separating salt from the water of oceans.
7. Heat helps to form clouds from the surface water that helps to rain.
8. Heat causes wind.
9. Heat is used for separating metals from their ores.
10. Heat is used for refining crude oil into petrol, diesel, kerosene, etc.
11. It is used in the manufacture of glass, cement, ceramics, etc.

70 MODERN GRADED SCIENCE CLASS - 10 HEAT

Temperature

When a stainless steel glass filled with boiled water is touched, it feels very hot. If a
piece of ice is touched, it feels cold. The sense of touch is the simplest way to distinguish
hot bodies from cold bodies. Our judgement regarding hotness or coldness of a body is
our sense of temperature. Temperature is a measure of hotness or coldness of a body. A
hot body is said to possess a higher temperature than a cold one. When two bodies, one
hot and the other cold, are kept in contact, heat always flows from the hotter to the colder
body. This flow continues till they are equally hot i. e. till they reach the same temperature.
Temperature is the thermal condition of a body which determines the flow of heat from
it to another body in its contact. It also can be said that temperature is the average kinetic
energy of the molecules contained in a body.

A device which is used for measuring the temperature of a body is called a
thermometer. A thermometer works on the basis of the principle that ''when a body is
heated, it expands and it contracts on cooling.'' It also can be said that thermometers are
based on the property of thermal expansion of liquids. Generally, a liquid is used in a
thermometer. The thermometer, in which mercury or alcohol is used as thermometric
liquids, is called a liquid thermometer.

Thermometric liquids: The liquids used in a thermometer are called thermometric liquid.
Mercury and alcohol are examples of thermometric liquids.

(a) Mercury
Mercury ever remains in the liquid state in quite a wide range of temperature

because it freezes at –39°C and boils at 357 °C. A mercury thermometer cannot be used
for measuring the temperature below –39 °C because it freezes below that temperature.
Therefore, a mercury thermometer cannot be used to measure the temperature in very
hot regions.

(b) Alcohol
Alcohol is also a thermometric liquid. The freezing point of alcohol is –117 °C and the

boiling point is 78 °C. Thus, alcohol remains liquid to a very low temperature but not at
a high temperature i. e. beyond 78 °C. Due to this reason, an alcohol thermometer is used
for measuring the temperature in very cold regions. Normal temperature of the human
body is 37 °C (98.6 °F). The SI unit of temperature is Kelvin.

Temperature scale

General thermometers are calibrated in centigrade scale, fahrenheit scale and kelvin scale.

a. Centigrade scale upper fixed
In this scale, the lower and the upper fixed points are points

0°C and 100°C respectively. The interval between these
two fixed points is divided into 100 equal parts. Each of the
parts is called one degree centigrade. It is denoted by 1°C.

b. Fahrenheit scale

The lower and the upper fixed points in the fahrenheit lower fixed
scale are 32°F and 212°F respectively. The interval between points
these two points is divided into 180 equal divisions. Each of
the divisions is called one degree fahrenheit. It is denoted celsius kelvin fahrenheit
by 1°F. Fig: 4.1 LFP and UFP
c. Kelvin scale

In the kelvin scale, the lower fixed point (LFP) is 273 K and the upper fixed point
(UFP) is 373 K. The interval between these two fixed points is divided into 100 equal
divisions. Each of these divisions is called one kelvin. It is denoted by 1 K.

HEAT CLASS - 10 MODERN GRADED SCIENCE 71

Relation between different temperature scales

To find the relation among different scales of temperature, the freezing point in each
scale is made equal and when the similar freezing point is divided by the difference
between the lower fixed point and the upper fixed point, they give the same value of
temperature. For it C–O, F–32 and K–273 give the similar freezing point. When they are
divided by 100, 180 and 100 respectively, they give a similar value as given below.

C – 0 F – 32 K – 273
100 = 180 = 100

Relation between heat and temperature

Consider, the tea is ready in a kettle by adding all the required things. Now, the tea is
poured in a cup from the kettle. If the tea from both of the vessels is tasted one by one,
which one of them will be sweeter? Certainly, both of them will have the same sweetness.
Think once about the amount of sugar in the kettle and in the cup. Where will be the
maximum amount of the sugar?

We can compare the sweetness with the
temperature and amount of sugar with the
amount of heat. As both of them have the same
sweetness, the temperature of the liquid in both
the vessels is the same. It is because both of
them have some average kinetic energy of the
molecules. If both of them are left to cool, which
one will be cooled faster and why? We will find
that the tea of the cup will be cooled faster than Fig: 4.2 tea at same temperature

the tea in the kettle. It is because the number of molecules in the kettle having the same
average kinetic energy is more than the number of molecules in the cup. Thus, the total
kinetic energy of the molecules will be more in the kettle. It means that the tea of the kettle
has more heat. The sweetness of the tea can be increased by increasing the amount of
sugar. Likewise, the temperature can be increased by heating it more and more.

Let's take another example. Suppose that we have a bucketful of lukewarm water and
a glowing matchstick. Which of them is hotter? Certainly, the glowing matchstick may
burn us because it has more average kinetic energy of the molecules (temperature). But
when they are left to cool, the matchstick will cool faster, because the number of vibrating
molecules is less in it, thus it has less heat. If the glowing matchstick and the bucket of
water are kept in thermal contact, what will be the direction of the flow of heat? It will be
either from the matchstick to the bucket or from the bucket to the matchstick? Discuss to
find the possible answer with reason.

Difference between heat and temperature

S. No. Heat Temperature
1.
It is a form of energy which gives the It is the degree of hotness or coldness
2. sensation of coldness and hotness. of a body.
3.
It is a cause of change in temperature. It is the effect of heat.
4. It is a measure of the total kinetic It is the measure of average kinetic

energy of all the molecules in a body. energy of the molecules in a body.

Its SI unit is joule (J). Its SI unit is kelvin (K).

5. Heat does not decide its flow itself. Temperature decides the flow of heat as it
flows from higher to lower temperature.

6. Calorimeter is used to measure it. Thermometer is used to measure it.

72 MODERN GRADED SCIENCE CLASS - 10 HEAT

Types of thermometer

There are different types of thermometers in use. Clinical thermometer, simple
thermometer, maximum-minimum thermometer are some examples of liquid
thermometers. Pyrometer, digital thermometers, etc. are some other examples. Some of
them are described below.

a. Clinical thermometer stem with scale

A clinical thermometer is used to measure the temperature of capillary with
human body. It is also called a daddy thermometer and doctor's mercury
thermometer. It is smaller in size and it has less range of scale i. e. constriction
35°C to 42°C or 94°F to 108°F compared to other thermometers. It mercury bulb
may be oral, armpit, rectal, or ear type. It may be mercury or digital.
You will study separately about digital thermometers. Fig: 4.3 clinical thermometer

The structure of a clinical thermometer shows a bulb filled with
mercury at its one end and a glass stem. The bulb has a thin wall
while the prismatic stem has a thick wall with very fine bore called
capillary. The capillary has a bent narrow part above the bulb which
is called constriction. It blocks the back flow of the raised mercury
level. The stem is calibrated with scale.

When the bulb is kept in contact with the body for one to
two minutes, the mercury in the bulb expands and rises up in the
capillary. The scale at the mercury level in the capillary shows
temperature. For further use of the thermometer, it is jerked to set
the mercury level below the normal body temperature.

b. Laboratory thermometer

Laboratory thermometer is also called a mercury bulb capillary with stem with scale
mercury

simple thermometer. It is quite similar to a clinical

thermometer in structure and working method. It 100
90
does not have a constriction in its capillary. It is used 80
to measure the temperature of materials. Usually, it 70
60
50
40
30
20
10
0
10
20
30

Fig: 4.4 simple thermometer

is longer than the clinical thermometer and it has more range of scale usually of 10°C to

110°C.

To measure the temperature of any substance, the bulb of the thermometer is kept
is contact with the substance. The changed mercury level in the capillary shows the
temperature of the substance.

c. Digital thermometer cap over
the battery
Digital thermometer is an electronic thermometer. It has a
wide range of scale on its LCD display. The thermometer has a LCD display
thermistor at its one end. It is also equipped with a beep alarm and
a memory function. A cap is found at the top of the thermometer.
It is made of thick, hard and smooth plastic which protects the
battery kept under it. When the thermometer is kept in contact
with a body, the temperature is displayed on the screen.

Nowadays, infra-red sensor is also used to make non-contact

electronic thermometer.

d. Maximum and minimum thermometer thermistor

A maximum-minimum thermometer is a device that records Fig: 4.5 digital thermometer

the highest and the lowest temperatures reached over a certain time period or in a day of

a given area. It is made up of U- shaped parallel glass tubes and two bulbs. One side of

HEAT CLASS - 10 MODERN GRADED SCIENCE 73

the U tube is registered for minimum temperature and the other for maximum temperature.

The U tube of the thermometer contains mercury which moves up or down on the basis

of expansion and contraction of alcohol which is filled just above the mercury.

When the temperature of the environment left bulb right bulbvacuum
rises, the alcohol expands and pushes the

mercury up in the maximum column and this °C alcohol °C
also pushes the mercury down in the minimum
column. When the temperature of the –25 40
environment decreases, the alcohol contracts –20 35 maximum
and pulls the mercury up in the minimum –15 30 temperature
column which also causes the mercury to fall –10
of 30°C
25

in the maximum column. The steel indexes are –5 20
located on the surface of the mercury and move 15
along with the flow of mercury up and down. –0 steel 10
indexes
5

When the temperatures reach their minimum 10 5
and maximum, the steel indexes remain at 0
that place. This allows for the readings of both maximum 15 –5
the minimum and maximum temperatures temperature 20 –10
simultaneously without constant monitoring 25 –15
of the thermometer. The steel indexes can of 24°C 30 –20
be returned to the surface of the mercury by –25
35 mercury
using small magnets.
40

Specific heat capacity

The same amount of two different types of substances is taken into two different
vessels of the same size. If both of them are heated by giving equal amount of heat for
5 minutes, they show different temperatures. Can you say, why? It is because of their
different specific heat capacities.

Q
or, S = m.dt ............ (iv)
The specific heat capacity of a body is defined as the amount of heat required to raise
the temperature of a unit mass of that body by 1°C (or 1K).

Different substances have different specific heat capacities. The bodies which have
more specific heat capacity change their temperature slowly and those which have less
specific heat capacity change their temperature faster. Water has the maximum specific
heat capacity among the known substances. The SI unit of the specific heat capacity is J/
kgK but J/kg °C is widely used.

Specific heat capacity of some materials

S.No. Materials 'S' in Jkg–1 °C–1 S.No. Materials 'S' in Jkg–1 °C–1
1. Lead 126 11. Aluminium 910
2. Gold 130 12. Air 993
3. Mercury 140 13. Petrol 1670
4. Silver 234 14. Water vapour 460
5. Brass 380 15. Olive oil 2000
6. Copper 380 16. Ice 2100
7. Steel 447 17. Paraffin 2200
8. Iron 470 18. Alcohol 2590
9. Glass 670 19. Water 4200
10. Sand 800 20. Kerosene 2090

74 MODERN GRADED SCIENCE CLASS - 10 HEAT

Some effects of specific heat capacity

1. Water is used to cool the engines of vehicles. Water has a very high specific heat
capacity (4200Jkg–1°C–1) compared with other liquids. Thus, it can absorb a large
amount of heat from the hot engines without any notable rise in the temperature of
water. It, thus, prevents the engines from getting excessively hot.

2. The specific heat capacity of water is very high. When water is cooled by the same
range of temperature, it gives the same amount of heat as it has absorbed. Hence,
a large amount of heat may be available by cooling hot water. That's why, water is
suitable for heating systems.

3. The night in a desert is very cold while the day is very hot. It is because the specific
heat capacity of sand is less; so its temperature changes fast. During days, the sand
gets heated which results in a very hot day. At night, the sand gets cooled faster
which causes a very cold night.

4. Sea breeze occurs light and heat from the sun

at day and land hot air cold air
breeze occurs

at night. It is

because water cold air hot air

has maximum sea land sea land
a. sea breeze b. land breeze
specific heat

capacity and Fig: 4.6

sand has less specific heat capacity. Due to it, at day, the sand of land is heated faster

than the water and the air above it becomes less dense by its expansion. Thus, the air

above the sea being denser blows towards the land and the sea breeze occurs. But at

night, the seawater cools slowly but the sand cools faster.

Now, the air above the seawater becomes less dense and the air above the land is
denser. Thus, the air over the land moves towards the sea and there occurs land breeze.

Heat equation

Generally, when a body is heated, it gains heat and its temperature rises. Unlike it,
when a body is cooled, it loses heat and its temperature falls.

Let the temperature of a body of mass 'm' raise from t1 to t2, when it is heated by
supplying the amount of heat Q.

Here, mass of the body = m

Initial temperature = t1
Final temperature = t2
Amount of heat supplied = Q

∴ Change in temperature (dt) = t2 - t1
It has been found that heat gained or lost by a body is directly proportional to (i) the
mass of the body and (ii) the change in its temperature.

∴ Q ∝ m ........ (i)

Q ∝ dt ......... (ii)

Combining equations (i) and (ii), we have

Q ∝ mdt

∴ Q = Smdt

HEAT CLASS - 10 MODERN GRADED SCIENCE 75

or, Q = mSdt .......... (iii)

Here, Q = Heat gained or heat lost

m = mass

s = specific heat capacity

dt = change in temperature

Where, s is a constant of proportionality and also called specific heat capacity of the
material of the body. It is constant for a given material whatever be its shape, size, mass,
etc. The equation (iii) is called heat equation. It states that the product of mass, specific
heat capacity and change in temperature is equal to the heat gained or heat lost.

Example 1: How much heat is supplied to raise the temperature of 100 g of water from 5°C
to 90°C? (sp. heat capacity of water is 4200 Jkg–1°C–1).

Solution:

Here, mass of water (m) = 100 g = 100 kg = 0.1 kg
1000

Initial temperature (t1) = 5 °C
Final temperature (t2) = 90 °C

∴ Rise in temperature (dt) = t2 - t1 = 90 - 5 = 85 °C
Specific heat capacity of water (S) = 4200 Jkg–1 °C–1.

∴ Amount of heat supplied (Q) = ?

We have,

Q = msdt

= 0.1 × 4200 × 85 = 38700 J

Hence, the amount of heat supplied to water is 35700 J.

Example 2: If 50 kJ of heat is transferred to 10 kg of water, what is the rise in its temperature?
The specific heat capacity of water is 4200 Jkg–1°C–1.

Solution:

Here, Heat supplied (Q) = 50 kJ = 50 × 1000J ( 1 kJ = 1000 J) = 50000 J

Mass of water (m) = 10 kg

Specific heat capacity of water (S) = 4200 Jkg–1 °C–1.

Rise in temperature (dt) = ?

We have,

Q = mSdt

or, dt = Q = 50000 = 1.19 °C
mS 10×4200

Therefore, the rise in temperature of water is 1.19 °C.

Principle of calorimetry The thermal capacity of a body is
defined as the amount of heat required
We know that when two bodies at different to raise the temperature of the body by
temperatures are kept in a thermal contact, heat 1°C. Mathematically, C = m×s.
flows from the hotter to the colder body. According
to the principle of conservation of energy, energy
cannot be destroyed or created. Thus, heat energy
lost from the body at the higher temperature is

76 MODERN GRADED SCIENCE CLASS - 10 HEAT

equal to the heat energy gained by the body at the lower temperature. It is assumed that
there is no loss of heat to the surroundings. That is, there will be exchange of heat between
hot and cold bodies only. Such exchange of heat continues until they attain the same
temperature. It is called the principle of calorimetry.

∴ Heat lost = Heat gained ......... (i)

(by a body at higher temperature) (by a body at lower temperature)

or, m1 × S1 × dt1 = m2 × S2 × dt2

Example 1: If 200 ml of tea at 90°C is mixed with 10 ml of milk at 15 °C, what will be the
final temperature of the mixture? Assume that the specific heat capacity of milk equals to
that of the tea (Suppose the mass of 1ml of milk = mass of 1ml of tea = 1 g).

Solution:

Here, mass of tea (m1) = 200 ml = 200g ( mass of 1ml of tea = 1g)
= 0.2 kg ( 1000 g = 1kg)

Temperature of tea (t1) = 90 °C

Mass of milk (m2) = 10 ml = 10 g ( mass of 1 ml of milk = 1g) = 0.01 kg

Temperature of milk (t2) = 15 °C.

Let 't' be the final temperature of tahcecomrdilikn.gIftoS1thanedquSe2 satrieonthSe1 specific heat capacities
of tea and milk respectively, then = S2.

Heat lost = Heat gained

m1 S1dt1 = m2 S2dt2
0.2 S1 (90 - t) = 0.01 . S2 (t - 15) [ specific heat capacity is same]
or, 18 - 0.2t = 0.01t - 0.15

18 + 0.15 = 0.01t + 0.2 t

18.15 = 0.21t

108.2.115 = t

∴ t = 86.43°C

Hence, the temperature of the mixture is 86.43°C.

Example 2: Calculate the final temperature when 2400J of heat is given to the iron of mass
2 kg at 20 °C. (The specific heat of the iron is 460 J/kg°C.)

Solution: Antimony behaves like water. It expands
Here, while freezing and contracts at first when
Heat supplied (Q) = 2400J warmed above its melting point. Because of
Mass of iron (m) = 2Kg such expansion, it makes sharp castings.
Initial temperature (t1) = 20°C
Final temperature (t2) = ?
Specific heat capacity (S) = 460J/Kg°C

We have,

Q = m× S × (t2–t1)
Q
∴ t2 - 20°C = m×s

HEAT CLASS - 10 MODERN GRADED SCIENCE 77

or, t2 - 20°C = 2400
2×460

or, t2 - 20°C = 2.60

or, t2 = 2.6 + 20 = 22.6°C
Therefore, the final temperature of the iron mass is 22.6°C.

S me Reasonable Facts

1. In summer, the atmospheric temperature goes up higher than our normal body
temperature (37°C). To maintain our body temperature fixed at 37°C, our body loses
the excess heat by excreting sweat. So, we sweat in summer.

2. In winter, the atmospheric temperature falls below our normal body temperature
(37°C). Due to this difference in temperature, our body cools losing its heat. To
prevent our body from excessive cooling, we need warm clothes in winter.

3. In an earthen pot, water oozes out through the pores which is evaporated. Evaporation
of the oozed water takes heat from the water inside the vessel and hence it cools the
water inside it. Such type of oozing does not occur in metal pots. So, water in an
earthen pot is cooler than the water in a metal pot.

4. Water cannot be used instead of mercury in thermometers. It is because of the
following reasons.
a. Water has maximum specific heat capacity thus it takes a long time for water to
change its temperature.
b. Water is invisible in the glass capillary as it is clear or colourless.
c. Water has irregular expansion. (anomalous expansion)

d. Water sticks on the glass surface.

5. A clinical thermometer has constriction in its capillary. It is because the constriction
blocks the raised mercury level from falling down when it is used. Due to this, the
scale can be read conveniently.

6. The stem of a clinical thermometer is made prismatic. The prismatic shape magnifies
the size of the capillary while reading the temperature. If it is not prismatic, the
mercury level cannot be seen easily inside the capillary of a clinical thermometer.

7. The bulb of a thermometer has a thin wall but the stem has a thick wall. We know
that glass is an insulator of heat. The thin layered wall of the bulb reduces the
resistivity to pass heat easily to the mercury. Unlike it, the thick wall of the stem
resists environmental heat to effect the expansion of raised mercury level.

Things To Know

1. Heat is a form of energy which flows from a hot body to a cold body when they are
placed in a thermal contact.

2. Temperature is a measure of hotness or coldness of a body.
3. The SI unit of heat is joule and the SI unit of temperature is kelvin.
4. Heat is measured by using a calorimeter and temperature is measured by using a

thermometer.
5. Thermometers are of many types. They are simple, clinical, digital thermometer, etc.
6. A clinical thermometer is used to measure body temperature.
7. Digital thermometers have thermistor, LCD display and plastic cap. It measures

temperature.

78 MODERN GRADED SCIENCE CLASS - 10 HEAT

8. The product of mass, specific heat capacity and change in temperature of a body is
equal to the heat gained or heat lost. It is called heat equation.

9. The amount of heat gained is equal to the amount of heat lost; the relation is called
principle of calorimetry.

10. 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 body by 1°C (or 1K).

11. Different substances have different values of specific heat capacity.
12. The substances having more specific heat capacity change their temperature slowly

and the substances having less specific heat capacity change their temperature fast.
13. Due to the maximum specific heat capacity of water, it is widely used to heat and cool

things.

Things To Do

Take two identical beakers, A and B. Pour thermometer thermometer
equal mass of water and edible oil in the beaker A
beakers separately. Record the temperature beaker B
of the water and edible oil by dipping water edible oil
two thermometers into them. Let θ1 be tripod stand
their common temperature. Heat them by tripod stand
two identical spirit lamps for about two burner burner
minutes. Record the temperatures of the
water and the edible oil again. Are they at
the same temperature in this case? Discuss.
Why is it so?

Test Yourself

1. Multiple choice questions (MCQs).

a. The average kinetic energy of the molecules contained in a body is …………….
of that body.

A. Heat B. Temperature

C. Specific heat capacity D. Heat equation

b. Which of the following is a heat equation?

A. m = Q×S×dt B. S = m × Q × dt

C. Q = m × s × dt D. dt = m × S × Q

c. 1 calorie heat is equivalent to:

A. 1.2 J B. 2.2 J C. 3.2 J D. 4.2 J

d. Which of the following is the SI unit of temperature?

A. Kelvin B. Degree Celsius C. Fahrenheit D. Joule

e. Thermistor is found at one end of:

A. Clinical thermometer B. Simple thermometer

C. Pyrometer D. Digital thermometer

HEAT CLASS - 10 MODERN GRADED SCIENCE 79

f. What is the range of temperature on the scale of a clinical thermometer?

A. 42°F to 100°F B. 98°F to 108°C

C. 94°F to 108°F D. 35°F to 42°F

g. Land breeze occurs during:

A. Day B. Night C. Morning D. Evening

h. Which of the following is the specific heat capacity of mercury?

A. 240 J/kg°C B. 140 J/kg°C C. 340 J/kg°C D. 410 J/kg°C

i. Which of the following is the specific heat capacity of water?

A. 4200 J/kg°C B. 4100 J/kg°C C. 2100 J/kg°C D. 2200 J/kg°C

j. Which set of the following are freezing and boiling points of alcohol?

A. -39°C and 78°C B. -39°C and 457°C

C. -117°C and 78°C D. -117°C and 457°C

2. Answer the following questions.
a. What is heat? How is it different from temperature?
b. On what factor does heat depend? What is the relation of heat with its factors?
c. Give the concept of heat and temperature on the basis of its molecular motion.
d. Define.
i. Specific heat capacity
ii. One calorie heat
iii. Heat equation
e. Clarify the following statements.
i. The specific heat capacity of water is 4200 Jkg–1°C–1.
ii. The specific heat capacity of mercury is 138 Jkg–1°C–1.
f. Describe the structure of a digital thermometer with a diagram.
g. Explain the structure of a clinical thermometer.

3. Differentiate between:

a. Heat and temperature.
b. Simple thermometer and clinical thermometer.

4. Give reasons.

a. Water is used to cool hot engines of vehicles.
b. During high fever, a piece of wet cloth is kept on the forehead of the person.
c. When a beaker filled with water at 4°C is cooled or heated, the water overflows

from the beaker.
d. Mercury is heated faster than water.
e. Clinical thermometer has constriction above the bulb.
f. When we get out of bed on a very cold morning, we feel the air of the room cold.

But when we come back staying out for some time, we feel the air of the same
room warmer.
g. Water is used in hot water bags.
h. Digital thermometer is more advance than mercury thermometer.
i. Water cannot be used as a thermometric liquid.
j. Temperature of boiling water cannot be measured by using an alcohol thermometer.

80 MODERN GRADED SCIENCE CLASS - 10 HEAT

5. Diagrammatic questions

a. Answer the questions on the basis of the given table. Sp. heat
i. If A, B and C are taken in equal mass and heated Substances capacity

by giving equal heat, which one of them gains the A 2100 J/kg°C
maximum temperature and why ? B 910 J/kg°C

ii. If all of them are liquid, which one of them is C 138 J/kg°C

suitable for cooling and heating purposes? Why?

iii. If equal mass of A and B are taken at 80°C and left
to cool, which will be cooled faster and why?

iv. If all of them are liquid, which is suitable as a thermometric liquid? Why?

b. Study the given table and answer the following questions.

i. If the equal mass of X, Y and Z has the same Substances Sp. heat
temperature, which one has maximum heat? capacity

ii. If three pieces of them have equal temperature and X 380 J/kg°C
equal amount of heat, which one of them has the Y 910 J/kg°C
maximum mass? Z 470 J/kg°C

iii. What do you mean by specific heat capacity of 'Z' is
470 J/kg°C?

iv. If the equal mass of the same shape and size of them at 100° C temperature is
kept over a wax slab, which of them will melt the wax for the maximum depth?

c. Answer the following questions on the basis of the given diagram.

i. Name the instrument and mention its one use. B
AC
ii. Name A, B and C.

iii. Write the functions of A, B and C.

iv. How is it different from a digital
thermometer?

6. Numerical problems

a. If the specific heat capacity of copper is 380 Jkg–1°C–1, what is the thermal capacity
of 5 kg of copper? [Hint : thermal capacity (C) = m×s]

b. The specific heat capacity of a substance is 0.5238 cal g–1°C1. Convert it into SI
system.

c. How much heat energy is required to raise 5 kg of water from 30 °C to 100 °C?
The specific heat capacity of water is 4200 Jkg–1°C1 .

d. Calculate the specific heat capacity of water if 2 kg water at 25 °C requires 2.1×105
J heat energy to increase its temperature to 50 °C.

e. One kilogram of paraffin requires 44000 J heat to raise its temperature by 20 °C.
Find the amount of heat energy required to raise the temperature of 5 kg paraffin
by 10 °C.

f. A pressure cooker of mass 2 kg is at the temperature of 25 °C. If 6400 J of heat
energy is supplied to it, what is its temperature? The specific heat capacity of the
material of the pressure cooker is 1000 J kg–1°C1.

g. Hot water at 100 °C is added to 300 g of water at 0 °C until the final temperature
is 40 °C. Find the mass of the hot water added. The specific heat capacity of water
is 4200 Jkg–1°C1.

HEAT CLASS - 10 MODERN GRADED SCIENCE 81

h. The temperature of 600 g of a certain metal is 100 °C. It is then placed in 300
g of water at 15 °C. If the final temperature is 20 °C, calculate the specific heat
capacity of the metal.

i. What is final temperature of the mixture if 100 g of water at 70 °C is added to
200 g of cold water at 10 °C? The specific heat capacity of water is 4200 Jkg–1°C1.
(Neglect heat absorbed by the container).

j. The mass of a block is 1 kg. An electric heater of power 48 W takes 5 minutes
to raise the temperature of the block from 20 °C to 50 °C. Find the specific heat
capacity of the block.

k. The temperature of water is 5 °C in winter season. If 20 liters water has to be
heated to 35 °C for taking a bath, calculate the amount of heat required for it.
(Specific heat capacity of water is 4200 Jkg–1°C1, mass of 1 litre of water = 1kg)

l. For taking a bath, water at 40 °C is required. Calculate the mass of cold water at
15 °C which is to be added to 60 kg water at 100 °C for bathing purpose.

a. 1. 9×103 J/°C b. 2.1×103 J kg–1°C–1 c. 1.47 × 106 J d. 4200 J kg–1°C–1
e. 1.1 × 105 J f. 28.2°C g. 0.2 kg h. 131.25 J kg–1°C–1
i. 30°C j. 480 J kg–1°C–1 k. 2.52 × 106 J l. 144 kg

Quartz : a kind of hard mineral, especially crystallized silica

Breeze : gently moving air

Lower fixed point : melting point of ice at sea level is called lower fixed point

Upper fixed point : temperature of steam at sea level is called upper fixed point

82 MODERN GRADED SCIENCE CLASS - 10 HEAT

Chapter LIGHT

5 Total estimated periods: 7 (T 5 + P 2)

give a brief introduction of lens and its types.
explain the refraction of light through lenses.
demonstrate the ray of light passing through a lens and sketch a ray diagram

related to convex and concave lenses.
describe the uses of lenses in daily life.
explain the defects of vision and their remedy.

Light is a form of energy which can be changed into other forms of energy. For
example, electric energy can be transformed into light energy and light energy can be
transformed into electric energy. So, we can say that light is a form of energy. Light energy
is formed from natural sources like the sun and also from the artificial sources like electric
bulbs, fluorescent lamps, etc. The visible light is a form of energy that affects our eyes and
produces the sensation of sight.

The objects around us can be seen only in the presence of light or at day time. But such
objects cannot be seen in the absence of light or at night. It shows that light is essential to
see any objects. Light makes the objects around us visible. However, light itself is invisible
energy. When light falls on any object, it is reflected from them. The reflected light reaches
the retina of our eyes and then to the brain through the optic nerves. Then it produces the
sensation of sight of the objects. The sensation of sight of the objects is formed in our eyes
and then we see the objects. Thus, light is an invisible form of energy that effects our eyes
and produces the sensation of sight. We cannot see such objects distinctly in many cases.
We can use various optical instruments to see objects in that case. Lenses and mirrors
are used in optical instruments. In this lesson, we are going to study about lenses and
their uses, defect of vision and their remedy and optical instruments such as microscope,
binoculars and telescope.

Lens c. Convex lens b. Concave lens
Fig: 5.1
Lens is a useful optical device which is used in various
optical instruments like microscopes, telescopes, binoculars,
cameras, etc. The human eye is one of the most wonderful
organs which contains a lens. The eye lens of the human eye
refracts the light coming from the objects. The refracted rays

LIGHT CLASS - 10 MODERN GRADED SCIENCE 83

fall on the retina and form the images of the objects. A lens is an optical device which is
made up of transparent refracting medium bounded by at least one spherical surface. It is
usually made up of glass. It is of two types: convex lens and concave lens.

Convex lens

A lens that is thicker

at its middle than at the focus F F

edges is called a convex lens.

Biconvex, concavo-convex

and planoconvex are different a. biconvex b. plano- c. concavo-
kinds of convex lenses. convex convex

When a parallel beam Fig: 5.2 Fig: 5.3 convex lens

of light is incident on a convex lens, it converges this beam to a point after the refraction

through the lens. So, a convex lens is also called a converging lens. Depending upon the

position of objects, convex lenses form real as well as virtual images and such images may

be diminished or magnified.

Concave lens

A lens that is thinner at Focus F F
its middle than at the edges is
called a concave lens. Biconcave, a. plano- b. biconcave c. convexo- Fig: 5.5 concave lens
plano-concave and convexo- concave Fig: 5.4 concave
concave lens are the different
types of concave lenses.

When a parallel beam of light is incident on a concave lens, this beam appears to
diverge from a point after the refraction through the lens. So, a concave lens is also called
a diverging lens. Concave lens always produces a virtual and diminished image wherever
be the position of the object.

Difference between convex lens and concave lens

Convex lens Concave lens

1. It is thicker at the middle and thinner 1. It is thinner at the middle and thicker at

at the edges. the edges.

2. It converges a parallel beam of light 2. It diverges a parallel beam of light after

after refraction through it. refraction through it.

3. Its focus is real. 3. Its focus is virtual.

4. Depending on the object distance, it 4. It forms a virtual image.

forms both real and virtual images.

5. The virtual image formed by a convex 5. The virtual image formed by a concave

lens is always magnified. lens is always diminished.

A lens in the form of sections of prism F F
LIGHT
A convex lens is said to be made up of many Fig: 5.6 converging lens
prisms, with a thick glass slab in the middle as in the
figure. The bases of the upper prisms face
downwards and the bases of the lower prisms face
upwards. As we move towards the edges of the lens

84 MODERN GRADED SCIENCE CLASS - 10

from the middle, the angle of the prism increases. As the angle of the prism increases, the

deviation of the incident rays also increases. Thus, if the parallel rays of light fall on a prism,

the rays incident near the centre are bent least and the rays incident at the edges are bent the

most. The ray of light passing through the centre of the lens (glass stab) is not deviated. In a

convex lens, all the refracted rays are deviated towards the centre and meet at point F. Thus,

the convex lens converges the parallel rays of light. As a result, all rays meet at a point i. e. F.

Similarly, a concave lens is also maximum deviation
said to be made up of many prisms or bending of ray

with a thin glass slab in the middle as

shown in the figure. The bases of the F F no deviation
upper prisms are directed upwards

and the bases of the lower prisms are

directed downwards. As we move

towards the edges from the centre, the

angle of prisms increases. As the angle Fig: 5.7 diverging lens maximum deviation
increases, the deviation of the incident or bending of ray

rays also increases. The rays of light passing through the centre, the refracted ray is not

bent. As we move towards the edges, the incident rays falling on the prisms get deviated

more than near the centres.

In a concave lens, the rays of light are deviated away from the centre and all the
refracted rays appear to come from point F. Thus, a concave lens diverges parallel rays of
light. As a result, all the refracted rays appear to come from point F.

The shape of lenses causes the difference in deviation of the rays of light at the centre
and at the edges.

Some terms related to lens

Centre of 1 2 1 2
curvature: The C2 C1
convex lens and X C1 C2 YX Y
the concave lens O
O

are assumed to

be bounded by r1 r2 Fig: 5.8 r1 r2
two spherical convex lens concave lens
surfaces with

centres C1 and C2 as shown in the figures (a) and (b). The centres (C1 and C2) of the spheres
whose parts form a lens are called centres of curvature. A lens has two centres of curvature

C1 and C2. Usually, C1 and C2 are denoted by 2F as they are found at double distance of the
focus. The radius of a sphere whose surface forms a part of the lens is called radius of

curvature. As a lens is bounded by two spherical surfaces, it has two radii of curvature r1
and r2.

Principal axis: It is an imaginary

straight line passing through two centres X O YX O Y
of curvature (C1 and C2) of a lens. In the
figures, the straight line XY represents the

principal axis. convex lens concave lens

Fig: 5.9

LIGHT CLASS - 10 MODERN GRADED SCIENCE 85

Optical centre: The geometrical centre of a lens is called its optical centre (O). If a ray
of light passes through the optical centre 'O', it does not deviate. Any distance related to
the lens is measured from this point.

Principal focus: When a parallel beam of light is incident on a lens they meet or
appear to meet at a point on the principal axis after refraction through it. This point is
called principal focus of the lens. It is denoted by F.

In the figure, a parallel beam of light incident on the convex lens, parallel to its
principal axis, actually meets at point F on its principal axis after refraction through it.
Hence, its principal focus is real.

In the figure, a parallel beam of light F F F F
incident on the concave lens, parallel to its F O O
principal axis, diverges after refraction
through it. If this divergent beam is convex lens Fig: 5.10 concave lens
produced back, it meets at point F on its
principal axis. Hence, its principal focus is F F F
virtual. O O

As light may pass through either
side of a lens, there are two principal foci
on either side of the lens as shown in the
figures. These two principal foci are at an
equal distance from the optical centre of the
lens.

Focal length (f): It is the distance ff Fig: 5.11 ff
between the principal focus (F) and the convex lens concave lens
optical centre (O) of a lens. It is usually
denoted by 'f '. It is positive (+) for a convex
lens and negative (–) for a concave lens.

Activity 5.1

To measure focal length of a lens

Materials required: A lens, white paper to use as screen.

Method:

1. To measure the rough focal length of a convex lens move a convex lens in front
of a piece of white paper until a sharp image of a distant object such as cloud is
obtained.

2. The distance between the lens and the paper is the approximate focal length.

A thick convex lens has shorter focal length than that of a thin convex lens of the
same diameter.

The process of making a sharp image white paper convex lens
on a screen with a convex lens by changing
the distance between the lens and its screen rays from distant cloud
is called focusing. Focusing is done in
microscopes to view the image of tiny objects focal length
clearly. Our eyes also do the same.
Fig: 5.12

86 MODERN GRADED SCIENCE CLASS - 10 LIGHT

Rules of refraction through lens

The position and nature of the images formed by a lens can be obtained either from a
ray diagram or by calculations. The following rules should be remembered for the
construction of ray diagrams in both convex and concave lenses.

i. An incident ray of light, passing F FF F
parallel to the principal axis, refracts
through the principal focus.

convex lensFig: 5.13 rule 1 concave lens

ii. An incident ray, passing through the FO FFO F
optical centre (O), emerges without
deviation after refraction in both convex
and concave lenses.

convex lens Fig: 5.14 rule 2 concave lens

iii. An incident ray passing through 2F F O F 2F FO F
the principal focus in convex lens,
refracts parallel to the principal convex lens concave lens
axis appearing to meet as it Fig: 5.15 rule 3
emerges parallel to the principal A' 2F
axis after refraction. P B'
F
While constructing a ray B
diagram, the above mentioned A 2F FO
types of rays are drawn from the u fv
same point of an object. This will
be clear from the example given 2f
below. Fig: 5.16 convex lens

In the given figure, F is the principal focus of a convex lens and O is its optical centre.
Thus, the line OF represents the focal length (f). The point 2F on the principal axis is at
a distance of 2f from the optical centre O. If f = 3 cm, the point 2F is at a distance of 6 cm
from O.

BP and BO are two incident rays drawn from the same point B of the object AB
which lie beyond 2F of the convex lens. The ray BP is parallel to the principal axis. After
refraction at P, it passes through F along PB'. The ray BO passing through O emerges
without deviation along OB'. Two refracted rays PB' and OB' meet at B'. Hence, A'B' is the
real and inverted image of AB. The image A'B' is found to lie in between F and 2F.

The distance between the object and the optical centre O is called object distance (u).
The distance between the image and the optical centre is called image distance (v).

In the figure given above, OA' = image distance (v) OF = focal length (f)
OA = Object distance (u)

LIGHT CLASS - 10 MODERN GRADED SCIENCE 87

Construction of ray diagrams by a convex lens

The size, location and nature of the image formed by a convex lens depend on the
position of the object. They are given below with illustrative diagrams.

1. When an object is at infinity, parallel rays image at focus

coming from it meet at the focus after refraction
through the convex lens. Thus, the image is object at ∝ 2F
formed at the focus. The image is real, inverted F O F 2F

and highly diminished. This type of image Fig: 5.17 an object at ∝ convex lens

formation denotes the action of the objective

lens of a camera and astronomical telescope.

2. When an object is placed beyond 2F, its image is formed between F and 2F on the
other side of the object in the convex lens. The image is real, inverted and diminished.
This type of image formation is used in a photographic camera.

B P

object FO F A' 2F
A 2F
image
Fig: 5.18 an object at 2F convex lens B'

3. When an object is placed at 2F, its image is formed at 2F on the other side of the object
in the convex lens. The image is real, inverted and of the same size as that of the object.
This type of image formation is used in terrestrial telescope and photocopier etc.

BP

object F A' 2F
A image
2F F O

Fig: 5.19 an object between 2F and F convex lensB'

4. When an object is placed between F and 2F, its image is formed beyond 2F on the
other side of the object in the convex lens. The image is real, inverted and magnified.
This type of image formation is used in side projectors to produce a magnified image
on the screen.

B P F 2F A'
O image
object F
2F A

Fig: 5.20 an object at F convex lens

B'

5. When an object is placed at F, the rays BP

coming after refraction through the A O F 2F
convex lens become parallel. These F
2F

parallel rays meet at infinity forming image at infinity

an image there. The image is real, Fig: 5.21 an object at F convex lens

inverted and highly magnified. This

type of image formation is used in making spot lights in theatres, search light, etc.

88 MODERN GRADED SCIENCE CLASS - 10 LIGHT

6. When an object is placed between F and O, the convex lens forms an image on the
same side as the object. The image is virtual, B'

erect and magnified. This type of image

formation is used in simple microscope. AB ⇒ object

The use of a convex lens in this position is image A'B' ⇒ image

also called simple microscope. B

From the given ray diagrams, it is clear 2F F F 2F
that as an object approaches the lens, the image A' AO
formed gradually increases in size and is
formed further away from the lens. Fig: 5.22 an object between F and O convex lens

Images formed by a convex lens: At a glance

S.No. Position of the object Position of the image Nature and size of the image

1 At infinity At focus (F) Real, inverted, highly diminished

2 Beyond 2F Between F and 2F on the Real, inverted, diminished
other side of the object

3 At 2F At 2F on the other side Real, inverted, same size as the

of the object object

4 Between F and 2F Beyond 2F on the other Real, inverted, magnified
side of the object

5 At F At infinity Real, inverted, highly magnified

6 Between F and O On the same side of the Virtual, erect, magnified
object

Construction of ray diagrams formed by a concave lens

1. Object between infinity and focus

The image formed by a concave lens for a real object is always virtual, erect and
diminished whatever is the position of the object.

When the object is between infinity and B

the focus of a concave lens, the image is formed B' O F 2F
between the focus (F) and the optical centre (O) on 2F F

the same side of the object in the lens. A A'

The image is virtual, erect and diminished. Fig: 5.23 an object in front of a concave lens
This type of image formation is used in spectacles
for correcting short sightedness.

2. Object at infinity object image at focus O F 2F
at ∝ 2F F
When the object is at infinity, the image
is formed at the focus (F). The image is
virtual, erect and highly diminished. It is
used in the eye lens in Galilean telescope.

Fig: 5.24 an object at ∞ concave lens

LIGHT CLASS - 10 MODERN GRADED SCIENCE 89

New Cartesian sign convention for lenses negative positive positive

The following New Cartesian sign of convention is used Lens
for measuring the distances of the ray diagrams of both
convex and concave lenses.

positive (+ve) direction negative positive negative
of incident rays F 2F Fig: 5.17
object height
object height O positive 2F F O
2F F
F 2F
–ve height

–ve distance +ve distance focal length of convex lens
is always negative
against the distance of along the side

incident rays of incident rays

focal length of convex lens is always positive

Fig: 5.25

1. All distances are measured from the optical centre of the lens.

2. The distance measured in the same direction as that of incident rays is taken as
positive.

3. The distance measured in the opposite direction of the incident rays is taken as
negative.

4. The distance measured upward and perpendicular to the principal axis is taken as
positive.

5. The distance measured downward and perpendicular to the principal axis is taken as
negative.

Real image and virtual image

An image that can be obtained on a screen is called a real image. It is formed by the
actual intersection of the refracted rays. A convex lens usually forms a real image.

An image which cannot be obtained on a screen is called a virtual image. Generally,
a concave lens forms a virtual image. It is formed by the apparent intersection of the rays.

Difference between real image and virtual image

Real image Virtual image

1. It is formed at the point where the 1. It is formed at the point where the

refracted rays meet. refracted rays appear to meet.

2. It is always inverted. 2. It is always erect.

3. It is usually formed on another side or 3. It is always formed on the same side

behind the lens. of the object in the lens.

4. Its size depends on the distance of the 4. Its size is larger in the convex lens

object from the optical centre of the lens. and smaller in the concave lens.

5. It can be obtained on a screen. 5. It cannot be obtained on a screen.

90 MODERN GRADED SCIENCE CLASS - 10 LIGHT

Magnification (m)

The size of the image produced by the lens depends upon the distance of the object
from the lens. If an object in placed near the lens, the image is magnified and if the object
is taken away from the lens, the size of the image is diminished. Therefore, magnification
of a lens is defined as the ratio of height of the image to the height of the object.

Mathematically,

Magnification = height of image (I)
height of object (O)

∴ m= I
O

We know, according to the definition of magnification,

Magnification = CD
AB

Magnification can also be calculated by the ratio of image distance (v) to object
distance (u).

or, m= distance of image from lens (v)
distance of object from (u)

i. e., m= v
u

Interpretation of magnification

1. If magnification (m) is equal to 1 (m =1), then the height of the image (I) is equal to the
height of the object (O) i. e. I = O.

2. If magnification (m) is less than one (m < 1), then the height of the image is smaller
than the height of the object.

3. If magnification (m) is greater than 1 (m>1), then the image is larger than the height
of the object.

4. If magnification (m) is negative, the image is virtual and erected.

5. If magnification (m) is positive, the image is real and inverted.

Thus, magnification shows how much smaller or larger an image is than the object.

To prove that I = v
O u

Let an object AB be placed on the principal axis of a convex lens and perpendicular

to its principal axis beyond 2F. A ray BP parallel to the principal axis passes through

F after refraction. Another ray BO passes straight through its optical centre O. These

two refracted rays PB' and OB' meet at B'. Hence, A' B' is the real image of object AB.

B Pv

O object FO F A' 2F
A 2F u image
I
B'

Fig: 5.26 convex lens

LIGHT CLASS - 10 MODERN GRADED SCIENCE 91

In ∆ABO and ∆A'B'O, we have

i. BAO = B'A'O [ both being 90º]

ii. BOA = B'OA' [ vertically opposite angles]

iii. ABO = A'B'O [ remaining angles of each triangle]

∴ ∆ABO and ∆A'B'O are similar.

Hence, we can write

∴ A'B' = OA'
AB OA

i. e., hheeiigghhtt of image = image distance
of object object distance

∴ I = v proved
O u

Relation between object distance, image distance and focal length

If u, v and f represent object distance, image distance and focal length of a lens
respectively, the relation between them is given by;

1 = 1 + 1
f u v

This is called lens formula for a lens.

We are not using the new sign convention. So we will use the negative sign for the
focal length of concave lens only. Other sign conventions will not be used.

Example 1: An object is placed at a distance of 25 cm from a convex lens of focal length 10
cm. Find the image distance and magnification.

Solution:

Here,

Object distance (u) = 25 cm

Focal length (f) = 10 cm

Image distance (v) = ?

We have,

1 = 1 + 1
f u v

or, 1 = 1 + 1
10 25 v

or, 1 – 1 = 1
10 25 v

or, 5–2 = 1
50 v

or, 3 = 1
50 v

92 MODERN GRADED SCIENCE CLASS - 10 LIGHT

or, 1 = 3
v 50

∴ v= 50 = 16.67 cm
3

We have,

∴ v = 16.67 = 0.6668
u 25

Therefore, the image distance is 16.67 cm and magnification is 0.6668.

Example 2: An object is placed 20 cm away from a concave lens. The focal length of the
lens is 10 cm. Find the image distance.

Solution:

Here,

Object distance (u) = 20 cm

Focal length (f) = –10 cm

Image distance (v) = ?

We have

1 = 1 + 1
f u v

or, 1 = 1 + 1
–10 20 v

or, 1 – 1 = 1
–10 20 v

or, 1 + 1 = 1
10 20 v

or, 1 = 3
v –20

∴ v = – 6.67 cm

Hence, the virtual image is formed at a distance of 6.67 cm from the lens on the same
side as the object.

Power of lens

The power of a lens is its capacity to converge or diverge light rays falling on it. In
other words, the power of lens is defined as the reciprocal of its focal length expressed in
metre.

Mathematically, Memorize

Power = 1 physical quantity SI unit
focal length of lens (in metre)
focal length metre (m)

i. e. P = 1 power of lens dioptre (D)
(m)
f P = 1
f

LIGHT CLASS - 10 MODERN GRADED SCIENCE 93

or, P = f 100
(cm)

∴ P = 1
f

The SI unit of power is diopter (D).

One diopter: A lens is said to have power one diopter if its focal length is one metre.

Some important notes

1. The focal length of a convex lens is positive, so its power is positive.
2. The focal length of a concave lens is negative, so the power of a concave lens is negative.
3. If the focal length is long, then the converging/diverging power of the lens is less and

vice-versa.

Example: A person uses a convex lens of focal length 10 cm in his/her spectacles. Find the
power of the lens.

Solution:

Here,

Focal length (f) = 10 cm = 0.1 m

Power (P) = ?

We have

∴ power = f 1 = 1 = 10 D
(m) 0.1m

Hence, the power of the lens is 10 D.

Uses of lens

Lenses are very useful to us. They are used for many purposes as mentioned below:

1. They are used in optical instruments like telescope, microscope, camera, slide
projector, film projector, etc.

2. They are used in spectacles for the correction of defects of vision.

3. Our eye lens is also a convex lens. It forms the image of an object on the retina.

4. A concave lens is used in the peeping hole of a door to see the person beyond the
door.

Eyes

The eyes are natural real image forming optical instruments which are concerned with
the sense of vision.

A human eye is somewhat spherical and slightly bulged in the front part. The wall of the
eyeball consists of three layers. They are sclera, choroid and retina.

1. Sclera: It is the outermost layer. It is tough, opaque and white in colour. Its front
portion is transparent; the curved part is called cornea. It gives the fixed shape to the
eyeball. It also protects the inner parts of the eyeball.

94 MODERN GRADED SCIENCE CLASS - 10 LIGHT

2. Choroid: The middle layer is called choroid. It is dark black in colour. It protects and
nourishes the retina. It also prevents the internal reflection of light and helps to form
a clear image of the object.

a. Iris and pupil: The front part of the choroid forms coloured iris. The iris has a
hole at the centre called the pupil. The iris controls the amount of light entering
the eye. The pupil allows light to pass in the eye.

b. Eye-lens: Behind the iris, there is a portentous crystalline, transparent convex
lens. It converges the light on the retina to form a real, inverted and diminished
image of an object.

c. Ciliary muscles and suspensory ligaments: The eye lens is held in the position by
the ciliary muscles and suspensory ligaments that are connected to the choroid.
The ciliary muscles also alter the focal length of the lens of the eye.

d. Aqueous humor and vitreous humor: The space between the lens and cornea is
filled with a transparent liquid called aqueous humor. It keeps the lens moist and
protects the lens from physical shock. It also maintains the curved shape of the
cornea. The space between the lens and the retina is filled with a jelly-like
transparent substance called vitreous humor. It maintains the shape of the eyeball
and protects the retina and its nerve ending.

3. Retina: The inner layer of the

eyeball is called retina. It is sensitive sclera

to light like the screen of a camera. ciliary muscle choroid
The retina prevents internal iris
reflection. The image of the object is retina
formed on the retina that is carried cornea convex lens
to the brain by the optic nerve. pupil yellow spot
vitreous humor
aqueous humor blind spot

suspensory optic nerve
ligaments

a. Yellow spot: The yellow spot Fig: 5.27 eye structure
is found at the centre of the

retina. It is sensitive to light. It is rich in rod and cone cells. The image of the

object is formed in it.

b. Blind spot: The blind spot is the point from where the optic nerves enter the eye.
It is not sensitive to light so no image of the object is formed in it.

Working: When light from an object enters the eye, it is refracted by the whole
lens system i. e. cornea, aqueous humor, eye lens and vitreous humor. However,
the major role in focusing is played by the eye lens. Due to this refraction, a real,
inverted and diminished image of the object is formed on the retina. Optic nerves
carry the impulses to the brain and then the brain inverts the image so that it is
seen as erect and in its proper size.

The size of the pupil can be varied with the help of the iris. The pupil controls the
amount of light entering the eye lens. In dim light, the iris expands the pupil to allow
more light to go in. But in a good light condition, it contracts the pupil to allow less light
to go in. In this way, the iris controls the amount of light entering the eye.

LIGHT CLASS - 10 MODERN GRADED SCIENCE 95

Range of vision and power of accommodation of human eye

A normal eye can see objects clearly at 25cm
infinity and nearer points. The nearest point

up to which an object can be seen clearly by

the eyes is called the near point of the eyes. retina

The distance of the near point from the eye near point

is called the least distance of distinct vision.

For a normal eye, it is 25 cm. In this case, the

ciliary muscles make the eye lens thick to Fig: 5.28 near point

decrease its focal length. The image of an object at a near point is formed on the retina as

shown in the figure.

The farthest point up to which objects infinity far point retina
can be seen clearly is called the far point. For Fig: 5.29 far point
a normal eye, the far point is at infinity. In
this case, the ciliary muscles make the eye
lens thin to increase its focal length. The eye
lenses are stretch in this case. The parallel
rays coming from the distant object are
focused on the retina as shown in the figure.

Ciliary muscles of eye play vital role in accommodation of the lens.

The range of vision of a healthy or normal human eye is between its near point and
far point i. e. from 25 cm to infinity.

Thus, the ability of the eye to change the focal length of the eye lens so as to always
obtain the image at the retina is called the power of accommodation.

Defects of vision

A normal eye can see clearly the The effect of vision due to which the impression of an
objects lying anywhere between 25 image lasts on the retina even after the removable of
cm and infinity from the eye. Thus, the object is called persistence of vision. For example,
the image of an object lying anywhere when light from an object falls in the eyes, the image
between 25 cm and infinity is formed formed on the retina lasts for 0.1 second even after
on the retina. the removal of the object.

An eye which does not form the

image of an object lying between the near point (i. e. 25 cm) and the far point (i. e. infinity)

on the retina is said to suffer from a defect of vision. When (i) the ciliary muscles fail to

contract and expand the lens properly or (ii) the eye balls get shortened or elongated, it is

unable to change the focal length of the eye lens. Due to this, the image is not formed at

the retina and the object is not seen clearly. This is the cause of the defects of vision.

The common defects of vision are long-sightedness or hypermetropia and
shortsightedness or myopia.

Long-sightedness (Hypermetropia)

It is defined as that defect of vision in which a person can see distant objects distinctly
but cannot see nearby objects clearly.

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