science 10

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

what happens there?

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

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

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

Mercury barometer Pressure due to Torricellian
Atmosphere vacuum
The barometer is an instrument which
is used to measure the atmospheric Pressure due to the
pressure. It uses mercury as the Mercury column
barometric substance. A mercury
barometer is constructed as follows: Trough
Mercury
A strong glass tube closed at one end
is filled with clean and dry mercury. Barometer
Then the open end of the tube is
closed with palm and is inverted.
The open end of the inverted tube is
carefully dipped in mercury taken in

47 Times' Crucial Science Book - 10

a trough. While inserting the tube into the mercury of the trough,
the atmospheric air should not get into the tube. Then the level of
mercury in the tube falls to a certain height and remains stationary.
This creates a vacuum inside the tube. This vacuum is called Torricelli
an Vacuum in honour of the Italian scientist E. Torricelli, who has
invented the mercury barometer. The tube is kept erect using a stand
and graduated scale is made in the tube. Since mercury is poisonous
in skin contact also, we should wear gloves while handling mercury.

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

Syringe Needle Piston

A syringe is a medical instrument

which is used to send medicine

into the body through blood. It is
also used to draw blood from the

body of a patient. It consists of Storage cylinder

three main parts – needle, storage cylinder and piston.

The piston is pushed up to the mouth of the storage cylinder and the
needle is inserted into a bottle of liquid medicine. When the piston
is pulled backward, a partial vacuum is created inside the storage
cylinder. Since the atmospheric pressure is higher than that inside
the cylinder, it pushes the medicine into the syringe. A liquid always
flows from the region of high pressure to the region of low pressure.

Air pump

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

Times' Crucial Science Book - 10 48

piston is pushed down, the air of the cylinder is pushed into the tube of the
bicycle. The tube of bicycle contains a valve to regulate the entry of air into it.
Water pump
A water pump is used to draw water from a well. It works on the
principle of atmospheric pressure.
A water pump has two main parts – barrel and piston. Both of them
consist of valves. The piston valve (V1) moves up and down along with
the piston. The foot valve (V2) is connected at the junction of barrel
and pipe. The pipe is long which is sent down to draw water from the
well. Two strokes, i.e. upstroke and down stroke are involved while
drawing water from the well.

When the handle of the pump is pushed downward, the piston moves
up and creates a partial vacuum between the valves V1 and V2. In such
case, the valve V1 gets closed whereas the valve V2 opens and water
rises into the barrel. This process is called upstroke. The upward
movement of water is due to the effect of atmospheric pressure upon
the water of well.
When the handle is pulled upward, the piston moves down. Then the
distance between the two valves decreases. In such case, the foot valve
experiences more pressure and gets closed but the piston valve opens
due to water pressure. Thus, water comes out through the spout. This
process is called down stroke.

49 Times' Crucial Science Book - 10

Solved Numerical Problem 2.4

When a block of wood having dimensions 50cm × 30cm × 20cm is
placed in water, what part of it remains above the water if the density
of the wood is 0.9 g/cc.

Solution:

Given, Density of the block of wood (d) = 0.9 g/cc

Volume of the wood (v) = 50cm × 30cm × 20cm = 30,000cm3

We have,

d= m Or, m = d × v = 0.9 × 30,000 = 27000g
v

∴ Mass of the block of wood (m) = 27000g

Since density of the wood is less than that of water, it floats on water.
According to the law the of flotation,

mass of the displaced water = mass of the wood

Mass of displaced water (m1) = 27,000g.

Density of water (d1) = 1 g/cc m1
d1
∴ Volume of the displaced water (v1) = = 27000 = 27000cc
Now, 1.0

The volume of the immersed part of the wood = volume of the

displaced water = 27000cc

∴ The immersed part of the wood = immersed volume of the wood
Total volume of wood

Again, part of the wood above water = 1 – 9 = 1 = 27000cc = 9
10 10 30,000cc 10

Here, the height of the wood is 20cm, therefore the height of the

block of the wood above the water

= 20 × 1 cm = 2 cm
10
Thus, the part of the wood which remains above water is 1 and the
10
height above water is 2cm. Density of water (d1) = 1 g/cc

∴ Volume of the displaced water (v1) = m1 = 27000 = 27000cc
d1 1.0
Now,

The volume of the immersed part of the wood = volume of the
displaced water = 27000c

Times' Crucial Science Book - 10 50

∴ The immersed part of the wood = immersed volume of the wood
Total volume of wood

Again, part of the wood above water = 1 – 9 = 1 = 27000cc = 9
10 10 30,000cc 10

Here, the height of the wood is 20cm, therefore the height of the

block of the wood above the water

= 20 × 1 cm = 2 cm
10 remains
Thus, the part of the wood which above water is 1 and the
height above water is 2cm. 10

Solved Numerical Problem 2.5

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

Solution:

Given,

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

Density of brick (d1) = 2.5g/cc

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

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

it sinks in water.

For sinking body

Volume of displaced water = volume of sinking body

(V2) = 800cc
Density of water (d2) = 1g/cc
Mass of displaced water (m2) = ?
Again, m2 = d2 × v2

= 1 × 800

= 800g.

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

51 Times' Crucial Science Book - 10

Solved Numerical Problem 2.6

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

Medium Weight
A 10N
B 15N
C 12N

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

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

c. Find the density of the stone.

Solutions:

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

b. Weight of the stone in air = 15 N

Weight of the stone in water = 12N

Weight of the stone in salt solution =10N

We have,

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

= 15 – 12 = 3 N.

∴ Upthrust given by water = 3N

Similarly, upthrust given by salt solution

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

15 – 10 = 5N.

Times' Crucial Science Book - 10 52

∴ The upthrust given by water is 3N and the upthrust given by the
salt solution is 5N.

c. When the stone is kept in water, it sinks completely. According to
Archimedes’ principle,

Weight of the displaced water = upthrust = (w) = 3N.

Mass of the displaced water (m) = weight = 3 = 0.3kg = 300g
g 10

Density of water (d) = 1g/cc

Volume of displaced water (v) = m = 300 = 300cc
d 1
For sinking bodies

Volume of the sinking body = volume of the displaced water (V2) = 300cc

Weight of the stone in air = 15N

Mass of the stone in air (m2) = 15 = 1.5kg = 1.5 × 100g = 1500g.
10

Density of the stone (d2) = m2 = 1500 = 5g/cc
v2 300

∴ Density of the stone is 5 g/cc.

Learn and Write

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

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

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

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

53 Times' Crucial Science Book - 10

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

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

Main points to remember

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

atmosphere.
3. The pressure exerted by a liquid is called liquid pressure.
4. Pascal’s law state that the pressure applied at a place on

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

Exercise

A. Choose the best alternative.

1. What kind of physical quantity is pressure?

a. Scalar b. Vector c. Both d. None

2. Which of the following is not the unit of pressure?

a. Pascal b. J/s c. N/m2 d. Atmosphere

3. Up to what height above the earth's surface is atmosphere found?

a. 10 km b. 100 km c. 1000 km d. 10000 km

4. The magnitude of liquid pressure at the bottom of container is
given by

a. P = Agh b. P = Vgh c. P = m x g d. P = dgh

5. Which of the following devices works on the principle of

Times' Crucial Science Book - 10 54

Pascal's law? b. Ship c. Hydraulic lift d. All of these
a. Hydrometer

B. Answer the following questions in brief.
1. What is pressure? What are the factors upon which pressure
depends? How do they affect?
2. What is one pascal pressure?
3. What is liquid pressure? What is the formula for liquid pressure?
4. Prove P = dgh where p, d, g, and h have their respective
meanings in liquid pressure.
5. Mention properties of liquid pressure.
6. State Pascal’s law.
7. What is hydraulic lift? On which principle does it work?
8. What is hydraulic garage lift? Where is it used?
9. What is upthrust? What are the factors which affect upthrust?
10. State Pascal’s law. Write down any two applications of this law.
11. Name the instruments that are constructed on the basis of Pascal’s law.
12. Name an instrument that works on the principle of
floatation. Also write its use.
13. What will be the effect in the weight of displaced water if load
is added into the ship floating in the ocean? Write with reason.
14. If an object is immersed in water, at what condition it can
float in water and at what condition it can sink in water?
15. The weight of any object decreases inside water. Why? How
much weight will an object lose in water?
16. A ship coming from sea enters the river, will its hull sink
more or less in river water? Give reasons.

C. Give reasons:
1. Deep sea divers wear diving suits.
2. A bucket is filled faster at the lower stair than at the upper stair.
3. Foundations of buildings are wider than the walls.
4. Camels can run faster than horse in deserts.
5. A nail is made pointed.
6. A hermit can sleep on a bed of nails.
7. It is easier to swim in ocean water.

D. Differentiate between:

1. Force and pressure

2. Thrust and pressure

55 Times' Crucial Science Book - 10

3. Archimedes' principle and law of floatation

4. Density and relative density

5. Thrust and upthrust

6. Liquid pressure and atmospheric pressure

15. Two tanks contain 1000 litres and 500 liters of water
respectively and level of water is equal in height.

a. Which one exerts more thrust at the bottom. Why?

b. If both contain equal amount of water, which one
exerts more pressure at the bottom. Why?

c. What happens to the pressure when salt is added to the water? Why?

E. Numerical Problems

1. The depth of water in a rectangular tank is 6m, find pressure
exerted by the water at the bottom of it.

(Given, g = 10m/s2, density of water = 1000kg/m3)

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

3. Calculate the mass of displaced water when 30cm thick iceberg
with surface area 1000cm2 floats on water.

(Given, Density of ice = 0.9 g/cc and density of water = 1g/cc)

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

5. A rectangular body is dipped in

to water as shown in the figure. 2m

The upper or lower surface area 6m
of it is 2m2. Find out the upthrust

acted on it by water. (Given,

Density of water is 1000kg/m3)

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

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

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

piston? B 375N
250N
7. Pistons A,B and C of the apparatus
given are frictionless. What is the area

A 20cm2 C 10cm2

Times' Crucial Science Book - 10 56

of the piston B? What force is exerted on the piston C?

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

Medium Weight
A 10N
B 15N
C 12N

i. Which medium is air and water? Why?

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

iii. If weight of 1 kg is 10N in water, what is the density of the
stone?

Answers E. 1.58800 Pascal 2. 6/8th part 3. 27000g 4.22N

5. 78,400N 6. 80N 7.30 cm2, 125N 8. (ii) 2N,5N (iii) 10g/cc

Project Work

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

Glossary : sink
: amount or quantity
• Submerge : an instrument that measures the
• Magnitude
• Barometer atmospheric pressure
: an instrument that measures the air
• Pressure gauze
pressure inside the tubes of tyres
• Graduated : divided into equal parts in a scale
• Fluid : liquid or gas, a substance that can
flow freely
• Thrust : force
• Floatation : act of floating on the surface of a liquid
• Perpendicular : at the angle of 90º

57 Times' Crucial Science Book - 10

Chapter

3 Energy

Albert Einstein

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

Estimated Periods: 5 (5T+0P)

ObAjetctthieveesnd of the lesson, students will be able to:

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

of energy.

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

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

Sources of energy

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

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

Times' Crucial Science Book - 10 58

Renewable Source of energy

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

Non- renewable source of energy

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

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

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

Fossil fuel

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

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

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

Coal

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

59 Times' Crucial Science Book - 10

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

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

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

Mineral oil

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

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

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

etc.

Advantages of fossil fuel

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

engines, generate electricity, etc.

Hydro-power

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

Times' Crucial Science Book - 10 60

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

Advantages of hydroelectricity

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

2. The hydroelectricity is
pollution free.

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

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

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

6. Modern lighting devices operate only with electricity.

Energy crisis

According to the present status of energy use in the world, 80 percent
of the necessary energy has been fulfilled from mineral oil, natural
gas, coal, etc. The reserve of such fuels is limited. But, the demand
of these fuels is increasing due to rapid population growth and use of
more number of vehicles. The consumption of coal is also increasing
in industrialized countries. In this way, the yearly demand of fuel
energy has been increasing in the world by 2.3 percent.

According to the survey conducted in 1984A.D., it was estimated that
these fuels will be used up within 25-30 years. New oil storage three
times bigger than the existing one has been considered to be present

61 Times' Crucial Science Book - 10

in Mid-Eastern Asia which can fulfil the demand of the fuel energy
for 50 years more. Thus, the reserve of fuels can fulfil the demand of
people for 2037 AD only if the demand increases by the same pace.
Thus, the world is facing the problem of scarcity of energy in near
future.

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

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

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

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

Alternative sources of energy

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

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

1. Bio-mass fuel

Biomass fuel is any organic matter which

can be used as energy source. Wood,

cattle dung, straw, bio-gas, etc are bio-

mass fuel. They are used in villages for

domestic purpose. Certain plants and

algae, including sugarcane and sea Bio-gas plant
weed can be grown especially for use as

biomass. Biomass fuel provides energy by

burning. In village areas, the cattle dung is dried to get dry mass

called ‘Guintho’ which is used to cook food.

Firewood is widely and commonly used source of fuel in Nepal.
Wood is converted to charcoal by burning it in insufficient amount of

Times' Crucial Science Book - 10 62

oxygen. Similarly, briquette cake is made from ‘Banmara jhar’ a type
of useless weed. Charcoal and briquette coal can be used as energy
source in place of coal.

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

Advantages of bio-gas

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

in crop fields.

2. Nuclear fuel

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

Nuclear fission

When a heavier nucleus of radioactive element is bombarded by slow
neutrons, it splits into smaller nuclei with the release of huge amount
of energy. It is called nuclear fission. For example, uranium nucleus
splits into krypton and barium nuclei when bombarded by neutrons.

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

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

63 Times' Crucial Science Book - 10

Nuclear fusion

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

21H1+ 21H1 → 2He4+ 2e°+ Energy.

Hydrogen bomb is based on the nuclear fusion reaction. This reaction
requires high temperature and pressure and cannot be controlled.

Advantages of nuclear fuel

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

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

be used as alternative source of energy.
Disadvantages

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

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

disposed or dumped inside the earth, in river or in the seas or
in open fields as other wastes.

4. Tidal energy

The surge and ebb of water level from oceans due to gravitational pull
of the moon and the sun is called tide. The water raised from oceans
or seas are collected in a dam made at the sea-shore. Pipes are kept
to return the water from the dam to the seas. This water is allowed
to fall on the turbines which drives generator to produce electricity.

5. Wind energy

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

Times' Crucial Science Book - 10 64

Advantages

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

Disadvantages

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

6. Geo-thermal energy

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

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

Advantages

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

7. Solar energy

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

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

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

65 Times' Crucial Science Book - 10

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

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

Surface temperature: 5700°C

Temperature at the core: 15 million degree Celsius

Energy radiation: 4 × 1026 Joule of energy per second

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

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

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

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

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

Advantages

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

Limitations

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

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

other forms.

Times' Crucial Science Book - 10 66

The sun: The ultimate source of energy

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

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

Wind energy is the outcome of solar energy

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

Bio-fuel and fossil fuel are outcomes of solar energy

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

Hydroelectricity is the outcome of solar energy

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

How is energy produced in the sun?

In the core of the sun, there is extremely high temperature of about 13
million degree Celsius. When temperature reaches 5 million degree
Celsius, electrons fly away from the atoms thus separating nucleus
and electrons. Hydrogen gas is present in the sun in abundant amount.
Therefore, hydrogen atoms ionize into protons (hydrogen nucleus) and
electrons. Although, there is strong repulsion force between protons,

67 Times' Crucial Science Book - 10

the free protons (1H1) fuse together to form deuteron or deuterium
(1H2) and positron due to high pressure and temperature. A proton
(1H1) and a deuteron (1H2) fuse to form a light helium (2He3). Then,
two nuclei of light helium fuse to form an ordinary nucleus of helium
(2He4) with some hydrogen nuclei. In each step of fusion reaction,
enormous amount of energy is released. Thus, one helium nucleus is
formed from the fusion of four hydrogen nuclei. The summary of the
nuclear reactions in the sun is as follows.

1H1+ 1H1 → 1H2 + 1e° + Energy
1H1 + 1H2 → 2He3 + energy
2He3 + 2He3 → 2He4 + 1H1 + 1H1 + energy
In short,
21H1+ 21H1 → 2He4 + 21e° + energy

The particles formed during nuclear fusion reaction in the sun

Hydrogen atom has atomic number one and atomic weight also one.
Therefore, it is represented by 1H1. Similarly, atomic number of helium
is 2 and its atomic weight is 4. Its nucleus is represented by 2He4. In
the same way, –1e° is the symbol for an electron and le° is the symbol
for a positron. Positron is the positively charged electron.

The nuclear reaction taking place in the sun in which one helium
nucleus is formed by the fusion of four hydrogen nuclei is called
thermonuclear fusion reaction.

Conditions required for nuclear fusion in the sun

The following conditions are required for nuclear fusion reactions in
the sun:

1. There is abundant amount of hydrogen gas in the sun.
2. There is huge amount of helium gas formed from the nuclear

fusion of hydrogen atoms.
3. There is the presence of extremely high temperature in the

core of the sun for the formation of free hydrogen nuclei.
4. There is high pressure required for the fusion of free hydrogen nuclei.

Present status of the energy use

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

Times' Crucial Science Book - 10 68

status of consumption of energy in the world is as shown below in the
table.

S.N. Source of energy Percentage

1. Mineral oil 35

2. Coal 27

3. Natural gas 17
4. Bio-fuel 13

5. Hydroelectricity 5

6. Nuclear fuel 3

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

Conservation of energy

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

Learn and Write

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

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

69 Times' Crucial Science Book - 10

2. Coal is a fossil fuel. Why?

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

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

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

4. Differentiate between nuclear fusion and nuclear fission

Nuclear fusion Nuclear fission

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

which two or more light nuclei which a heavier nucleus

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

nucleus. light nuclei.

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

Therefore, energy cannot be Therefore, energy can be

converted into useful form. converted into useful form.

3. It produces comparatively more 3. It produces comparatively

amount of energy. less amount of energy.

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

principle. principle.

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

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

Main points to remember

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

which they are present in the nature are called primary sources
of energy.
3. The sources of energy which are obtained from primary sources
are called secondary sources.
4. The sources of energy which can be replenished within short
period of time are called renewable source of energy.

Times' Crucial Science Book - 10 70

5. The sources of energy which cannot be replenished easily when
their sources get finished are called non-renewable source of
energy.

6. The sources of energy which do not get exhausted are called
perpetual sources of energy.

7. The sources of energy which are obtained from the dead
remains of plants and animals of millions of years ago are
called fossil fuels.

8. The energy due to flowing water is called hydropower.
9. The shortage or scarcity of energy sources on the earth due to

overuse of the non-renewable source of energy is called energy
crisis.
10. The sources of energy which can be used instead of conventional
non-renewable source of energy are called alternative source of
energy.
11. Bio-mass is any organic matter which can be used as a source
of energy.
12. The elements which produce large amount of energy as a result
of nuclear reactions are called nuclear fuels.
13. When a heavier nucleus splits into smaller nuclei due to
bombardment of neutrons, it is called nuclear fission.
14. The nuclear reaction in which two or more nuclei fuse to form a
heavier nucleus is called nuclear fusion.
15. The energy contained in the wind of high speed is called wind
energy.
16. The heat energy contained in the interior part of the earth is
called geo-thermal energy.
17. The energy obtained from the sun is called solar energy.
18. The sun is the ultimate source of energy of the earth.

Exercise

A. Choose the best alternative.

1. Which of the following is the perpetual source of energy?

a. Water b. Biomass c. Sunlight d. All of these

2. Which of the following is the best quality coal?

a. Lignite b. Bituminous

c. Sub-bituminous d. Anthracite

3. Which of the following natural phenomena provide evidence
for the existence of geo-thermal energy?

71 Times' Crucial Science Book - 10

a. Hot water spring b. Volcanic eruption

c. Storm d. Both a and b

4. What kind of reaction produces enormous amount of energy in the sun?

a. Thermonuclear fission reaction

b. Thermonuclear fusion reaction

c. Chemical fusion

d. Rearrangement reaction

5. Which of the following is the most commonly used source of energy?

a. Mineral oil b. Bio-fuel c. Natural gas d. Hydroelectricity

B. Answer the following questions in brief.

1. What are primary and secondary sources of energy?

2. What is renewable source of energy? Give some examples.

3. What is non-renewable source of energy? Give some examples.

4. What is fossil fuel? Give some examples.

5. What is coal? Mention its uses.

6. What is mineral oil? What are the uses of mineral oil?

7. What is hydro-power? Mention the advantages of hydro- electricity.

8. What is energy crisis? What are the various ways to solve the
problems of energy crisis?

9. What is alternative source of energy? What are the various
alternative sources of energy?

10.What is bio-gas? How is it obtained?

11.What are the advantages of bio-gas?

12. What is nuclear fuel? What are various types of nuclear reactions?

13.Mention the advantages of nuclear fuel.

14. What is wind energy? Mention the advantages of wind energy.

15.What is geo-thermal energy? Mention its advantages.

16.What is solar energy? What are the uses of solar energy?

17. What are the two natural processes obtaining geo-thermal energy.

C. Give reasons.
1. Wind energy is the outcome of solar energy.
2. Hydro-electricity is the outcome of solar energy.

Times' Crucial Science Book - 10 72

3. Mineral oil is a fossil fuel.
4. Bio-gas is the best energy source in Nepal.
5. Production of hydroelectricity should be encouraged.

D. Differentiate between:
1. Renewable and Non-renewable source of energy
2. Nuclear fusion and Nuclear fission

Project Work

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

Glossary : everlasting, unending

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

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

• Crisis : complete combination
• Alternative
: break down

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

: shortage, problem

: something different that performs the
same job, substitute for

73 Times' Crucial Science Book - 10

Chapter

4 Heat Anders Celsius

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

Estimated Periods: 6 (5T+1P)

ObAjetctthieveesnd of the lesson, students will be able to:

• explain heat as a form of energy;
• differentiate between heat and temperature;
• describe specific heat capacity and derive heat equation;
• solve simple numerical problems related to the heat.

Introduction

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

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

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

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

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

Temperature

When we touch warm water, we feel hot. But we feel hotter when
we touch boiling water. The warm water and boiling water are not
equally hot. We need numerical quantity to express degree of hotness
or coldness. This quantity is called temperature. Temperature is

Times' Crucial Science Book - 10 74

defined as a quantity which measures degree of hotness or coldness of
a body. The hotter objects have more temperature than colder objects.

When a body is heated, the vibration of the molecules contained in
it increases then the temperature, too. The increase in vibration of
molecules causes the increase in temperature. Thus, temperature
measures the average speed of vibration of molecules. In other words,
temperature is defined as the average kinetic energy of the molecules
of a body.

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

Heat and temperature

Heat contained in an object is the sum of kinetic energy of all the
molecules contained in it. Kinetic energy of a molecule depends upon
its mass and average speed. The objects having more heat are hotter
and the objects having less heat are colder. But heat is contained in
all types of objects either hotter or colder. Ice also has heat because its
molecules contain kinetic energy. A body becomes colder or hotter due
to difference in temperature. Hotter bodies have more temperature.
The molecules contained in hotter bodies vibrate with higher speed.
Thus, heat and temperature are very much related. A hotter body
has more temperature than the colder one but may not have more
amount of heat.

Differences between heat and temperature

Heat Temperature

1. I t i s a f o r m o f e n e r g y 1. It is the degree of hotness
w h i c h produces sensation of or coldness of an object.
warmth.

2. Heat contained in an object 2. Temper ature of a
depends upon the mass of sub st a nce depends upon
total molecules and the speed the speed of vibration of
of vibration of the molecules. molecules but not on the
mass of the molecules.

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

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

75 Times' Crucial Science Book - 10

Types of thermometer

a. Clinical thermometer

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

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

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

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

doctors and nurses to measure the temperature of patients.

Alcohol capillary tube Glass tube

Bulb constriction temperature scale

Clinical thermometer

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

b. Digital thermometer

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

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

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

c. Laboratory thermometer

A thermometer which is used to measure the temperature of different

chemicals, solvents and reagents in the laboratory is called laboratory

thermometer. It consists of a thick walled glass tube having uniform

and fine bore. The capillary tube is connected to bulb containing

mercury at one end. The other end of the capillary is sealed. The

capillary tube and the bulb are covered by a glass cover. The capillary

tube is straight and it does not have any constriction.

Mercury capillary tube Glass tube

Bulb Laboratory thermometer temperature scale

The stem of the laboratory thermometer is graduated in degree

between –10°C to 110°C. When the bulb of the thermometer is placed

in contact with a hot body, the mercury of the bulb begins to rise

and reaches a constant value. The temperature of the body can be

measured by keeping the thermometer in contact with the body.

Differences between clinical and laboratory thermometer

Clinical thermometer Laboratory thermometer

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

of human body. different objects.

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

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

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d. Minimum and maximum thermometer

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

Limb A is connected to the bulb X containing
alcohol. The change in volume of alcohol with
temperature is responsible for registering
the minimum and maximum temperature.
Limb B is connected to the bulb 'Y' partially
filled with alcohol. There is an empty space
in the bulb 'Y' to allow the further expansion.
When the alcohol of the bulb X expands due
to increasing temperature, it pushes the
mercury column of limb B sending the steel
index above the level C. This index enables
to record the maximum day temperature
that can be read on the scale marked along
the limb.

The temperature of a place falls down at
night. When the temperature falls, the
alcohol of the bulb at X contracts and the
mercury is drawn back pushing steel index
above the level D. The index, therefore gives
the minimum temperature reached at night.

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Specific heat capacity

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

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

upon the nature of the substance

Materials required:
Two beakers of same size, two thermometers, oil, water, spirit lamp,
stand, stop watch, etc.

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

water in another beaker.

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

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

Thermometer

Beaker

Water Alcohol

Tripod
stand

Burner

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

minutes.

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

water.

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

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

79 Times' Crucial Science Book - 10

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

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

1. Lead 126
2. Mercury 138
3. 234
4. Silver 380
5. Brass 390
6. Copper 447
7. Steel 470
8. Iron 900
9. Aluminium 670
10. Glass 800
11. Sand 2200
12. Kerosene 2100
13. 4200
14. Ice 1670
15. Water 2000
16. Petrol 2400
Vegetable oil
Alcohol

Heat capacity

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

Consequences of the highest specific heat capacity of water

1. Water is used to cool the engines of vehicles.
Water is the substance having the highest specific heat capacity,

Times' Crucial Science Book - 10 80

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

2. Water is used in hot water bag.

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

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

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

When a body is heated, its temperature increases. On the other hand,
when a body is cooled, its temperature decreases. Suppose, a body of
mass m having initial temperature t1 is heated by supplying heat Q
then its temperature rises to t2. Now, the change is temperature:

dt = t2– t1.
It has been proved that the heat gained or lost (Q) by a body is directly
proportional to (i) mass of the body (m) and (ii) change in temperature.
Mathematically,
Q α m ……… (i)
Q α dt ……….. (ii)

Combining equations (i) & (ii), we get:
Q α mdt

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

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

81 Times' Crucial Science Book - 10

Solved Numerical Problem 4. 1

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

Solution:
Given,Mass (m) = 5kg
Initial temperature (t1) = 10°C
Final temperature (t2) = 30°C
Change in temperature (dt) = t2–t1 = 30–10 = 20°C
Specific heat capacity (S) = 4200J/kg°C
Amount of heat gained (Q) = ?
We have,
Q = m s dt = 5×4200×20 = 420000J.
∴ The amount of heat required is 420000 Joule.

Principle of calorimetry

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

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

Solved Numerical Problem 4. 2

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

Solution:

Given, Mass of iron (m1) = 2 kg
Initial temperature of iron (t1) = 90°C
Specific heat capacity (s1) = 470 J/kg°C
Mass of water (m2) = 3kg

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Initial temperature of water (t2) = 10°C
Specific heat of water (s2) = 4200 J/kg°C
Let, the final temperature be ‘t’ after keeping the iron ball in the
water. According to the principle of calorimetry,

Heat lost by iron = Heat gained by water

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

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

84600 - 940t = 12600t - 126000

13540t = 210600 Or, t = 15.55°C
∴ The final Temperature is 15.55°C

Solved Numerical Problem 4. 3

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

Solution:

Given, Mass (m) = 5 kg

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

Amount of heat supplied (Q) = 2400J

Final temperature (t2) = ?
We have,

Q = msdt Or, Q = ms(t2– t1)

Or, 2400 = 5 × 470 (t2 – 40)

Or, 2400 = 2350t2 – 94000

Or, 2350t2 = 96400 Or, t2 = 41.02°C

∴ The final temperature of the iron is 41.02°C

Learn and Write

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

In the earthern pots, there are tiny pores. Water oozes out
through the tiny pores of the earthern pots. Evaporation of oozed

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water takes heat from the water inside the pot making it colder.
Such oozing does not take place in metal pots. Thus, the water
in the earthen pots becomes colder.

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

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

3. Which one is heated faster: 1kg iron or 1kg mercury when equal
amount of heat is supplied ?

One kilogram mercury gets heated faster than 1kg iron because mercury
has less specific heat capacity (138°J/kg) than the iron (470 J/kg°C).

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

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

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

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

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

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

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

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Main points to remember

1. Heat is a form of energy which is produced due to vibration of molecules.
2. SI unit of heat is Joule.
3. Temperature is the degree of hotness or coldness of a substance.
4. Abnormal expansion of water in which it contracts from 0°C

to 4°C then expands vigorously after 4°C is called anamolous
expansion of water.
5. Anamolous expansion of water is responsible for the survival of
aquatic life of cold regions in winter seasons.
6. The amount of heat required to increase the temperature of an object
of mass 1kg by 1°C is called the specific heat capacity of the object.
7. The amount of heat lost or gained by an object is equal to the
product of mass of the object, specific capacity and change in
temperature.

Exercise

A. Choose the best alternative.
1. The instrument which measures the heat is
a. Thermometer b. Calorimeter c. Hydrometer c. All of these
2. What is the SI unit of temperature?
a. °C b. °F c. K d. J
3. Water has highest density and least volume at
a. O°C b. 100°C c. 32°C d. 4°C
4. The amount of heat required to raise the temperature of a body through
1°C is called
a. Specific heat capacity b. Heat capacity
c. Calorimetry d. None of these
5. Heat equation
a. Q = sdt b. Q = msdt c. Q = s/t d. 4200J/kg°C

B. Answer these questions.
1. What is heat? On which factors does amount of heat depend?
2. Give the concept of heat and temperature on the basis of molecular motion.
3. Mention effects of heat.
4. What is temperature? How is it measured?
5. Differentiate between heat and Temperature .
6. What is a thermometer? What are the parts of a thermometer?
7. Define
i. Specific heat capacity
ii. One calorie heat

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iii. Heat capacity
8. What do you mean by the fact that specific heat capacity of water is 4200 J/kg°C ?
9. Mention some effects of highest specific heat capacity of water.
10. What is heat equation?

C. Give reasons:
1. Water is used to cool the engines of vehicles.
2. A thick glass tumbler cracks when hot water is poured into it.
3. Honey solidifies from the bottom whereas water solidifies from the top.
4. It is very hot during day and very cold during night in the desert.
5. Temperature of coastal region remains fairly constant.

D. Answer the following questions on the basis of following table.
1. If substances A, B and C have equal mass and equal
temperature, which one has maximum heat in it? Why?

Substance Sp. Heat capacity

A 2100J/kg°C

B 470J/kg°C

C 380J/kg°C

2. If all three substances have equal mass and equal temperature,
which one needs more heat to raise equal temperature? Why?

3. If all of them are solid and have equal mass, which penetrates
the deepest in the wax slab when they are heated to 200°C and
kept on the wax slab. ? Why?

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

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

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

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

Project Work

What sources of energy are being used at your home? Keep
the record of amount of money spent for LPG, diesel, petrol,
kerosene, electricity, coal, firewood, etc for a month. Present
your results in a pie-chart.

Glossary : rapid to and fro motion of a body

• Vibration : change
• Transformation
• Kinetic : related to motion
• Digital thermometer
: a thermometer that displays
• Frozen temperature in digits, an
electronic thermometer
• Breeze
• Ooze : covered by or made into ice,
solidified into ice

: gentle and cool wind

: to exude a liquid substance
slowly and in small quantities

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Chapter

5 Light
Willebord Snell

He is known for the Snell's Law.

Estimated Periods: 7 (5T+2P)

ObAjetctthieveesnd of the lesson, students will be able to:

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

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

Lens

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

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

i. Convex lens ii. Concave lens

Convex lens Thin

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

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

Concave lens

The lens which is thin in the middle and thick

at the edge is called concave lens. O

When parallel rays of light fall on a concave F

lens, they get diverged after refraction. They

appear to diverge from a point as shown in

the figure. Therefore, a concave lens is called

diverging lens.

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

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

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

convex lens to the paper for about 5 minutes.

Observation:
The paper starts burning after sometime.

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

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

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

So, it does not burn.

Lens as a group of prisms

A lens is assumed to be made by the

combination of many prisms in correct

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

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

lens are like those shown in the figure.

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

lens bends the incident light downward,

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

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

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

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

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

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Optical Centre

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

Principal focus

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

principal axis converge after refraction

through a convex lens or appear to diverge

after refraction through a concave lens. It F

is denoted by F.

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

after refraction through convex lens meet

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

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

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

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

concave lens is virtual.

Focal length

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

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

Materials required:

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

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

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

image becomes clear and distinct.

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

Rules of refraction through a lens

F FF F F F

(a) (b) (c)

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

through the principal focus after refraction through a lens.

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

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

becomes parallel to the principal axis.

Rules for drawing ray diagrams in a convex lens

Following rules are to be followed while drawing ray diagrams:

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

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

lens. M

2F F Ov
u F 2F

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

Images formed by a convex lens

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

1. When an object is at infinity

When an object is at infinity, the

parallel rays coming from the

object meet at the principal focus F

after refraction through the lens.

Thus, the image is formed at the

principal focus.

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

highly diminished.

2. When an object is beyond 2F

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

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3. When an object is at 2F

When an object is placed at 2F of a B

convex lens, the image is formed at A F O F A'
2F at the another side. The image 2F 2F
B'
is real, inverted and of same size as
A'
that of object. 2F

4. When an object is at any point between F and 2F B'

When an object is at any point
between F and 2F of a convex B

lens, the image is formed A F

beyond 2F at another side. The 2F F O

image is real, inverted and

bigger in size, i.e. magnified.

5. When an object is at ‘F’

When an object is placed at ‘F’, the B

rays coming after refraction through A F

the lens become parallel to each 2F F O 2F

other. These rays meet at infinity. To image at
Thus, the image is formed at infinity infinity

at the next side of the lens. The image is real, inverted and highly

magnified.

6. When an object is kept between ‘F’ and ‘O’

When an object is placed B'

between ‘F’ and ‘O’ in front

of a convex lens, the image is

formed on the same side of the A' B O F O
object. The image is virtual, 2F FA 2F

erect and magnified.

In this position, convex lens
can be used as a magnifying
glass.

From the above ray diagrams, it can be said that when the object
approaches the lens, the images formed are gradually larger and
farther away from the lens.

Rules to draw ray diagram in a concave lens

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a. A ray parallel to the principal axis from the tip of the object is
drawn which gets diverged after refraction through the lens.
The diverged ray meets the principal axis after producing at
point ‘F’.

b. A ray from the tip of the object should be drawn passing
through the optical centre. It passes without deviation.

A O O F
BF F

(a) (b)
c. An image is formed at a point where the ray passing through

the optical centre and the produced ray meet.

Images formed by a concave lens

A concave lens always forms virtual, erect and diminished image. The
image is always formed on the same side.

1. When the object is at infinity

When an object is at infinity, the image is formed at F O
the principal focus (F) on the same side of the lens.
The image is virtual, erect and highly diminished.

2. When an object is at a point between infinity and principal focus (F)

When an object is placed at a point between F O
infinity and principal focus (F), the image is A
formed between the lens and the principal
focus (F) on the same side of the lens. The

B

image is virtual, erect and diminished.

Differences between real image and virtual image

Real image Virtual image

1. The image which can be 1. The image which cannot

obtained on a screen is be obtained on a screen

called real image. is called virtual image.

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

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3. It is formed by a convex lens, 3. It is formed by a concave

camera, projector, etc. lens, microscope,

telescope, etc.

4. It is formed by meeting the 4. It is formed by the

actual refracted rays. apparent meeting of

refracted rays.

5. It is usually formed at the 5. It is generally formed

another side or behind the on the same side of the

lens. object in the lens.

Magnification

Images formed by a lens will be larger or smaller than the size of the

object depending upon the positions of the objects. The variation in

the size of the image with respect to the size of object is expressed by

the term magnification.

Magnification is defined as the ratio of height of image to the height of object.

Mathematically,

Height of image ⇒ m = I
Magnification = Height of object O

Relation of magnification with the size of image

1. When the magnification is less than 1, the size of image is
smaller than the size of object, i.e. I < O when m < 1.

2. When magnification is more than 1, the size of image is larger
than the size of object, i.e I> 0 when m > 1.

3. When magnification is equal to 1, the size of image is equal to
the size of the object, i.e, I = 0 when m = 1

Method of measuring magnification

M

A

B F O F B'
2F 2F

A'

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