Blooming Science-10-2077- final press

Natural gas is used as fuel in industries and homes. In homes it is used for heating and in industries
it is used to extract hydrogen gas.

Advantages of Fossil Fuels

Coal or mineral oil is very important fuel. Its advantages are as follows:
A) It is cheaper and more abundant in comparison to other types of fuels.
b) It can be easily transported.
c) It is a multipurpose fuel thus, it is used to run different vehicles and engines.
d) It is also used in industries and factories that require abundant heat supply and also in

generation of electricity.

Disadvantages of Fossil Fuels

They cause air pollution as they release pollutants like sulphur dioxide, carbon dioxide ,carbon
monoxide etc. They cause change in global climate.

c. Hydro Energy and Hydro Electric Energy (Hydropower)

The water in motion possess kinetic energy and hence it can be used directly to rotate or move an
object. In other cases, water stored at height can be made to flow down to lower place. In doing
so potential energy stored with water is converted into kinetic energy. The kinetic energy of the
moving water can be used to rotate the turbine of the generator thereby producing electricity.
Thus electricity produced by using energy of water is called hydroelectricity or hydroelectric
energy.

It has been long time, of potential and kinetic energy of water, being used successfully to rotate
wheels of water mills, locally called as Pani Ghatta, which are still in use in remote hilly regions
of our country.

Principle of Generation of Hydroelectricity

The flowing water rotates the wheel about a shaft kept on
the axis of rotation of the wheel. The shaft is connected to an
electric generator.

When the wheel rotates, it also rotates the armature of the
generator in a magnetic field and electricity is generated.

If the flow of water is not sufficient to rotate the wheel directly, Power station
it is collected in a dam and is used later on.

Use and production of hydroelectricity is increasing day by day due to following reasons:

1. Producing hydroelectric power does not pollute environment

2. Hydroelectric energy is renewable and hence it cannot be exhausted.

3. Small power to high power can be generated according to size of volume flow per second
of water.

4. The hydroelectric power plant can be set up anywhere at a suitable place.

5. It is cheep in long run.

Blooming Science Book 10 51

Disadvantages of Generating Hydroelectric Power

1. The construction of dam covers large area of land rendering the fertility of the soil to be
wasted.

2. The natural habitat of large number of organism is sunk by the dam. The flora and fauna
are affected.

Alternative Sources of Energy

The major portion of the world’s energy consumption comes from the petroleum products like
petrol, diesel, coal, natural gas etc. At the present rate of consumption of such energy, it may be
exhausted in two or three decades. On the other hand, it takes millions of years for the formation
of such fuels. This depicts the probable dearth of such fuels in the near future as vicious problem.
To avoid such a crisis we have to search other renewable and inexhaustible forms of the energy.
The renewable inexhaustible sources of energy used instead of non-renewable or depletable
sources of energy to avoid energy crisis are called the alternative sources of energy.

The following are the alternative sources of energy:

a) Nuclear Energy b) Bio-mass Energy
c) Geothermal Energy d) Wind Energy
e) Tidal Energy f) Solar energy

a) 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.

There are two types of nuclear energy.
i. Nuclear fusion ii. Nuclear fission

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

U235 + 0n1 56Ba141 + 36Kr92 + 3on1 + Energy

92

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, where c is the velocity of light in
vacuum (3×108m/s). Mass-energy relationship was propounded by Albert Einstein in 1905 A.D.
The enormous amount of heat energy released in nuclear fission is used for producing steam that
rotates the blades of generators. It then generates electricity.

When two or more light nuclei combine to form a heavy nucleus, 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.

21H1 + 21H1 2He4 + 21eO + energy

The fusion reaction does not take place under ordinary conditions. It occurs only under

52 Blooming Science Book 10

extremely high pressure and 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, 1g of uranium can
produce energy equal to that produced by three tons of coal.

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

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

Disadvantages of using Nuclear Fission 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.

3. Radiations produced by nuclear reaction cause long term genetic effect.

b) Bio-mass Energy

The domestic waste of grabage which consists of

decomposable organic matter is called the biomass.

Biomass includes cow dung, wood, poultry excreta,

pig excreta, human excreta, crop residues, vegetable

wastes and sewage etc. These garbages can be used

to produce electricity which in turn prevents water

and atmospheric pollution. The biomass can be

converted into biogas in a biogas plant. This biogas

is renewable source of energy. Biogas is produced

by anaerobic micro-organism in the presence of Fig: A Bio gas Plant

water but in the absence of oxygen. The biomass

contains the compounds like carbohydrates, proteins and fats. The anaerobic bacteria decompose

these carbon compounds into methane gas, carbon dioxide, hydrogen, hydrogen sulphide etc.

This mixture of gas is called biogas. The main constituent of biogas is methane gas which is

about 65% The methane is very efficient fuel. The biogas is used to generate electricity, cooking

and lighting. Biogas is smokless and produces large amount of heat. The by products formed in

the production of biogas is a good manure because it is rich in nitrogen & phosphorous.

Dung cake are burnt directly to get heat in rural areas of Nepal. Actually in remote rural areas
more than 80% of the total energy need is supplied by bio-mass energy like firewood, crop
residues, animals excreta etc.

Application of Bio-mass Energy

The bio-mass energy can be used for heating and cooking in houses and in food industries. The
bio-gas like gobar gas is used for lighting purpose as well.

Blooming Science Book 10 53

Advantages of Using bio-mass energy

1. It is renewable source of energy

2. It is the cheapest energy

3. It is available very easily

4. Very useful by-products are obtained when bio-mass are transformed or used to generate
electricity.

Disadvantages

1. Huge amount of energy cannot be obtained at once.

2. Burning of wood, cow dung, cakes, crop residues etc. causes air pollution by releasing smoke.

c. Geothermal Energy
The core of the earth is extremely hot. During the eruption of volcano, the molten matters
present inside the earth come out of it. In the areas where earthquake and volcano are active the
temperature increases by about 80oC per kilometer depth. In other normal places the temperature
increases by about 30oC per kilometer depth. In some places, extremely hot rocks are found
within some depth below the earth’s surface. When water is sent to that place, water is readily
vaporized. The steam power is used to run steam engine and electricity is generated, this type of
energy lying inside the earth is called geothermal energy.
Application of Geothermal Energy
i) Geothermal energy can be used to generate electricity.
ii) It can be used for boiling water in large amount.
iii) It can be used to heat space in houses and building.
Advantages
i) This is a renewable source of energy.
ii) It causes no air pollution in the environment
iii) It can be used for various purposes.
Disadvantages
i) It is very difficult to construct geothermal power plant. This is because the holes are to be

drilled deep into the earth which is very painstaking.
ii) It is useful only in volcanic region.

d. Wind Energy

The fast moving air is called wind. Thus, the wind possesses kinetic energy. We can see the
destruction caused by wind frequently. For example; breaking huge trees, destroying the
buildings, fences, electric poles and wires etc. We can turn that destructive power of wind to
constructive power and solve the energy crisis in the world.

In the earlier days, wind energy was utilized for many purposes

i) For sailing a boat.

ii) For drawing under ground water.

iii) For grinding grains to make flours.

54 Blooming Science Book 10

Application of wind Energy

i) The wind energy can be used to pump up underground water.

ii) It can be used to grind grains to get flour.

iii) It can be used for transportation by sail boat.

iv) It can be used to generate electricity

Advantage: It is the cheapest renewable source of energy.

Disadvantage: Wind is not available every time we need.

e. Tidal Energy

The rise and fall of ocean water due to the gravitational force of the moon is called tide. The rise
in level of water is called high tide and the fall in water is called low tide. The rise and fall in
water occurs twice a day in the oceans. The occurrence of high tide and low tide in the coastal
region provides movement of huge amount of water to utilize this energy of tide. A tidal barrage
called tidal dam is constructed. These days, technology has also been developed to generate
electricity from the tides and surge of the sea. A big dam is constructed at the seashore near water.
In the sea, when tides surge up, water enters the dam after crossing it and is trapped in the dam.
Again when the tides ebb, water from the seashore moves to the middle part of the sea. Pipes are
placed for the trapped water to return to the body of the sea. This water collected in dam is used
to run turbines and electricity is generated.
f. Solar energy
The sun is a big ball of fire. It is not a solid body. About 75% mass of the sun is hydrogen and less
than 25% is helium. The sun is the most abundant and everlasting source of energy on the earth.
The energy given by the sun is called solar energy.

Application of solar energy

Solar Heater
In a solar heater, the sun light is reflected into a black pipe containing water by means of a plane
reflector. When the sun light falls on the plane reflector, it reflects the sunlight onto the black
pipe. The black pipe absorbs the heat and uses it for heating the water in it.
Application: It is used to heat water for domestic supply. It is also used to heat rooms by
circulating the hot water heated by the heater.

Solar Power Plants
In a solar power plant, the solar energy is converted into electricity by using solar cell. The solar
cell is a device which converts solar energy in electricity. To increase the amount of current, the
solar cells are joined together to from solar panel.
Application: It is used to generate moderate power electricity.
The output potential difference from the solar battery is connected to a chargeable battery which

Blooming Science Book 10 55

is charged by it. The e.m.f. from the battery can be used to drive any electrical device.

The solar batteries have several applications:
a. Solar batteries are used to light houses.
b. They are used to heat houses.
c. They are used to operate electrical devices.
d. They are used to operate electronic devices like watch and calculator
e. Solar cells are used in powering artificial satellites.
f. They are used in street lighting.

g. They are used in running light vehicles like tempo, cars, micro-buses etc.

Traditional use of solar energy

The solar energy is used for drying cloth, drying grains, etc. in the past. It has some limitations
like it is not available every time. It is not uniformly distributed everywhere in the earth.

Present Status of World Energy Consumption

According to the present status of energy use in the world, 35 percent of the necessary energy
has been fulfilled from the mineral oil. With the pace of development the use of means of
transportation has been increased and consequently consumption of mineral oil is higher in the
world. The consumption of coal is more in industrialized countries. Coal has been taken as a
main source of energy after mineral oil. About 27 percent of worlds energy supply is of coal. In
this way, the early demand of fuel energy has been increasing in the world by about 2.3 percent,
if no step is taken for its proper use and conservation in time, the energy crisis will arise.

Energy Percentage consumption
Mineral oil 35%
27%
Coal 17%
Natural gas 13%
5%
Biomass 3%
Hydroelectricity
Nuclear energy

Wood, firewood, cattle dung, hay etc. are bio fuel. It has been found that the use of these types of
energy sources is more in the third world and the under developed countries. Though the population
in the undeveloped and poor countries is more, the demand of fuel is still less. The bio mass fulfills
about 13 percent of the world’s demand for fuel. It is known that of the world’s total consumption
of fuel, about 5 percent is supplied by Hydropower and 3 percent by nuclear reaction.

Energy Crisis

Energy is the most important part of our life which is obtained from various sources. For
sustaining our lives, the major sources of energy are the sun, water, coal, natural gases, LP
gases, petroleum products, wood etc. Out of these energies, some are renewable and some are
nonrenewable. At present world, more energy has been used up due to over population. In the
one hand, there has been an over exploitation of the non-renewable sources of energy; and on
the other hand, there has not been enough search for renewable sources of energy. For example,
the petroleum products, coal etc. are being finished in their mines; the forests are being reduced;

56 Blooming Science Book 10

and the ground water level has decreased. The rate of consumption of the fuel energy is so
rapid that they can not last for long time. If the present rate of consumption of fuel energy is
continued and no alternative sources of energy are brought in, the fuels like coal, petroleum etc.
will be evacuated. As a result, the world is likely to face a serious problem of energy scarcity.
Such scarcity of energy in future due to industrializations, urbanization and population growth
is called energy crisis.

Conservation of Energy: The remedy to the probable energy crisis of the world are:

a) to make judicious (or economic) use of present energy and
b) to make use and to search for alternative sources of energy as far as possible.
The economical use of energy is called the conservation of energy.
The following steps are to be taken to conserve the energy:
1. The main cause of energy crisis is the over population. The step to conserve energy should

be to control the world’s population.
2. The use of non-renewable sources of energy should be avoided as much as possible.
3. The renewable sources of energy should be used in place of non-renewable one.
4. Devices based on solar energy should be encouraged.
5. Bio-gas should be used for cooking and other domestic applications.
6. More efficient devices which consume less energy should be used.
7. The fuel running vehicles should be replaced by solar and electrical vehicles as far as possible.
8. Automobiles should be serviced regularly so that they consume less fuel.
9. Deforestation should be stopped and afforestation should be implemented.
10. Awareness about the importance of energy conservation should be created in people.

Let's Learn

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

2. Coal and mineral oil are fossil fuels as they are formed from the dead bodies of plants
and animals.

3. Scientists are busy to design solar equipment. It is because, solar energy is perpetual
energy source and by using it, non-renewable energy sources can be conserved for the
future. It will push further the energy crisis.

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

5. Energy crisis is the burning issue at present, it is because lengthy crisis may bring the
existence of human beings in hazard, as in the absence of major energy they cannot survive.

6. Tidal energy is not possible in our country, it is because we don’t have access to seas and
oceans.

Blooming Science Book 10 57

Caloric Values of Fuels: The caloric value of a fuel is defined as amount of heat produced by
burning unit mass of the fuel completely. That is if by burning of a fuel of mass (m) produces
Q joules of heat energy, then

CV=Q/m J/Kg

The following table presents the caloric value of some fuels.

Fuel Caloric value (KJ/gm) Fuel Caloric value (KJ/gm)
Dung cake 6-8 Kerosene 48
Wood 17 50
Coal 30 Petrol
Bio-gas 35 to 40
Alcohol 30
Charcoal 33 Natural gas 33 to 50
Diesel 45 Hydrogen gas 150
55
Methane

Points to Remember

1. Energy is needed for all organisms to survive. It is also needed for the entire works
performed by the human beings.

2. There are different sources of energy around us. Energy can be obtained very easily from
some sources but it is very difficult and expensive to obtain from the other sources.

3. The source of energy from which energy can always be obtained or can be easily detained
again is called renewable source.

4. The energy source that cannot be regained once it is depleted is called non-renewable
energy source.

5. E = mc2 is the relation given by Albert Einstien in 1905 AD and is called Einstien’s mass
energy relation.

6. The only source of energy in this earth is the sun. The sources of energy like wood, coal,
mineral oil, Hydropower, wind, etc are the outcome of solar energy.

7. Atomic energy or reaction is the only source in solar energy. When Hydrogen atoms
undergo fusion reaction; a vast amount of heat energy is produced. So, the sun can also be
called an atomic power plant.

8. Hydropower is the pollution free and renewable energy source.
9. Since, mineral oil and coal are formed from plants, they are called fossil fuel.
10. Fossil fuel is the source of 80 percent of the world’s total fuel consumption. A big energy

crisis will arise in the world after these fuel stores are depleted.
11. The habit of conserving fuel should be practiced in order to solve energy crisis and

alternative energy sources should be developed.

58 Blooming Science Book 10

Project Work

1. Visit a nearby hydroelectricity project and observe how is electricity being
generated there. Discuss with your friends about the type of energy produced.

2. Visit a biogas (gobar gas) plant in your locality. Observe the mechanism of
producing energy in gobargas plant. Discuss the advantages and disadvantages of
using biogas as a source of energy. Write down in points and show to your teacher.

Exercise

Answer the following questions in short.
1. What is a renewable source of energy? Give two examples,
2. What is nuclear fusion reaction? Give its equation.
3. For what works is the solar energy being used in the nature?
4. “Hydropower is also the product of solar energy”. Clarify the statement.
5. Why is coal called the fossil fuel?
6. What are the four advantages of Hydropower?
7. What is energy crisis? Write down any two methods to solve it.
8. What is nuclear fission reaction? Write down its equations.
9. What is Einstein’s mass energy relation?
10. Distinguish between i) nuclear fusion and nuclear fission. ii) renewable and non-renewable

energy.
12. How can we conserve energy?
13. Write any three evidences of nuclear reaction going on the sun.

14. Write short notes on:
i) Nuclear energy
ii) Fossil fuel
iii) Geothermal energy
iv) Biomass
v) Alternative sources of energy

15. Give reasons:
a) The use of Hydropower should be increased more than that of coal and mineral oil for
the energy.
b) Fossil fuel has been used more in the existing world.
c) The sun is considered as a major source of fuel energy.
d) Nuclear energy is harmful to living beings.
e) It is difficult to generate hydroelectricity in our country.
f) Government encourages villagers to plant bio-gas by providing subsides in our country.
g) Urbanization brings energy crisis.

Blooming Science Book 10 59

16. Choose the correct alternatives from the following options.

i. Coal is an example of which source of energy?

a. Renewable

b. Alternative

c. Non-renewable

d. None of the above
ii. Who established the formula, E = Mc2 ?

a. Pascal

b. Laplace

c. Albert Einstein

d. Isaac Newton

iii. Which is the best alternative source of energy in our country?

a. Wind

b. Nuclear energy

c. Hydroelectricity

d. Cool

iv. The scarcity of energy is called:

a. Energy crisis

b. Adequacy of energy

c. Opulence of energy

d. All of the above
v. How much solar energy falls per m2 on the surface of the earth?

a. 1.4 kw

b. 2 kw

c. 14 kw

d. 140 kw

60 Blooming Science Book 10

Chapter Heat

4

Learning Outcomes Estimated Periods : 5

On the completion of this unit, the students will be able to:
 distinguish between heat and temperature.
 explain specific heat capacity and derive heat equation.
 describe and use various devices of measuring temperature.
 solve simple numerical problems related to heat equation.

Introduction

Boiled tea and coffee are hot whereas beverages kept in refrigerator are cold. It is common
experience to all that some bodies are hot and others are cold. The hotness or coldness of a body
can be felt when touched. Burning wood, electric heater, etc provide us heat. Those things which
provide heat are called the sources of heat. The sun is the natural source of heat.

Heat is a form of energy because it has capacity to do work as can be seen in kettle in which water
is being boiled. The lid of kettle moves because of steam which is formed owing to heat. On the
same basis the steam engines are made which has capacity to do mechanical works.

When we go near source of heat we feel warm. So heat gives us the sensation of warmth.

Hence, heat is a form of energy which gives us the sensation of warmth. The SI unit of heat is
Joule and CGS unit is calorie.

One Calorie of heat is defined as the amount of heat required to raise the temperature of one gram
of pure water by 1oC.

Joule is small unit of heat. 4200 Joules of heat energy is required to raise the temperature of 1kg
of water through1oC. i.e. 4.2 joule of heat raises temperature of 1gm of water by 1oC.

1 Calorie = 4.2 Joules

The larger units of heat are kilo joule (KJ) and Mega joule (MJ). Scan for practical experiment

and 1KJ = 1000 J = 103J

1MJ = 1000 KJ = 103 KJ

1MJ = 1000000 J = 106J

What happens when we touch hot or cold bodies? visit: csp.codes/c10e04

When we touch hot objects heat energy transfers to our body and hence we feel the object to be hot.

If we touch cold object, heat of our body moves to the object and we feel cold.

Blooming Science Book 10 61

Heat as energy (Molecular view of Heat / Kinetic molecular theory)

Every material 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, molecules
in a motion possess kinetic energy. There is 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
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 is analogous to rain and water in a lake is
analogous of internal energy. Basically, they are same thing, namely, energy, but when internal
energy is in flow it is called heat. Thus, heat is a energy in transit. It is measured by using
caloriemeter and the units used for this purpose are calorie and joule. Amount of heat on the body
(i.e., internal energy) depends on following:

1. Number of molecules of the body i.e., mass of the body. It means larger the number of
molecules, higher the value of internal energy of a body is.

2. Average kinetic energy of the molecules.

∴ Amount of heat ∝ Number of molecules of the body × temperature

i.e., Amount of heat ∝ Mass of the body × Average kinetic energy of molecules

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 in different speed than a molecule of another body at the same temperature.

When a body is heated, the magnitude of vibration of molecules of the body increases. This
consequently increases kinetic energy of molecules and hence the internal energy of the body rises.
When the body is allowed to cool, the magnitude of vibration of molecules begins to decrease and
thus its internal energy decreases. That is, a hot body has more internal energy than other identical
cold body. If two bodies: one hot and other cold are kept in thermal contact, the internal energy
flows from the hot body to the cold body. This flow of energy is 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 thermal contact.

Thus, heat is defined as total KE of the molecules contained in a body.

Temperature

When a steel container filled with boiled water is touched, we feel it to be hot. If a piece of
ice is touched, we feel it to be 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 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 flow of heat from it to another body in its contact. It also can be said
that temperature is the average kinetic energy of molecules.

On heating a body the heat energy is absorbed by its molecule and the heat content as well as the
temperature (except during change of state of the body increase).

Similarly, on cooling, the temperature of a body decreases. So the heat is the cause of temperature
i.e. temperature is the effect of heat.

62 Blooming Science Book 10

Temperature and the Direction of Flow of Heat

We can compare the flow of heat with the flow of water taking a system of two cylinders of
different sizes which are connected by a narrow tube having stopper.

Closing the tube with the help of stopper if same volume of water is kept in two cylinders A&B,
the water will be at higher level in B than in A. If the stopper is opened certainly water will flow
from B to A until the level of water becomes same in A & B.

So water flows from higher level to lower level irrespective of the volume of water, i.e. level of
water determines the direction of water flow.

Similar to this, heat flows from object at higher temperature to object at lower temperature,
irrespective of the amount of heat.

Thus, temperature determines the direction of flow of heat energy from one body to another body
when two bodies are at thermal contact. The greater the difference in temperature between two
objects, the faster is the flow of heat between them.

∴ Temperature in thermodynamics is analogous to level of water in hydrodynamics
and amount of heat is analogous to volume of water.

Activity -1
To observe flow of heat from higher temperature to lower temperature.
Materials required:
three beakers, thermometers, hot water, cold water.
Method:
a. Take about 200 ml of hot water in a beaker and same quantity of cold water in another
beaker.
b. Measure the temperature of both water by using the thermometers and note down.
c. Now mix water of both beakers in a bigger beaker and measure the temperature.

Thermometer

Beaker with hot Beaker with Beaker with
water cold water mix water

Observation:

The temperature of water after mixing is found more than in cold water and less than in
hot water.

Conclusion:

Heat flows from a body of higher temperature to the one at lower temperature until the
temperature of both becomes the same.

Blooming Science Book 10 63

Differences between heat and temperature

S.N. Heat S.N. Temperature

1. It is a form of energy which gives 1. It is the degree of hotness or coldness of
sensation of coldness and hotness. a body.

2. It is a cause of change in 2. It is an effect of heat.
temperature.

3. It measures total kinetic energy of all 3. It measures average kinetic energy of
molecules of a body. the molecules of the body.

4. Its SI unit is joule (J) 4. Its SI unit is Kelvin (K)

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

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

Measurement of Temperature

The science of temperature and its measure is called thermometry. An instrument used to measure
temperature is called a thermometer. Thermometers use the facts that some property of matter
changes with change in temperature. For example, in a mercury glass thermometer, the mercury
expands with increasing temperature; in a gas thermometers, the pressure of the gas changes with
change in temperature etc.

A device which is used for measuring the temperature of a body is called thermometer. The
thermometer works on the principle that “When a body is heated, it expands and contracts on
cooling.” Generally liquid is used in a thermometer. The thermometer in which mercury and
alcohol are used as thermometric liquids, then it is called liquid thermometer.

Mercury remains in liquid state for a quite wide range of temperature because it freezes at -39oC
and boils at 357oC.

The mercury thermometer cannot be used for measuring the temperature below -39oC because it
freezes below that temperature. Therefore, mercury thermometer cannot be used to measure the
temperature in very cold regions.

Alcohol is also a thermometric liquid. The freezing point of alcohol is -117oC and the boiling
point is 78oC. Thus, alcohol remains liquid to a very low temperature but not at high temperature
i.e., beyond 78oC. Due to this reason, alcohol thermometer is used for measuring the temperature
in very cold regions. Alcohol thermometer can’t be used to measure the temperature of boiling
water as it boils at 78°C before boiling water.

Normal temperature of the human body is 37oC (98.6oF). The SI unit of temperature is Kelvin.

Types of thermometer 2. Digital thermometer
1. Clinical thermometer 4. Maximum - minimum thermometer
3. Laboratory thermometer

64 Blooming Science Book 10

1. Clinical Thermometer

Clinical thermometer is a special kind of mercury thermometer. It is used to measure the
temperature of the human body. It is available in both Celsius and Fahrenheit scales. It is smaller
in size with a short range of temperature from 35°C to 42°C (94°F to 110°F).

It is so graduated because the temperature of a normal human body is 37°C (98.6°F) which is
indicated on the thermometer by a special mark which is usually a small arrow head in red.

Average body temperature (98.6°F)

Mercury Constriction

The clinical thermometer has a narrow constriction in the capillary tube near the bulb. It stops
the mercury thread running back into the bulb, when the thermometer is taken out from the body.
Hence, the temperature of a person can be read even long after it has been taken out. To use the
thermometer again, it is given a few smart and vigorous jerks to force the mercury in the capillary
tube through the constriction, back into the bulb.

The body of the thermometer is not cylindrical but prismatic in shape. This space magnifies very
thin thread of mercury in the thermometer. This makes the reading easy and correct. The thick
prismatic shape body of the thermometer also prevents the transfer of heat of user’s hand to the
mercury in the capillary tube. Each degree is divided into five equal parts so that temperature
corresponds to one fifth of a degree can be read on it.

2. Digital thermometer

These days the use of clinical thermometer is being replaced by digital thermometer as these are
more convinient to use.

The electronic device which measures the temperature of the
objects with the help of temperature sensing instruments usually
resistance or thermister and show the reading in digital display is
called digital thermometer. The digital thermometer is of different
types, depending upon the types of sensor used in it. It can display
the reading in any scales of the temperature i.e. Fahrenheit or
Celsius or both and works on the principle that resistance changes
as the temperature is changed.

Similarly, on the basis of amount of resistance of the resistor Digital thermometer
or thermister, amount of current flows through it and reading is
displayed.

The digital thermometer is widely used in hospital and laboratory in those days due to its
efficiency. However, it has also the disadvantage that it measures the temperature moment to
moment which makes sometime difficult to take an accurate reading.

Blooming Science Book 10 65

3. Laboratory or General Thermometer

General thermometer is an ordinary thermometer used to measure the temperature of different
substances in the laboratory. It is generally long, round and cylindrical in shape with thin wall.
It has marking from -10°C to 110°C. Its capillary tube is very thin. The part of the capillary
tube above the mercury is vacuum. This makes the mercury raise easily in the tube. It has no
constriction.

Bulb Thread

Mercury

4. Maximum and Minimum Thermometer

It is a special type of thermometer used to measure both the maximum and minimum temperature
within twenty four hours at any places. It’s one arm records the maximum temperature and the
other arm records the minimum temperature. Meteorologists use this type of thermometer.

It consists of a U-shaped tube which is partially filled with mercury.

Its one end is connected with a cylindrical bulb B and the other end A1 A2
with spherical bulb ‘S’. Both the bulbs are filled with alcohol. The

spherical bulb is not completely filled with alcohol. There a small -10 I2 130
space left above the alcohol in the spherical bulb for the expansion of 0 120
alcohol in the arm. 10 I1 110
20 100
There are temperature scales in both the arms. The temperature 30 90
scales in arm A1 is increasing downwards while in arm A2, increasing 40 80
upwards. 50 70
60 60
A small iron or steel index provided with spring is fitted over the 70 50
80 40
90 30
20
100 10
110 0
120 -10
130

surface of mercury in each arm of the U-tube. These iron indexes do Maximum and Minimum
not fall freely.
Thermometer
When the temperature of the surrounding increases, the alcohol in the

cylindrical bulb B expands and pushes the mercury in arm A1 down. This causes the mercury in
the arm A2 to move up pushing the index I2 up.

The index is pushed upto the maximum limit depending upon the maximum temperature reached
on that day. The lower end of the index I2 shows the maximum temperature of the day. When
the temperature of the surrounding decreases, the alcohol in the spherical bulb S contracts. This

causes the mercury in the arm A2 to move and pushes the index I1 to maximum height depending
upon the minimum temperature of the surrounding. The lower end of the index I1 shows the
minimum temperature of the day.

The indexes do not fall freely, once they are set in the positions of maximum and minimum
temperatures. The corresponding temperatures are recorded. For the use of the thermometer next
time, small magnet is used to bring indexes back.

66 Blooming Science Book 10

Specific Heat Capacity

The substance which heated faster also gets cooled faster. When two substances of different
types of same amount are taken into different vessels of same size and both of them are heated
by giving equal amount of heat from 3 minutes, they show different temperatures, can you say
why? It is because of their different specific heat capacities.

Specific heat capacity of a body is defined as the amount of heat required to raise the
temperature of unit mass of it through 1oC (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 fast. Water has the maximum specific heat capacity of the
known substances. SI unit of specific heat capacity is J/KgOC. The specific heat capacity of water
is 4200 J/KgOC.

Some effects of specific heat capacity

1. Water is suitable for heating systems. When water is cooled by the same range of
temperature, the same amount of heat is given out as it has absorbed. Hence, a large
quantity of heat may be available by cooling the hot water. That’s why water is suitable
for heating systems.

2. Water is used to cool engines of vehicles. Water has very high specific heat capacity as
compared with other liquids. Thus, it can absorb a large amount of heat from the hot engines
without appreciable rise in temperature of water. It thus prevents the engines from getting
excessive hot.

3. The night of desert is very cold and day of that is very hot. It is because specific heat
capacity of sand is less, so its temperature changes fast. At days, sand gets heated which
results very hot day. At nights, sand gets cooled which results very cold night.

Heat from Sun

Colder air Hotter air

Hotter air Land Colder air Land
Sea Sea

4. Sea breeze occurs at days and land breeze occurs at nights. It is because water has
maximum specific heat capacity and sand has less specific heat capacity. Due to it, at
days sand of land is heated faster than water and the air above it becomes less dense by
its expansion. Thus, air above the seas being denser blow towards the land and sea breeze
occurs. But at nights the sea water cools slowly but sand cools faster. Now, the air above
sea water becomes less dense and that above land is denser. Thus, air over lands moves
towards the seas and land breeze occurs.

Blooming Science Book 10 67

Specific heat capacity of some materials: J/KgoC
130
S.N. Materials 134
1. Lead 138
2. Gold 234
3. Mercury 380
4. Silver 400
5. Brass 447
6. Copper 460
7. Steel 670
8. Iron 800
9. Glass 900
10. Sand 1010
11. Aluminium 1670
12. Air 1974
13. Petrol 2000
14. Water vapour 2100
15. Olive oil 2200
16. Ice 2400
17. Paraffin 4200
18. Alcohol
19. Water

Heat Equation

Generally, when a body is heated, it gains heat and its temperature increase and when a body is
cooled, it loses heat and its temperature decreases.

Let the temperature of a body of mass ‘m’ raises from t1 to t2 when it is heated by supplying
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) mass of the body
and (ii) change in its temperature.

∴ Q ∝ m ………. (i)

Q ∝ dt ……….. (ii)

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

Q ∝ mdt

∴ Q = smdt

or, Q = msdt ……….. (iii)

68 Blooming Science Book 10

Where, 's' proportionality constant and called as specific heat capacity of the material of the
body. It is constant for a given material. 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.

∴ Heat gained (or lost) = mass x specific heat capacity × change in temperature

From (iii), we have,

∴ Q = m × s × dt

Here, m = mass of the body

s = specific heat capacity

dt = change in temperature

Solved Problems

Example 1: How much heat is supplied to raise the temperature of 100g of water from 5oC to
Solution: 90oC? (sp. heat capacity of water is 4200J/KgOC).

Here, mass of water (m) = 100g = 100 kg = 0.1 kg
Initial temperature (t1) 1000

= 5oC

Final temperature (t2) = 90oC

∴ Rise in temperature (dt) = t2 - t1 = 90 - 5 = 85oC

Sp. heat capacity of water (s) = 4200 J/KgOC

∴ Amount of heat supplied (Q) = ?

We have,

Q = msdt = 0.1 × 4200 × 85 = 38700 J or 3.57×104 J.

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

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 4200J/KgOC.

Solution: Here, Heat supplied (Q) = 50KJ = 50 × 1000 (∴ 1kJ = 1000J)

= 50000J

Mass of water (m) = 10 kg

Sp. heat capacity of water (s) = 4200J/KgOC.

Rise in temperature (dt) =?

We have, Q = msdt

Q 50000
or, dt = ms = 10 × 4200 = 1.19oC.
Principle of Caloriemetry

When two bodies at different temperature are in thermal contact, heat flows from hotter to
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 equal to the heat

Blooming Science Book 10 69

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 principle of
caloriemetry.

∴ Heat lost = Heat gained

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

or, m1 × s1 × dt1 = m2 × s2 × dt2

Final Temperature of a Mixture

Before

Consider a hot iron ball whose initial temperature is T1, mass m1& specific heat capacity s1.
Let this ball be immersed in cold water kept in a vessel. Let mass of water be m2, specific heat
capacity s2 and its initial temperature be T2.
When the iron ball is kept in water it will lose heat and water will gain it. This will continue until
the temperature of both water & iron ball becomes same ie. T (thermal equilibrium). Let the final
heat lost by hot ball (Q1) = m1s1dt
or, Q1 = m1s1(T1-T)...........(1)
& Heat gained by water (Q2) = m2s2(T-T2) ............(2)
Now, using principle of calorimetry, we have

Q1 = Q2
or, m1s1(T1-T) = m2s2 (T-T2)

Solved Problems

Example: 1 Hot water of mass 7kg at 98oC is cooled for taking bath by mixing 14 kg of water
at 10oC. What is the final temperature of water?

s = 4200 J/kgoC for water

Neglected heat taken by bucket

Solution:

For hot water For cold water

mass (m1) = 7kg mass (m2) = 14 kg

initial temperature (T1) = 98oC initial temperature (T2) = 10oC

Specific heat of water (s) = 4200 j/kgoC

70 Blooming Science Book 10

Let T be one final temperature of the mixture. Then, we have
m1s1(T1-T) = m2s2 (T-T2)

7×4200 (98–T) = 14×4200 (T–10)
T = 39.33oC

Hence, the final temp. of mixture (water) is = 39.33ºC.

Example 2: If 200ml of tea at 90oC is mixed with 10 ml of milk at 15oC, what will be the final
temperature of the mixture? Assume that the specific heat capacity of milk equals
that of tea. (mass of 1 ml milk = mass of 1 ml tea = 1g)

Solution: Here, mass of tea (m1) = 200 ml

Example 3 : = 200g (∴ mass of 1 ml tea = 1g)
Solution:
= 0.2 kg (∴ 1000g = 1 kg)
Temperature of tea (t1)
Mass of milk (m2) = 90oC

= 10 ml = 10g (∴ mass of 1 ml milk = 1 g)

= 0.01 kg

Temperature of milk (t2) = 15oC.
Let ‘t’ be the final temperature of the milk. If s1 and s2 are the specific heat
capacities of tea and milk respectively, then according to question s1 = s2.
Heat gained by milk = Heat lost by tea

∴ m2s2 (t - t2) = m1s1 (t1 - t)

or, 0.01 × (t - 15) = 0.2 (90 - t)
0.02 (90 - t)

or, t - 15 = 0.01
or, t - 15 = 20 (90 - t)

or, 21t = 1785

∴ t = 85oC

Hence, the temperature of mixture is 85oC.

Calculate the final temperature when 2400 joules of heat is given to iron of mass
2kg at 20oC. (The specific heat of iron is 460J/kgoC.)

Here,

Heat supplied (Q) = 2400J

Mass of iron (m) = 2 kg

Initial temperature (T1) = 20oC
Final temperature (T2) =?
Specific heat capacity (S) = 460J/kgoC

Blooming Science Book 10 71

We have,

Q = m × s × (T2 - T1)
Q

∴ T2 - 20oC = m × s

2400 or, T2 - 20oC = 2.60
or, T2 - 20oC = 2 × 460
or, T2 = 22.6oC

Therefore, the final temperature of iron mass is 22.6oC.

Let's Learn

1. In winter, the atmospheric temperature falls below our normal body temperature (37oC).
Due to this difference in temperature, our body loses heat and so our body cools. To
prevent our body from excessive cooling, we need warm clothes in winter.

2. Different substances have different specific heat capacity. It is because they are made up
of different kind of molecules. Some molecules vibrate fast and some vibrate slow with
same amount of heat energy supplied.

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

4. In earthen pot, water oozes out through the pores and is evaporated. Evaporation of oozed
water takes heat from 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 the earthen pot is cooler than that
in a metal pot.

5. Water is not used as a thermometric liquid because it has not uniform expansion. Water
contracts on heating from 0oC to 4oC and then from 5oC, it starts to expand.

6. Alcohol thermometer is used in cold region to measure the temperature because it can
measure upto -117oC. But mercury freezes at -39oC.

Points to Remember

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

2. Temperature is a measure of hotness or coldness of a body.

3. 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.

4. The amount of heat gained is equal to the amount of heat lost, thus the relation is called
principle of calorimetry.

5. The specific heat capacity of a substance is defined as the amount of heat required to raise
the temperature of unit mass of it through 1oC (or 1K).

6. Different substances have different values of specific heat capacity.

7. The electronic device which measures the temperature of the objects with the help of
temperature sensing instruments usually resistance or thermistor and show the reading in
digital display is called digital thermometer.

72 Blooming Science Book 10

Project Work

Collect different types of thermometers and measure the following:
a. Human body temperature of five different persons and find the average body
temperature.
b. Temperature of hot water.
c. Temperature of tap water.
d. Temperature of ice cubes.
e. Maximum and minimum Temperature of a day.

Exercise

1. What is heat? How is it different from temperature?
2. On what factor does heat depend? What is the relation of heat with these factors?
3. Give the concept of heat and temperature on the basis of molecular motion.
4. Differentiate between heat and temperature in points.
5. Define:

a. Specific heat capacity b. One calorie heat
6. What does it mean?
a. The specific heat capacity of water is 4200 JKg-1oC-1.
b. The specific heat capacity of mercury is 138 JKg-1oC-1.
7. What is the principle of calorimetry?
8. Answer the questions on the basis of given table.

Substance Sp. heat capacity
A 2100 J/kgoC
B 910 J/kgoC
C 138 J/kgoC

a. If A,B,C are taken in equal mass and heated by giving equal heat, which one of
them gains the maximum temperature and why?

b. Which one of them is suitable for cooling and heating purposes? Why?

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

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

Blooming Science Book 10 73

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

Substance Sp. heat capacity

X 380J/kgoC

Y 910 J/kgoC

Z 470 J/kgoC

a. If equal mass of X, Y and Z have same temperature which one has maximum heat in it?

b. If three pieces of them has equal temperature and equal amount of heat, which of
them has the maximum mass?

c. What do you mean by specific heat capacity of ‘Z’ is 470 J/kgoC?

d. If equal mass of same shape and size of them at 100oC temperature of them are kept
over a wax slab, which of them will melt the wax for the maximum depth?

10. What is digital thermometer?

11. What is the use of laboratory thermometer?

12. Which thermometer is used to measure body temperature and why?

13. Give reason:

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

b. During high fever, a piece of wet cloth is kept on the forehead of the person.

c. Temperature differences in the places near the sea is low.

d. Mercury is heated faster than water.

e. Water is used in hot water bags.

f. When we get out of bed in the very cold morning, we feel the air of room cold. But
when come back staying sometimes out, we feel the air of the same room warm.

Numerical Problems

1. How much heat energy is required to raise the temperature of 5 kg of water from 30oC to
100oC? The specific heat capacity of water is 4200 Jkg-1oC-1. [1.47 × 106J]

2 Calculate the specific heat capacity of water if 2kg water at 25oC requires 2.1 × 105J heat
energy to increase its temperature to 50oC. [4200 Jkg-1oC-1]

3. A mass of block is 1kg. And electric heater of power 48W takes 5 minutes to rise the
temperature of the block from 20°C to 50°C. Find specific heat capacity of the block.
[480J/kg°C]

4. A pressure cooker of mass 2 kg is at temperature of 25oC. If 6.4KJ of heat energy is
supplied to it, what is its temperature? The specific heat capacity of the material of the
pressure cooker is 1000 Jkg-1oC-1. [28.2oC]

5. What quantity of heat is required in calorie for 5kg water to raise its temperature from

20oC to 100oC? [Ans: 400000cal

6. The temperature of 20 kg water in the radiator of engine of a car is 30oC. If the temperature

of the water increases up to 100oC after the engine is heated , what quantity of heat will the

water absorb? [Ans: 5.88 × 106 J]

74 Blooming Science Book 10

7. What will be the quantity of heat required to raise the temperature of 2kg paraffin by

10oC if 44000 joules of heat energy is required to raise the temperature of that paraffin by

20oC? [Ans: 2.2 × 104 J]

8. What is the final temperature of mixture if 100g of water at 70oC is added to 200g of milk

at 10oC. The specific capacity of water and milk are supposed equal. [Ans: 30oC]

9. The normal temperature of human body is 98.6ºF. What is temperature in ºC? [Ans. 37ºC]

10. Determine the amount of heat to be taken out from hot iron block of mass 800gm to cool

it from 300ºC to 20ºC? Specific neat of iron = 480J/kgºC. [Ans. 40320 J]

11. A pressure cooker having 6.5 liters capacities has mass 4 kg and temperature of 30ºC. If

the alloy of pressure cooker has specific heat capacity of 1000 J/kgºC, what will be its final

temperature when 400gm of water at 10°C is added in it? [Ans. 24.04ºC]

12. 3 Kg water at 10°C is mixed with 2 Kg. of metallic block of specific heat capacity 600J/
Kg°C to attain temperature of mixture 40°C. Calculate temperature of metal the block.

[Ans: 355°C]

Multiple Choice Questions

Choose the correct alternatives from the following options.

i. In molecular theory, heat is defined as:

a. Total KE of molecules b. 1/2 KE of molecules

c. Average KE of molecules d. All of the above

ii. 1 calories is equal to:

a. 4.4 Joules b. 4.3 Joules

c. 4.2 Joules d. 2.4 Joules

iii. SI unit of specific heat capacity is:

a. J/Kg°C b. Kg°C

c. J/Kg 1°C d. Kg/J°C

iv. Which is the best for cooling the hot engines of vehicles?

a. Kerosene b. Petrol

c. Oil d. Water

v. The value of specific heat capacity of water is:

a. 4200 J/Kg°C b. 1000 J/Kg°C

c. 420 J/Kg°C d. 500 J/Kg°C

Blooming Science Book 10 75

Chapter

5 Light Willebrord Snell
1580 AD-1626 AD

Learning Outcomes Estimated Periods : 5+2

On the completion of this unit, the students will be able to:
 explain refraction of light through lenses.
 describe types of lenses.
 draw ray diagrams and write the nature of images formed by convex and concave lenses.
 tell about defects of vision and their correction.

Introduction Scan for practical experiment

Light is a form of energy producing sensation of vision in our eyes.
The branch of Physics which deals with the study of light is called
Optics.

Lens

A lens is a portion of transparent medium (usually glass) bounded by
two refracting surfaces at least one of which is spherical. Lenses are
used in many optical instruments such as cameras, picture projectors,
telescopes, microscopes, binoculars and several other optical devices. visit: csp.codes/c10e05

In face our eye also contains a convex lens. Lenses produce images by refracting light.

Types of Lenses

There are two types of lenses

a. Convex lens and

b. Concave lens. Parallel Rays Principal focus
Principal axis
a. Convex lens: A convex lens is thicker F
at the middle and gradually becomes f
thinner towards the edges. That is, it is
a lens which bulges out in the middle. It
converges a parallel beam of light after
refraction through it and is therefore also
called a converging lens.

Convex lenses and concave lenses are available in following shapes as shown in figure below.

b. Concave Lens: A concave lens is thinner at the middle O
and gradually becomes thicker towards the edges. That f
is, a concave lens bends inwards in the middle. It diverges
a parallel beam of light after refraction through it and is F
therefore also called a diverging lens. It always gives a
diminished image. So all objects appear smaller when
seen through a concave lens.

76 Blooming Science Book 10

Biconvex Plano convex Convex Biconcave Plano Concave

meniscus concave meniscus

Some terms related to lens

1. Optical centre: The optical centre of a lens is a geometrical centre (centre point) on the
principal axis within the lens through which the rays of light pass straight without any
deviation. All distances are measured from the optical centre. The distance from the optical
centre to the object is called object distance and is denoted by ‘u’. Similarly distance from
the image to the optical centre is image distance. It is denoted by ‘v’.

Centre of curvature of the A Centre of curvature of the Centre of curvature of the A Centre of curvature of the
second surface ADB first surface ACB first surface ACB second surface ADB

Optical Optical
centre centre

C2 CO C1 C1 C O D C2
D

BB

2. Centre of curvature: The centre of curvature of a spherical surface is the centre of the
sphere of which lens is a part. There are two centres of curvatures for two surfaces. ie. c1
and C2.

3. Principal axis: A straight line joining the centre of curvatures C1 and C2 of the two surfaces
of the lens is called the principal axis.

Principal axis Principal axis

C1 O C2 C1 O R2 C2
R1 R2 R1

(a) Convex lens (c) Concave lens

4. Radius of curvature: The radius of curvature of a lens is the radius of the spheres of which
the lens is a part. There are two radii of curvature of two surfaces. ie. R1 and R2.

5. Principal focus: It is the point on the principal axis where rays of light parallel to the
principal axis after refraction through the lens, converge to a point on the axis (in case
of convex lens) or appear to diverge from a point on the axis (in case of concave). It is
usually denoted by F.

OF FO
f
f
77
Blooming Science Book 10

6. Focal length: The distance between the optical centre and the principal focus of a lens is
called its focal length. A thinner lens has a greater value of focal length than a thicker lens.
Since there are two focal points of a lens, there are two focal lengths for a lens too. We
generally use the second focal length and is denoted by ‘f’. The focal length of a convex
lens is considered as positive and the focal length of a concave lens as negative.

The focal length of a lens depends on the curvature of each surface. In general, the more
highly curved the surfaces, the shorter is the focal length of a lens. The thick lens has a
short focal length and the thin lens has a long focal length.

Activity -1

To determine rough focal length of a convex lens.

Materials required:

convex lens, optical bench, screen made of white sheet of paper, candle, match stick,

etc.
Method:

a. Adjust the screen, lens and buring candle on the optical bench as required.

b. Keep the candle at least 3cm away from the lens and screen at 20-30cm from it.

c. Adjust the distance between the screen and the lens by moving anyone close to

each other and find the clearest image formed on the screen.

Burning Optical plane lens
candle
Paper sheet

(20-30)cm

Optical bench
Observation:
When the clearest image is observed on the screen note down the distance from the
lens. The point is the principal focus (F) of the lens.
Conclusion:
The image of the distant object is formed at 'F' by a convex lens.

Ray Diagrams

The rays coming from a point object generally change their directions after passing through a
lens and the emergent rays so produced actually converge to or appear to diverge from a second
point which is known as the image of the first. If the rays actually converge to the second point,
then the second point is said to be the real image of the first but if the emergent rays appear to
diverge from a second point, then it is called a virtual image.

Information about the image formed by a lens can be found by drawing ray diagram. For
simplicity, rays are usually shown bending along the upright line through the middle of the lens,
though in reality bending takes place at each surface. Any number of incident rays may be taken

78 Blooming Science Book 10

to locate the image of an object. But any two of the following three rays starting from a point of
the object are sufficient to fix the position and size of the image.

1. A ray parallel to the principal axis after refraction passes through the second focus in a
convex lens or appears to diverge through the second focus in concave lens.

2. A ray of light passing through the optical center of a lens goes straight without deviation.

3. A ray of light passing through the first focus in a convex lens or appearing to pass through
the first focus of a concave lens, after refraction emerges out parallel to the principal axis
after refraction.

Construction of ray diagrams in convex lens
(Three rules for light rays passing through convex lenses)

Ray Incident Refracted Path of light rays
F
Through principal (a)
focus
1. Parallel to the
principal axis

(b)

2. Passing through Goes straight O
optical centre O

(c)

3. Passing through the Parallel to the

principal focus principal axis F

The two rays chosen actually meet at a point to form a real image or appear to diverge from a
point form a virtual image.

Image Formed by a Convex Lens

The nature, position and size of the image formed by a convex lens depend on the position of
the object on the principal axis with respect to the lens. The images formed by concex lens with
different possible positions of objects are given below with illustrative diagrams

1. Object at infinity: When an object is placed at infinity in front of the convex lens, the
image is formed at principal focus(F) on the other side of the lens. The image thus formed
is real, inverted and highly diminished.

OF

f
Blooming Science Book 10 79

2. Object beyond 2F: When an object is placed beyond 2F, its image is formed between
Fand 2F on the other side of the lens. The image thus formed is real, inverted and
diminished in size.

A

F B 2F

B 2F F O

A

3. Object at 2F: When an object is placed at twice the focal length (2F) from the lens, Its
image is formed 2F on the otherside of the lens. The image thus formed of a convex
lens.

A

B F B'
2F
2F F O

A'

4. Object between F and 2F: When an object is placed F and 2F, the image formed is
beyond 2F on the other side of the lens. The image thus formed real, inverted and
magnified.

A B'
2F B F O F 2F

u v A'

5. Object at Principal focus F1: When an object is placed at F, the image is formed at
infinity on the other side of the lens since the emergent rays are parallel to each other. The
image thus formed real is inverted and highly magnified.

A F 2F
2F F O



A lens in this position is used as eye piece in a telescope and in search light. In search

80 Blooming Science Book 10

light a powerful lamp is placed at the focus of the convex lens that gives through
powerful parallel beam of light.
6. Object between the optical center (O) and focus F1: When the object is placed
between the lens and F, the image is formed beyond the object on the same. The image
formed is virtual, erect and magnified.

A’

A

B’
2F F B O F 2F

From the above diagram, it can be observed clearly that as the object approaches the
lens, the image formed moves gradually away from the lens. When the objects is placed
nearer to the lens, the size of image gradually increases.

Position and Nature of Image Formed by a Convex Lens

Position of object Position of image Size of image Nature of image
Real and inverted
An infinity At focus Highly diminished Real and inverted
Real and inverted
Between infinity and 2F Between F and 2F Diminished
Real and inverted
At 2F At 2F Same size as the object
Real and inverted
Between 2F and F Between 2F and Magnified
infinity Virtual and erect.

At F At infinity Highly magnified

Between F and the On the same side Magnified
optical center beyond the object

Image Formed by a concave lens

Construction of ray diagrams in concave lens

(Three rules for light rays passing through concave lenses)

Ray Incident Refracted Path of light rays

1. Parallel to the Appears to diverge F O
principal axis from principal focus

2. Passing through Goes straight O
optical centre O

Blooming Science Book 10 81

3. Appears to pass Parallel to the OF
through the principal principal axis
focus

1. Object at infinity
When an object is placed at infinity infront of a concave lens, its image is formed at principal

focus (F) on the same side of the lens. Thus formed image is virtual, erect and highlt diminished.

A
A'

B' O F

F

2. Object between infinity and optical centre
When an object is placed at any position between infinity and optical centre, the image is

formed between focus and optical centre on the same side of the object in the lens. Thus formed
image is virtual, erect and diminished.
In case of concave lens also, as the object is gradually moved towards the optical centre
of the lens the size of image gradually increases but remains always lesser than the size of the
object.

A
A'

B F B' O

Activity -2

To observe the formation of image by a convex lens.
Materaials required:
Convex lens, optical bench, screen (paper sheet), candle, match stick.
Method:
a. Find the focal length of the given lens as in Activity 1.
b. Adjust the given apparatus to observe the image formation.
c. Find the position of F, 2F, O on both sides of the lens by finding the focal length.
d. Similarly shift the candle towards the lens in different positions and observe.
Observation:
When the burning candle is shifted towards the lens, the image is shifted away from the lens.
Now, draw the ray diagram to show the formation of image by a convex lens when the
object is kept at various positions in front of a convex lens.

82 Blooming Science Book 10

Differences between Real and Virtual Images

Real Image Virtual Image

1. It can be obtained on the screen. 1. In cannot be obtained on the screen.

2. It is always inverted with respect to the 2. It is always erect with respect to the object.
object.

3. In real image the rays of light actually 3. In virtual image the rays of light do not

intersect to form image. actually intersect but they intersect only

when produced backwards.

4. Convex lens produces real images for all 4. A concave lens always forms a virtual
positions of an object from the lens except image.
when an object is placed within the focal
length. i.e. focus and optical centre.

Uses of Lenses

Convex lenses form real images, so have many uses. They are used in the optical instruments
such as camera, microscope, telescope, binocular etc. They are used as a magnifying glass or eye
glass to make things look bigger. They are also used as a burning glass to lit fire from the sun’s
rays. Long sighted person uses convex lenses in his spectacles.

Concave lenses have limited uses. It is used to correct short-sightedness. It is used in the peep
hole of the door which enables the person looking outside to have a clear view of the outside. The
reasons why a concave lens is used in a peep hole are;

(i) If forms only virtual images for all positions of the object.

(ii) It has a wider field of vision.
Magnification

For the different positions of the object, a convex lens forms images of different sizes. It forms
bigger image when the object is near to the lens, smaller image when the object is far from the
lens and same size image when the object distance and image distance is equal. The variation in
the size of the image formed by a lens can be explained as the magnification.

Magnification of a lens is defined as the ratio of the size or height of the image to the size or
height of the object. If ‘I’ is the height of an image and ‘O’ is the height of the object, then
magnification ‘m’ produced by the lens is given by;

Size(height) of image (I)
Magnification (m) = Size (height) of the object (O)

Image distance (v)
or Magnification (m) = Object distance (u)

Power of Lens

Focal length is an important physical characteristic of a lens. The power of a lens is its ability to
converge (in convex lens) or diverge (in concave lens) the incident beam of light. The ability of
lens to converge or diverge is inversely proportional to its focal length. Shorter the focal length,
the greater is the convergence or divergence produced by a lens. Thus the power of the lens is
measured by the reciprocal of the focal length expressed in metre.

Power (P) = 1

f(m)

Blooming Science Book 10 83

The power of a lens is measured in dioptre (D). A lens is said to possess a power of one dioptre
if it has a focal length of one meter. Thus,

1
Power (P) in dioptre = focal length (f) in metre

100
or, Power (P) in dioptre = focal length (f) in cm

Thus, smaller the focal length of a lens, greater is its magnifying power and vice versa. The
power is positive for convex lens and negative for concave lens.

O F O FF O

f f f
(a) (b)
(c)

Solved Numericals

Example 1: An object is placed at a distance of 4cm from a convex lens. If a real image
forms at a distance of 16 cm from the lens. How much does the lens magnify
the object?

Given, Image distance (v) = 16 cm
Object distance (u) = 4 cm
Magnification (m) = ?

According to the formula,

v 16cm

m = u = 4cm =4

∴ The lens magnifies the image 4 times the object.

Example 2: Calculate the power of the lens whose focal length is 20 cm.

Given: Focal length (f) = 20 cm

Power (P) =?

According to the formula,
100 100

P = f (cm) = 20 = 5D

Hence, the power of a lens is 5D.

84 Blooming Science Book 10

Example 3: Calculate the focal length of the lens whose power is +0.5 D. What type of
lens is this

Given, Power (P) = 0.5 D Scan for practical experiment

Focal length (f) = ?

According to the formula,
100

P = f (cm)

100 100 visit: csp.codes/c10e06
f = P = 0.5 = 200 cm = 2m

∴ The focal length of the given lens is 2m. Since its power is positive, it is a convex lens.

Example 4: Calculate the focal length of lens whose power is -1.25 D. What type of lens
is this?

Given, Power (P) = - 1.25 D

Focal length (f) = ?

According to formula,
100

P = f (cm)

100
f = -1.25 = –80 m
Hence, the focal length of the lens is -0.8 m and it is a concave lens because its power is
negative.

Human Eye

The human eye resembles a camera in having an elaborate lens system on one side and sensitive
screen called the retina on the other. In its action and performance, the eye is an automatic optical
instrument.

The structure of the eye is shown in the figure below. The most important parts of the eye are as
follows:

1. Sclera: It is the outermost covering of the eye. It is made of white tough tissue. It maintains
the shape of the eye and protects its delicate parts from injuries.

2. Cornea: It is the front bulged portion of the sclerotic layer. It is transparent and allows the
light to enter into the eye.

3. Choroid: It is a dark grey membrane attached to the sclerotic layer. It contains blood
vessels to supply blood to the eye. It’s function is to darken the eye from inside so as to
avoid internal reflections.

4. Iris and Pupil: It is the coloured portion of the eye. It lies behind the cornea and in front of the
lens in the aqueous humour. It is an opaque diaphragm with the circular opening at the center

Blooming Science Book 10 85

called the pupil. The light enters the eye through the pupil. The pupil looks black because all
the light entering the eye is absorbed and none is reflected. The muscles in the iris can increase
or decrease the size of the pupil in response to the intensity of light falling on it.

Cornea Sclera
Pupil Choroid
Lens Retina
Iris
Fovea

Optic disc
blind spot

Ciliary body Blood vessels
Suspensory ligament Optic nerve
Vitreous body

In dim light the size of the pupil increases to allow more light. But in bright light it becomes
smaller to reduce the amount of light entering the eye. Thus, the iris prevents the damage of the
retina.

Lens: It is a transparent structure made of a jelly-like substance. It is held in place by suspensory
ligaments and ciliary muscles. The eye lens is more curved on the posterior side (i.e. the inner
side). Unlike the lens of a camera, the eye lens is flexible and can change its shape as the muscles
attached at its periphery contract or expand. The function of the eye lens is to focus the image of
a distant object. It forms a real, inverted and diminished image of the object on the retina.

The lens divides the inner portion of the eye into parts- the aqueous humour and the vitreous humour.
The aqueous humour is the anterior chamber, which lies between the cornea and the lens. It is filled
with watery fluid. The vitreous humour is the posterior chamber, which lies between the eye lens
and the retina. It is filled with a dense, jelly-like fluid and is larger than the aqueous humour.

Retina: It is the internal lining at the back of the eye and consists of thousand of light sensitive
cells called rods and cones. When light falls on these cells they change light energy into electrical
impulses which travel through the optic nerve to the brain. The brain interprets the inverted
image as being erect. The function of the retina is to receive the optical image of an object and
then to convert it into optical impulses.

Action

The working of the eye is very similar to that of a camera. In the case of a camera, the amount of
light is controlled by a shutter, but in human eye this action is performed by iris. The size of the
pupil changes by the action of tiny ciliary muscles.

In a camera, the focusing of the image is done by moving the lens. But the lens of the human
eye does not move and forth. Instead the eye lens changes its focal length by changing its shape
brought about by relaxing or stretching of the ciliary muscles.

86 Blooming Science Book 10

Accommodation of Eye

The human eye can produce a focused image of either a near object or a distant object on the
retina. This is achieved by the change in the focal length of the eye lens. The automatic ability
of the lens to change its focal length is called accommodation. This change in the shape or focal
length of the lens is brought about by contraction or relaxation of the ciliary muscle which runs
in a circular round the lens.

To focus a near object the ciliary muscles contract and cause the lens to become thick. This
reduces the focal length of the lens but increases its converging power. This shape is better able
to refract the rays from the close object. The closer object would require more power of the lens.

In old ages, the lens loses some of its elasticity. Thus, it is more difficult to accommodate for
close objects and we have to use spectacle for reading and other close work.

To focus a distant object the ciliary muscle relaxes and stretches the lens to its thin shape. In this
condition the focal length of the lens increases which is equal to the retina-lens distance. In this
condition the eye is more relaxed and the lens has the least power. So the parallel rays coming
from a distance object are focused on the retina.

Normal Vision

The power of accommodation possessed by the eye with normal vision is such that it can clearly
see the object placed anywhere between infinity and its near point. The closest or least distance at
which any object can be seen clearly is called the least distant of distinct vision. In this condition,
the power of the eye-lens is maximum. This distance is usually 25cm for a normal eye. Thus the
near point for a normal eye is at about 25cm. It is the minimum distance for the eye beyond which
an eye can accommodate and focus objects clearly.

The farthest point from where an object can be seen clearly is called the far point of the eye. The
far point is infinity for the normal vision.

If the object is placed at a distance less than 25cm from the normal eye, the power of
accommodation is not able to help and the image is formed beyond retina.

Defects of Vision

The eye lens becomes thick to focus the image of the nearby object and thin to focus the image
of the distant object on the retina. Thus in normal life, eye lens changes its shape according to the
distance of the objects to be viewed. If the lens in the eye cannot change its shape according to
the distance of objects to be viewed, such eye problem is called defect of vision.

Normal human eye has a wide range of vision from 25cm to infinity, but a defective eye is not
able to see objects over such a wide range of distance. There are two common defects of vision
that can be corrected by the use of spectacles having either concave lens or convex lens.

(i) Short-sightedness (myopia), and

(ii) Long-sightedness (hypermetropia)

Blooming Science Book 10 87

(i) Short-sightedness (Myopia): A person suffering from this defect cannot see distant object

clearly; but can see the near objects object at far point Retina
clearly. Such a person has either
elongated eye ball or thick eye lens. The I

focal length of the relaxed eye is slightly

less than retina-lens distance. So the defective eye

image of the distant object comes to O Retina
focus in front of the retina and the image I
on the retina itself is blurred.

In order to remove this defect, a defective far point

diverging lens of proper power is used. Short sightedness and its correction

In fact, the focal length of the concave

lens should be such that the parallel rays after refraction through the lens should appear to

come from the far point of the effective eye to form a sharp image on the retina.

For a normal eye, the far point is infinity. An object at infinity forms a clear image on the
retina. In a normal eye the size of the ball exactly match the power of the lens.

(ii) Long-sightedness (Hypermetropia): A person suffering from this defect cannot see the
near objects distinctly. The near point moves away beyond 25 cm. The person has no
difficulty in seeing the distant object.

Such a person has either short eye ball or thin Retina
eye lens. The focal length of the relaxed eye

is more than the retina-lens distance. The light O I

from a distant object is brought to focus on the

retina but from a near object it is focused behind Defective eye
the retina and the image.

object at Retina

In order to remove this defect, a convex lens of near point I
proper power is used. The focal length of the O

convex lens should be such that the rays from O Convex
(the near point of a normal eye) should appear to
come from the far point of the defective eye to Long sightedness and its correction

form a clear image on retina.

Differences between Long sightedness and Short Sightedness

Long Sightedness Short Sightedness

1. A person with this defect can see distant 1. A person with this defect cannot see

object clearly but is unable to see object distinctly the object at long distance but

nearby. can see object nearby.

2. Eye lens may be too thin i.e. its focal length 2. Eye lens may be too thick i.e. its focal

may be large. length may be small.

3. The image of the nearby object is formed 3. The image of the nearby distant object is

behind the retina. formed in front of the retina.

4. Convex lens is used to correct this defect. 4. Concave lens is used to correct this defect.

88 Blooming Science Book 10

Let's Learn

1. A convex lens is called converging lens because all the parallel beam of light focus at
a point after refraction through it. A concave lens is called diverging lens because the
parallel beam of light incident on it, appears to diverge from the point after refraction.

2. Convex lens is used to correct long sighted eye. The convex lens converges the light rays
coming from object at normal near point so the rays appear to come from defective near
point and are again converged by eye lens to from image on the retina.

3. Concave lens is used to correct short sighted eye. The concave lens diverges the rays
passing from normal far point and appears to come from defective far point then the eyes
converses the ray into retina to form an image.

Points to Remember

1. A lens is a portion of a transparent medium bounded by two surfaces, at least one of which
is a curved surface.

2. Convex lens is a converging lens.

3. Concave lens is a diverging lens.

4. An image formed by a convex lens may be small or same or larger than the object
depending upon the position of the object.

5. A convex lens forms a virtual image when an object is placed within its focal length.

6. A concave lens always forms diminished and virtual image.

7. In real image the rays of light actually intersect to form image.

8. In virtual image the rays of light do not actually intersect but they intersect only when
produced backward.

9. The magnification produced by the lens is the ratio of the height of the image to the height
of the object. It has no unit.

10. The extent to which an image is magnified or diminished depends upon the type of the
lens used and the position of the object.

11. Power of lens is measured by the reciprocal of the focal length expressed in meter.

12. An optical instrument is an arrangement, which makes use of combination of lenses,
mirrors or prisms.

13. The automatic ability of the lens to change its focal length is called accommodation.

14. The minimum distance upto which an eye can see clearly is the distance of distinct vision.
It is 25cm for a normal eye.

15. Myopia and hypermetropia are two common defects of vision.

Blooming Science Book 10 89

16. Convex lens is used to correct farsightedness. It converges the ray of light coming from
the object and forms the image.

17. Concave lens is used to correct short sightedness. It diverges the ray of light coming from
object and forms image on retina.

18. The process of adjusting the distance between the eyepiece and the objectives such that
clear image of the object can be seen is called focusing.

Project Work
To observe mammalian eye

Collect a goat eye from the bucther's shop of your locality. Observe the external parts.
Cut it with the help of sharp blade and observe the internal parts. An eye of a goat is
similar to that of human eye. Draw the diagram of human eye in your practical copy.

Exercise

1. What is lens? What are the two types of lenses.
2. Write the uses of lens.
3. Explain the terms optical centre, principal axis and principal focus of a lens.
4. Does a convex lens always form a real image? If not when does it form a virtual image?

Show with diagram.
5. What is meant by the power of a lens? In what unit is it expressed? Calculate the power of

lenses having the focal lengths; i. 1m ii. 0.5 m iii. 25 cm.
6. If an object is placed at a distance of twice the focal length from a lens. Use a ray diagram

to show where the image will be formed?
7. Distinguish between the virtual image formed by a convex lens and concave lens.
8. What is magnification? Write down its formula.
9. What is meant by the focal length of a convex lens? A convex lens has a focal length of

3cm. If a candle is placed at a distance of 7cm from the lens.
(i) How far from the lens will the image form? Use ray diagram.
(ii) What is the magnification?
(iii) What is the power of the lens?

10. An object is placed at a distance of 4cm from a convex lens. If a real image forms at a

distance of 16 cm from the lens on the other side. How much does the lens magnify the

object? (Ans: 4)

11. In the given diagram in the case of the diverging lens, AB is the position of an object.

(i) What is the size of the image formed? A
(ii) What is the nature of the image?
B 2F O
F F 2F

90 Blooming Science Book 10

12. What will be the focal length of a lens whose power is –1D dioptre and what type of lens

will that be? (Ans: 1m)

13. An object is placed 3 cm from a convex lens of focal length 2 cm. Calculate the

image distance. Show by means of ray diagram. Find the power of the lens and the

magnification. (Ans: V = 6cm; P = 50 D; m = 2)

14. An object is placed at 1 cm in front of convex lens whose focal length is 2cm. Find the

image distance. Show image position with diagram. (Ans: –2 cm)

15. Mr. Sen sees the distant object comfortably but wears spectacles to read a newspaper.

(i) What is the defect of vision?

(ii) What is the cause of this defect?

(iii) What type of lens is used in his spectacles?

(iv) Draw a diagram of the corrected eye defect. A

16. An object AB is placed between F and 2F as shown 2F B F O F 2F
in the diagram. With the help of ray diagrams, locate

the image and mention its nature and size.

17. Nita sitting at the back of her class has difficulty in reading the blackboard but she reads
her book easily.

(i) What is her defect of vision?

(ii) Mention the cause of defect.

(iii) How can her defect be corrected? Show by means of a diagram.

18. (i) In the given diagram of the eye, what kind of defect N I
is shown?

(ii) Give reasons for this defect.

(iii) What type of lens is used in the spectacle to correct the above defect? Explain.

19. The given diagram shows the ray of light entering the eye of
a short sightedness from the distant object.

i) Use the diagram to explain why a short sighted person
cannot see the distant object?

ii) Draw second diagram to correct short sightedness.

20. Write differences between:

i) Near point and far point ii) Long sighted and short sighted eye

iii) Convex and concave lens

iv) Focal length and principle focus

Blooming Science Book 10 91

21. Give reason:

i) Concave lens has negative focal length

ii) Convex lens has positive power

iii) Lenses are made up group of prisms

22. Write a function of :

i) Lens of eye ii) Ciliary muscles iii) Pupil

iv) Retina v) Iris

vi) Aqueous and vitreous humor

23. Choose the correct alternatives from the following options.

i. The concave lens has which type of focal length?

a. Positive b. infinite

c. negative d. Zero

ii. The lens of a person’s eye is thick, his defect of vision is:

a. Short sightedness b. Long sightedness

c. Both ‘a’ and ‘b’ d. None of the above

iii. The SI unit of power of lens is:

a. Watt b. Dioptre

c. Kwh d. KW

iv. A convex lens has 10 cm focal length, its power is:

a. 1D b. 0.1 D

c. 1/10 D d. 10 D

v. Formula to calculate magnification is:

a. v/u b. u/v

c. f/u d. 1/f

92 Blooming Science Book 10

Chapter Electricity
and
6
Magnetism
Michael Faraday
1791 AD-1867 AD

Learning Outcomes Estimated Periods : 8+3

On the completion of this unit, the students will be able to:
 describe various effects of current
 explain various electromagnetic appliances like electric motor, dynamo and transformer

and their uses.
 list out saftely mesaures while using electricity.
 solve simple numericals related to power and cost of electricity.

Electricity has an important role for the present civilization in our earth. Radio, fans, heaters,
computers, electric lamps, televisions, refrigerators are some of the electrical appliances which
are common in use today. Electricity is used in our homes, in industry and in transport.

Effects of Current

From our previous chapter, we come to know that there are different sources and forms of energy.
According to the law of conservation of energy, energy can be transformed from one form to the
other. Electrical energy can also be converted into light energy, heat energy, chemical energy and
mechanical energy. A bulb converts electric energy into heat and light energy. A heater converts
electric energy to heat energy. An electric motor or a fan uses electric energy to do mechanical
work. When electric current is passed through electrolyte such as dilute sulphuric acid, a chemical
reaction takes place and the electric current is converted into chemical energy.

Thus, the electric current has various effects like Scan for practical experiment

(i) Heating effect (ii) Lighting effect

(iii) Chemical effect and (iv) Magnetic effect

A. Heating and Lighting Effect of Current

The production of heat by passing electric current through a wire of visit: csp.codes/c10e07
high resistance is called heating effect of current. When conductor is
red hot it emits light which is called lighting effects of current.

Most of the household appliances like electric heaters, electric irons, toasters etc. are based on
heating effect of current. This appliances convert electrical energy into heat energy.

(R)

I

Blooming Science Book 10 93

When an electric current is passed through a conductor of high resistance there is opposition
to the flow of electrons: therefore work has to be done to overcome the resistance. This work
is transformed to heat energy. Thus it is the resistance offered by the conductors to the flow of
electron that causes the heating effect. when a body is extremely heated if emits light energy.

Heating Element: A heating element is a coil having very high resistance and high melting point
which is used to convert electrical energy into heat energy.

Nichrome wire of suitable length is a mostly used as a heating element. Nichrome is an alloy of
nickel (60%) and Chromium (40%). Manganin which is the alloy of manganese (84%), copper
(12%) and nickle (4%) is also sometime used as a heating element. Nichrome is a good heating
element because:
1. it has very high resistance
2. it has high melting point
3. it does not react with oxygen even at very high temperature of 900oC produced by it.

1. Heating Devices: The household electrical appliances which are used for the heating
purpose in our daily life are called heating devices. These are electric heater, electric
kettle, immersion heater, electric iron, electric stove. etc.

(i) Electric Heater: In an electric heater, the heating Filament
element is made of nichrome wire which is thick,
long and well exposed to air. The heater has a shining
metal surface behind it to reflect back and heat
radiation.

(ii) Electric Iron: The heating element is a nichrome wire Filament
wound on a mica sheet. The mica sheet is insulator of
electricity but provides the conduction of heat. There is a M
base plate which is made of iron plate. B

(iii) Electric Kettle: In an electric kettle, the heating element
is a fine nichrome wire wound on a mica. The element is Filament
insulated from the inner wall by thin mica sheets and the
element is sealed inside the metal tube.

2. Lighting Devices: The household electrical appliances which are used for the lighting

purpose in our daily life are called lighting devices. These are the different electric bulbs

like filament lamps and fluorescent lamp. Glass bulb

(i) Filament Lamp: The filament lamp is the most L
common and cheapest form of lighting device. F
It has a short and thin filament in a coiled form.

The filaments are enclosed in a glass bulb. The P L
two ends of filament are connected by thick nickel P Filament

wires, to the two terminals. Filament lamp

The filament is a tungsten wire having a high resistivity and whose melting point
is about 3400oC. At this temperature filament would react with oxygen so air is
removed from the bulb and in vacuum tungsten evaporates quickly.

94 Blooming Science Book 10

The glass bulb contains inert gas like argon or nitrogen which prevents the
tungsten filament from evaporation. When electric current is passed through the
filament it becomes white hot and emits light and other parts of filament do not
glow because other parts don’t have high resistance.

Thus, in a bulb, electrical energy is converted into heat energy first and then
into light energy. Such type of lamp converts about 10% of the electrical energy
into light and the remaining 90% of the total electrical energy is wasted as heat
energy so, it is inefficient. It’s life is about 1000 hours.

(ii) Fluorescent Lamps: A fluorescent lamp is considered to be three times more
efficient than the filament, i.e. it converts 30% of the total energy into light and
the remaining 70% of the total electrical energy is wasted as heat. Its life span is
for 3000 hours. Although its purchase price is more expensive than that of the
filament lamp, it consumes less electricity.

It consists of a long thin tube made of glass, containing mercury vapour. The
inner side of the tube is coated with a fluorescent powder such as calcium
magnesium tungstain or zinc and cadmium silicates. Two electrodes are fitted at
two ends of the tube. The tube is filled with mercury vapour which collides with
electrons released by electrodes to produce UV rays which produces an ultraviolet
ray when current passes through it. The UV rays so produced are invisible to our
eyes.

Electrodes

E Glass Tube E’
Mercury vapour Fluorescent powder

Fluorescent tube

When the ultraviolet rays so produced are allowed to fall on the fluorescent powder,
the powder emits visible light. These days CFL and LED bulbs are also in use
widely because they are efficient and consume less electric energy but they are
expensive compared to filament and fluorescent lamp.

CFL (compact fluorescent lamp) is a condensed type of fluorescent lamp which is
more efficient than simple fluorescent lamp. It can convert 90% of electrical energy
unto light energy.

A LED (light emitting diode) is a semiconductor device that emits visible light
when an electric current passes through it. LED lamps have a life span and electrical
efficiency which are several times greater than filament and fluorescent lamps.
They are used in applications like automative headlamps, general, lighting, traffic
signals, camera flashes, etc. It converts 90% of electrical energy into light.

B. Chemical Effect of Current

When an electric current is passed through a metal their molecules do not actually move, the
electrons move and there is no chemical change in the conductor through which the current is
passed.

Solution of salts, acids and bases dissociate the molecules of the solute into ions. When a current

Blooming Science Book 10 95

is passed through the solution, two types of ions are formed in the solution move in opposite
directions and produce chemical changes in the solution. This is called chemical effect of current.
The process of dissociation of a compound in a solution by passing the electricity is known as
electrolysis. The solution whose electrolysis is being carried is known as electrolyte. The vessel
in which the electrolysis is carried out is called electrolytic cell or voltameter. The metallic or
non–metallic conductors which are used to pass current or apply p.d. in the electrolyte are called
the electrodes. The electrode through which current enters into the solution is known as anode
and the electrode through which current leaves the solution is known as cathode.

C. Magnetic Effect of Current
It is discovered by Hans Christain Oerested in 1819 AD that when a current carrying wire is
placed over a compass needle, the needle gets deflected. If the direction of current is reversed,
the direction of deflection of the needle is changed toward the opposite direction.
This experiment clearly indicates that a magnetic field is set up around a wire when current is
allowed to pass through it. This magnetic field deflects the compass needle. This is the magnetic
effect of current.

S NS N

Magnetic effect of electric current

The application of magnetic effect of current is applied in various electric appliances like
microphone, radio, television, loudspeaker, etc. A temporary magnet is formed when electricity
is passed around a magnetic material with the help of solenoid. Thus formed magnet is called
electromagnet and it is called for various propose.
The strength of the magnetic field can be varied by the following methods:
i) The increase in the number of the turns of the wires in the solenoid results in an increase

in the strength of the electromagnet. Similarly, a decrease in the number of turns of wires
in the solenoid results in a decrease in the strength of the electromagnet.
ii) The increase in the current flowing through the solenoid results in an increase in the
strength of the electromagnet. Similarly, the decrease in the current flowing through the
solenoid results in a decrease in the strength of the electromagnet.
iii) By inserting a soft iron bar inside the solenoid, the strength of the electromagnet increases.
iv) By decreasing the distance between the poles of magnet.
Applications: In these days, electromagnets are used in electric bells, in telegraph circuit, the
receiver part of the telephone apparatus and in industries to left heavy load. In garbage industry
to separate magnetic and non magnetic materials.

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Electric Bell

An electric bell is a device which converts electric energy into sound energy. It is an application
of an electromagnet. Electric bells are used in homes, offices and schools. It is also used in
telephones.

Construction: The construction of an electric bell is shown in the given fig.

It consists of a horse shoe shaped electromagnet. + - Switch

There is an armature A (made of a soft iron bar) in

front of the poles of the electromagnet. At one end of

which a soft iron piece with a hammer H is connected. Electromagnet
There is a bell gong near the hammer. The armature A

acts as a spring. A contact screw S makes contact with

another steel strip B attached to the armature A.

Working: When the electric circuit is completed, AB Armature
the current flows through the electromagnet. The
electromagnet is magnetized and it attracts the Steel strip
armature A. Due to the movement of armature the
hammer strikes the gong G and the bell rings. As soon S

Screw

as the armature A is attracted by the electromagnet, Hammer

there is a disconnection between the contact screw S

and the steel strip B. As a result, no current flows and H
the electromagnet loses its magnetism and the strip B Gong

returns to its original position and touches the contract (G)

screw again. Thus the circuit is again completed and Fig: An Electric Bell
the hammer strikes the gong again. This process of

contact and break of the circuit goes on. Due to this repeated action, the bell rings continuously.

Direct current and Alternating current

Direct current: The current which always flows in the same direction is called direct current.
The current flowing from a cell or battery is direct current, because it always flows in the same
direction. The source of direct current has positive and negative terminals.

Direct current

Io
Current

Time
Direct current

Alternating Current: The current which changes its direction a number of times in one second
is called alternating current. In our country Nepal, we use the alternating current of frequency
50 Hz which means that its polarity becomes positive 50 times in a second and also negative 50
times in a second. Thus, the polarity of alternating current changes many times in a second.

Blooming Science Book 10 97

Current+ Time
-

Alternating current

Advantage of AC over D.C.

1. Voltage of A.C. can be increased or decreased as desired by using transformers.

2. A.C. can be converted to D.C. by using rectifiers.

3. A.C. can be transmitted to long distances without much loss of energy:

Disadvantages of A.C. over D.C.

1. The peak voltage of A.C. is much higher and so it is dangerous to use and needs better
insulation.

2. A.C. cannot be used in electrolytic process, such as electrolysis, electroplating etc. only
D.C. is required in such process.

3. A.C. gives huge & sudden shock but D.C. gives repelling shock.

Electromagnetic induction
When a coil, made up of insulated copper wire is brought in a magnetic field is allowed to move,
an induced emf is produced in a coil. Such a phenomenon is called as electromagnetic induction.
There must be changed in magnetic flux in the closed circuit in order to produce induced current.
Generator and dynamo are based in principle of electromagnetic induction.

Faraday’s laws of electromagnetic induction
i) When magnetic flux linked with closed circuit changes, an emf is produced.

ii) Induced emf last as long as change in magnetic flux.

iii) Induced emf is directly proportional to rate of change of magnetic flux.

Activity -1

To observe induced current / Electromagnetic induction.
Materials required:
cover of a math box, insulated copper wire, bar magnet, galvanometer.
Method:
a. Take a cover of match box and coil the insulated copper wire arround it with
about 50-60 turns.
b. Connect two ends of the wire with the galvanometer.
c. Keep a bar magnet inside the match box and move it in and outside of the box.

Match box

Magnet wire

Galvanometer

98 Blooming Science Book 10

CoilObservation:
What would you observe in the galvanometer? Write down.
Conclusion:
Why does the galvanometer show the deflection? Discuss with your friends and write
down the conclusion.

1. Dynamo or Generator
A device that converts mechanical energy into electrical energy is called dynamo. But the dynamo
that produces fairly large amount of current (power) is called a generator.

Magnetic lines of force

Direction of motion

Magnet

Fig: AC Generator
Principle
The working principle of both dynamo and generator is same. They work on the principle of
electromagnetic induction that when magnetic flux linked with the closed circuit coil changes, an
emf is induced in it resulting induced current.
The generator consists of a rectangular coil (armature) of wire capable of rotating between the
poles of strong magnet . The coil is of many turns and is wound on a soft iron core. It is connected
to slip rings against which carbon brushes press. As the coil rotates, it cuts lines of force and
current is induced. The current induced is supplied by wires connected to carbon brushes.
The coil is rotated by kinetic energy of water in Hydroelectricity using turbine.

Bicycle Dynamo

A very small and simple dynamo is used in bicycle to light its bulb.

It consists of a bottle which is provided with a movable lid which
is connected with a bar magnet as shown in figure. A coil insulated
copper wire is kept in the magnetic field of the magnet. The magnet is
connected with the help of magnet with an axle.

When bicycle runs, the lid of dynamo connected with bicycle tyre also
rotates, because of which the coil crosses the magnetic lines of force,
hence EMF is induced in it. The induced current flows through the
wire and the bulb glows. We have seen that when the bicycle moves
faster the brightness of the bulb increases and the bulb glows dim if

Blooming Science Book 10 99

speed of bicycle decreases. This is because when the bicycle moves faster, this in turn rotates
the magnet faster which increases the rate of change of flux linked with the coil thereby
increasing the magnitude of induced EMF.

The magnitude of induced EMF in a generator or bicycle dynamo can be increased by:

1) Increasing the speed of rotation of magnet.

2) Increasing the number of turns in coil.

3) By decreasing the distance between the coil and the magnet.

4) By using strong magnet.

Activity -2

To observe the working of Dynamo.
Materials required:
A bicycle with dynamo
Methods:
a. Take a bicycle with dynamo and adjust it in proper way.
b. Keep the bicycle in stationary position with its stand.
c. Then rotate the paddle slowly and increase its rotation.
Observation:
What do you observe on rotating the paddle and on increasing the speed of rotation?
Conclusion
a. Dinrcarweasyeouorn cinocnrcelaussiinogn,thwehsypedeodesofthreotbautilobng?lows and why does the brightness
b. Draw the diagram of bicycle dynamo.

2. Transformer

Transformer is an electrical device, which is used to increase or decrease the voltage of alternating
current. It cannot be used with a direct current.

Principle: It works on the principle of electromagnetic induction. When the A.C. current is
passed through the primary coil as in given figure with changing in magnitude and direction,
the magnetic field surrounding the coil also changes. This changing strength and direction of the
magnetic field in the primary coil induces an alternating e.m.f. in the secondary coil.

Current Alternating current Primary coil Secondary coil
+ T. P S

- Input AC Output AC

AC current Soft iron Transformer
laminated core

Construction: A transformer consists of a rectangular metallic frame in which thin rectangular

100 Blooming Science Book 10