Optional Science Book Class 10

From the overflow tank, the spent slurry is removed gradually. The spent dung slurry, left
after the extraction of biogas, is rich in nitrogen and phosphorus compounds and hence
forms good manure. The initial gas in the gas dome is allowed to escape. The gas is again
accumulated for three days, now the biogas will burn with a blue flame and is ready for use.
This gas will not have any odor while burning. It takes about 50 to 60 day for the new gas-
plant to start functioning.

Know the Reason

The initial mixture of gases collected in the dome of a biogas plant is allowed to escape
by opening the outlet, why?
The initial gas has more carbon dioxide (CO2) and less of methane (CH4). It has oxygen as
well as some other gases. So the initial mixture of gases collected in the dome of a biogas
plant is allowed to escape by opening the outlet.

Factors affecting the amount of gas generated

a. Quality and quantity of feed: The amount of gas generated changes with the quality
and quantity of the feed. For example, to generate 1 m3 of biogas, 25 kg of cow dung or
3 kg of oil cake are needed.

b. Temperature: The suitable temperature for biogas production is 35oC to 38oC. It
decreases below 20oC and stops below 8oC. In winter, the production of biogas from
a biogas plant decreases. Sometimes people add warm water while making slurry to
increase the production of biogas.

c. pH value: The suitable pH range for bacterial growth is 6.8-7.8.
d. Stirring digester contents: Stirring the input material and water to make slurry affect

its decomposition inside the digestive tank. If the slurry is made perfect, the production
of gas is increased.

Applications of biogas

a. Cooking fuel: A biogas pipe is connected to a biogas stove to use it as a cooking fuel.
b. Lighting fuel: Biogas can be burnt in lighting mantles for lighting purpose.
c. To generate electricity: Biogas can be converted directly into electricity by using fuel

cells.
d. Fuel for combustion engine: Biogas is used as fuel in combustion engines, which

convert it to mechanical energy.

Know the Reason

Biogas is one of the suitable sources of energy for villages in Nepal, why?
In the villages, many people are engaged in farming.
i. Raw materials like animal dung, plant waste are easily available.
ii. Bulbs can glow from the electricity generated from biogas for lighting purpose.
So biogas is a suitable source of energy for villages in Nepal.

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Advantages of biogas

a. It produces more heat when burnt.
b. It is a non-polluting fuel as it does not produce smoke. So it helps in controlling air

pollution.
c. The spent dung slurry, left after the extraction of biogas, is rich in nitrogen and

phosphorus compounds. It provides nutrient-rich manure for plants.
d. Biogas can also be used to produce electricity for lighting purpose.

Know the Reason

The spent dung slurry, left after the extraction of biogas, is used as good manure, why?
The spent dung slurry, left after the extraction of biogas, is rich in nitrogen and
phosphorus compounds and hence forms good manure.

Limitations of biogas plants

a. Initial cost of installation of the plant is high.
b. Limited biogas is produced in winter.
c. In case a farmer owns only a few cattle, animal dung is insufficient to feed a biogas

plant.
d. Construction of a biogas plant occupies some space. So it is not possible to construct

anywhere in the cities.
Briquette

The term briquette comes from the French language and is related to bricks. Briquettes are
solid fuel often used for domestic purposes in developing countries. They are used for heating
purpose in industries, too. Briquettes are formed by compacting solid material. Compaction
increases the heating value per unit volume. A briquette is a compressed block of coal dust
or other combustible biomass material, such as charcoal, sawdust, wood chips, peat or paper
used for fuel. Using waste to form briquettes allows the utilization of low-grade combustible
materials. Himalayan Green Energy Pvt. Ltd., Shubha Biomass Briquette Pvt. Ltd., R.S.
Briquette Factory Pvt. Ltd., etc. are producing briquettes to replace firewood and fossil fuels
in Nepal.

Know the Reason

Briquettes are densified, why?
Compaction, or densification, of briquettes is to increase the heating value per unit
volume so that it is comparable to coal.

Types of briquettes

There are different types of briquettes. They are coal briquette, charcoal briquette, biomass

briquette, paper briquette, etc. Although you will be learning about biomass briquettes in this

unit, there is information about charcoal briquettes, too because in Nepal these days charcoal

briquettes are produced on a commercial scale.

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Charcoal briquettes _,..·.,i,,.f.r
___, :,
A charcoal briquette is a compressed block of charcoal. It is produced by
burning a carbon-rich material such as wood in a low-oxygen atmosphere
and then mixing it with other binders. Charcoal briquettes are not actual
charcoal lumps, but a combination of charcoal and other ingredients
molded into easy-to-light lumps.

Biomass briquettes fig:Charcoal briquettes

Biomass densification is known as briquetting of biomass. Briquetting of
saw dust and other agro residues has been practiced for many years in
several countries. A block of flammable matter prepared by densification
of woody biomass, leafy biomass and agricultural residues is known
as biomass briquette. They are renewable source of energy. Their use fig:biomass briquettes
replaces the use of fossil fuels, such as oil or coal. It also substitutes the use of charcoal.

Raw materials

In developing countries, biomass briquettes are made from agricultural waste. Agricultural
wastes, like groundnut shells, castor seed shells, almond shells, coconut shells, rice husks,
cotton stalks, sunflower stalks, pine needles, coffee husks, jute waste, and forestry waste like
fallen leaves, tree bark, sawdust can be used to make biomass briquettes.

Types of biomass briquettes

Wood briquette
Wood briquettes are made from dry untreated wood chips. They are pressed at high pressure
without any binder. When wood is compressed at high pressure, then it changes into fuel like
brown coal. Wood briquettes have a lower ash and sulphur content than in fossil fuels.

Know the Reason

Burning a wood briquette is far more efficient than burning firewood, why?

The amount of water in a log determines how well it will burn and how much heat it
will produce. The moisture content of a briquette can be as low as 4%, whereas for green
firewood, it may be as high as 65%. Also the compactness of briquettes increases the
heating value per unit volume. So burning a wood briquette is far more efficient than
burning firewood.

Sawdust briquette

Sawdust is a byproduct of woodworking places like saw mills, furniture workshop, etc. It is a
cluster of fine particles of wood. Sawdust briquetting is a useful way of managing wood waste.
In order to make sawdust briquettes, the saw dust is fed into a briquetting machine. In such
machines, the sawdust is compressed by using hydraulic cylinders to make a reconstituted log
that can replace firewood. It is a process similar to forming a wood pallet but on a large scale.
No binders are involved in the manufacture of sawdust briquettes, too. The natural lignin in
the wood particles binds the dust together to form a solid.

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Rice husk briquettes

Rice husk contains cellulose, lignin, etc. The combustibility of rice husk is high, but the density
of rice husk is low. The combustion rate increases when it is converted into briquettes by
briquette machines. To make briquettes, rice husk is crushed into small pieces. Such crushed
rice husk is fed into a rice husk briquette machine to make briquettes.

Straw or hay briquettes

Whether the straw waste is from hay, corn, wheat or other agricultural crops, it can be used
to make briquettes. Instead of storing straw waste in bins, briquetting offers an option for
eliminating the need for storage, disposal and landfill fees. To briquette straw waste, the
moisture content is eliminated by drying it and crushed into pieces of a size two inches or less.
It is then fed into a straw briquetting machine for compaction. Briquettes produced from straw
waste are easy to store and transport.

Paper briquettes

Paper briquettes are made by compressing shredded paper materials into a small cylindrical
form. Paper briquetting minimizes the volume of paper waste and provides us a fuel for
heating purpose.

Manufacture of biomass briquettes: Briquetting technology

A briquetting plant is the technology to convert all types of agro forestry waste into solid
fuel. A briquette is manufactured by a briquetting machine, also known as a briquette maker
or briquette press. Briquetting plants with both small and high production capacities can be
found in Thailand. They have been successful in briquetting rice husk commercially. The
briquetting sector is growing moderately in India despite some failures. In other countries,
briquetting is not popular due to lack of advanced technology.

In Nepal, biomass briquetting was introduced in the year 1986 through a demonstration
program organised by a Japanese private company with the support of the Japanese Embassy.
The technology used for the demonstration, was based on the extruder principle and
manufactured by Fuji Conveyor. This program fostered a growth in the briquette manufacturing
industry. In 1987/88, four extrusion type briquetting machines were imported from Sun Chain
Company, Taiwan and established in Simara, Hetauda, Chitwan and Parwanipur. Nowadays
small briquetting machines are in operation. These can pave the way for large commercial
production of briquettes, which could make use of the huge quantity of agro-residue available
in our country..

Briquetting processes

The following processes are involved in a briquetting unit/ plant:

Raw Crusher Drier biomass
material briquette -machine

Finished • Cooling
Product

Production Processes Flow Chart

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1. Wood/straw crusher: The crushing machine prepares suitable size raw materials for a
briquetting plant. The choice of crushing machines depends on the type and size of raw
materials. A hammer mill is used to crush small-size materials.

2. Briquetting dryer: It is done in the hot sun or by passing hot air to reduce the moisture
content from 25% to 10 %. In the case of large-scale production in a briquetting plant, the
choice of the dryer model depends on the original moisture content of raw materials.

3. Briquetting press: To manufacture biomass briquettes, the raw materials are directly
supplied into the biomass briquette machine. It makes the biomass material into
briquettes. All materials containing lignite and cellulose are suitable for densification in
a briquetting machine.

There are mainly two types of briquetting machines; piston press briquetting machine and
screw extrusion briquetting machine. The briquettes produced by screw briquette extruder
have a concentric hole and have a larger specific area. Because of this, screw press briquettes
have better a high combustion rate. They do not disintegrate easily and are also homogeneous.
While briquettes of piston press are completely solid.

Screw extrusion briquetting machine: The screw extrusion type briquetting plant basically
consists of the following components:

1. The electric motor, which drives the pulley for rotating the screw

2. The hopper, for feeding the raw material

3. The die heater and muff

4. The screw, which densifies the raw material to produce briquettes.

The electric motor drives the briquetting screw, which is housed inside the die, through a V-
belt and pulley arrangement. Biomass raw material is fed to the screw through the hopper.
The electric die-heater softens the lignin in the raw material as it passes through the die, which
acts as a binding material. A smoke trapping system traps and removes the smoke from the
vicinity during the briquetting process. The produced briquettes are collected at the opening
provided at the bottom of the smoke collection box.

Uses of biomass briquette

Biomass briquettes can replace fuels like diesel, kerosene, lignite, coal, firewood. In our
country, where huge amounts of agro-residue are available, this technology can handle a wide
range of biomass to give good quality briquettes.

1. It is used for room heating purpose.

2. It is used as a fuel for cooking purpose in the kitchen, picnic spot, etc.

3. People use briquettes for keeping the baby and new mother warm.

4. It can be used instead of wood and loose biomass in small scale industries, like rubber,
leather processing, brick kilns, etc.

Memory Plus

Using biomass briquettes: First, ignite one end of it with gas until it turns
red. Transfer that briquette into the stove made up of ceramic, iron or steel.
The burnt part of the briquette is kept downward.

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Advantages of biomass briquette

1. It is a renewable source of energy
2. It is cheap and readily available.
3. It is much cleaner to handle than charcoal or coal.
4. It reduces air pollution and helps to conserve the environment.
5. It replaces the use of fossil fuels like kerosene and LPG and traditional sources of energy

like, firewood, coal and dung cakes.
6. Ash after complete combustion of a briquette can be used in the garden and field.
7. Biomass briquettes are uniform in size and composition. So they are easier to store and

use for cooking than wood.
8. Loose biomass causes problem in handling, transportation and storage. Briquettes are

easy to handle and eliminate problems of transportation and storage of loose biomass.

Disadvantages of biomass briquettes
Biomass briquettes have some drawbacks:

1. It requires a huge investment for its initial production.
2. In production, there is a high energy requirement.
3. In the rainy season, the drying process needs another heat source other than the sun.
4. Biomass briquettes must be stored in a waterproof place otherwise the briquettes will

not burn when lit.
5. The quality of the raw materials and wear of the die, piston and screw are frequent

operational problems associated with briquetting technology.

Solar energy

You have already learnt about the sun as the ultimate source of energy and thermonuclear
fusion in optional science of class 9. You had learnt about applications, advantages and
disadvantages of solar energy too. Solar energy is the light and heat received by the earth
from the sun. It is the main source of energy on the earth. Life on Earth is possible due to solar
energy. The earth receives an enormous amount of energy directly from the sun each day. Sun
is an enormous source of energy. This energy is due to the nuclear fusion reaction taking place
in the core of the sun. It is understood that the energy given by the sun to the earth in one day
is 50,000 times more than the total energy consumed by the world in one year.

Solar energy can be harnessed using a range of technologies, such as solar heating,
photovoltaics, solar thermal energy. Solar cooker, solar furnace, solar water heater, solar
electric power plant are examples of devices that convert solar energy into other forms of
energy. Those devices which convert solar energy into other forms of energy are called solar
devices. In the last dacade, solar energy has also been used to power earth-orbiting satellities,
cars and even planes.

056 Optional Science - 10

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Know the Reason

Solar energy is a useful source of energy in the context of our country, why?

On average Nepal has 6.8 sunshine hours per day. In most parts of our country, there
are about 300 sunny days in a year. It is estimated that if proper technology is employed,
then we can harness 20 MW of solar energy per square kilometer. So solar energy is a

useful source of energy in the context of our country.

Wind energy

Energy received from the movement of the wind across the earth is called wind energy. This
energy is a result of unequal heating of the earth’s surface and the atmosphere. The wind can
be obtained continuously. We never run out of it. So the wind is a renewable source of energy.
It is one of the oldest and cleanest forms of energy. The kinetic energy of the wind has been
traditionally used to pump water and to run flour mills and to propel sailing boats. A large
amount of energy can be produced from a windmill. Nowadays, the improved versions of the
windmill are used to generate electricity. It depends upon the weather and location to capture
wind energy.

Memory Plus

Nepal has capacity to generate more than 10,000 MW of electricity from wind energy.
Places above the Churiya range in Nepal get 15-17 hours of wind every day. Places like
Mustang, Dhading, Galchhi, Borlini are suitable for electricity generation from the wind.

Answer writing skill I

1. Name the alternative sources of energy which are suitable in our context.

Alternative sources of energy like solar energy, wind energy, biogas, biomass briquettes
and hydroelectricity are suitable in our context.

2. What raw materials are used to make biomass briquettes?

Any type of agro-forestry waste or bio-mass like sawdust, groundnut shell, caster seed
shell, sugar-cane bagasse, cotton stalk, bamboo dust, rice husk can be used as raw
materials to make biomass briquettes.

3. Write two advantages of biomass briquette over loose biomass

1. The direct burning of loose biomass in the conventional way and gives less heat
and creates more air pollution.

2. Loose biomass causes problems in handling, transportation and storage.

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4. Fossil fuels are called non-renewable energy sources.

Coal, crude oil and natural gas are formed from the fossilized remains of prehistoric
plants and animals. They cannot be regenerated in a short period of time. It takes
millions of years for fossil fuel to form. Their supply is limited, and they cannot be used
again and again.

5. Suggest two alternative sources of energy for rural areas in our country.

The following two sources of energy are suitable alternative sources of energy for rural
areas of our country.
1. Solar energy: In places where there are no hydropower transmission lines, solar

panels can provide electricity.
2. Biogas: It is a suitable fuel for the people in the villages of our country. The raw

materials to produce biogas are easily available in the villages.

6. Write two uses of biomass briquettes.

The two uses of biomass briquettes are:
1. It is used as a fuel for cooking purpose in the kitchen, picnic spot, etc.
2. People use briquettes for keeping the baby and the new mother warm after

delivery.

7. Relate the traditional and advanced ways of capturing wind energy

The kinetic energy of the wind has been traditionally used to pump water and to run
flour mills and to propel sailing boats. Large amounts of energy can be produced from a
windmill. Nowadays, improved versions of the windmill are used to generate electricity.

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Exercise

1Step

Define 2. wind energy 3. biomass
5. alternative sources of energy
1. energy
4. biofuel

Very short answer questions
1. Name alternative sources of energy which are suitable in the context of our country.
2. What is hydroelectricity?
3. What is biogas?
4. What is a briquette?
5. What raw materials are used to make biomass briquettes?
6. Write two advantages of biomass briquette over loose biomass.
7. Name some of the suitable places for generating electricity from the wind in our country.

2Step

Short answer questions
1. 'Hydroelectricity is the most suitable source of energy in our country.' Justify this

statement in two points.

2. What are the input materials for biogas production?

3. Write in short about the factors affecting the amount of biogas produced in a biogas
plant.

4. Write in short about the different types of biomass briquettes.

5. Write about the biomass briquetting processes.

6. How does the method of compaction in briquetting increase the heating value?

7. 'Solar energy is a suitable source of energy in our country.' Justify this statement in two points.

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Differentiate between
1. Renewable sources of energy and non-renewable sources of energy
2. Charcoal briquette and wood briquette

Give reason
1. A tremendous amount of energy is released during a nuclear reaction.
3. Hydroelectricity is a multipurpose source of energy.
4. Biomass is a renewable energy source.
5. Biogas is one of the suitable sources of energy for the villages in Nepal.
6. The initial mixture of gases collected in the dome of a biogas plant is allowed to escape

by opening the outlet.
7. The spent dung slurry, left after the extraction of biogas, is used as manure.
8. Burning wood briquette is far more efficient than burning firewood.

3Step

1. Write three uses of hydroelectricity and biogas.

2. Write three advantages and three disadvantages of the following:

1. Hydroelectricity 2. biogas

3. biomass briquettes

3. Draw a diagram to show the construction of a biogas plant.

4Step

1. Explain the technology to generate hydroelectricity.
2. Explain the construction of a biogas plant.
3. Explain the method of biogas production from a biogas plant.
4. Explain biomass briquetting technology.

Multiple choice questions (MCQs)

1. Which of the following is a renewable source of energy?

a. wood b. petroleum

c. natural gas d. uranium

2. Which of the following is not an alternative source of energy?

a. coal b. briquette

c. solar energy d. tidal energy

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PHYSICS

3. Biogas is a mixture of gases like

1. ethane, sulphur dioxide, hydrogen and hydrogen sulphide
2. ethane, nitrogen dioxide, oxygenand hydrogen sulphide
3. methane, carbon dioxide, oxygen and hydrogen sulphide

4. methane, carbon dioxide, hydrogen and hydrogen sulphide
4. Which of the following briquette is most popular in Nepal?

a. coal briquettes b. peat briquettes

c. biomass briquettes d. paper briquettes

5. The main possible source of energy in our country is

a. hydroelectricity b. nuclear energy

c. tidal energy d. geothermal energy

6. Normally the rate of increase in the temperature below the earth's surface in the first 10

km is

a. 80 °C per km b. 30 °C per km

c. 20 °C per km d. 40 °C per km
7. Biomass includes

a. wood, cow dung, paper plastic b. wood, cow dung, agricultural waste
c. Paper, plastic, old wires d. unused metal pieces, paper, straw

Project Work

1. Make a model of a turbine and rotate it with the help of the wind at high speed.
2. To make a paper briquettes
3. Place four handfuls of shredded paper into a standard-size mixture. Pour in about 2 cups

of water.
4. Run your mixture5-10 seconds until the paper is pulpy.
5. Pour the mixture into a sieve over the sink. Allow it to strain out most of the water. Apply

a great deal of pressure to remove excess water and properly compress the pulp. Place
it in a sunny, dry location. Allow it to dry thoroughly; which may take as long as a week.

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UNIT

4 HEAT

Joseph Black (1728-1799) was a Scottish physician and chemist, known for his discoveries
of magnesium, latent heat, specific heat and carbon dioxide. He attended grammar school
in Belfast at the age of 12 and entered the University of Glasgow in 1746 when he was 18,
studying there for four years before spending another four at the University of Edinburgh,
furthering his medical studies. He was Professor of Anatomy and Chemistry at the University
of Glasgow for 10 years from 1756, and then Professor of Medicine and Chemistry at the
University of Edinburgh from 1766, teaching and lecturing there for more than 30 years.

In 1761, he deduced that the application of heat to ice and boiling water caused a volume
change without change in temperature. He concluded that the heat applied must have
combined with the ice particles and boiling water and become latent.

Key terms and terminologies of the unit
Key terms and terminologies of the unit

1. Melting/fusion: The change from a solid to a liquid phase at a constant
2. temperature is called melting or fusion.
3.
4. Boiling/vaporization: The change of state of a material from liquid to gas on
5. heating at a constant temperature is called boiling or vaporization.
6.
7. Condensation: The change of a substance from its gaseous form (e.g. water vapor)
8. into liquid form (e.g. water) is called condensation.

062 Solidification/freezing: The change of a substance from its liquid form (e.g.
water) to solid form (e.g. ice) is called solidification or freezing.

Sublimation: The change from a solid to a vapor state without passing through
the liquid state is called sublimation.

Deposition: The change from a gaseous to a solid state without passing through
the liquid state is called deposition.

Phase change diagram: A phase change diagram is the representation of phase
transformation and also the conditions of transformation of a matter.

Latent heat: Latent heat of a substance is the amount of thermal energy absorbed
or released by a substance during a change in its physical state that occurs without

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PHYSICS

changing its temperature.

9. Specific latent heat: The amount of heat energy absorbed or released to change
the phase of a unit mass of a substance without changing its temperature is called
specific latent heat.

10. Latent heat of fusion: The heat energy required to convert a solid at its melting
point into liquid without any rise in the temperature is called latent heat of fusion.

11. Specific latent heat of fusion: The heat required to change a unit mass of a solid
to liquid at its melting point is called specific latent heat of fusion of the solid.

12. Latent heat of vaporization: The heat energy required to convert a liquid at its
boiling point into gas without any rise in the temperature is called latent heat of
vaporization.

13. Specific latent heat of vaporization: The quantity of heat required to change a
unit mass of a liquid to a gaseous phase at constant temperature is called specific
latent heat of vaporization of the liquid.

14. Latent heat of sublimation: The heat absorbed by a substance, which directly
changes from a solid to a gaseous state, or the heat given out by a gaseous substance
when it directly changes into a solid is called the latent heat of sublimation.

15. Heat equation: The amount of heat gained or lost by a body (Q) is equal to the
product of its mass (m), specific heat capacity (s) and change in its temperature
(∆t). i.e. Q = ms∆t, This is called the heat equation.

16. Principle of calorimetry: According to the principle of calorimetry, if there is no
loss of heat while mixing two substances at different temperatures, the amount of
heat lost by the body at high temperature is equal to the amount of heat gained by
the body at low temperature.

Introduction

All matters have atoms or molecules with a certain molecular Liquid
attraction called intermolecular force of attraction. Such a force of

attraction is called a bond that holds the atoms together. At normal Gas
room temperature, it possesses certain kinetic energy too. Whenever

the kinetic energy of atoms or molecules in a substance increases, Solid
it becomes warmer. Heat is a form of energy caused by the motion

of atoms and molecules. Actually the substances do not contain

heat but have internal energy. The internal energy is the sum of the fig:states of water

kinetic energy of all the molecules in a substance. When heat flows, the internal energy of a hot

substance decreases, and the internal energy of a cold substance increases.

Memory Plus

The sum of the kinetic energy of all the atoms or molecules in a substance gives heat.
But the average kinetic energy of all the atoms or molecules in a substance gives its
temperature. When matter gets warmer, the atoms or molecules in the matter move faster.

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External source of energy can change the configuration of atoms or molecules in a matter. If
enough heat is transferred to a body, then the bonds will break and the molecules will change
the state of the body. In this unit, we will learn about the hidden heat during the change of
phase of a matter, called latent heat, heat equation to calculate the heat gained or released by
a body, and the principle of calorimetry.

Memory Plus

The SI unit of heat is the joule (J) and a more common unit of heat is the calorie. The
amount of heat required to raise the temperature of 1 gram of water by 10C is called
one calorie heat. Kilocalorie (kcal) is the bigger unit of heat. One kilocalorie is the heat
required to raise the temperature of 1 kg of water by 10C.
1 calorie = 4.2 joules
1 kilocalorie = 1000 calories = 4200 joules

Change of state

Matter can exist in three states, namely solid, liquid .I I Heat Heat
and gas. These states are also called phases. When a
solid substance changes from a solid phase to a liquid Ice (s) Water (l) Steam (g)
phase, energy must be supplied in order to overcome the
molecular attraction between the constituent particles of fig:change of state of water
the solid. That is, energy is needed to break the bonds
when a substance melts or boils. The state of matter can
be changed by changing the temperature or pressure.

When heated, the rigid molecular structure in a solid breaks down to liquid state, and the
liquid to gaseous state on further heating. Conversely, when energy is released, then gas
changes into liquid, and liquid changes back to solid. For example, water exists in all three
states. On heating, ice changes into water. Water changes into steam on further heating. In
reverse order, steam changes into water on cooling. On further cooling, water changes into ice
again.

Types of phase transition

The following phase changes can be observed in matter:

1. Melting or fusion: When heat is given to ice, it melts and Solid
forms water. The change from a solid to a liquid phase at
a constant temperature is called melting or fusion. The Melting
temperature at which melting occurs is called the melting
point of a solid. Heat is released from the solid in this process.

Memory Plus

The temperature at which a solid melts at 1 atmosphere of pressure (760 mmHg) is
called normal melting point. The normal melting point of ice is 0°C.

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PHYSICS

2. Boiling or vaporization: On further heating of water, it changes into steam (vapor). The
change of state of a material from liquid to gas on heating at a constant temperature is
called boiling or vaporization. The temperature at which boiling occurs is called the

boiling point of liquid. Heat is absorbed by a liquid from outside in this process.

Memory Plus

The temperature at which a liquid boils at 1 atmosphere of pressure (760 mmHg) is
called the normal boiling point. The normal boiling point of water is 100 °C.

If the pressure decreases, then water will boil at a temperature lower than 100°C. The
boiling point of water decreases by 1°C every 300 m of height increase above sea level.

Differences between melting and boiling

I- Melting I Boiling -1

~

It is the change of state of matter from solid It is the change of state of matter from

to liquid. liquid to gas.

Heat is released from the solid in this Heat is absorbed by the liquid from

process. I outside in this process. __J

I

3. Condensation: On cooling steam or water vapor, it changes into water. The change of a
substance from its gaseous form (e.g. water vapor) into liquid form (e.g. water) is called
condensation.

Know the Reason

There are water droplets on the surface of a cold soda bottle on a humid day, why?

On a humid day, water molecules in the air, as vapor, hit the colder surface of the
soda bottle and get condensed to form water droplets. So there are water droplets on
the surface of a cold soda bottle on a humid day.

4. Solidification or freezing: On cooling water, it changes into ice.The change of a
substance from its liquid form (e.g. water) into solid form (e.g. ice) is called solidification
or freezing.

5. Sublimation: The change from a solid state to a vapor state without passing through
the liquid state is called sublimation, and the substance is said to sublime. For example,
dry ice, camphor and iodine. Energy is required for a solid to sublime into gas.

6. Deposition: The change from a gaseous state to a solid state without passing through
the liquid state is called deposition. For example, in subfreezing air, water vapor changes
directly to ice without first becoming a liquid. The process of deposition releases energy.

Latent heat

There is change in the temperature of ice and water when they are heated. But when ice
changes into water or water changes into steam, there is just change in the volume of water
or steam without rise in the temperature although heat is supplied continuously. Contrary to
this, when steam is cooled to water and then into ice, there is no fall in the temperature during
the change of state. Such hidden heat is termed as latent heat.

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The term ‘latent heat’ was introduced around 1761 by Scottish chemist Joseph Black. It is
derived from the Latin word, ‘latere’, meaning ‘to lie hidden’. In 1761, Black deduced that the
application of heat to ice at its melting point does not cause a rise in the temperature of the
ice/water mixture, but rather an increase in the amount of water in the mixture. Additionally,
Black observed that the application of heat to boiling water does not result in a rise in the
temperature of the water/steam mixture, but rather in an increase in the amount of steam.
From these observations, he concluded that the heat applied must have combined with the ice
particles and boiling water and become latent.

Latent heat of a substance is the amount of thermal energy absorbed or released by the
substance during the change in its physical state that occurs without changing its temperature.
The SI unit of latent heat is joule (J).

Explanation of latent heat on the basis of kinetic theory

There are many atoms or molecules in a substance. They have intermolecular forces of
attraction. To increase the temperature of a substance, there should be an increase in kinetic
energy of the molecules. But the heat supplied during phase transition does not increase the
kinetic energy of the molecules, and hence there is no rise in the temperature of the substance
during phase transition. Thus, the latent heat is used up in overcoming the force of attraction
among the molecules of the substance instead of giving a rise in the temperature.

Know the Reason

The temperature of a material does not change during the change of phase, why?
Heat supplied during the change of phase is used up in overcoming the force of attraction
among the molecules of the substance instead of giving a rise in the temperature. So the
temperature of a material does not change during the change of phase.

Calculation of heat absorbed or given out during change of state of a substance
Let ‘Q’ be the amount of heat absorbed during the change of state of a substance of mass ‘m’. It
is experimentally found that the quantity of heat absorbed or given out by a substance during
its change of state is directly proportional to the mass of the substance. That is,

Qαm
∴ Q=L×m
Or, Q = mL

Where, L is a constant called ‘specific latent heat’ of the substance. Its value depends only on
the nature of the substance.
∴ Amount of heat absorbed or given out during phase transition = Mass of the substance
× Specific latent heat of the substance
Specific latent heat

The amount of heat energy absorbed or released to change the phase of unit mass of a substance
without changing its temperature is called specific latent heat. It is generally represented by
the letter L and expressed as

L= Q , where ‘Q’ is the heat supplied and ‘m’ is the mass of the substance.
m
066
Optional Science - 10

PHYSICS

Units of specific latent heat

Since L = Q , the SI unit of heat is the joule and that of the mass is the kilogram. So the SI unit
m

of specific latent heat is joules per kilogram (J/kg).

Other common units are calories per gram (cal/g), kilocalories per gram (kcal/g) and joule per
gram (J/g).

Types of latent heat

Latent heat can be categorized as latent heat of fusion, latent heat of vaporization and latent
heat of sublimation.

Latent heat of fusion

The heat energy required to convert a solid at its melting point into liquid without any rise in
the temperature is called latent heat of fusion. It is required to break the bonds between solid
molecules. Latent heat of fusion is associated with melting of a solid or freezing of a liquid.
During melting of a solid, latent heat of fusion is absorbed from outside, and at the time of
freezing the same amount of heat is given out.

Specific latent heat of fusion

The heat required to change a unit mass of a solid to liquid at its melting point is called specific

latent heat of fusion of the solid. It is denoted by Lf or simply by L.

Thus, if Q is the quantity of heat required to melt a mass of ‘m’ of a solid substance completely

into liquid, then the latent heat of fusion for the substance is

Lf or L = Quantity of heat absorbed during melting
mass of the solid substance melted
Q
L= m

The SI unit of specific latent heat of fusion is joule per kilogram (J/kg).

Values of the specific latent heat of fusion and melting point for some common substances

Substance Specific latent heat of fusion Melting point ( in oC)

In SI units (J/kg) In cal/g

Aluminium 3.97× 105 94.52 658

Ice 3.36× 105 80 0

Copper 1.80 × 105 43 1083

Zinc 1.34× 105 31.9 420

Silver 0.882 × 105 21 961

Gold 0.644 × 105 15.33 1063

Lead 0.25 × 105 6 327.3

Solved numerical: Temperature (oC)
420
The phase change diagram for a solid of mass 200g is shown in the D

given figure. Find 320 B C
1. Melting point

2. Specific latent heat of fusion of the substance. A 900 Heat (J)
1500 1800

PHYSICS 0Optional Science - 10 67

Solution:

Here, mass of the substance (m) =200 g = 200 = 0.2 kg
1000

i. Form the phase change diagram, BC represents the melting of the solid at 320oC.

Thus, the melting point of the given solid is 320oC.

ii. Heat at ‘B’ = 900 J, Heat at ‘C’ = 1500J

The amount of heat absorbed during melting (Q) = 1500-900 = 600 J

Now, the latent heat of fusion is given by

Q = mL

Or L= Q = 600 = 3000J/kg
m 0.2
Therefore, the latent heat of fusion of the given solid is 3000 J/kg.

1. Specific latent heat of fusion of ice:

Activity:

i. Take some crushed ice in a beaker and note its temperature with the help of a
laboratory thermometer. Is it 00C? Otherwise let the ice melt first.

ii. Now heat these pieces of ice by using a small flame. Note the temperature after
every minute. Do you find a constant temperature?

On heating of ice at 0oC continuously, it melts to form water, but the thermometer shows the same
temperature. Hence, there is no rise in the temperature during melting of ice even if heat
is being supplied continuously.

The energy that a unit mass of ice at 0oC absorbs from its surrounding in order to overcome the
attractive forces between its molecules when it changes from ice to water at 0oC is called specific
latent heat of fusion of ice. It is experimentally found that 3.36× 105 joules of heat energy is
required to convert 1 kg of ice at its melting point to water at the same temperature. That is, the
specific latent heat of fusion of ice is 3.36×105 J/kg.

Memory Plus

When ice at 0°C melts, the water formed with the latent heat of fusion of ice is also at 0°C.

Specific latent heat of fusion of ice is 3.36 × 105 J/kg means that 3.36 × 105 J of heat is required
to change one kilogram of ice at 0°C into water at the same temperature, or 3.36×105 J of
heat is given out when 1 kg of water at 0°C freezes into ice at the same temperature.

Solved numerical

Calculate the amount of heat required to convert 500g of ice at 0oC into water at the same
temperature. (Specific Latent heat of ice = 3.36× 105 J/kg)

Solution:

Here, mass of ice (m) = 500 g = 500 = 0.5 kg
1000
Specific latent heat of ice (L) = 3.36× 105 J/kg

The heat absorbed during phase transition is given by,

68 Optional Science - 10

PHYSICS

Q=m×L

Or, Q = 0.5× 3.36× 105 = 1.68× 105 J

∴ The heat required is 1.67× 105 J.

2. Phase change diagram

In this graph, point A represents ice. When heat is 20 C
supplied to this ice, its temperature does not rise even
though heat is continuously supplied. Change of phase 10
in which ice changes into water is represented by AB.
The temperature of a mixture of water and ice is 0oC. At -Ice melting
B, all ice has melted. On further heating beyond B, the
temperature of water increases along BC. (solid + liquid)
A Time of heating (in minButes)
Temperature in ᵒC
water (liquid)

3. Consequences of high specific latent heat of fusion of ice

Ice has high specific latent heat of fusion (i.e., 3.36×105 J/kg). The following are the
consequences of the high specific latent heat of fusion of ice:

i. Snow is formed due to freezing of water vapor in the atmosphere. During freezing,
every 1 kg of water at 0oC gives out 3.36×105 J of heat to the surrounding. It makes
the surrounding weather moderate during snowfall.

ii. Snow in the Himalayan region melts slowly. Therefore, water flows continuously
in perennial rivers.

Know the Reason

Ice at 0°C appears to be colder in our mouth than water at the same temperature, why?
When ice at 0°C changes into water at 0°C, it absorbs heat equivalent to the specific latent
heat of fusion of ice (3.36× 105J/kg). But this extra heat is not absorbed by the ice at 0°C. So
ice at 0°C appears to be colder in our mouth than water at the same temperature.

Latent heat of vaporization

The heat energy required to convert a liquid at its boiling point into gas without any rise
in the temperature is called latent heat of vaporization. It is required to break the bonds
between liquid molecules. Latent heat of vaporization is associated with boiling of a liquid
or condensation of a liquid. When boiling a liquid, the latent heat of vaporization is absorbed
from outside, and at the time of condensation, the same amount of heat is given out.

Specific latent heat of vaporization

The quantity of heat required to change a unit mass of a liquid to the gaseous phase at constant
temperature is called specific latent heat of vaporization of the liquid. It is denoted by Lvap or
simply by L.

Thus, if Q is the quantity of heat required to boil a mass ‘m’ of a liquid substance completely
into vapor, then the latent heat of vaporization for the substance is

Lvap or L = Quantity of heat absorbed during boiling = Q
mass of the solid substance vaporized m

The SI unit of specific latent heat of vaporization is joule per kilogram (J/kg).

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Values of the specific latent heat of vaporization and boiling point for some common substances

- Specific latent heat of vaporization Boiling point ( in oC)

Substance In SIunits (J/kg) I In cal/g

Water 22.68× 105 540 1000C

Ethyl alcohol 8.5 × 105 202.38 780C

Sulphuric acid 5.1 × 105 121.43 3380C -
92.86 34.50C -
Ether 3.9 × 105 I 66.67 3570C
2.8 × 105 -
~

Mercury

1. Specific latent heat of vaporization of water:

Activity:

i. Put some amount of water in a beaker and heat it to a boil. Measure the temperature
of the water with the help of a laboratory thermometer as it starts boiling. Is it
1000C?

ii. Note the temperature of boiling water after every minute. Do you find a constant
temperature?

iii. On heating water at 100oC continuously, it boils to form steam, but the thermometer
shows the same temperature. Hence, there is no rise in temperature during boiling
of water even if heat is being supplied continuously.

The energy that a unit mass of water at 100oC absorbs from its surrounding in order to overcome
the attractive forces between its molecules when it changes from water to steam at 100oC is
called specific latent heat of vaporization of water. It is experimentally found that 22.68× 105
joules of heat energy is required to convert 1 kg of water at its boiling point to steam at the same
temperature. That is, the specific latent heat of vaporization of water is 22.68×105 J/kg.

Memory Plus

When water at 100°C boils, the steam formed with the latent heat of vaporization of water
is also at 100°C.

Specific latent heat of vaporization of water is 22.6×105 J/kg means that 22.68×105 J of heat
is required to change one kilogram of water at 100oC into steam at the same temperature,
or 22.68×105 J of heat is given out when 1 kg of steam at 100°C gets condensed into water
at the same temperature.

Know the Reason

When water in a pot is kept boiling, the temperature remains at 100 °C (212 °F) until the
last drop get vaporized, why?

All the heat being added to the boiling water is absorbed as latent heat of vaporization
and carried away by the escaping vapor molecules. So when water in a pot is kept boiling,
the temperature remains at 100 °C (212 °F) until the last drop get vaporized.

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2. Phase change diagram

In this graph, AB represents water and point B 150 D
represents water at 100oC. When heat is supplied
to this water, its temperature does not rise even Temperature (inᵒC)t
though heat is continuously supplied. Change (liquwiadt)er
of phase in which water changes into steam is (vapsotuera)m100
represented by BC. The temperature of a mixture Vaporization
of water and steam is 100oC. At C, all water has B (Or boiling) C
vaporized. On further heating beyond C, the
temperature of steam increases along CD. Liquid + vapour

20 A

Time of heating (in minutes)

3. Some consequences of high specific latent heat of vaporization of water

Water has high specific latent heat of vaporization (i.e., 22.68×105 J/kg). The following
are the consequences of high specific latent heat of vaporization of water:

i. Evaporation of water from a surface removes excess heat energy. This causes the
cooling effect of leaves, removes body heat through sweating and protects an
organism from sudden temperature fluctuations (i.e., thermal shock).

ii. Every 1kg of water at 100°C absorbs 22.68×105 J of heat when it changes into steam
at the same temperature. So when water is used as a cooling agent for a combustion
engine, steam absorbs a large amount of heat energy from a hot engine.

Know the Reason

Skin burn from 100oC steam is more severe than a burn from water at 100oC, why?

When water at 100°C changes into steam at 100°C, it absorbs heat equivalent to the specific
latent heat of vaporization of water (22.68× 105 J/kg). But this extra heat is not absorbed
by water at 100°C. So skin burn from 100°C steam is more severe than a burn from water
at 100°C.

Latent heat of sublimation

Sublimation is a physical process in which a solid directly converts into a gaseous (vapor)
state without going through a liquid state. The heat absorbed by a substance, which directly
changes from a solid to a gaseous state, or the heat given out by a gaseous substance when it
directly changes into a solid, is called the latent heat of sublimation.

Heat equation

Change in temperature of a body is either due to absorption of heat or due to the release of
heat. On heating body, it absorbs heat and its temperature rises but when a body gets cooled,
it gives out heat. This results in decrease in temperature of the body.

Let us consider the temperature of a body of mass ‘m’ and specific heat capacity 's' rises from

initial temperture (t1 )to final temperature (t2), when it is heated by supplying ‘Q’ amount of
heat.

Experimently it has been proved that, the heat gained or lost (Q) is directly proportional to:

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

Qαm ………. (i)

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Change in the temperature (∆t), i.e.,

Q α ∆t ………. (ii)

Combining relation (i) and relation (ii)

Q α m ∆t

Q = ms∆t, where ‘s’ is a proportionality constant called specific heat capacity.

Thus, the amount of heat gained or lost by a body (Q) is equal to the product of its
mass (m), specific heat capacity (s) and change in its temperature (∆t), i.e. Q= ms∆t,
This is called the heat equation.

Solved numerical- 4.

Specific heat capacity of water is 4200 J kg-1oC-1. Calculate the amount of heat energy given out
when 5 kg of water cools down from 80oC to 30oC.

Solution:

Here, Specific heat capacity of water (s) = 4200 J kg-1oC-1

Mass of water (m) = 5 kg

Initial temperature (θ1) = 80o C
Final temperature (θ2) = 30o C
Change in temperature (∆t) = θ1 – θ2 = 80o C – 30o C = 50o C
Now, heat energy lost, Q = ms ∆t

Or, Q = 5 × 4200 × 50

∴ Q = 1050000J = 1.05 × 106 J

Principle of calorimetry

The branch of physics which deals with the measurement of heat energy is called calorimetry.
When two bodies of different temperatures are kept in thermal contact, heat flows from a
body at high temperature to a body at low temperature. Heat lost or gained is measured using
a calorimeter. A calorimeter is acylindrical container made ofcopper or aluminium. Its outer
surface is kept shining to prevent the exchange of heat between the container and the
surrounding. The arrangement is kept in a hollow wooden box. The gap between the container
and the box is filled in by non-conductors of heat like wool, cotton, etc. The stirrer kept in the
device is used to stir the mixture for uniform distribution of heat.

In winter, people mix hot water with cold water for bathing. The Thermom e te r
temperature of the mixture is less than the temperature of hot water
and more than the temperature of cold water. In this process, the hot Stirrer
water loses heat, and the cold water gains the heat lost, and finally the
mixture attains a common temperature. According to the principle of vessel
calorimetry, if there is no loss of heat while mixing two substances at
different temperatures, the amount of heat lost by the body at high Reaction
temperature is equal to the amount of heat gained by the body at low mixture
temperature.
fig:Calorimeter

Thus, heat lost by the body at high temperature = heat gained by the body at low temperature

Or, m1 s1 (θ1– θ) = m2 s2 (θ– θ2)

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Where, θ = final temperature of the mixture

m1 = mass of the hotter body m₂ = mass of the colder body

s1 = specific heat capacity of hotter body s₂ = specific heat capacity of colder body

θ1 = initial temperature of the body at high θ2 = initial temperature of the body at low

temperature temperature

Solved numerical- 4.

100g of water at 1000C is mixed with 200g of water. If the temperature of the mixture becomes

500C, find the initial temperature of the cold water.

Solution

Here, mass of hot water (m1) = 100g = 100 kg = 0.1 kg
1000

Initial temperature of hot water (θ1) = 1000C
Mass of the cold water (m2) = 200g = 0.2kg
Final temperature of the mixture (θ) = 500C

Initial temperature of cold water (θ1) = ?
From the principle of calorimetry

Heat lost by hot water = Heat gained by cold water

Or, m1s (θ1 – θ) – m2s (θ2 – θ)
Or, 0.1 × (100 – 50) = 0.2 × (50 – θ2)

Or, 0.1 × 50 = 50 – θ2
0.2

∴ θ2 = 50 – 25 = 25°C
Therefore, the initial temperature of cold water is 250C.

Answer writing skill I

1. What causes latent heat?

The intermolecular force of attraction in different substances is the cause of latent heat.
2. What is a calorimeter?

A device which measures heat lost or heat gained on mixing two substances at different
temperatures is called a calorimeter.
3. How much heat is given out by 1kg steam at 100oC when it gets condensed into water
at the same temperature?
22.68 × 105 J of heat is given out when 1kg of steam at 100°C gets condensed into water

at the same temperature.

PHYSICS 0Optional Science - 10 73

4. Write the relation between the heat energy absorbed and mass of a body.

Heat energy absorbed by a body is directly proportional to the mass of the body. If ‘Q’
is the heat absorbed and ‘m’ be the mass of the body then, Q α m.

5. What is meant by

1. The specific latent heat of fusion of ice is 80 cal/g?

The specific latent heat of fusion of ice is 80 calories per gram means that 80
calories of heat are absorbed by each gram of ice at 0oC on melting or 80 calories
of heat are given out by 1g of water at 0oC during freezing.

2. The specific latent heat of vaporization of water is 540 cal/g?

The specific latent heat of vaporization of steam is 540 calories per gram means
that 540 calories of heat are absorbed by each gram of water at 100oC on boiling or
540 calories of heat are given out by 1g of steam at 100oC during condensation.

6. Write the differences between evaporation and vaporization

Evaporation Vaporization
It occurs at all temperatures. It occurs only at boiling point
It is a slow process. It is a rapid process.
It occurs only on the surface of a liquid. It occurs over the entire volume of a liquid.
It causes cooling of the atmosphere. It does not cause cooling of the atmosphere.

7. How does the temperature of air remain moderate during snowfall?

Snow is formed due to freezing of water vapour in the atmosphere. During freezing,
every 1kg of water at 0°C gives out 3.36×105 J of heat to the surrounding. It makes the

surrounding weather moderate during snowfall.

8. What are the different phases of water shown in the given phase diagram?
---rwater at 100oC temperature.
T(ᵒC)

1. First phase: Water at temperature less than 100oC to steam

100ᵒC water + steam

2. Second phase: Water at 100oC to steam at 100oC water

temperature. 0

3. Third phase: Steam at 100oC to steam greater than -10 t (in min)

100oC temperature.

9. Calculate the heat energy released when an iron ball of 5 kg mass is cooled down from

430°C to 30°C? How much mass of water can be heated from 30°C to 100°C by this heat

energy? The specific heat capacity of iron and water are 460 J/kg°C and 4200 J/kg°C

respectively.

Here, mass of iron ball (m) = 5kg

Initial temperature (t1) = 430°C
Final temperature (t2) = 30°C
Change in temperature (dt) = 430°C– 30°C = 400°C

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PHYSICS

Specific heat capacity of iron (s) = 460 J/kg°C
According to the heat equation, the heat released is given by
Q = ms dt
Q = 5 × 460 × 400 = 920000 J

Again, specific heat capacity of water (s) = 4200 J/kgoC
Mass of water (m) = ?
Initial temperature (t1) = 30°C
Final temperature (t2) = 100°C
Change in temperature (dt) = 100 – 30 = 70
Heat supplied (Q) = 920,000 J
Now, heat energy supplied, Q = ms dt
Or, 920,000 = m × 4200 × 70

920000
Or, m = 4200 × 70 = 3.129 kg

10. Prove that the amount of heat gained or lost by a body (Q) is equal to the product of its

mass (m), specific heat capacity (s) and change in its temperature (dt), i.e., Q = md dt.

The heat gained or lost (Q) is directly proportional to: mass of the body (m),
i.e.,

Qαm ……. (i)

change in temperature (∆t), i.e.,

Q α ∆t ……. (ii)

Combining relation (i) and relation (ii)

Q = ms ∆t, where ‘s’ is a proportionality constant called specific heat capacity.

Thus, the amount of heat gained or lost by a body (Q) is equal to the product of its mass
(m), specific heat capacity (s) and change in its temperature (∆t), i.e. Q = ms ∆t, This is
called the heat equation.

PHYSICS 0Optional Science - 10 75

Exercise

1Step

Define

1. One calorie heat 2. melting 3. freezing
4. vaporization 5. condensation 6. sublimation
7. deposition 8. latent heat of fusion 9. latent heat of vaporization
10. latent heat of sublimation

Very short answer questions

1. Name three states of matter.

2. Write two factors which can change the state of matter.

3. Write SI unit of the following

a. latent heat b. specific latent heat of fusion

4. How much heat is required to change one kilogram of ice at 0oC into water at the same
temperature?

5. How much heat is required to change one kilogram of water at 100oC into steam at the
same temperature?

6. Write the formula to calculate the heat absorbed or given out during the change of state

of a substance.

2Step

Short answer questions

1. Write an example of each of the following

a. melting b. freezing c. vaporization d. condensation

e. sublimation f. deposition

76 Optional Science - 10

PHYSICS

2. During melting, there is no increase in the temperature of ice even though heat is being
supplied continuously. Where is the applied heat used?

3. Write some consequences of high specific latent heat of fusion of ice.
4. Write some consequences of high specific latent heat of vaporization of water.
5. What is a heat equation? Write its mathematical form, too.

Differentiate between
1. Melting and boiling
2. Normal melting point and normal boiling point
3. Latent heat of fusion and latent heat of vaporization
4. Sublimation and deposition

Give reason
1. There are water droplets on the surface of a cold soda bottle on a humid day.
2. Temperature of a material does not change during change of phase.
3. Ice at 0°C appears to be colder in our mouth than water at the same temperature.
4. When a pot of water is kept boiling, the temperature remains at 100°C (212°F) until the

last drop evaporates.

5. Skin burn from 100°C steam is more severe than a burn from water at 100°C.

Answer the questions with the help of the given figure

1. The phase change diagram shows how the temperature of a pure substance changes on

heating. 150 B D
C
1. What is the initial value of temperature of t
the substance?
100
2. At what temperature does the substance
boil? Temperature (inᵒC)
(liquid)
3 Label the parts of the graph which represent 20 A
different phases of the substance.
--Time of heating (in minutes)

2. The phase change diagram for a solid of mass 500g r130oC ---------C-----D~
is shown in the given figure. Find
1. melting point Temp (T)
2. Specific latent heat of fusion of the
substance. 10oC AB
3. boiling point O
4. Specific latent heat of vaporization '' '' 9.7
3.3 6.7
PHYSICS
Heat energy (kJ)

0Optional Science - 10 77

3Step

Numerical problems

1. The specific latent heat of fusion of ice is 336000 J/kg. Find the mass of the ice at 0°C
which requires heat of 168000 J to melt completely into water at 0°C. Ans: 0.5 kg

2. Calculate the quantity of heat required to convert 500g of ice at 0°C into water at a
temperature of 0°C. (Specific latent heat of fusion of ice = 336000 J/kg )Ans: 1.008× 105 J

3. Calculate the quantity of heat required to convert 450g of water at 100°C into water

vapor at the same temperature. (Specific latent heat of vaporization of water = 22.68 × 105

J/kg) Ans: 10.206×105 J

4. How much heat must be added to raise the temperature of 200g of water from 15°C to

85°C? Ans: 58800 J

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

100C if 44000 joules of heat energy are required to raise the temperature of the paraffin

by 20°C? Ans: 4.4× 104 J

6. An electrical kettle contains 200g of water at 200C. How much heat is to be supplied in
order to boil the water? (Assume that the kettle takes no heat) Ans: 67200 J

7. A brass rod of 0.4 kg mass at 100°C is dropped into 1.0 kg of water at 20°C. The final

temperature is 23°C. Calculate the specific heat capacity of brass. (Specific heat capacity

of water = 4200 J/kg°C) Ans: 409.09 J/kgoC.

8. A bucket contains 8kg of water at 25°C. Calculate the final temperature of the water if

2kg of water at 80°C is poured into it. (Neglect the heat loss) Ans: 36°C

10. When a 200g hot iron sphere is kept in 500g of water at 10°C, then the final temperature
of water becomes 30°C. Find the initial temperature of the iron sphere. (Specific heat

capacity of iron = 470 J/kg°C) Ans: 476.8°C

Draw a diagram

1. To show the phase change which occurs when ice is heated and completely changes into
steam.

2. To show different types of phase transitions.

4Step

1. Derive heat equation, Q = ms ∆t , where the symbols have their usual meaning.
2. Explain an experiment to demonstrate latent heat of fusion.
3. How can you demonstrate latent heat of vaporization? Explain.
4. Explain latent heat on the basis of kinetic theory.
5. Which type of changes do you observe on complete heating of ice into steam? Explain.

6. Explain the factors which affect amount of heat energy.

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Multiple choice question (MCQs)

1. During change of state, which of the following is true for molecules of a substance?

a. kinetic energy increases b. potential energy increases

c. both increases d. both remain constant
2. Snowballs are formed due to b. freezing of moisture

a. melting of ice

c. sublimation d. deposition
3. Specific latent heat of fusion of ice is b. 80 cal/g

a. 80 kcal/g

c. 80 J/g d. 80 J/kg

4. Specific latent heat of vaporization of water is

a. 405 cal/g b. 504 cal/g

c. 540 cal/g d. 450 cal/g

5. The SI unit of specific latent heat of fusion is

a. J b. J/kg

c. cal d. cal/g

6. The heat energy contained in a body depends upon:

a. mass b. change in temperature

c. specific heat capacity d. all the above

7. 1kg of paraffin requires 44000 J heat energy to increase its temperature by 20oC, then its

specific heat capacity is b. 2200 J /kgoC
a. 2400 J /kgoC

c. 2100 J /kgoC d. 2000 J /kgoC

Project Work

1. Take frozen ghee and ice cubes from a freezer and put them in two separate beakers.
Insert a laboratory thermometer inside both of them to measure their initial temperature.
Now heat them with a small flame and note their temperature every 30s. Find the melting
point of ice and ghee.

2. Take three glass tumbler. Label them as A, B and C. Put 200ml of cold water in A and 200ml
of hot water in B. Measure their temperatures with the help of a laboratory thermometer.
Mix both of them in a thermos flask. Record the temperature of the water after mixing
them. Use the heat equation to make a rough estimate of the heat lost by the hot water
and the heat gained by the cold water. (Neglect the loss of heat to the surrounding).

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UNIT

5 LIGHT

Christiaan Huygens (1629-1695) was a Dutch mathematician, astronomer and physicist,
who founded the wave theory of light, discovered the true shape of the rings of Saturn, and
made original contributions to the science of dynamics. Huygens came from a wealthy and
distinguished middle-class family. His father was a diplomat and poet. From an early age,
Huygens showed a marked mechanical bent and a talent for drawing and mathematics. In
1645, Huygens entered the University of Leiden, where he studied mathematics and law. He
introduced correction in the construction of the telescope with his new method of grinding
and polishing lenses, which made it possible to discover the true shape of the rings of Saturn
in 1659. Using his improved telescope, he discovered a satellite of Saturn in March 1655 and
distinguished the stellar components of the Orion nebula in 1656.

Key terms and terminologies of the unit

1. Lens: A lens is a piece of transparent optical material bounded by two refracting
surfaces.

2. Convex lens: A lens which is thicker in the middle and thinner at the edges is
called a convex lens.

3. Concave lens: A lens which is thinner in the middle and thicker at the edges is
called a concave lens.

4. Power of a lens: The power of a lens is defined as the reciprocal of its focal length
(in metres).

5. One dioptre power: The power of a lens whose focal length is one metre is called
one dioptre.

6. Magnification: The ratio of the height of the image formed by a lens to the height
of the object placed in front of the lens is called magnification of the lens.

7. Optical instrument: Optical instruments are devices which process light wave to
enhance an image for a clearer view.

8. Compound microscope: An optical instrument for forming magnified images of
very small objects is called a compound microscope.

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9. Astronomical telescope: Telescopes to get a clear view of the heavenly bodies are
called astronomical telescopes.

10. Terrestrial telescope: A telescope which is used to observe distant objects on the
earth's surface is called a terrestrial telescope.

11. Binoculars: Binoculars are hand-held optical instruments with two barrel
chambers mounted on a single frame side by side to provide a magnified view of
distant objects.

Introduction

Light is essential to see the colorful world around us. You had already learnt in lower classes
that the light which enters our eyes may be directly from a self-luminous object or it may be
the reflected light from a non-luminous object. Such light when falls on our eye's retina, then
we have a sensation of vision. Thus, light is a form of energy which produces the sensation of
vision. Light, when it passes from one medium to another, its direction gets changed. Such a
phenomenon in which light bends on passing from one medium to another medium is called
refraction of light. In class 9 optional science, you learnt about refraction of light and its cause.
You learnt about the bending of light through a glass slab, too. Similar bending of light occurs
through some other media like a glass prism, lens, etc. Here in this unit, you will learn about
lenses, magnification, power and optical instruments like a compound microscope, telescope
and binoculars.

Memory Plus

Light travels along a straight line. This property is called rectilinear propagation. The
straight line along which light travels is called a ray.

Refraction of light through lenses

Optical instruments like the camera, microscope, telescope work on the basis of refraction of
light through a lens. A lens is a piece of transparent optical material bounded by two refracting
surfaces. Generally, the transparent medium in a lens is of glass or clear plastic. The refracting
surfaces of lens are usually spherical. Lenses are used in making optical instruments like
cameras, compound microscopes, telescopes. A person with a vision defect wears spectacles
with a lens of a certain power.

Types of lenses

Lenses are made of different kinds of glass and come in a variety of shapes, but they belong to
either the converging group or diverging group. That is, lenses are of two types:

i. Convex lens or converging lens ii. Concave lens or diverging lens

Convex lens

A lens which is thicker in the middle and thinner at the edges is called a convex lens. It bulges
out at the center. It helps the light rays to converge.

PHYSICS 0Optional Science - 10 81

Memory Plus

Types of convex lenses: There are three types of convex lenses.

1. Biconvex or double convex lens: Both of its surfaces are convex.

2. Plano-convex lens: It has one surface convex and the other surface a Double convex Plano convex Concavo convex

plane.

3. Concavo- convex: It has one convex surface and the other surface a concave.

A convex lens as a group of prisms: Converging action of a convex lens

A convex lens is regarded as being made up of a large number of prisms.
The prisms on the upper half of the lens have their bases downward and the
prisms on the lower half have their bases upward. The angle of the prisms
goes on decreasing from the central line. The central part of the prism is just
like a glass slab.

In the case of a prism, the lesser the angle of a prism, the greater is the bending of light. Thus,
the light rays falling on the edges of a convex lens bend maximum. Finally, all the rays meet
at a point after refraction. This point is called the focus of the convex lens.

Know the Reason

A convex lens is called a converging lens, why?
A convex lens bends all the light rays falling on it inward. Therefore, all
the parallel beams of light converge at a point. So it is called a converging
lens.
A convex lens has a real focus, why?
The refracted rays from a convex lens actually meet at a point. So a convex lens has a real
focus.
It is dangerous to look through a convex lens at the sun or a bright light, why?
A convex lens converges the sunrays at one point. It increases the temperature at that
point. If we look through a convex lens at the sun on a bright sunny day, it can damage our
eyes. So it is dangerous to look through a convex lens at the sun or a bright light.

Uses of convex lens
1. Convex lenses are used in optical instruments like compound microscopes, telescopes,

etc.

2. MemCoonrvyexPlleunsses are used for image formation on film and slide projectors.

3. It is used to make searchlights and spotlights in theaters. A convex lens with a powerful
Tleynpseleassm: opf acot nitcsafvoeculesnpsreosd:uTcheesrae paorewtehrrfeuel ptyapraelsleolfbceoanmcaovfelight.

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TI ] ]Concaavreelceonnscave.

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A concave lens as a group of prisms: Diverging action of a concave lens

A concave lens is regarded as being made up of a large number of prisms. The prisms on the
upper half of the lens have their angles facing downward and the prisms on the lower half
have their angles facing upward. The angle of the prism continuously increases from the
central line towards the edges of the lens.

The light rays passing through a prism deviate towards its base. The lesser
the refracting angle, the greater the deviation produced. Thus, the light
rays near the edges of a convex lens deviate less while the light rays around
the central line of the lens deviate more. When all the diverged light rays
are produced backward, then they meet a point. This point is called the
principal focus of a concave lens.

Know the Reason

A concave lens is called a diverging lens, why?

A concave lens bends all the light rays falling on it outward. All the
parallel beams of light diverge, and the diverged rays appear as if they
are coming from a point. So it is called a diverging lens.

A concave lens has a virtual focus, why?

The refracted rays from a concave lens appear to diverge from a point. When we produce
them backward, they meet at a point which is the virtual focus. So a concave lens has a
virtual focus.

Memory Plus

Those lenses that are thicker in the middle converge parallel rays and those which are
thicker at the edges diverge parallel rays. Hence, light rays are always bent toward the
thicker part of a lens.

Uses of concave lens
1. It is used as an eye lens in the Galileo Telescope.
2. A concave lens is used for correction of short sightedness.

Power of a Lens

The power of a lens is a measure of the degree of convergence or divergence produced by it

when light rays fall on it. If a convex lens converges a parallel beam of light closer to its optical

center, then the focal length is less, but the converging power is high. Thus, the power of a lens

depends on its focal length. The power of a lens is defined as the reciprocal of its focal length

(in meters). Mathematically,

Power of lens (P) = focal 1 (in metre) = 1
length f

If the focal length is measured in centimeter

Power of lens (P) = f 100
(in cm)

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A convex lens has a positive focal length. So, the power of a convex lens is considered to be
positive. Whereas the focal length of a concave lens is negative. So, the power of a concave lens
is considered to be negative.

Know the Reason

The power of a lens is measured as a reciprocal of its focal length, why?
A thick lens with short focal length has more power to bend the light rays. But a thin lens
with long focal length has less power to bend the light rays. The power of a lens of short
focal length is more and vice-versa. So the power of a lens is measured as a reciprocal of
its focal length.

Unit of power of a lens
If the focal length of a lens is measured in meters, then the unit of power is meter-1, which is
called the diopter and denoted by the symbol 'D'.

One dioptre power: The power of a lens whose focal length is one meter is called 1 diopter.

Memory Plus

A lens of short focal length has more power whereas a lens of long focal length has less
power.
The power of a lens can be measured directly by using an instrument called adioptometer.
Opticians use it to measure the power of spectacle lenses.
A lens of power 3 D (diopter) means that the lens brings parallel rays of light to focus at  1/3
meters.

Solved numerical- 5.

A convex lens forms a real and inverted image of an object at a distance of 50 cm. How far
is the object from the lens if the image is equal to the size of the object? Also calculate the
power of the lens.
Solution: Here, image distance = 50 cm

If the image size is equal to the object size, then the object is at a distance double the
focal length (i.e., 2f)

In this case, image distance = object distance = 2f = 50 cm

f = 25cm = 25 m = 0.25 m
100 lens, P =
1 1
Now, power of a f = 0.25 = 4 D

Thus, the power of the convex lens is +4 D.

Combination of thin lenses

If two or more thin lenses are kept in contact, then the reciprocal of their equivalent

focal length is equal to the sum of the reciprocal of individual focal lengths of all

lenses. That is, combination
of lenses

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1 = 1 + 1 + 1 + .........
f f1 f2 f3

Where f1, f2, f3, ……. are the focal length of the thin lenses in contact.

Therefore, the power of the combination of thin lenses is given by

P = P1 + P2 + P3 + ..........

Where, P1= 1 , P2= f12, P3= 1 .........
f1 f3

Magnification

Magnification is the degree to which the object is enlarged during formation of its image by
using optical instruments like binoculars, microscope, etc.

Some examples of magnification
1. A magnifying glass when placed close to an object produces an erect and magnified

image. Palmists use it to study the lines of a palm. It is also used as a reading glass.
2. A telescope forms a magnified image of distant objects.
3. A compound microscope makes a very small object appear much larger.
4. In the case of a slide projector, it projects a large image of a small slide on a screen.

Calculation of magnification

When an image is longer than the object, we say it is magnified, and when it is smaller we

describe it as diminished. The ratio of the height of the image formed by a lens to the height of

the object placed in front of the lens is called the linear magnification of the lens.

Mathematically,

magnification (m) = height of the image
height of the object
I
Or m = O

To obtain a relation involving object distance (u) and image distance (v):

In the given figure, Convex lens

Height of the object (AB) = O B

Height of the image (DC) = I 2F A F O D
Object distance (OA) = u Object F 2F
Image distance (OD) = v
In triangles AOB and COD: u C
v

S.N. Statement Reason

i. AOB = COD Vertically opposite angles are equal.

ii. OAB = CDO Both of them are right angles

iii. ABO = OCD If two angles of a triangle are equal to two angles of another
triangle, then the third pair of angles are equal.

The triangles AOB and COD are similar.

In the case of the two similar triangles, the ratio of corresponding sides is equal, i.e.,

PHYSICS 0Optional Science - 10 85

DC = OD
AB OA

I = v = m
O u

So the magnification produced by a lens is also defined as the ratio of the image distance (v)

to the object distance (u).

magnification (m) = Image distance
Object distance
v
Or m = u

Magnification has no unit, as it is a ratio of two distances.

Magnification (m) Image size
'm' greater than 1 (m>1) Magnified: larger than the object
'm' equal to 1 (m>1) Image is of same size as the object
'm' less than 1 (m<1) Diminished: smaller than the object

Solved Numerical- 5.

A convex lens forms an image of an object at a distance of 12 cm when the object is 6 cm from
the lens. Find the magnification of the image.

Solution: Here, object distance (u) = 6 cm

Image distance (v) = 12 cm 12
v 6
Now, magnification = u = = 2

Solved numerical- 5.

An object of height 2 cm is placed at a distance double that of the focal length of a convex lens.
Find the size of the image and its magnification.

Solution Here, size of the object (O) = 2 cm

When the object is placed at a distance double that of the focal length of a convex lens, then

Size of object (O) = Size of the image (I) = 2 cm

Now, magnification of image (m) = I = 2 = 1
O 2

Memory Plus

Magnification of a lens is expressed in number times (e.g. 10×). A lens has magnification of
10× means that the image formed is 10 times bigger than the object.

Optical instruments

Optical instruments are devices which process light waves to enhance an image for a clearer
view. For example, cameras, magnifying glasses, microscopes, telescopes, binoculars are
optical instruments. Such instruments form an image of an object with the help of the lens or
a number of lenses.

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Compound microscope

An optical instrument for forming magnified images of Focusing Additional
very small objects is called a compound microscope. A high knob
power compound microscope achieves higher levels of (interchangeable)
magnification and is used to observe or analyze microscopic Objective objective lenses
objects or organisms. lens

Construction: It consists of two convex lenses placed co- Light
axially with two sliding tubes. The lens towards the object source
is called objective, and it has a short focal length and a small
aperture. The lens through which the final image is observed fig:Compound microscope
is called eye lens or eyepiece. The focal length of the eyepiece
is comparatively longer and the aperture is larger, too. The
objective lens and the eyepiece are arranged in such a way
that the distance between them can be adjusted.

Working: The object AB to be magnified is placed in between F and 2F of the objective lens. A
real, inverted and magnified image A'B' is formed in front of the eyepiece. A'B' acts as the
object for the eyepiece. The position of the eyepiece is adjusted to bring A'B' within its focus.
Finally, the eyepiece forms the magnified image A''B'' on the same side as A'B'.

Eyepiece

Object Objective I1-Real, inverted Fe
A and enlarged Eye
O Fo Fe B'
B''
B A'

I2-Virtual, erect and
highly magnified
final image

A''
fig: ray diagram to show the formation of image in a compound microscope

Uses of compound microscope:

1. Diagnosis: A compound microscope is of great use in pathology labs to examine the
stool, blood and urine of patient. It helps to identify disease-causing agents and the
corresponding disease.

2. Use in study and research: A compound microscope is used in a biology lab to observe
the cell structure and its organelles. Study through a microscope helps in research work,
too.

Telescope

The angle subtended at the eye by an object very far from our eye is small. So the object appears

small. To see the object nearer, the angle subtended by it at our eye should be increased. Such

a function is that of a telescope. A telescope is a device that is used to observe distant objects

either on the earth or in the heavenly bodies like the moon and planets. There are two types of

telescopes, namely astronomical telescope and terrestrial telescope.

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Astronomical telescope
A telescopes for getting a clear view of the heavenly bodies is called an astronomical telescope.
It has the same construction as that of a compound microscope, but the focal length of the
object lens is larger than the eye lens used in it. We are not going to draw the ray diagram of its
image formation in an astronomical telescope here rather will do it for the terrestrial telescope.

Terrestrial telescope

An astronomical telescope is not used to see objects on the earth's

surface. The demerit of an inverted final image in an astronomical

telescope led to the invention of the new telescope, in which the final

image is erect. This was achieved by adding one more lens between the

objective lens and eye lens in the model of the astronomical telescope.

The telescope which is used to observe distant objects on the earth's

surface is called a terrestrial telescope. fig:terrestrial telescope

Construction: In a terrestrial telescope, there is an auxiliary lens, or erecting lens, in between

the objective lens and eye lens. A larger aperture of the object lens increases the quality of

image. So the aperture of the object lens in a terrestrial microscope is made larger. The eye lens

is smaller than the object lens and the auxiliary lens has the shortest focal length. Its main

purpose is to invert the image formed by the object lens.

The length of the telescope is adjusted in such a way that, L = fo + 4fa +fe. Where, fo is the focal
length of the object lens, fa the focal length of the auxiliary lens and fe the focal length of the
eye lens.

To infinity
fo

A1
2fa

Fo Fe
OB B1

A Auxiliary lens 2fa fe

(erecting lens) Eye piece

Objective

fig:ray diagram to show the formation of image in a terrestrial telescope

Working: The image, 'AB' of a distant object, is formed by the object lens at its focus (i.e., Fo).
The position of the auxiliary lens is so adjusted that 'AB' is at its center of curvature (2Fa). The
auxiliary lens forms a real image A1B1 of AB. A1B1 is inverted with respect to AB but erect with
respect to the object. Since the object is at the center of the curvature of the auxiliary lens, their

size is equal (i.e., size of A1B1= size of AB). Their distance from the center of the lens is 2fa. The
position of the eyepiece is so adjusted that the image A1B1 is on the focal plane of the eyepiece.
So the final image is formed at infinity. The final image is erect with respect to A1B1. Hence the
final image is erect with respect to the object.

Uses of a terrestrial telescope: A terrestrial telescope is a telescope used for viewing objects

on land. It is used for seeing armies from a greater distance, seeing signal flags on a distant

hill, etc.

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Drawbacks
1. The length of the terrestrial telescope is longer than the astronomical telescope.
2. Due to one extra lens added in this telescope, the intensity of the final image decreases.

Know the Reason

An astronomical telescope cannot be used to view distant objects on the earth's surface,
why?
In an astronomical telescope, the final image formed is inverted with respect to the object.
So an astronomical telescope cannot be used to view distant objects on the earth's surface.

Binoculars

Binoculars are a handheld optical instrument with two barrel chambers mounted on a single

frame side by side for providing a magnified view of distant objects. The two barrel chambers

in the binoculars are like two small eyepiece
telescopes mounted together. When

we view through binoculars, a

perception of depth and 3-dimensional

structure is obtained on the basis of

visual information from the two eyes. Light
Two prisms are used in binoculars to precision
produce an erect final image by total
graphics

internal reflection. fig:construction and working of binoculars

Construction

Modern binoculars consist of two barrel chambers. One is fixed with an objective lens and
the other with the eyepiece. There is a pair of prisms inside each barrel chamber. The prisms
reflect and lengthen the light, while the objective lenses enhance and magnify the images. The
tubes of the binoculars are made from aluminium and coated with silicon. Their lenses and
prisms are made from glass and coated with an anti-reflective coating.

Working

Binoculars are simply two telescopes placed side by side, one for each eye. The front lens
catches the light rays from a distant object and creates a focused image a short distance behind
the lens. This lens is called the objective. Binoculars have a pair of prisms inside them to rotate
the image 180 degrees. One prism rotates the image through 90 degrees, then the next prism
rotates it through another 90 degrees. The two prisms effectively turn the image upside down.
The final image of the object is obtained through the eyepiece. There is no loss of light energy
while propagating through the prisms due to total internal reflection. The two prisms used
in binoculars can lengthen the light path between the objective lens and the eyepiece (ocular
lens). Such an arrangement increases the magnification.

Uses of binoculars

Binoculars are a necessity for an astronomer, hunter, saltwater fisherman, boater, sports fan,
and experienced traveler. The following are some uses of binoculars:

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a. General use: Binoculars of different categories like 3×10, 7×50, etc. are available in the
market. People use such binoculars to obtain a closer view for different purposes as in a
theater, while travelling, etc. Some binoculars are also equipped with night scope vision
to see objects that are far away even at night.

b. Land surveys and geographic data collection: Geographers and other geoscientists use
binoculars for data collection.

c. Bird watching: It is a popular hobby among nature and animal lovers. They use
binoculars of high magnification like 7×, 10×.

d. Hunting: Hunters use binoculars to see animals which are difficult to spot with the
naked eye. They use binoculars of high magnification as well as of wider objective lens
to capture light in low light conditions.

e. Military purposes: Binoculars have a long history of military use. One of the military
uses is to find the movements of the enemy in a specific territory. Military binoculars
are fitted with filters blocking LASER beams used as weapons.

f. Astronomical use: Astronomers use binoculars for general observations in the sky. It
is used for locating astronomical objects in the night sky. Binoculars for astronomical
viewing have larger aperture objective lenses like 80 mm or larger. This increases the
total amount of light captured.

g. To watch sport events live: To get a magnified and clear view while watching sports
like football on the ground, people use good quality binoculars.

Advantages of binocular vision

The following are some good features of binoculars vision:

a. Binoculars are small and light in weight to carry.

b. Seeing through a pair of binoculars with two eyes is suitable for observation of far
objects.

c. Binoculars allow people to watch in three dimension.

d. Binoculars are cheaper than a telescope.

Disadvantages of binocular vision

a. Binoculars do not have wide visual field.

b. The magnification of a pair of binoculars is less in comparison to a telescope.

Answer writing skill I

1. Name an optical instrument in which there is use of the phenomenon of total internal
reflection?

Prism binoculars.

2. What is the relation between the power of a lens and its focal length?
The power of a lens is inversely proportional to its focal length. That is a lens with shorter
focal length has more power to converge or diverge the light rays falling on it.

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3. Define magnification.
Magnification is the degree to which the object is enlarged during formation of its image
by using optical instruments like binoculars, a microscope, etc.

4. How does a terrestrial telescope form an erect final image?
In a terrestrial telescope, there is use of an auxiliary lens in between the objective lens
and eye lens. It forms an inverted image. Finally there is formation of an erect image
when we observe through the eyepiece.

5. Why is the power of a concave lens measured in the negative?

A concave lens has a virtual foci. Such a distance of the virtual focus is measured in
1
negative. The power of a lens is reciprocal to its focal length, i.e., P = f

As the focal length of a concave lens is negative, its power measured is also negative.

6. If a student wears lenses of power -4.5 D in his spectacles for correction of vision,

a. What is the focal length of the corrective lens?

b. What is the nature of the corrective lens?

a. Focal length, f = 1
P
1
Or, f = —4.5 = —0.222 m

Or, f = — 0.222 100 cm = —22.2

The focal length of the corrective lens is 22.2 cm.

b. The lens is a concave lens.

7. An astronomical telescope forms an inverted image but a terrestrial telescope forms

the final image erect. Present the mechanism behind it.

In an astronomical telescopethere are two lenses which form an inverted image of the
object at infinity. But terrestrial telescope forms the final image erect with respect to the
object due to a third lens between objective and eyepiece. The third lens of short focal
length f is placed at 2f which forms an inverted image of the object. This image serves as
the object for the eyepiece and final image becomes erect. The lens placed in the center
of the telescope which actually erects the image is called as the erecting lens.

8. Explain the construction of a terrestrial telescope.

In a terrestrial telescope, there is an auxiliary lens, or erecting lens, in between the

objective lens and eye lens. The larger aperture of the objective lens increases the quality

of the image. So the aperture of the objective lens in a terrestrial microscope is made

larger. The eye lens is smaller than the objective lens, and the auxiliary lens has the

shortest focal length. Its main purpose is to invert the image formed by the objective

lens.

The length of the telescope is adjusted in such a way that, L = fo + 4fa +fe where, fo is the
focal length of the objective lens, fa is the focal length of the auxiliary lens and fe is the
focal length of the eye lens.

PHYSICS 0Optional Science - 10 91

Exercise

1Step 2. angular magnification 3. power of a lens

Define
1. linear magnification

Very short answer questions
1. What is a lens?
2. Write the SI unit of the following

a. focal length
b. power of a lens
3. Give the formula
a. to calculate the magnification of a lens in terms of object distance and image distance.
b. to calculate the power of combination of thin lenses.
4. What is an optical instrument?
5. What is a compound microscope?
6. What is a telescope?
7. What is an astronomical telescope?
8. What is a terrestrial telescope?
9. What are binoculars?

2Step

Short answer questions
1. Write two uses each of the concave lens and convex lens.

2. 'The power of a lens gives the conversing and diverging capacity of a lens', justify this
statement.

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3. Write some examples of magnification
4. Write the meaning of the following:

a. Power of a lens is 3D.
b. Magnification of a lens is 7.
5. Write two uses of a compound microscope.
6. Write two uses of a terrestrial telescope.
7. Write two uses of binoculars.
9. Write two advantages and two disadvantages of binocular vision.
12. Distinguish between the nature of image formed by the eye lens and objective lens of
telescope.

Differentiate between
1. Concave lens and convex lens
2. Compound microscope and terrestrial telescope.

Give reasons
1. Convex lens is called converging lens.
2. Concave lens is called diverging lens.
3. A convex lens has a real focus.
4. It is dangerous to look through a convex lens at the sun or bright light.
5. A concave lens has a virtual focus.

3Step

Numerical problems

1. An object is placed at a distance of 3 cm from a convex lens. It forms a real image at a

distance of 12 cm from the lens, then how many times does the lens magnify the size of

the object while forming its image? Ans: 4 times

2. A convex lens magnifies the image of an objects 3 times the size of the object. If an object
is placed at a distance of 4 cm from the lens, then how far is the image formed from the

convex lens? Ans: 12cm

3. Find the magnification of the lens which forms a real image of an object placed at a
distance of 4 cm in front of the lens on a screen 20 cm behind it. Ans: 5

5. An object of height 20 cm is placed at a distance of 2m from a convex lens. If a real image

of the candle is formed at a distance of 1m from the lens, then find the height of the image

formed. Ans: 0.1 m

6. The height of object placed in front of a convex lens is 20 cm and that of the image formed

is 10 cm. Find the magnification of the lens. Ans: 0.5

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7. A convex lens of power 10 D is used to burn a piece of paper by converging the solar
beam. What should be the distance between the lens and the paper to burn it? Ans:10 cm

8. How far should a hand lens of power 20 D be placed from a book to read it clearly?
Ans: 5cm

9. A lens has a power of –0.5 D. What are the focal length and the nature of the lens?
Ans:–2 m

10. If a lens is of focal length 50 cm, then calculate its power. Ans: +2.0 D

12. A concave lens has a focal length of 20 cm. Calculate its power. Ans: -5D

13. A person having a myopic eye uses a concave lens of focal length 10 cm. What is the

power of the lens? Ans: -10 D

14. The power of a lens is + 5 D. Find its focal length in centimeters. Ans: 20 cm

Draw a ray diagram

1. to show image formation in a compound microscope.
2. to show image formation in a terrestrial telescope.

4Step

1. Derive the formula to calculate the magnification of a lens involving object distance (u)

and image distance (v) i.e. m = v
u

2. Explain the construction and working of a compound microscope.

3. Explain the construction and working of a terrestrial telescope.

4. Explain the construction and working of binoculars.

5. Explain individual eyepiece focus and center focus methods of binocular focusing.

Multiple choice questions [MCQs]

1. If a lens is thicker at the center than at the edges, then it is

a. convex lens b. concave lens

c. plano concave lens d. plano convex lens

2. If a lens is thinner at the center than at the edges then it is

a. convex lens b. concave lens

c. plano concave lens d. plano convex lens

3. 4. A lens forms an image that is 5 cm tall when the object is 10 cm tall. The magnification

is

a. 0.2 b. 0.5

c. 2 d. 5

4. The SI unit of power of a lens is

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a. dioptre/meter b. meter

c. meter-1 d. meter/dioptre

5. The length of a terrestrial telescope is given by

a. L = fa+ 4 fo +fe b. L = fo + 4fa +fe

c. L = fo + 4fe +fa d. L = 4fe + fa +fo

Project Work

1. Collect convex lenses of different thickness. Put those lenses in between two rectangular
blocks. Measure the gap between those two blocks to measure the thickness of lenses.
Let the sunlight refract through those lenses one after another. Converge the refracted
light on the ground. Move the lenses until you find a 'sweet spot' on the ground. As you
find such a sharp pointed image on the ground, measure the distance of the lenses from
the ground. This gives a rough estimate of the focal length of the lens. Fill your data in a
table as shown below:

S.N. Type of lens Focal length Power
1.

II

What is the relation between the thickness of the lenses and their focal length? Write
your conclusion.

2. Make a telescope using a cardboard sheet, 2 lenses of
different thickness from magnifying glasses, black tape,
and a hot glue gun or super glue. Roll two cardboard
sheets to form two pipes in such a way that one pipe
can slide inside the other. Then tape the edges of the
cardboards. Glue the thicker lens on the outer edge of the
bigger pipe and thinner lens on the edge of the small pipe.
Your telescope is ready now. Hold the telescope with the inner tube facing your eye. Aim
it at an object in the night sky but not at the sun during day time, and adjust the distances
between the two lenses to obtain a sharp image.

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UNIT

6 CURRENT
ELECTRICITY

AND MAGNETISM

Michael Faraday (1791-1867) was an English scientist who contributed to the study
of electromagnetism and electrochemistry. His main discoveries include the principles
underlying electromagnetic induction, diamagnetism and electrolysis. Although Faraday
received little formal education, he was one of the most influential scientists in history.
Faraday established the basis for the concept of the electromagnetic field in physics. He
discovered the principles of electromagnetic induction and diamagnetism, and the laws
of electrolysis. His inventions of electromagnetic rotary devices formed the foundation
of electric motor technology, and it was largely due to his efforts that electricity became
practical for use in technology.

Key terms and terminologies of the unit

1. Magnetization: The process of making a substance temporarily or permanently
2. magnetic by inserting it in a magnetic field is called magnetization.
3.
4. Diamagnetic substance: Substances which when placed in a strong magnetic
5. field acquire feeble magnetization opposite to the direction of the magnetic field
6. are called diamagnetic substance.

096 Paramagnetic substance: Substances which when placed in a strong magnetic
field acquire feeble magnetization in the direction of the applied magnetic field
are called paramagnetic substances.

Ferromagnetic substance: Substances which when placed in an external magnetic
field acquire strong magnetization in the direction of applied magnetic field are
called ferromagnetic substance.

Magnetic flux: Magnetic flux is defined as the total number of magnetic lines of
force passing normally through any surface inside the magnetic field.

Electromagnetic induction: The process of obtaining an induced emf in a closed
circuit by changing the magnetic flux linked with it is called electromagnetic
induction.

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7. Fleming’s right hand rule: According to the Fleming’s right hand rule, “if we
stretch the right-hand thumb, fore finger and middle finger perpendicular to one
another and point the thumb in the direction of the motion of the conductor, the
fore finger in the direction of the magnetic field, then the middle finger indicates
the direction of the induced current from electromagnetic induction.”

8. Electric generator: An electric generator is a device that converts mechanical
energy into electric energy.

9. Alternating current (AC.): The current whose direction changes periodically and
whose magnitude also varies continuously is called an alternating current.

10. Direct current (DC): If the electrons always flow in the same direction and the
direction of a current remains unchanged in a conductor, it is called direct current

11. Motor effect: When a freely suspended current carrying conductor is kept in a
magnetic field, then it comes in motion. This effect is called motor effect.

12. Electric motor: An electric motor is the device which converts electric energy into
mechanical energy.

13. Mutual induction: This mechanism of obtaining an induced emf in a secondary
coil by passing an alternating current in the neighboring coil (primary coil) is
called mutual induction.

14. Transformer: A transformer is a device for increasing or decreasing AC voltage.

15. Step-up transformer: A transformer which converts the applied low voltage AC
to high voltage AC is called a step-up transformer.

16. Step-down transformer: A transformer which converts the applied high voltage
AC to low voltage AC is called a step-down transformer.

17. Resistor: A resistor is a two-terminal electrical component that adds electrical
resistance to the limit or regulates the flow of electrical current.

18. Series connection: When two or more resistors in a circuit are connected from
end to end in order to get the same current flowing through each of them in
succession, then the resistors are said to be connected in a series.

19. Parallel connection: When two or more resistors in a circuit are connected
between two common points in order to get different currents, then the resistors
are said to be connected in a parallel.

20. Diode: A diode is a two terminal electric component that allows electric current
to flow in only one direction.

21. Semiconductor: A semiconductor is a solid substance that has conductivity
between that of an insulator and most metals.

22. Intrinsic semiconductor: A semiconductor in its pure form is known as an
intrinsic semiconductor.

23. Doping: The process of adding impurity to pure semiconductors is called doping.

24. Extrinsic semiconductor: The doped semiconductors are called extrinsic
semiconductors.

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25. N-type semiconductor: An extrinsic semiconductor which is created by doping
an intrinsic semiconductor with a pentavalent impurity like phosphorous (P), in
which electrons are the majority carriers and holes the minority carriers, is called
a N-type semiconductor.

26. P-type semiconductor: An extrinsic semiconductor, which is created by doping
an intrinsic semiconductor with trivalent impurity like boron (B), in which holes
are the majority carriers and electrons the minority carriers, is called a P-type
semiconductor.

27. P-N junction diode: A P-N junction diode is a two terminal device, whose one
side is made a P-type semiconductor and another side an N-type semiconductor
by doping on an intrinsic semiconductor.

28. Triode: Triode is a vacuum tube device with three elements - cathode, grid and
anode.

29. Transistor: A transistor is a three terminal semiconductor electronic device with
a P-type or N-type semiconductor region sandwiched in such a way that there is
formation of two P-N junctions.

Introduction

In today’s world, electricity is an important source of energy. It is a convenient form of energy
to convert into other forms of energy like heat, light, etc. Electricity is used in our homes
for lighting tubes and bulbs, operating fans, heating purposes, etc. All electric devices have
electric circuit at their heart. The electric loads are connected in series or parallel in an electric
circuit. Such loads convert electrical energy into other forms of energy. The way to connect
resistors in a series or parallel in an electric circuit and their features are described in this unit.

The magnetic property of a magnet is known as magnetism. Different types of substances
show variation in their behavior in an external magnetic field. Magnetic behavior of a material
depends upon its composition. In this unit, you will learn about magnetic property in matter.
Until the early 19th century, people treated electricity and magnetism independently. But
electricity and magnetism are interrelated. When a charged particle is in motion then there
is also a magnetic field around the particle. In 1820, Hans Christian Oersted discovered the
magnetic effect of current. Similarly, in 1831, Michael Faraday discovered electromagnetic
induction and the production of electricity from a magnetic field. This is a revolutionary
invention in the field of electricity generation. Transformers are used to increase the emf of
the electricity generated for its transmission. This unit also describes about electromagnetic
induction and principle of the generator, electric motor and transformer.

Magnetic property in matter

Spinning electrons in the atoms of matter have magnetic property. When a material is placed
in an external magnetic field, the magnetic forces of the material’s electrons will be affected.
Different types of materials can react quite differently to the presence of an external magnetic
field. This reaction depends on a number of factors, such as the atomic and molecular structure
of the material, and the net magnetic field associated with the atoms.

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Molecular theory of magnetism

The molecular theory of magnetism was given by Wilhelm Eduard Weber and modified by
James Alfred Ewing. According to the molecular theory of magnetism:

a. Each molecule of a magnetic substance is an independent magnet.

b. In a magnetized substance, all molecules are arranged in the form of an open chain so
that all north poles point towards one direction and all south poles towards the other.
In this way, one end becomes the north pole and the other end the south pole.

c. In an unmagnetized substance, the molecular magnets are in the form of closed chains
and thereby cancel the magnetic fields of one another.

Magnetization

The magnetic behavior of a magnet is characterized by the alignment of the atoms inside a
substance. When an iron bar is brought under the application of a strong external magnetic
field, then the atoms inside the bar experience a turning effect, and they align in the direction
of the magnetic field applied. Iron gets strongly magnetized in the direction of the magnetic
field.

The process of making a substance temporarily or permanently magnetic by inserting in
a magnetic field is called magnetization. The response of a material to the application of a
magnetic field depends on the level of magnetization that the material undergoes.

Magnetic permeability

Magnetic permeability measures the degree of penetration of a magnetic field through the
substance kept in an external magnetic field. In other words, magnetic permeability measures
the capacity of the substance to accept magnetization.

Magnetic susceptibility

The measure of the extent to which a substance becomes magnetized when it is placed in an
external magnetic field is called magnetic susceptibility. It measures how easily or strongly a
material is magnetized by the applied external magnetic field.

Classification of magnetic substances

According to the behavior of the substance in the magnetic field, Pierre Curie and Michael
Faraday divided substances into three categories. They are diamagnetic substances,
paramagnetic substances and ferromagnetic substances.

Diamagnetic substances

Michael Faraday discovered that a specimen of bismuth was repelled by a strong magnet. He
coined the word diamagnetism for such properties of substances like bismuth. Substances
which when placed in a strong magnetic field acquire feeble magnetization opposite to the
direction of the magnetic field are called diamagnetic substance. For example, bismuth (Bi),
phosphorous (P), antimony (Sb), copper (Cu), zinc (Zn), silver (Ag), gold (Au), diamond,
graphite, water (H2O), common salt (NaCl), , hydrogen (H2), nitrogen (N2), etc.

Memory Plus

Two of the strongest diamagnetic materials are graphite and bismuth.

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Properties of diamagnetic substances

1. Diamagnetic substance are somewhat repelled when brought near the ends of a

powerful magnet. Suspended
2. When a rod of diamagnetic material is suspended

freely between two magnetic poles, it slowly turns N S
to set itself perpendicular to the direction of the
magnetic lines of force. The poles produced on S
the two sides of the rod are similar to the nearer
magnetic pole. diamagnetic substance
fig: diamagnetic substance in a magnetic field

3. In a non-uniform magnetic field, the diamagnetic substance is repelled towards the
weaker part of the field. That is, a diamagnetic substance moves from a stronger part to
a weaker part of the field.

For example, when a diamagnetic liquid taken in a watch glass

or glass crucible is placed over two nearby magnetic poles, then

the liquid accumulates on the sides causing a depression at the
center where the field is strongest as shown in the given figure.

When the distance between the pole pieces is increased, the fig:diamagnet liquid in a
effect is reversed. That is the liquid rises in the middle, because magnetic field
now the field is stronger near the poles.

4. When a diamagnetic material is placed within a magnetic field, the lines of force tend

to go away from the material.

Paramagnetic substances:

Substances which when placed in a strong magnetic field acquire feeble magnetization in
the direction of the applied magnetic field are called paramagnetic substances. For example,
aluminium (Al), sodium (Na), platinum (Pt), chromium (Cr), manganese (Mn), cupric chloride
(CuCl2), oxygen (O2), etc.

Properties of paramagnetic substances

1. These substances are attracted feebly towards the magnet when brought close to the
ends of a powerful magnet.

Know the Reason

Diamagnetic substances are repelled when brought near the ends of a powerful
magnet, why?

The electrons in a diamagnetic material rearrange their orbits slightly when kept
in a strong magnetic field. It induces a weak magnetic moment in the direction
opposite to the applied field. So diamagnetic substances are repelled when brought
near the ends of a powerful magnet.

Paramagnetic substances are attracted towards the magnet, why?

When paramagnetic substances are subjected to a magnetic field, these are feebly
magnetized in the direction of the field. So paramagnetic substances are attracted
towards the magnet.

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