CBSE Class 10 Science Sources Of Energy Notes

A black body is a good absorber of heat radiation. Thus, the purpose of the blackened surface is to absorb the maximum amount of heat and to minimize heat loss due to reflection.

Glass cover plate

The purpose of glass cover plate is to trap heat radiation. It possesses the property of selective transmission of heat radiation. It allows infrared radiation of short wavelengths to pass through it, but reflects infrared radiation of long wavelengths. So, solar radiation, which passes through the glass, heats up the surface inside the solar cooker. These warm surfaces radiate heat. Since these surfaces are at much lower temperature than the sun, this radiation mainly consists of long wavelength infrared radiation. The glass walls reflect these and thus heat is trapped inside the solar cooker.

Thus, the glass cover placed on the box, produces a kind of greenhouse effect.

Reflector {Mirror} Its main purpose is to increase the effective area for the collection of solar energy.

If a plane mirror is replaced by a concave mirror, then solar energy can be concentrated from over a large area into a small area

2 ) SOLARCELL

(a) A solar cell is a device which converts solar energy directly into electricity.A typical solar cell consists of a 2 cm square piece. A 2cm x 2 cm single solar cell can produce about 0.7 watt of electricity

(b) Silicon and selenium are two materials used for fabricating solar cells. Silicon is best suited for making solar cells as its efficiency is higher than that of selenium. .

(c) Silver wire is used to connect solar cells in a solar panel, because silver is the best conducting material. If some other metal is used then a substantial fraction of electricity generated by solar cells will get used up in overcoming the resistance of the connectors as solar cells already have low efficiency.

(d) Advantages

(i) The principal advantages associated with solar cells are that they have no moving parts, require little maintenance and work quite satisfactorily without the use of any focusing device.

(ii) They can be set up in remote and inaccessible hamlets or very sparsely inhabited areas in which laying of a power transmission line may be expensive and not commercially viable.

Disadvantages

(i) Availability of the special grade silicon for making solar cells is limited. (ii) The entire process of manufacture is still very expensive. Silver used for interconnection of the cells in the panel further adds to the cost.

The two uses of solar cells are:

(i) In spite of the high cost and low efficiency, solar cells are used for many scientific and technological applications.

(ii) It is used in used in artificial satellites and space probes

(iii) Radio or wireless transmission systems or TV relay stations in remote locations have solar cell panels.

3) NUCLEAREN ERGY

A) Nuclear fission: In this process the nucleus of heavy atom (such as uranium, plutonium) when bombarded with low energy neutrons split into lighter nuclei with the liberation of large amount of energy.

This energy can be controlled & converted into electricity in the nuclear power plant

B) Nuclear fusion: In this process the two light weight nuclei combine to form a stable heavier nucleus with the liberation of large amount of energy.

The conditions for the occurrence of a nuclear fusion reaction are:

(a) Temperature of fusing nuclei should be raised to 107 K.

(b) Fusing nuclei be accelerated to high speeds using particle accelerators.

Advantages of nuclear energy system over fossil fuel energy system.

(i) On equal mass basis, nuclear energy system produces more energy than fossil fuels.

(ii) Nuclear energy systems consume very little fuel. Once loaded, a nuclear reactor operates for years together. Fossil fuel systems need a few hundred tons of coal everyday.

Disadvantages of nuclear energy system over fossil fuel energy system

(i) Pollution caused by radiation leak from a nuclear reactor is much more serious than the pollution caused by burning fossil fuels.

(ii) There is a very serious problem regarding storage and disposal of nuclear waste. There is no serious problem for the disposal of ash produced in fossil fuel energy systems

4) BIOGAS

Slurry of cow dung and water is made in the mixing tank from where it is fed into the digester. The digester is a sealed chamber in which there is no oxygen. Anaerobic microorganisms that do not require oxygen decompose or breakdown complex compounds of the cow dung slurry. It takes few days for the decomposition process to be complete and generate gases like methane, carbon dioxide, hydrogen and hydrogen sulphide. The biogas is stored in the gas tank above the digester from which they are drawn through pipes for use.

The slurry left behind is removed periodically and used as excellent manure, rich in nitrogen and phosphorus.

Bio-gas is an excellent fuel as it contains up to 75% methane. It burns without smoke, leaves no residue like ash in wood, charcoal and coal burning. Its heating capacity is high. Bio-gas is also used for lighting.

5) Wind Energy

Cause

Unequal heating of the landmass and water bodies by solar radiation generates air movement and causes winds to blow. This kinetic energy of the wind can be used to do work.

Uses of windmills in the past

to do mechanical work. For example,

1. In a water-lifting pump, the rotatory motion of windmill is utilized to lift water from a well

2. In awind mill, the rotatory motion of windmill is utilized to grind grains

Uses of windmills in modern times

Wind energy is used to generate electricity. A windmill essentially consists of a structure similar to a large electric fan that is erected at some height on a rigid support. To generate electricity, the rotatory motion of the windmill is used to turn the turbine of the electric generator 

Wind energy farm

The output of a single windmill is quite small and cannot be used for commercial purposes. Therefore, a number of windmills are erected over a large area, which is known as wind energy farm. The energy output of each windmill in a farm is coupled together to get electricity on a commercial scale.

It is estimated that nearly 45,000 MW of electrical power can be generated if India’s wind potential is fully exploited. The largest wind energy farm has been established near Kanyakumari in Tamil Nadu and it generates 380 MW of electricity.

Advantages

Wind energy is

1. an environment-friendly

2. efficient source of renewable energy

3. Requires no recurring expenses for the production of electricity.

Limitations in harnessing wind energy.

1. Wind energy farms can be established only at those places wherewind blows for the greater part of a year.

2. The wind speed should also be higher than 15 km/h to maintain the required speed of the turbine.

3. there should be some back-up facilities (like storage cells) to take care of the energy needs during a period when there is no wind.

4. Establishment of wind energy farms requires large area of land. For a1 MW generator, the farm needs about 2 hectares of land.

5. The initial cost of establishment of the farm is quite high.

6. Moreover, since the tower and blades are exposed to the vagaries of nature like rain, Sun, storm and cyclone, they need a high level of maintenance

6) Geothermal Energy

Due to geological changes, molten rocks formed in the deeper hot regions of earth’s crust are pushed upward and trapped in certain regions called ‘hot spots’. When underground water comes in contact with the hot spot, steam is generated. Sometimes hot water from that region finds outlets at the surface. Such outlets are known as hot springs. The steam trapped in rocks is routed through a pipe to a turbine and used to generate electricity. Advantages

1. The cost of production would not be much.

2. available 24 hrs. a day

Only source of energy which is not derived from sun

Limitations

There are very few commercially viable sites where such energy can be exploited. There are number of power plants based on geothermal energy operational in New Zealand and United States of America.

ENERGY FROMTHESEA :

7) TIDAL ENERGY

Due to the gravitational pull of mainly the moon on the spinning earth, the level of water in the sea rises and falls. This phenomenon is called high and low tides and the difference in sea-levels gives us tidal energy.

WORKING:-Tidal energy is harnessed by constructing a dam across a narrow opening to the sea. A turbine fixed at the opening of the dam converts tidal energy to electricity. Limitations:-

1. There are very few sites suitable for harnessing tidal energy.

2. The rise and fall of water during tides is not very large. so large scale generation of electricity is not possible
Because of the above two limitations the tidal energy is not likely to be a potential source of energy.

SOURCES OF ENERGY

• Energy comes in different forms and one form can be converted to another. For example if we drop a plate from a height, the potential energy of the plate is converted mostly to sound energy when it hits the ground.
• If we light a candle, the process is highly exothermic so that the chemical energy of the wax is converted into heat energy and light energy on burning.

A Good Source of Energy would be one

• which has high calorific value
• be easily accessible
• be easy to store and transport
• be economical
  
   
• Amont the sources of energy, some of them get exhausted (Non-Renewable) While some of them do not get exhausted, therefore called as Renewable source of energy. Examples

1. Non Renewable source of energy-> Coal, Petroleum, Natural Gas

2. Renewable Source of energy®->Air, Water, Solar radiation, Geothermal Energy, ocean waves etc.

CONVENTIONAL SOURCE OF ENERGY

1. Fossil Fuels : Fuels developed from the fossils. Eg. Coal & Petroleum.

•  Formed over million of years ago have only limited reserves
• These are non-renewable source of energy
• India has about 6% share in the world reserved coal, that may last 250 years more at the present rate of consumption.

Disadvantages of Burning Fossil Fuels

• released oxides of Carbon, Nitrogen, Sulphur (acidic in nature) cause Air pollution & Acid rain & green-house effect.

It consists of a rectangular box which is made up of material such as plastic or wood.
Box is covered with black sheet and its inner walls are painted black to increase heat absorption.
Solar cookers are covered with glass plate and have mirros to focus the rays of the sun and achieve a higher temperature.

KEY CONCEPTS & GIST OF THE LESSON

• Characteristics of a good fuel:

(iv) High calorific value

(v) Less smoke

(vi) Less residue after burning

(vii) Easy availability

(viii) Inexpensive

(ix) Easy to store and transport

• Fossil fuels: were formed millions of years ago, when plants and animal remains got buried under the earth and were subjected to high temperature and pressure conditions. E.g.: Coal,

Petroleum, etc. These fossil fuels are non renewable sources of energy and cause environmental problems due to pollution.

• Thermal power plants: 

(i) Use coal, petroleum and natural gas to produce thermal electricity.

(ii) Electricity transmission is very efficient.

(iii) The steam produced by burning the fossil fuels runs the turbine to produce electricity

• Hydro power plant:

(Refer to figure 14.3, page no. 246 of N.C.E.R.T Text book)

(i) It is the most conventional renewable energy source obtained from water falling from a great height.

(ii) It is clean & non polluting source of energy.

(iii) Dams are constructed to collect water flowing in high altitude rivers. The stored water has a lot of potential energy.

(iv) When water is allowed to fall from a height, potential energy changes to kinetic energy, which rotates the turbines to produce electricity.

• Disadvantages of Hydro power plant:

(i) Highly expensive to construct.

(ii) Dams cannot be constructed on all river sites.

(iii) Large areas o human habitation and agricultural fields get submerged.

(iv) People face social and environmental problems.

• Non conventional sources:

(1) Bio mass: It is the source of the conventionally used fuels that are used in our country. E.g.: Cow dung cakes, fire-wood, coal, charcoal

o Bio gas: It is a mixture of gases produced during decomposition of bio mass in the absence of Oxygen. (Anaerobic Respiration). Methane is the major component of bio gas.

o Bio gas plants: Animal dung, sewage, crop residues, vegetable wastes, poultry droppings, etc. are used to produce Bio gas in Bio gas plants.

o (Refer to figure 14.4, page no. 247 of N.C.E.R.T Text book)

(2) Wind energy: 

o It can be converted into mechanical and electrical energy.

o Kinetic energy of the wind is used in running of wind mills, which are used to lift water, grind grains, etc.

o Wind mill-(Refer to figure 14.5, page no. 247 of N.C.E.R.T Text book)

o Advantages: (i) Eco friendly (ii) Renewable

o Disadvantages: (i) Wind speed not uniform always.

(ii) Needs a large area to erect series of wind mills.

(iii) Big amount of investment is needed.

(iv) Out put is less as compared to investment

(3) Solar energy:

o Solar radiations can be converted electricity through solar cells (photovoltaic cells).

o Photovoltaic cells convert solar radiations directly into electricity through silicon solar cells.

o Solar cells arrange on a large flat sheets form a solar panel.

o Solar cookers are painted black from outside and a large glass plate to trap solar radiations by green house effect.

o (Refer to figure 14.6, page no. 249 of N.C.E.R.T Text book)

o Advantages of Solar cookers:

(i) Eco friendly

(ii) Renewable

(iii) Used in rural areas.

(iv) Retains all the nutrients in food due to slow cooking.

o Disadvantages of solar cooker:

(i) Silicon cells are expensive.

(ii) Solar radiations are not uniform over earth‘s surface.

(iii) Cannot be used at night or on cloudy days.

(iv) Cannot be used to make chapattis for frying as these require a temperature of 1400C or more.

(Maximum temperature of 1000C only can be achieved in a solar cooker)

o Other solar devices- Solar water heater, Solar furnace

(4) Geo thermal energy:

(i) Energy harnessed from the heat of the sun is called Geo thermal energy.

(ii) Magma is formed when this heat melts the rocks. The molten rocks and hot gases are called magma

(iii) The magma gets collected at some depths below the earth‘s surfaces. These places are called ‗Hot spots‖

(iv) When underground water comes in contact these hot spots, it changes into steam, which can be used to generate electricity.

o Advantages of Geo thermal energy:

(i) Renewable

(ii) Inexpensive

o Disadvantages of Geo thermal energy:

(i) Only few sites available for harnessing energy.

(ii) Expensive

(5) Nuclear energy:

(i) Energy released when some changes take place in the nucleus of the atom of a substance, is called Nuclear energy.

(ii) It is used for heat generation, fuel for marine vessels.

o Advantages of Nuclear energy:
(i) Alternative source of energy due to depletion of fossil fuels.
(ii) From a small amount of fuel, a large amount of energy is released.

o Disadvantages of Nuclear energy:
(i) Risk of nuclear waste leakage
(ii) High cost of setting up of nuclear plant
(iii) Pollution of environment.

(6) Energy from the sea-
(A) Tidal energy: Locations in India – Gulf of Kutch, Gujrat & W. Bengal
(i) Depends upon harnessing the rise and fall of sea level due to tidal action.
(ii) Dams are constructed across a narrow part of sea and turbine converts tidal energy into electrical energy.

Disadvantages: Uniform tidal action is not seen

(B) Wave energy:
(i) Kinetic energy of the waves of sea are used to rotate turbines..
(ii) These turbines generate electrical energy
 

SOURCES OF ENERGY

ENERGY: - Whenever a body is capable of doing work, the body is said to possess energy. Thus energy is defined as the ability of a body to do work and the amount of energy possessed by a body is equal to the amount of work it can do when its energy is released.

Energy comes in different forms and one form can be converted to another. For example, if we drop a plate from a height, the potential energy of the plate is converted mostly to sound energy when it hits the ground.

Units of energy: - On S.I. system, energy is measured in the units of joules or in calories, and on C.G.S. system in ergs. However, the commercial unit of energy is kilowatt-hour. The energy is said to be one kilowatt-hour, when a body consumes one kilowatt of energy in one hour.

Sources of energy : a source of energy is that which is capable of providing enough useful energy at a steady rate over a long period of time.

A good source of energy should be :

i) Safe and convenient to use, e.g., nuclear energy can be used only by highly trained engineers with the help of nuclear power plants. It cannot be used for our household purposed.

ii) Easy to transport, e.g., coal, petrol, diesel, LPG etc. Have to be transported from the places of their production to the consumers.

iii) Easy to store, e.g., huge storage tanks are required to store petrol, diesel, LPG etc.

Characteristics of an ideal or a good fuel:

♦ It should have a high calorific or a heat value, so that it can produce maximum energy by low fuel consumption.

♦ It should have a proper ignition temperature, so that it can burn easily.

♦ It should not produce harmful gases during combustion.

♦ It should be cheap in cost and easily available in plenty for everyone.

♦ It should be easily and convenient to handl Øe, store and transport from one place to another.

♦ It should not be valuable to any other purpose than as a fuel.

♦ It should burn smoothly and should not leave much residue after its combustion.

INTEXT QUESTIONS 

1. What is a good source of energy?
Ans. A good source of energy fulfils the following criteria:

♦ It produces a lot of heat per unit mass.

♦ It does a huge amount of work per unit mass.

♦ It is easily accessible.

♦ It is easy to store and transport.

♦ It is economical.

♦ It produces less amount of smoke.

2. What is a good fuel?

Ans. A good fuel produces a huge amount of heat on burning, does not produce a lot of smoke, and is easily available.

3. If you could use any source of energy for heating your food, which one would you use and why?

Ans. Natural gas can be used for heating and cooking food because it is a clean source of energy. It does not produce huge amount of smoke on burning. Although it is highly inflammable, it is easy to use, transport, and it produces a huge amount of heat on burning.

CLASSIFICATION OF SOURCES ENERGY :

The sources of energy can be classified as follows : (i) Renewable

(ii) Non-Renewable.

1. Renewable sources of energy :-

Renewable sources of energy are those which are inexhaustible, i.e., which can be replaced as we use them and can be used to produce energy again and again. These are available in an unlimited amount in nature and develop within a relatively short period of time.

Examples of Renewable Sources of Energy.

(i) Solar energy, (ii) Wind Energy, (iii) water energy (hydro-energy), (iv) geothermal energy,

(v) ocean energy, (vi) biomass energy (firewood, animal dung and biodegradable waste from cities and crop residues constitute biomass).

Advantages of Renewable Sources of Energy :

(i) These sources will last as long as the Earth receives light from the sun. (ii) These sources are freely available in nature.

(iii) These sources do not cause any pollution.

2. Non-Renewable Sources of Energy :

Non-renewable sources of energy are those which are exhaustible and cannot be replaced once

they have been used. These sources have been accumulated in nature over a very long period of million of years.

Examples of Non-renewable sources of Energy :

(i) Coal (ii) Oil and (iii) Natural gas. All these fuels are called fossil fuels.

Disadvantages of Non-renewable sources of Energy :

(i) Due to their extensive use, these sources are fast depleting.

(ii) It is difficult to discover and exploit new deposits of these sources.

(iii) These sources are a major cause of environmental pollution.

CONVENTIONAL AND NON-CONVENTIONAL SOURCES OF ENERGY :

Sources of energy are also classified as :

(i) Conventional sources of energy

(ii) Nonconventional sources of energy.

CONVENTIONAL SOURCES OF ENERGY

FOSSIL FUELS

Fossil fuels are the remains of prehistoric plans and animals which got buried deep inside the early millions of years ago due to some natural processes.

The energy of fossil fuels is in fact, that solar energy which was trapped by natural processes a very long time ago. Coal, petroleum and natural gas are fossil fuels.

Formation of Fossil Fuels: During its formation, an entire organism or its parts often get buried in sand or mud. These, then decay and disintegrate leaving no signs of their existence. Infact, the harder parts of organisms after their death, settle down and are covered by sediments and subjected to extreme pressure and temperature of the earth converts them into fossil fuels, the process being referred to as fossilization.

Disadvantages of Fossil Fuels:

1. The fossil fuels are non-renewable sources of energy and once used cannot be renewed.

2. Burning of fossil fuels causes air pollution.

3. The fossil fuels reserves in the earth are limited and may get exhausted soon.

THERMAL POWER PLANT

The term thermal power plant is used since fuel is burnt to produce heat energy which is converted into electrical energy.

Large amount of fossil fuels are burnt every day in power stations to heat up water to produce steam which further runs the turbine to generate electricity. The transmission of electricity is more efficient than transporting coal or petroleum over the same distance. Therefore, many thermal power plants are set up near coal or oil fields.

HYDRO POWER PLANTS

Hydro power plants convert the potential energy of falling water into electricity. Since there are very few water-falls which could be used as a source of potential energy, hydro power plants are associated with dams.

In order to produce hydel electricity, high-rise dams are constructed on the river to obstruct the flow of water and thereby collect water in larger reservoirs. The water level rises and in this process the kinetic energy of flowing water gets transformed into potential energy. The water from the high level in the dam is carried through pipes, to the turbine, at the bottom of the dam (see the above Fig.). Since the water in the reservoir would be refilled each time it rains (hydro power is a renewable source of energy) we would not have to worry about hydroelectricity sources getting used up the way fossil fuels would get finished one day.

Advantages of generating hydro electricity :- The main advantages of using water energy for the generation of hydro electricity are listed as under :

1. It is readily and abundantly available everywhere free of cost.

2. It is eco-friendly and does not produce any kind of environmental pollution.

3. It is a renewable source as water itself is a renewable and inexhaustible resource.

4. It is a cheap source of energy, as it does not involve any costly investment.

BIO-MASS

Biomass is defined as living matter or its residue and is a renewable sources of energy.

The biomass includes (i) all the new plant growth (ii) agricultural and forest residues (like biogases, bark, sae dust, wood shavings, roots, animal droppings etc.) (iii) Carbonaceous wastes (like sewage, garbage, night-soil, etc.) (iv) Biodegradable organic affluent from industries.

Biogas:- Biogas is a mixture of gases produced by anaerobic degradation of biomass in the presence of water but in the absence of oxygen. It is a renewable source of energy on account of its production from vastly and continuously available organic wastes.

Advantages of Biogas:-

i) A biogas plant, being quite simple, can easily be built in rural areas. A small plant using dung from 3 to 4 heads of cattle is capable of supplying biogas for 6 hours daily for cooking purposes.

ii) Biogas is a clean fuel that burns without smoke and leaves no ash.

iii) The main constituent of biogas, i.e., ethane has a higher calorific value (55kj/g) that of petrol (50kj/g).

iv) The spent slurry, being rich in nitrogen and phosphorus, is good manure.

v) By using biogas, firewood is saved and deforestation is reduce

Composition of Biogas:- Biogas is mainly composed methane (up to 75%), CO2 (25%) and traces of other gases such as nitrogen and hydrogen. Whereas methane is a high value calorific fuel, carbon dioxide is an inert gas.

Biogas is prepared in biogas plants which are of two types: (i) Fixed Dome Type (ii) Floating Gas Holder Type.

Fixed Dome Type Biogas Plant:-

The main parts of fixed of dome type of biogas plants are:

1. Digester. It is well shaped underground tank made of bricks. Its roof is some-shaped which acts as a storage tank for biogas.

2. Mixing tank. It is constructed on the ground level where cattle dung and water are mixed.

3. Inlet tank. It is constructed underground below the mixing tank.

4. Overflow tank. It is constructed slightly below the level of mixing tank.

5. Outlet tank. It is constructed below the overflow tank.

Working of Biogas Plant:-

Cattle dung and water are mixed in equal proportion in the mixing tank to form slurry. This slurry is fed into the digester tank through inlet tank when the digester tank is filled about 2/3rd of its capacity, the dome is left free for collection of biogas. The slurry undergoes anaerobic fermentation and biogas is produced after 50 to 60 days. As biogas is collected in the dome it exerts pressure due to which spent slurry go to the overflow tank through outlet tank and fresh slurry is fed into the digester and continuous supply of biogas is obtained spent slurry is used as manure.

WIND ENERGY

Wind Energy: -When large masses of air move from one place to another it is referred to as wind. During this process kinetic energy gets associated with it which is referred to as wind energy.

Principle of utilisation of wind energy: -

Wind energy is efficiently converted into electrical energy with the aid of a windmill. A windmill is a large fan having big blades, which rotate by the force exerted by moving wind on them. These blades remain continuously rotating as long as wind is blowing and can be used to drive a large number of machines like water pumps, flour mills etc. But these days a windmill is used to generate electric current which is used for various purposes and therefore wind power stations are established all over the world which convert wind energy directly into electrical energy.

Uses of wind energy: -

1. The important uses of wind energy are;

2. It is used to drive windmills, water lifting pumps and flour mills etc.

3. It is used to propel sale boats.

4. It is used to fly engine less aeroplanes or gliders in the air.

5. It is used to generate electricity used for various purposes like lightening, heating etc.

Advantages of generating wind energy: -

1. The chief advantages of using wind energy are;

2. It is readily and abundantly available at every place of the earth free of cost.

3. It is eco-friendly and does not produce any kind of environmental pollution.

4. It is a renewable source as air itself is a renewable and inexhaustible resource.

5. It is a cheap source of energy, as it does not involve any costly investment.

INTEXT QUESTIONS PAGE NO. 248

Question. What are the disadvantages of fossil fuels?
Answer: The disadvantages of fossil fuels are as follows:
(a) Burning of coal and petroleum produces a lot of pollutants causing air pollution.
(b) Fossil fuels release oxides of carbon, nitrogen, sulphur, etc. that cause acid rain, which affects the soil fertility and potable water.
(c) Burning of fossil fuels produce gases such as carbon dioxide that causes global warming.

Question. Why are we looking at alternate sources of energy?
Answer: Fossil fuels, which have been traditionally used by human beings as an energy sources, are non-renewable sources of energy. These sources of energy are limited and cannot replenish on their own. They are being consumed at a large rate. If this rate of consumption continues, then the fossil fuels would be exhausted from the Earth. Therefore, we have to conserve the energy sources. Hence, we should look for alternate sources of energy.
How has the traditional use of wind and water energy been modified for our convenience?
Traditionally, waterfalls were used as a source of potential energy which was converted to electricity with the help of turbines. Since waterfalls are few in number, water dams have been constructed in large numbers. Nowadays, hydro-dams are used in order to harness potential energy of stored water. In water dams, water falls from a height on the turbine, which produces electricity.
Earlier, the windmills were used to harness wind energy to do mechanical work such as lifting/drawing water from a well. Today, windmills are used to generate electricity. In windmills, the kinetic energy of wind is harnessed and converted into electricity. The rotatory motion of the blades turns the turbine of the electric generator to generate electricity.

ALTERNATIVE OR NON-CONVENTIONAL SOURCES OF ENERGY

Non-conventional sources of energy : Non-conventional source of energy are those which are not used as extensively as the conventional ones and meet our energy requirement only on a limited scale. Solar energy, ocean energy (tidal energy, wave energy, ocean thermal energy, OTE), Geothermal energy and nuclear energy belong to this category. These sources of energy which have been tapped with the aid of advances in technology to meet our growing energy needs are also called alternative sources of energy.

SOLAR ENERGY
The energy produced by the sun in the form of heat and light energy is called as solar energy.
Principles of utilisation of Solar Energy: - Solar energy is utilised by the involvement of two main principles:
(i) In the appliances requiring a moderate temperature, the incident sun rays are reflected by a plain mirror on a black container which absorbs the solar energy and gets heated .
(ii) In the appliances requiring a high temperature, the incident sun rays are reflected and concentrated by using a large concave reflector which focuses all the sun rays at a single point called focus and any object kept at the focus gets strongly heated.

Harnessing or utilisation of Solar energy:- The sun is the ultimate source of energy having a remarkable capacity to produce energy in the form of heat and light. The energy produced by the sun in one day is about 50,000 times more than the energy consumed in the whole world in one year. But solar energy has certain limitations, which does not facilitate its large-scale utilisation. However, solar energy can be put to use in two differ ways Viz.

1) Direct utilization: - Directly the solar energy can be used either by collecting it as heat energy or by converting it into electricity.

2) Indirect utilization: - Indirectly the solar energy can be utilized by converting it into chemical energy like biomass or by utilising the energy obtained from wind, sea waves, tides etc.

Solar Heating Devices: - A device that gets heated by absorbing solar energy radiated by the sun in the form of heat and light energy is called a solar heating device. For eg. Solar cooker, solar water heater, solar furnace and solar cells are solar heating devices.

Solar cooker: - A solar cooker is a device which utilises solar energy for cooking food material. It consists of an insulated wooden box (B) painted with black from inner side. The lid of the box is provided with a plane mirror reflector (R) and a glass sheet (G). The food to be cooked is placed in a metal container (C) painted with black from outer side and kept in the box .The container is covered with the glass sheet. The box is then kept in direct sunlight and its reflector is adjusted in such away that a strong beam of sun light falls over it. 

Working: - When the solar cooker is kept in direct sunlight, the reflector (R) reflects both visible and infrared rays of the sunlight on to the top of the box in the form of a strong beam of light. The black surface of the box and the vessel absorbs it. When the inner black surface becomes quite hot, it also starts radiating heat energy in the form of infrared rays, but the upper glass sheet (G) does not allow these rays to pass through it and go outside the box. As a result, these infrared rays get absorbed in the box, which increases its internal temperature up to about 1000C. This high temperature cooks the food material kept in the metallic container inside the box.

Limitations of solar Cooker:-
1. It can not be used during night.
2. On a cloudy day, it can not be used.
3. The direction of the reflector has to be adjusted according to the position of the sun.
4. It can not be used for making ‘chappatis’.
5. It can not be used for frying.

Solar cell: - A solar cell is a device which converts solar energy (light energy) directly into electricity. It is made of semi-conducting material like silicon, germanium, selenium or gallium. A modern solar cell is made from wafers of semi conducting materials containing impurities in such away that a potential difference gets generated when light falls on them. A 4 cm 2 solar cells produces a potential difference of about 0.4--0.5volts and generate about 60 milli-amperes of current. To generate a large amount of current a number of solar cells are arranged together in a definite pattern in a solar panel. The energy (electric current) generated in a solar panel is stored in a battery connected to it and can be used for various purposes.

Uses of a solar cell: - The solar cells are used effectively in various fields, but some of its important uses are:
Solar cells are used for production of electricity for lighting, houses, streets etc. Solar cells are used for production of electricity to run electronic appliances like televisions, radios, watches, calculators, toys, toy games etc. Solar cells are used to develop electricity for offshore oil drilling platforms etc. Solar cells are used to generate electricity in artificial satellites, rockets, and space vehicles etc.

ENERGY FROM THE SEA

Energy from Oceans : - The oceans acquire almost 71% of the surface of the earth and the enormous amount of water present in them not only act as a big collector of solar heat energy, but also store large amount of it due to its high specific heat. Thus ocean water can be used as a renewable resource of energy. The main forms of ocean energy are described as under;

i) Ocean Thermal energy : - The energy available due to the temperature difference between the deeper levels and surface of an ocean is called as ocean thermal energy.

ii) Ocean Tidal energy : - The rise of ocean water due to attraction of the moon is referred to as high tide and its fall as low tide. The enormous movement of water due to high and low tide provide a large amount of energy known as ocean tidal energy. This tidal energy can be utilised by constructing a tidal barrage or dam.

iii) Sea wave energy: - The energy obtained from the high speed sea waves is referred to as sea wave energy. Infact these high speed sea waves have a lot of kinetic energy associated with them, which can used to drive dynamos which convert kinetic energy into electrical energy.

iv) Energy from Nuclear deuterium of oceans: - The ocean water contains unlimited amount of heavy hydrogen isotope called deuterium which is isotope hydrogen having one proton and one neutron in its nucleus. Scientists are working hard to produce energy by carrying by out controlled nuclear fission of deuterium isotope. The process is still in its experimental stage.

v) Energy from Salinity gradient in seas: - The difference in the concentration of salts in the water of the two or more seas is called as salinity gradient. This salinity gradient is now a day used to obtain energy with the involvement of suitable techniques.

vi) Energy from sea vegetation or biomass: - Sea vegetation or biomass is another direct source of energy because the enormous amount of sea weeds present in the sea water provides an endless supply of methane fuel.

Limitations of Energy from Oceans :-
The limitations of energy that can be obtained from the oceans are:
i) Tidal Energy for which very few suitable sites are available for construction of dams and the power generation is intermittent and not very large.
ii) Wave Energy where power output is variable and the presently available technologies are very expensive.
iii) Ocean Thermal Energy where the conversion efficiency is low (3% - 4%) and a lot of capital investment is required.

GEOTHERMAL ENERGY

Geothermal energy is the heat of the earth and is the naturally occurring thermal energy found within rock formations and the fluids held within those formations.

Geothermal energy is one of those few sources of energy that do not come directly or indirectly from the solar energy.

The underground hot water in contact with hot spots changes into steam. As the steam is trapped between the rocks, it gets compressed to high pressure. At some places, hot water and steam gush out from the Earth’s surface after making their way through large cracks between the rocks and form natural geysers. Geothermal energy carried by natural geysers is utilized for generating electricity.

Merits if geothermal Energy:-
1. Geothermal energy is the most versatile and least polluting renewable source of energy.
2. It can be harnessed for 24 hours throughout the year.
3. Geothermal energy is relatively inexpensive.
4. As compared to solar energy and wind energy, the power generation level of geothermal energy is higher.
5. Geothermal energy can be used for power generation as well as direct heating. In USA, water is pumped from underground hot water deposits and is used to heat houses.

Limitations of Geothermal Energy:
1. Geothermal hot spots are scattered and usually some distance away fro the areas that need energy.
2. The overall power production has a lower efficiency (about 15%) as compared to that of fossil fuels (35% to 40%).
3. Though as a whole, geothermal energy is inexhaustible, a single by ore has a limited life span of about 10 years.
4. Noise pollution is caused by drilling operations at geothermal sites.

NUCLEAR ENERGY
A reaction in which the nucleus of an atom undergoes a change to form a new atom and releases an enormous amount of energy is called as nuclear energy. There are two distinct ways of obtaining nuclear energy. a) Nuclear fission b) Nuclear fusion.

Nuclear Fission reaction:
This type of nuclear reaction was first of all reported by Otto Hahn in 1938. He stated that when an unstable heavy nucleus is bombarded with slow speed thermal neutrons, it splits into two small stable nuclei liberates an enormous amount of heat and light energy.

Nuclear Fusion reaction:
This type of nuclear reaction was first of all reported by Hans Bethe in 1939. The word ‘fusion’ means ‘to combine together’. So, nuclear fusion means combining together of two or more nuclei to form a single nucleus. Thus, a process in which two lighter nuclei fuse (combine) together to form a stable heavier nucleus with a simultaneous release of a very large amount of energy is called nuclear fusion. The energy produced in a fusion reaction is much higher than that produced in a nuclear fission reaction.

Advantages of Nuclear Energy:-
The advantages of nuclear energy are that:
1. It produces a large amount of useful energy from a very small amount of a nuclear fuel (like uranium-235).
2. Once the nuclear fuel (like uranium-235) is loaded into the reactor, the nuclear power plant can go on producing electricity for two to three years at a stretch. There is no need for putting in nuclear fuel again and again.
3. It does not produce gases like carbon dioxide which contributes to greenhouse effect or sulphur dioxide which causes acid rain.

Disadvantages of Nuclear Energy :-
1. The major hazard of nuclear power generation is the storage and disposal of spent or used fuels – the uranium still decaying into harmful subatomic particles (radiations).
2. Improper nuclear-waste storage and disposal result in environmental contamination.
3. There is a risk of accidental leakage of nuclear radiation.
4. The high cost of installation of a nuclear power plant, high risk of environmental contamination and limited availability of uranium makes large-scale use of nuclear energy prohibitive.

INTEXT QUESTIONS PAGE NO. 253

Question. What kind of mirror − concave, convex or plain − would be best suited for use in a solar cooker? Why?
Answer: A solar cooker uses heat of the sunlight to cook and heat food. A mirror is used in order to reflect and focus sunlight at a point. A concave mirror is used in a solar cooker for this purpose. The mirror focuses all the incident sunlight at a point. The temperature at that point increases, thereby cooking and heating the food placed at that point.

Question. What are the limitations of the energy that can be obtained from the oceans?
Answer: The forms of energy that can be obtained from the ocean are tidal energy, wave energy, and ocean thermal energy. There are several limitations in order to harness these energies.
(i) Tidal energy depends on the relative positioning of the Earth, moon, and the Sun.
(ii) High dams are required to be built to convert tidal energy into electricity.
(iii) Very strong waves are required to obtain electricity from wave energy.
(iv) To harness ocean thermal energy efficiently, the difference in the temperature of surface water (hot) and the water at depth (cold) must be 20ºC or more.

Question. What is geothermal energy?
Answer: Geothermal power plants use heat of the Earth to generate electricity. This heat energy of the Earth is known as geothermal energy.
When there are geological changes, the molten rocks present in the core of the earth are pushed to the earth’s crust. This forms regions of hot spot. Steam is generated when the underground water comes in contact with these hot spots forming hot springs. This trapped steam is used to generate electricity in the geothermal power plants.

Question. What are the advantages of nuclear energy?
Answer: The advantages of nuclear energy are as follows:
(a) Large amount of energy is produced per unit mass.
(b) It does not produce smoke. It is a clean energy.
(c) Fission of one atom of uranium produces 10 million times the energy released by burning of one atom of carbon.
(d) Fusion of four hydrogen atoms produces huge amount of energy approximately equal to 27 MeV.

ENVIRONMENTAL CONSEQUENCES
1. The combustion for fossil fuels is producing acid rain and damaging plants (crops), soil and aquatic life.
2. The burning of fossil fuels is increasing the amount of greenhouse gas carbon dioxide in the atmosphere.
3. The cutting down of trees from the forest (deforestation) for obtaining fire-wood is causing soil erosion and destroying wild life.
4. The construction of hydro-power plants is disturbing ecological balance.
5. Nuclear power plants are increasing radioactivity in the environment.

INTEXT QUESTIONS PAGE NO. 253

Question. Can any source of energy be pollution-free? Why or why not?
Answer: No source of energy can be pollution-free. It is considered that solar cells are pollution-free. However, even their making causes environmental damage indirectly.
Also, in the case of nuclear energy, there is no waste produced after the fusion reactions.
However, it is not totally pollution-free. To start the fusion reactions, approximately 107 K temperature is required, which is provided by fission reactions. The wastes released from fission reactions are very hazardous. Hence, no source of energy is pollution-free.

Question. Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel than CNG? Why or why not?
Answer: Hydrogengas is cleaner than CNG. CNG contains hydrocarbons. Therefore, it has carbon contents. Carbon is a form of pollutant present in CNG. On the other hand, hydrogen is waste-free. The fusion of hydrogen does not produce any waste. Hence, hydrogen is cleaner than CNG.

INTEXT QUESTIONS PAGE NO. 254

Question. Name two energy sources that you would consider to be renewable. Give reasons for your choices.
Answer: Two renewable sources of energy are as follows:
(a) Sun: The energy derived from the Sun is known as solar energy. Solar energy is produced by the fusion of hydrogen into helium, fusion of helium into other heavy elements, and so on. A large amount of hydrogen and helium is present in the Sun.
Therefore, solar energy can replenish on its own. The Sun has 5 billion years more to burn.
Hence, solar energy is a renewable source of energy.
(b) Wind: Wind energy is derived from air blowing with high speed. Wind energy is harnessed by windmills in order to generate electricity. Air blows because of uneven heating of the Earth. Since the heating of the Earth will continue forever, wind energy will also be available forever.

Question. Give the names of two energy sources that you would consider to be exhaustible. Give reasons for your choices.
Answer: Two exhaustible energy sources are as follows:
(a) Coal: It is produced from dead remains of plants and animals that remain buried under the earth’s crust for millions of years. It takes millions of years to produce coal.
Industrialization has increased the demand of coal. However, coal cannot replenish within a short period of time. Hence, it is a non-renewable or exhaustible source of energy.
(b) Wood: It is obtained from forests. Deforestation at a faster rate has caused a reduction in the number of forests on the Earth. It takes hundreds of years to grow a forest. If deforestation is continued at this rate, then there would be no wood left on the Earth.
Hence, wood is an exhaustible source of energy.

EXERCISE QUESTIONS PAGE NO. 254

Question. A solar water heater cannot be used to get hot water on
(a) a sunny day (b) a cloudy day (c) a hot day (d) a windy day
Answer: (b) A solar water heater uses solar energy to heat water. It requires bright and intense sunlight to function properly. On a cloudy day, the sunlight reflects back in the sky from the clouds and is unable to reach the ground. Therefore, solar energy is not available for the solar heater to work properly. Hence, solar water heater does not function on a cloudy day.

Question. Which of the following is not an example of a bio-mass energy source?
(a) wood (b) gobar gas (c) nuclear energy (d) coal
Answer: (c) Bio-mass is a source of energy that is obtained from plant materials and animal wastes. Nuclear energy is released during nuclear fission and fusion. In nuclear fission, uranium atom is bombarded with low-energy neutrons. Hence, uranium atom splits into two relatively lighter nuclei. This reaction produces huge amount of energy. In nuclear
fusion reaction, lighter nuclei are fused together to form a relatively heavier nuclei. This reaction produces tremendous amount of energy. Hence, nuclear energy is not an example of bio-mass energy source.
Wood is a plant material, gobar gas is formed from animal dung, and coal is a fossil fuel obtained from the buried remains of plants and animals. Hence, these are bio-mass products.

Question. Most of the sources of energy we use represent stored solar energy. Which of the following is not ultimately derived from the Sun’s energy?
(a) Geothermal energy
(b) Wind energy
(c) Nuclear energy
(d) Bio-mass
Answer: (c) Nuclear energy is released during nuclear fission and fusion. In nuclear fission, uranium atom is bombarded with low-energy neutrons. Hence, uranium atom splits into two relatively lighter nuclei. This reaction produces huge amount of energy. In nuclear fusion reaction, lighter nuclei are fused together to form a relatively heavier nuclei. The energy required to fuse the lighter nuclei is provided by fission reactions. This reaction produces tremendous amount of energy. These reactions can be carried out in the absence or presence of sunlight. There is no effect of sunlight on these reactions. Hence, nuclear energy is not ultimately derived from Sun’s energy.
Geothermal energy, wind energy, and bio-mass are all ultimately derived from solar energy. Geothermal energy is stored deep inside the earth’s crust in the form of heat energy. The heating is caused by the absorption of atmospheric and oceanic heat. It is the sunlight that heats the atmosphere and oceans.
Wind energy is harnessed from the blowing of winds. The uneven heating of the earth’s surface by the Sun causes wind.
Bio-mass is derived from dead plants and animal wastes. Chemical changes occur in these dead plants and animal wastes in the presence of water and sunlight. Hence, bio-mass is indirectly related to sunlight.

Question. Compare and contrast fossil fuels and the Sun as direct sources of energy.
Answer: Fossil fuels are energy sources, such as coal and petroleum, obtained from underneath the Earth’s crust. They are directly available to human beings for use. Hence, fossil fuels are the direct source of energy. These are limited in amount. These are non-renewable sources of energy because these cannot be replenished in nature. Fossil fuels take millions of years for their formation. If the present fossil fuel of the Earth gets exhausted, its formation will take several years. Fossil fuels are also very costly.
On the other hand, solar energy is a renewable and direct source of energy. The Sun has been shining for several years and will do so for the next five billion years. Solar energy is available free of cost to all in unlimited amount. It replenishes in the Sun itself.

Question. Compare and contrast bio-mass and hydro electricity as sources of energy.
Answer: Bio-mass and hydro-electricity both are renewable sources of energy. Bio-mass is derived from dead plants and animal wastes. Hence, it is naturally replenished. It is the result of natural processes. Wood, gobar gas, etc. are some of the examples of bio-mass.
Hydro-electricity, on the other hand, is obtained from the potential energy stored in water at a height. Energy from it can be produced again and again. It is harnessed from water and obtained from mechanical processes.

Question. What are the limitations of extracting energy from −
(a) the wind? (b) waves? (c) tides?
Answer:
(a) Wind energy is harnessed by windmills. One of the limitations of extracting energy from wind is that a windmill requires wind of speed more than 15 km/h to generate
electricity. Also, a large number of windmills are required, which covers a huge area.
(b) Very strong ocean waves are required in order to extract energy from waves.
(c) Very high tides are required in order to extract energy from tides. Also, occurrence of tides depends on the relative positions of the Sun, moon, and the Earth.

Question. On what basis would you classify energy sources as
(a) renewable and non-renewable?
(b) exhaustible and inexhaustible?
Are the options given in (a) and (b) the same?
Answer:
(a) The source of energy that replenishes in nature is known as renewable source of energy.
Sun, wind, moving water, bio-mass, etc. are some of the examples of renewable sources of energy.
The source of energy that does not replenish in nature is known as non-renewable source of energy. Coal, petroleum, natural gas, etc. are some of the examples of non-renewable sources of energy.
(b) Exhaustible sources are those sources of energy, which will deplete and exhaust after a few hundred years. Coal, petroleum, etc. are the exhaustible sources of energy.
Inexhaustible resources of energy are those sources, which will not exhaust in future. These are unlimited. Bio-mass is one of the inexhaustible sources of energy.
Yes. The options given in (a) and (b) are the same.

Question. What are the qualities of an ideal source of energy?
Answer:
An ideal source of energy must be:
(i) Economical
(ii) Easily accessible
(iii) Smoke/pollution free
(iv) Easy to store and transport
(v) Able to produce huge amount of heat and energy on burning

Question. What are the advantages and disadvantages of using a solar cooker? Are there places where solar cookers would have limited utility?
Answer: Solar cooker uses Sun’s energy to heat and cook food. It is inexhaustible and clean renewable source of energy. It is free for all and available in unlimited amount. Hence, operating a solar cooker is not expensive.
Disadvantage of a solar cooker is that it is very expensive. It does not work without sunlight. Hence, on cloudy day, it becomes useless.
The places where the days are too short or places with cloud covers round the year, have limited utility for solar cooker.

Question. What are the environmental consequences of the increasing demand for energy? What steps would you suggest to reduce energy consumption?
Answer: Industrialization increases the demand for energy. Fossil fuels are easily accessible sources of energy that fulfil this demand. The increased use of fossil fuels has a harsh effect on the environment. Too much exploitation of fossil fuels increases the level of green house gas content in the atmosphere, resulting in global warming and a rise in the sea level.
It is not possible to completely reduce the consumption of fossil fuels. However, some measures can be taken such as using electrical appliances wisely and not wasting electricity. Unnecessary usage of water should be avoided. Public transport system with mass transit must be adopted on a large scale. These small steps may help in reducing the consumption of natural resources and conserving them.

Key Learning:

1. A windmill is a simple machine that works with the energy of the wind. The windmill can be used to run a pump to draw water from the ground. It can also be used to run a flour mill to grind grain..

2. The moving water also possesses kinetic energy. The water – wheel is a device used for obtaining energy from flowing water.

3. The energy of naturally flowing water in high rivers is generally stored behind dams as potential energy and then further used to generate electricity. The electricity produced by using the energy of flowing water is known as hydro – electric power. The most important advantage of water energy is that like wind energy, it does not cause any pollution.

4. Renewable sources of energy: The renewable sources of energy are those sources which are being produced continuously in nature and are inexhaustible. The important examples of renewable source of energy are: wood, falling water, geothermal power, sun, wind,, tides, gobar gas etc. The renewable source are also known as non – conventional sources of energy.

5. The non – renewable sources are those sources which got accumulated in nature over a long time and can’t be quickly replaced when they get exhausted. The important examples of non – renewable sources of energy are: Coal, natural gas, petroleum, uranium, etc. The non – renewable sources are also known as conventional sources of energy.

6. The energy radiated by the sun is called solar energy. It is interesting to note that the solar energy recovered on the earth in one day is about 50, 000 times more than the total energy consumed by all the nations of the world in one year.

7. There are two limitations of solar energy:

a. The solar energy that reaches the earth is in a very diffused form

b. The solar energy is not uniformly available at all the time and all the places.

8. Any device that gets heated by the sun’s energy is called a solar heating device. Such a device helps in collecting as much solar energy as possible. It is done by using a black pointed surface, a glass sheet cover and a reflector.

9. A solar cooker is a heating device which is used to cook food by utilizing the energy radiated by the sun. A solar heater is used to heat water by utilizing the energy radiated by the sun. The solar power plants are used to produce electricity by using the solar energy. The solar cells are used to convert solar energy directly into electrical energy. The solar cells are made from semi- conductor elements like silicon and germanium.

10. The oceans act as a storehouse of solar heat energy. Because the oceans covers almost 71% collector of solar heat energy. The energy collector of solar heat energy. The energy from the oceans is available in different forms. They are:

a. Ocean Thermal Energy (OTE)

b. Sea – waves energy

c. Tidal energy

d. Energy from salinity gradient in seas 

e. Energy from sea – vegetation

f. Energy from the nuclear fusion of deuterium that is present in oceans

11. There is always some significant temperature difference between the water ‘at the surface of ocean’ and ‘at deeper levels’. This temperature difference is even upto 2°C at enable the flow of heat. The energy available as a result of difference in the temperature water at the surface of the ocean and at deeper levels is known as ocean thermal energy. It is also abbreviated as OTE.

12. The rise of ocean water as a result of moon’s attraction is called ‘high tide and the fall of water is called ‘low tide’. The rise and fall of tidal waves takes place twice in a day. This gives rise to enormous movement of water between high tides and low a tides and is an excellent source of energy in many coastal areas of the world. The tidal energy can also be harnessed by constructing some tidal barrier or tidal dam.

13. Fuels are substances which are used for producing heat energy. Fuels are sued for producing electricity.

14. The waste material of plants and animals are called biomass. When plants and animal die, their biomass can be used as fuel.

15. The remains of plant and animals buried under the earth millions of years ago are known as fossils. These fossils are excellent fuels and are called fossil fuels. The common examples of fusil fuels are coal, petroleum and natural gas.

16. Petroleum is a mixture of several hydrocarbons with traces of salt, rock particles and water.

17. Fuels used directly to produce heat are called primary fuels and fuels derived from primary fuels are known as secondary fuels.

18. The nuclear reactor (commonly known as atomic reactor or atomic pile) is a specially designed furnace for carrying out the controlled fission of a radioactive material line U – 235 for producing atomic power. The heat energy produced from fission reactions is ultimately converted into electricity.

19. Nuclear fuel is the fissionable material used in the nuclear reactor for producing energy by the process of fission. The nuclear fuel used is enriched which slows down the speed of neutrons so as to cause the fission of uranium 235 effectively. Heavy water is an important moderator.

20. The process in which an unstable nucleus of a heavy atom (like U – 235) splits up into two medium - sized nuclei with the liberation of an enormous amount of energy is called nuclear fission.

₉₂U²³⁵ + ₀n¹ → ₅₆Ba¹⁴¹ + ₅₆Kr⁹² + 3(₀n¹) + energy

Why Conservation of Energy?
Energy and its sources
• Energy is the ability to do work or the total power derived from our natural resources. The energy exists in many forms and can be converted from one form of energy into another. Energy in usable form is dissipated to the surroundings as less usable form. • Energy is mainly derived from natural sources like the sun, oceans, fossil fuels, wind etc and is converted into electrical energy that we consume for our daily needs and benefits.

What Is a Good Fuel?
A good source of energy
• Sources can be classified as renewable and nonrenewable sources of energy.
• Any source of energy that does not get depleted or gets exhausted is considered a good source of energy and are usually called as renewable. Good fuel A good source of energy/fuel should be:
• Easily available
• Easy to store and transport
• Able to perform large amounts of work per unit mass and volume
• Economical

Fossil Fuels and Thermal Energy
Conventional sources of energy
• Sources of energy that have been in use for a very long time or have been used extensively worldwide are known as conventional sources. Eg: wood as a common source of heat energy or the usage of coal after the Industrial Revolution.
• Examples: Fossil Fuels, Hydro Power

Fossil fuels
• Fossil fuels are formed due to compression of dead organic matter over millions of years, buried deep under the earth. Eg. coal or natural gas.
• We rely heavily on fossil fuels for most of our work.
• Fossil Fuels are non-renewable sources of energy as they have limited reserves and hence finding alternative sources is essential to avoid an energy crisis.

Disadvantages of burning fossil fuels
• Fossil fuels also produce byproducts due to combustion which causes air pollution.
• Burning coal and petroleum produces harmful oxides of carbon, nitrogen, and sulphur which pollute the air leading to acid rain and greenhouse effect.
• Gases emitted by the combustion of fossil fuels are the main contributor to global warming.

Thermal power plants
• Thermal power plants use steam produced by burning fossil fuels (mainly coal) to move the turbines to generate electricity.
• The burning of coal heats up the water and forms steam that is used to run the turbine.
• Usually, Thermal power plants are located near coal or oil fields as it is easier to transmit electricity than transport coal.

Hydropower Plants
Turbines
• A turbine is a rotatory mechanical device that extracts kinetic energy in various forms and converts it into useful work. It uses a dynamo to convert this mechanical energy into electrical energy.
• Various uses of this have been implemented in power plants where the shaft of the dynamo is made to rotate by mechanical meAnswer :

Hydropower plants
• Another traditional source of energy is from the kinetic energy of flowing water or harnessing potential energy of water falling from a height.
• The falling/flowing water moves the turbine, which with the help of a dynamo converts mechanical energy into electrical energy.
• Hydropower plants are usually constructed near dams or waterfalls. (Image 3)

Dams
A dam is a barrier that impounds water or underground streams. The water is ejected upon requirement by the power plant to generate hydroelectricity.

Fuels Biomass
• Biomass is the source of energy derived from living things (organic matter). For a long time, we relied on wood for the source of heat energy. In India, we make fuel out of biowaste such as cow dung due to the availability of a thriving population of livestock.
• When wood is burnt in a limited supply of oxygen and water until volatile materials are removed, the residue left behind is charcoal. Charcoal has good heat generating efficiency. It also burns without flames.(Image)

Bio-gas plant
• In India cow dung, sewage waste, plant matter are decomposed in absence of oxygen to produce biogas. Since it has cow dung it is often termed as gobar gas.
• A biogas plant is a dome-like structure built with bricks where cow dung and other biowaste are mixed with water to form a slurry and put into a digester.
• The digester is a sealed chamber with anaerobic bacteria which breaks down the slurry.
• This decomposition process releases gases like methane, CO2, hydrogen sulfide and hydrogen.
• These gases are drawn via pipes which are transmitted to a turbine for the production of electricity.

Wind Energy
Wind energy
• Environment-friendly efficient source of energy.
• Wind is the natural phenomenon caused by pressure differentials due to unequal heating of land and water masses on the surface of the earth. It is harnessed in the form of kinetic energy. (Image 5)

Windmill
• Wind Energy is harnessed by rotatory structures known as windmills.
• They have huge blades or fans attached very high on a rigid support that is attached to turbines that rotate due to high speeds of wind and generate electricity.
• A single windmill has a low output and therefore, the wind farms are built that comprises of several windmills. Advantages and limitations of wind energy

Advantages: Environment-friendly, efficient, renewable source, with no recurring cost for production of electricity.
Limitations:
• Wind speeds should be constant and > 15 km/h.
• Must have back up storage facilities like cells.
• Requires large land area.
• High initial cost and regular maintenance are required.

Solar Energy
Solar energy
• Light energy and heat energy from the sun are known as solar energy.
• Sun has been radiating energy from the past 5 billion years and will continue to do so at the same rate for another 5 billion years or more.
• We must find ways to harness the energy with maximum efficiency, although only a small fraction of the solar energy reaches the earth’s surface.

Solar cooker
• Solar cookers and water heaters use solar energy to operate.
• Black surfaces absorb more energy than other surfaces and solar cookers use this property by coating their insides black.
• They use reflecting surfaces like mirrors to focus the sun’s rays.
• The device is covered with a glass plate thereby establishing the greenhouse effect by trapping heat inside the cooker.

Solar cell
• A device that converts solar energy into electricity is known as a solar cell.

• A typical solar cell produces a voltage of 0.5 1 V and 0.7 W of electrical power. A large number of such cells can combine to form a solar panel which can generate power large enough for practical uses.

• Advantages:
(i) no moving parts
(ii) require little maintenance
(iii) can be set up in remote areas without the hassle and expenses of transmission lines.

• Disadvantages:
(i) Requires a special grade silicon which is not easily available
(ii) Usage of silver for interconnections makes it expensive.

• Uses: traffic signals, calculators, artificial satellites and space probes.  (Image 7)

Energy From the Sea

Energy from sea The seas and oceans and other water bodies are a source of kinetic and potential energy due to the immense volume of water and the motion of waves.

Tidal energy
• Tides are variations in the level of water due to the gravitational pull of the moon.
• The phenomenon of rise and fall of water level or high and low tide gives tidal energy.
• Tidal energy is obtained by constructing dams near the narrow openings of the sea. When the tide sets in, it moves the turbine which directly produces electricity.
• It is limited to places near the sea.

Wave energy
• Waves possess a lot of kinetic energy that can be harnessed to produce electricity.
• Waves are produced by strong winds blowing over the sea.
• Limited to places with strong winds. Devices have been designed to capture this energy.

Ocean thermal energy
• The difference in surface temperatures of water and water at a certain depth in oceans is exploited to harness this form of energy.
• Temperature difference must be 20∘between the surface and water up to depths of 2 km
. • Warm water is used to boil volatile ammonia to form vapours that move the turbine. Cold water is used to condense the vapour back to liquid. (Image 8)

Geothermal Energy
Geothermal energy
• There is a huge amount of heat trapped inside the earth. Molten rocks from Earth’s core sometimes come up due to geological changes and get trapped in hotspots. Harnessing this heat energy is called geothermal energy.
• Any underground water present gets heated due to the hotspots and gets converted to steam which escapes from the surface of the earth as hot springs.
• This steam is used to rotate turbines and generate electricity.

Nuclear Energy
Nuclear energy
• Nuclear energy can be used to generate electricity by means of nuclear fission.
• In a nuclear reactor, the nuclear fuel is used to carry out sustained fission chain reaction to produce electricity at a controlled rate.

Nuclear fission
Nuclear fission is the process where a heavy atom (uranium or plutonium) is bombarded with neutrons that split the atom to give lighter nuclei. This process releases tremendous amounts of energy. For example, Fission of 1 atom of uranium gives 10 million times the energy of combustion of 1 atom of carbon from coal. (Image 10)

Disadvantages of using nuclear energy
• Nuclear waste is hazardous as heavy atoms decay into harmful subatomic particles.
• High setup and maintenance cost
• Limited availability of uranium
• Can be used for destructive purposes

Nuclear fusion
• Fusion means combining lighter nuclei to produce a heavier nucleus, usually hydrogen or hydrogen isotopes to create helium.
• It releases a huge amount of energy. The mass deficit between the product and reactants are given by Einstein’s equation. E=mc2 (Image 11)

SOURCES OF ENERGY

ENERGY

The ability of a body to do work is called energy. It is measured by the amount of work hat a body can do. It is a scalar quantity and its SI unit is Joule.

FUEL AND ITS USES:

A fuel is defined as any substance which burns easily to produce energy. e.g., coal, petrol, kerosene, natural gas,charcoal, wood, etc are fuels. A good fuel is defined as any substance which burns easily to produce adequate amount of heat energy without giving too much undesirable by-products. e.g., petrol, natural gas, LPG, CNG, Biogas, etc are good fuels. In fact, fuels are the concentrated store of energy. The characteristics of a good fuel are:

i. It should produce large amount of useful energy per unit volume or mass.

ii. It should be easily accessible

iii. It should be easy and safe to transport, handle and store.

iv. It should most importantly be economical.

v. It should not produce too much objectionable byproducts like harmful or poisonous gases.

vi. It should have proper ignition temperature.

vii. It should have moderate rate of combustion

SOURCE OF ENERGY and ITS CHARACTERISTICS

Any system from which useful energy can be obtained or tapped is called a source of energy. Any system from which adequate amount of useful energy can be obtained or tapped at a constant rate at a constant rate without giving too much objectionable byproducts is called a good source of energy. The characteristics of a good source of energy are:

i. It should produce large amount of useful energy per unit volume or mass.

ii. It should be easily accessible

iii. It should be easy and safe to transport, handle and store.

iv. It should most importantly be economical.

v. It should not produce too much objectionable byproducts like harmful or poisonous gases.

FOSSIL FUELS

Fossil fuels are defined as the fuels preserved under the earth’s crust as the remains of plants and animals. They are the rich compounds of carbon which were originally made by the plants with the help of solar energy. Typical examples of fossil fuels are petroleum, coal and natural gas.
Fossil fuels are believed to be formed over millions of years by the burial of plant and animal remains due to unusual sequence of geological processes operating within the earth’s crust. The remains of plants and animals which died millions of years ago were gradually buried deep in the earth and got covered with sediments like sand and mud and were thus locked away from the reach of oxygen. The covers of the sediments prevented their oxidation and decay while the weights of sediments squeezed out water and other volatile materials. In the absence of oxygen and under the continuous combined effect of high pressure, heat and bacteria, these buried remains of plants and animals are converted into fossil fuels like petroleum, coal and natural gas. The buried remains of large plants and animals are believed to form petroleum and natural gas.

Disadvantages of burning fossil fuels:

i. The burning of fossil fuels causes air pollution.

ii. The acidic oxides released on burning fossil fuels cause acid rain, which affects our water and soil resources.

iii. The burning of fossil fuels produces green house gases and hence cause green house effect.

iv. These fuels are non-renewable formed over millions of years.

v. The fossil fuels are limited in nature.

Pollution caused by burning fossil fuels can be reduced as follows:

i. Increasing the efficiency of the combustion process

ii. Using various techniques to reduce the escape of harmful gases and ash to the surroundings. e.g, using electrical precipitators, efficient engines, etc.

iii. Planting more and more trees.

iv. Using ecofriendly renewable sources of energy.

TURBINE: Turbine is a system consisting of a rotor with assembled blades, which are free to rotate. It is used for converting the kinetic energy of a fluid such as flowing water or a gas into the mechanical energy which inturn is used to run the shaft of the dynamo to generate electricity.

THERMAL POWER PLANT:

Thermal power plant is a power plant in which fuel is burnt to produce heat energy which is then converted nto electrical energy. Large amount of fossil fuel are burnt everyday in power stations to heat up water to produce steam, which further runs turbine to generate electricity. The transmission of electricity is more efficient than Transporting coal or petroleum over same distance, therefore many thermal power plants are set up near coal or oil fields.

BIOMASS AS A FUEL.

Greek words ‘bio’ means living and ‘mass’ means material. Thus, the waste material contained in the bodies of the living organisms like pants and animals is called BIOMASS. When organisms like plants die, their biomass can be used as a fuel for domestic purposes. In fact biomass like wood, agricultural wastes and cowdung together supply about 80% of our domestic energy needs in the rural areas. Wood and some agricultural wastes are also used as a fuel in industries. For e.g. bagasse is used in industrial boilers. Bagasse is the sugarcane from which juice has been extracted. In most our villages, biomass like wood are used as fuel in open furnace called chulhas.

The disadvantages of using biomass as a fuel are:

i. These fuels do not produce much heat.ie, they have low calorific value.

ii. A lot of smoke is given when they are burnt.

iii. Technological inputs are necessary to improve the efficiency of these fuels.

DESTRUCTIVE DISTILLATION OF WOOD (OR)FORMATION OF CHARCOAL

Burning or heating of a substance in limited supply of oxygen is called destructive distillation.

Charcoal is a form of carbon obtained by the destructive distillation of wood. i.e, by burning wood in the insufficient supply of air. When wood is strongly heated in the insufficient supply of air, the volatile substances present in it are expelled leaving behind a black residue called charcoal. When a solid organic compound is heated in a controlled supply of air, it leaves behind a grey residue commonly called charcoal. Charcoal is used as a fuel in house hold, as a reducing agent and in decolourizing sugarcane juice. It is a good composite fuel.

CHARCOAL IS A BETTER FUEL THAN WOOD

Charcoal is a better fuel than wood because of the following reasons:

i. Charcoal burns without flame.

ii. Charcoal has high calorific value than wood i.e., Charcoal produces more heat on burning than an equal mass of wood.
For example, 1 gram of charcoal produces 33 kJ of heat energy on burning whereas 1 gram of wood produces only about 17 kJ of heat energy.

iii. Charcoal is a smokeless fuel i.e., it does not produce any smoke while burning and hence does not cause any pollution on doors. On the other hand, burning of wood produces a lot of smoke which pollutes air.

iv. It is a compact fuel and therefore is easy to handle and store and convenient to use than wood.

BIOGAS AND ITS FORMATION

Biogas is a mixture of gases like methane, carbon dioxide, hydrogen and hydrogen sulphide. The major constituent of biogas is methane gas. In fact, biogas contains about 65% of methane.

Animal and plant wastes are easily degraded by anaerobic microorganisms like anaerobic bacteria in the presence of water to form gases such as methane, CO2, H and H2S. This mixture of gases is called biogas. The process of formation of biogas by the action of anaerobic microorganisms on animal and plant wastes is called fermentation. When obtained from animal dung, then it is called gobar gas and when it is obtained from sewage, it is called sewage gas. This gas can be used for various purposes.

Biogas an ideal fuel and a boon to the farmers: This is because of the following reasons:

i. Biogas is an excellent fuel as it contains upto 75% methane.

ii. It burns without producing any harmful or poisonous gases and smoke.

iii. It has high calorific value (30-40 KJ/g) i.e., it produces more heat on burning.

iv. It has proper ignition temperature i.e., its ignition temperature is neither be too high nor too low.

v. The slurry left behind in the biogas plants is rich in nitrogenous and phosphorous compounds and hence is used as manure by farmers in their fields.

vi. It is also used as a fuel to run engines of water pumping sets required for irrigation.

vii. It has moderate rate of combustion. i.e., it burns smoothly.

viii. It can be used by the farmers for lighting in homes as well as for street lighting in some places.

ix. It is safe to transport, easy to handle and convenient to store. It is supplied by pipes directly from the biogas plant.

x. Biogas plant provides a safe and efficient method of waste disposal.

BIOGAS PLANT:

Biogas plant is a system where biogas is produced on a large scale. A large sized biogas plant which supplies biogas to many houses in a village is called Community Biogas Plant. The cost of installing such a biogas plant is shared by the people in the village community. All the cattle dung from many houses in the villages is used to feed the plant. The biogas is then supplied to all the houses in the village through pipes.

There are two designs of community biogas plants: Fixed Dome Type Biogas Plant and Floating Gas Holder Type Biogas Plant.

Construction And Working Of Fixed Dome Type Biogas Plant:

A fixed dome type biogas plant is a unique type of biogas plant in which a gas holder and the digester are as one single unit and can be constructed by laying bricks. It has a longer life than the floating type biogas plant.

It consists of following parts:

i.. A well shaped underground tank called digester. It is made up of bricks.

ii. A dome shaped roof which acts as a storage tank for biogas. It is also made up of bricks.

iii. A mixing tank connected to the left of the digester through a sloping inlet chamber made of bricks.

iv. An overflow tank connected to the right of the digester through an outlet chamber made of bricks.

v. A Gas outlet at the top of the dome having gas valve ‘V’.

Slurry is made by mixing cattle dung and water in equal proportions in a mixing tank. This slurry is fed into the digester through the inlet chamber. The slurry is left in the digester for 50-60 days in the digester. During this period, anaerobic bacteria and other microorganisms present in the dung carry out its degradation. As a result a mixture of gases called biogas is evolved which begins to collect in the dome. Due to increased pressure of the gas, the spent slurry is forced out through outlet chamber into the overflow tank. This spent slurry is taken out and used as manure because it is rich in Nitrogen and phosphorous compounds. The gas is taken out through the gas outlet and is distributed to the houses through the gas pipes. When pressure falls the fresh slurry is added to the digester and continuous supply of biogas is maintained.

Construction and working of the Floating Gas Holder type biogas plant.

A Floating Gas Holder type biogas plant is a type of biogas plant in which a gas holder and the digester are not as one single unit. It consists of following parts:

i. A well shaped underground tank called digester. It is made up of bricks.

ii. A cylindrical drum shaped gas holder roof which acts as a storage tank for biogas. It is also made up of steel. It floats in the inverted position over the slurry.

iii. A mixing tank connected to the left of the digester through an inlet pipe made of steel.

iv. An overflow tank connected to the right of the digester through an outlet pipe made of steel.

v. A Gas outlet at the top of the gas holder having gas valve ‘V’.

vi. A partition wall which divides the digester into two parts.

The working of Floating Gas Holder type biogas plant is same as the working of fixed dome type biogas plant.

HYDROELECTRICITY

The electricity generated by using the kinetic energy of flowing water in rivers or the potential energy of water stored in a dam is called is called hydroelectricity. The arrangement in which the kinetic energy of flowing water is converted into electricity is called hydro power plant. Hydroelectricity is an indirect source of solar energy.

Principle of generation of Hydroelectricity: in order to produce hydroelectricity, high rise dams are constructed on the river to obstruct the flow of water and thereby collect the water in the reservoirs. The water level rises and in the process kinetic energy of the flowing water gets transformed

into potential energy. The water from the high level is carried through the pipes to the turbine at the bottom of the dam. When the fast flowing water

falls on the turbine, the kinetic energy of the water rotates the turbine rapidly, which in turn rotates the coil of the generator to generate electricity.

Advantages of hydroelectricity:

i. Hydroelectricity is a natural and renewable source of energy and hence we need not to worry about hydroelectric sources getting used up.

ii. It is ecofriendly source of energy if proper precautions are taken while constructing dams.

iii. It is cheap source of energy except for initial investment.

Disadvantages of hydroelectricity:

i. The dams can be constructed only in a limited number of places, preferably in hilly areas.

ii. Large areas of agricultural land and human habitation are to be sacrificed as they get submerged.

iii. Large ecosystems are destroyed when submerged under the water in dams.

iv. The vegetation which is submerged rots under anaerobic conditions and gives rise to large amounts of methane which can cause green house effect.

v. It also creates the problem of satisfactory rehabilitation of displaced people. Opposition to the construction of Tehri dam on the river Ganga and Sardar Sarovar project on the river Narmada are due to such problems.

WIND ENERGY

The moving air is called wind. The kinetic energy associated with the wind is called wind energy. Wind is caused due to unequal heating of the land masses and the water bodies by the solar radiation. The uses of wind energy are:

i. The kinetic energy of the wind can be used to do work. This energy was harnessed by windmills to do mechanical work like to lift water from a ell, to grind grains in flour mills, etc.

ii. It is also used these days to generate electricity with the help of wind mill.

iii. It is also used to run sail boats.

Advantages of wind energy:

i. It is an ecofriendly source of energy.

ii. It is an efficient source of renewable energy.

iii. It requires no recurring expenses for the production of electricity.

Disadvantages of wind energy:

i. Wind energy farms can be established only at those places where wind blows for greater part of a year.

ii. The wind speed should also be higher than 15 km/h to maintain the required speed of the turbine.

iii. Furthermore, there should be some backup facilities like storage cells to take care of the energy needs during a period when there is no wind.

iv. Establishment of wind energy farms requires large areas of land. For a 1 MW generator, the farm needs about 2 hectares of land.

v. The initial coat of establishment of the farm is quite high.

vi. Moreover, since the tower and the blades are exposed to the vagaries of nature like sun, storm and cyclone, they need a high level of maintenance.

Wind mill and wind energy farm:

A windmill is an arrangement which uses wind energy to do mechanical work or to generate electricity. It essentially consists of a structure similar to a large electric fan that is erected at some height on a ground. To generate electricity, the rotatory motion of the windmill is sued to run the turbine of the electric generator. The out put of a single wind mill is quit small and cannot be used for the commercial purposes. Therefore, a number of windmills are erected over a large area to get the energy output on a commercial scale. This arrangement of a number of windmills erected over a large area to get the energy output on a commercial scale is called wind energy farm. Denmark is called country of winds because more than 25% of their electricity is generated through windmills.

SOLAR ENERGY

The energy radiated by the sun in the form of electromagnetic radiations is called solar energy. It includes light radiations, infra red radiations and ultra violet radiations. Two thirds of solar energy consists of infra red rays. The sun has been radiating an enormous amount of energy at he present rate for nearly 5 billion years and will continue to do so for about 5 billion years more. Only a small part of the solar energy reaches the outer layer of the earth’s atmosphere. Nearly half of it is absorbed while passing through the atmosphere and the rest reaches the earth’s surface.

It is the inexhaustible source of energy. The various functions performed by the sun’s energy on the earth are:

i. Solar energy powers the flow of winds.

ii. It powers the flow of water cycle on earth.

iii. It provides energy for preparing food by the process of photosynthesis by plants and hence sustains life on earth. iv. Sun’s energy created the fossil fuels over a period of millions of years.

iv It provides light and warmth on earth.

v. Nowadays, it is used in devices like solar cookers, solar heaters and solar cells.

 Does all th e energy radi ated b y the sun reach th e earth ’s surface: .

Every square metre of the earth’s upper atmosphere receives 1.4 kJ of energy per second. This is called solar constant. The solar energy reaching unit area at the outer edge of the earth’s atmosphere exposed perpendicularly to the rays of the sun is known as solar constant. It is estimated to be approximately 1.4 KJ/S/m2 or 1.4kw/ m2. But all this energy does not reach the surface of earth. Some of the sunlight is reflected back into space, some is absorbed by water vapour, ozone, dust, CO2 and other gases present in the atmosphere. In fact, only 47% of the sun’s energy that strikes the earth’s atmosphere reaches the earth’s surface and is absorbed. Even this fraction of sun’s energy is quite substantial.

SOLAR COOKER

Solar cooker is a solar heating device which uses solar energy for cooking food. It consists of an insulated metal box or wooden box, which is painted black from inner side. Black surface absorbs as well as radiates more heat. The top of the ox is covered by a thick glass sheet so that the radiated heat does not escape from the box. This effect is called green house effect. A mirror adjusted at a suitable angle acts as a reflector so that more and more heat enters the box. The food to be cooked is placed in steel containers painted black from outside. When the solar cooker is kept in the sunlight, the solar energy enters the box and is absorbed by the box. The glass sheet does not allow the radiated heat to escape from the box and as a result the temperature inside the ox rises to about 100oC -150 oC. Thus, the food in the containers gets cooked.

The limitations of solar cooker  :

i. Solar cookers are useful only at certain times during the day.

ii. They cannot be used efficiently on a rainy or foggy day.

iii. They cannot be used during night.

iv. The reflector needs to be adjusted all the time.

SOLAR CELL

Solar cell is a device that converts solar energy into electrical energy. It is also called photovoltaic cell. A typical soar cell develops a voltage of 0.5 to 1 volt and can produce about 0.7 watts of electricity when exposed to sun. Solar cells are made of semi conducting materials like silicon, selenium, germanium, etc with added impurities. The arrangement of a large number of solar cells to deliver energy for practical use is called solar panel.

Advantages of solar cells:

i. They have no moving parts.

ii. They require little maintenance.

iii. They work quite satisfactorily without the use of focusing device.

iv. They can be set up in remote and inaccessible hamlets or very sparsely inhabited areas in which laying of power transmission line may e expensive and not commercially viable.

v. They use a renewable source of energy.

Disadvantages of solar cells

i. Silicon, which is used for making solar cells, is abundant in nature but the availability of the special grade silicon needed for making the solar cells is limited.

ii. The entire process of manufacture is very expensive.

iii. Silver is used for interconnection of cells in the solar panel, which further adds to the cost.

SOURCES OF ENERGY

DIFFERENCE BETWEEN NUCLEAR REACTION AND CHEMICAL REACTION: 

FORCES OPERATING INSIDE THE NUCLEUS

There are two types of forces operating inside the nucleus of an atom. These forces are electrostatic force and nuclear force.
Electrostatic force of repulsion: It is the force of repulsion that exists between the positively charged protons present in the nucleus. Due to this force protons in the nucleus repel each other and hence tend to make the nucleus unstable. It is a long-range force.
Nuclear force: It is the force of attraction that exists between two protons or two neutrons or a proton and a neutron present in the nucleus. It is a very strong attractive force which tends to make the nucleus stable. Nuclear force is a short rang force. i.e., it is effective only when the nucleons are very close together as in a small nucleus.
In a small nucleus, the protons and neutrons are very close together due to which the nuclear force of attraction is very strong than the electrostatic force of repulsion. Since the nuclear force of attraction dominates in a small nucleus, the small nucleus is very stable.
On the other hand, in a large nucleus like U-235 the distances between the nucleons (protons and neutrons) are greater. Due to this the electrostatic force of repulsion tends to dominate nuclear force of attraction. This makes the heavy nucleus unstable. Thus, the atoms having large nucleus like U-235 atom is held in delicate balance and can rupture even when slightly disturbed.

What happens when U-235 is bombarded with a slow moving neutron? Explain with the help of a diagram.
The nucleus of U-235 is large and is highly unstable. It is held in a delicate balance and can rupture even when slightly disturbed. A slow moving neutron colliding with a U-235 nucleus can rupture the nucleus completely to form smaller nuclei of barium and krypton with the emission of 3 neutrons and a large amount of energy. The rupture of nuclear fission of U-235 nucleus is diagrammatically shown as follows:
When an extra neutron enters the highly unstable nucleus of a U-235 atom, it completely upsets the delicate balance between the electrostatic and nuclear forces. The nucleus gets elongated and develops a slight depression (waist) and then a neck develops. The coulombic repulsion between the two portions helps in breaking the nucleus. Finally, the nucleus of U-235 atom splits into smaller fragments like Barium, krypton and 3 neutrons. A huge amount of energy is also released in this process.  

NUCLEAR RESEARCH CENTRES AND NUCLEAR POWER PLANTS IN INDIA.
In India, the major nuclear research centres are located at: (i) Baba atomic research centre at Trombay in Maharashta, (ii) atomic research centre at Gulmarg in Kashmir and (iii) nuclear research centre in Calcutta.
In India, the atmic power stations are located at : (i) Tarapur in maharashtra, (ii) Kota in Rajasthan, (iii) Kalpakkam in
Tamil Nadu and (iv) Bulandshahar in Uttar Pradesh

DIFFERENCE BETWEEN NUCLEAR FISSION AND NUCLEAR FUSION.

NUCLEAR FISSION:

i. The process in which a heavy unstable nucleus splits into two lighter nuclei of medium weights with the liberation of large amount of energy is called nuclear fission. The two new nuclei formed are called fission fragments and the heavy nucleus which splits is called parent nucleus and the particle which brings about fission is called projectile.
ii. Nuclear fission is carried out by bombarding the heavy nucleus like U-235 with suitable projectiles like a slow moving neutron.
It takes place at ordinary temperature.
iii. For example when U-235 atoms are bombarded with slow moving neutrons, the heavy U-atom first changes into highly unstable U-236 and then splits up into two fragments like Ba-139 and Kr-94 with the emission of two or three neutrons.
₀n¹ + ₉₂U²³⁵ → ₅₆Ba¹³⁹ + ₃₆Kr⁹⁴ +3₀n¹ + energy
iv. It is a chain reaction and such a reaction takes place in an atom bomb.
v. It produces large quantities of radioactive wastes which are harmful and hence causes more dangerous radioactive pollution.

NUCLEAR FUSION:

i. The process in which two lighter nuclei combine to form more stable nucleus with the liberation of enormous amount of energy is called nuclear fission. The two new nuclei which fuse together are called daughter nuclei and the heavy nucleus formed is called product nucleus.
ii. Nuclear fusion can be carried out at very high temperature (~ 10⁷K).
iii. For example, when two deuterium atoms are heated to a very high temperature of about 107K , then the two deuterium nuclei fuse together to form more stable and heavy helium nucleus with the liberation of enormous amount of energy.
₁H² + ₁H² → 2He³ + enormous energy
iv. It is not a chain reaction and such a reaction takes place in hydrogen bomb and in the sun.
v. It does not produce radioactive wastes and hence does not cause radioactive pollution.

DIFFERENCE BETWEEN ATOM BOMB AND HYDROGEN BOMB: 

Labelled and schematic diagram of a nuclear reaction taking place in U-235 nucleus.
A reaction in which a particle which starts the reaction is produced during the reaction to continue the reaction further and further is called chain reaction. It is a self sustaining or self propagating reaction because once started it goes on propagating by itself until one of the reactants is all used up. For example, fission of U-235 by means of slow moving neutrons is a chain reaction 

CONTROLLED FISSION REACTION
A critical fission reaction is that fission reaction in which fission of U-235 is allowed to retain just enough neutrons to ensure that the number of uranium atoms undergoing fission remains constant. It is also called controlled fission reaction because energy is released at a slow, steady and manageable rate.
Critical fission reaction is carried out by removing or absorbing two of the three neutrons produced in fission with help of boron and cadmium rods. Enrico Fermi carried out the first critical fission reaction on 2nd December 1942 at the Chicago University in U.S.A.

SCHEMATIC DIAGRAM OF A NUCLEAR POWER PLANT.
Nuclear power plant is a power plant where electricity is generated by using nuclear energy. It consists of nuclear reactor, heat exchanger, turbines and a condenser with a pump.
The nuclear reactor further consists of shield, fuel rods, coolant and moderator.
First the fuel rods of cadmium and boron are put fully inside the core of reactor and the fuel rods of U-235 are inserted into the reactor. The reactor is then sealed. The enriched uranium core is then subjected to nuclear fission by slow moving neutron. The fissionable material is stacked with heavy water or graphite which is used as moderators to slow down the neutrons produced in the fission. The control rods absorb the excess neutrons produced. They are slowly raised till criticality is achieved. The controlled fission produces a lot of heat which is transferred to heat exchanger by coolant like heavy water or liquid sodium. In heat exchanger the energy is used to convert water into steam. The steam produced is used to run a turbine to generate electricity. The spent steam is then cooled in the condenser. The shield around the reactor prevents heat or radiation leakage. 

HARMFUL EFFECTS OF NUCLEAR ENERGY:
i. Nuclear fission emits harmful nuclear radiations which can cause irreparable damage to tissue cells and cause skin cancers, leukemia, etc.
ii. The nuclear wastes generated during nuclear fission are radioactive and cannot be disposed off easily. Nowadays, they are being disposed in leak-proof lead containers. No such problem arises in using fossil fuels.
iii. Nuclear power plants cannot be located near the population.
iv. A nuclear power plant has to have number of safety measures, which makes the construction of the plant expensive.

Methods To Minimize The Harmful Effects Of Nuclear Radiations
i. Transporting nuclear materials in thick leak-proof containers.
ii. Reactor walls are made of thick lead surrounded by concrete walls.
iii. Nuclear radiation proof dress is used by the workers working in the nuclear power plant.
iv. Special gloves are used by technicians working in nuclear medicine laboratories.
v. Empty containers are disposed off with utmost care by burning them deep inside the earth in remote places.

The advantages of the energy systems using nuclear fission over those using fossil fuels are:
i. Nuclear fission can be controlled and used for constructive purposes like generating electricity on a very large scale.
ii. Nuclear power plants consume very little fuel. Once nuclear fuel like U-235 is loaded into the nuclear power plant, then it will go on releasing energy for 2 to 3 years at a stretch. On the other hand, fossil fuels have to be fed in regularly in the power plant.
iii. If properly used, nuclear power plants produce less atmospheric pollution than the thermal power plants.
iv. A sizeable amount of fuel (U & Pu) can be reclaimed by processing the spent fuel material. In contrast, fuels like coal cannot be reclaimed once they have been used.

What actually powers the sun?
The correct explanation for the source of sun’s energy was provided by a German scientist Hans Bethe in the year 1939. He suggested that the sun contains hydrogen nuclei in its core which are moving at very great speeds. Whenever these hydrogen nuclei fuse to form helium nucleus at high temperature of the order of 106 K, a tremendous amount of energy is released. Thus, it is the energy produced by nuclear fusion of hydrogen into helium that actually powers the sun.

GEOTHERMAL ENERGY
'Geo' means 'earth' and 'thermal' means 'heat'. Thus, geothermal energy is the heat energy from hot rocks present inside the earth.
This heat can be used as a source of energy to produce electricity. Geothermal energy is one of the few sources of energy that do not come directly or indirectly from solar energy (or sun's energy). The places where very hot rocks occur due to collection of hot magma at some depth below the surface of earth are called 'hot spots' and are sources of geothermal energy. This energy heats the underground water. The regions where hot underground water comes out of the earth’s crust are called hot springs or geysers.

Harnessing of Geothermal Energy:
The extremely hot rocks present below the surface of earth heat the underground water and turn it into steam. As more and more steam is formed between the rocks, it gets compressed to high pressures. A hole is drilled into the earth upto the hot rocks and a pipe is put into it. The steam present around the hot rocks comes up through the pipe at high pressure. This high pressure steam turns the turbine of a generator to produce electricity.
Sometimes two holes are drilled into the earth in the region of hot rocks and two pipes are put into them. Cold water is pumped in through one of the pipes. This cold water is turned into steam by the hot rocks. The steam thus formed comes out through the other pipe and used to generate electricity.

Advantages Of Geothermal Energy: Some of the advantages of using geothermal energy are as follows:
i. It is economical to use geothermal energy. This is because the cost of electricity produced by using geothermal energy is almost half of that produced from conventional energy sources.
ii. Another advantage is that the use of geothermal energy does not cause any pollution. So, it is a clean and environment friendly source of energy.
iii. It can be used 24 hours throughout the year.

Disadvantages Of Geothermal Energy: Some of the disadvantages of geothermal energy are as follows:
i. Geothermal energy is not available everywhere.
ii. It is available only in those areas where there are hot rocks near the earth's surface.
iii. Another disadvantage is that deep drilling in the earth to obtain geothermal energy is technically very difficult and expensive.
Note: In our country there are a very limited number of places where geothermal energy can be harnessed on a commercial scale. Two places where geothermal energy can be exploited on commercial scale are located in Madhya Pradesh and Himachal Pradesh. In USA and Newzealand, a number of geothermal energy power plants are working successfully.

ENERGY FROM THE SEA
The energy from the sea can be obtained mainly in three forms:
(i) Tidal energy,
(ii) Wave energy, and
(iii) Ocean thermal energy.

TIDAL ENERGY:
Tidal Energy the alternate rise and fall of sea water twice a day are known as tides. Tides are caused due to the gravitational pull of the moon and to some extent of sun. The rise of sea water is called "high tide" whereas the fall of sea water is called "low tide." The tidal waves in the sea build up and recede (rise and fall) twice a day. The kinetic energy of moving water of oceans during tides is called tidal energy.

Harnessing tidal energy: The tidal energy can be harnessed by constructing a tidal barrage or tidal dam across a narrow opening to the sea.During high tide, when the level of water in the sea is high, sea-water flows into the reservoir of the barrage and turns the turbines. The turbines then turn the generators to produce electricity. And during the low tide, when the level of sea-water is low, the sea-water stored in the barrage reservoir is allowed to flow out into the sea. This flowing water also turns the turbines and generates electricity. Thus, as sea-water flows in and out of the tidal barrage during high and low tides, it turns the turbines to generate electricity.

Limitations Of Tidal Energy: The tidal energy is not likely to be a potential source of energy in future because of the following reasons:
(i) There are very few sites around the world which are suitable for building tidal barrages (or tidal dams).
(ii) The rise and fall of sea-water during high tide and low tide is not enough to generate electricity on a large scale.

SEA WAVE ENERGY:
Due to the blowing of wind on the surface of sea, very fast seawaves (or water waves) move on its surface. Due to their high speed, sea-waves have a lot of kinetic energy in them. The kinetic energy of moving sea-waves is called sea wave energy. It can be used to run the turbine of a generator to generate electricity. A wide variety of devices have been developed to trap sea-wave energy to turn turbines and drive generators for the production of electricity.
(i) One idea is to set-up floating generators in the sea. These would move up and down with the sea-waves. This movement would drive the generators to produce electricity.
(ii) Another idea is to let the sea-waves move up and down inside large tubes. As the waves move up, the air in the tubes is compressed. This compressed air can then be used to turn a turbine of a generator to produce electricity.

Limitations Of sea wave energy are:
i. Models have beep made to harness this energy but it will be many years before full-size wave-energy generators can be built to harness the sea-waves energy on a large scale.
ii. The harnessing of sea-waves energy would be a viable proposition only at those places where sea-waves are very strong.

OCEAN THERMAL ENERGY (OTE):
A very large area of sea is called an ocean. The water at the surface of an ocean gets heated by the heat of the sun and attains a higher temperature than the colder water at deeper levels in the ocean. So, there is always a temperature difference between the water "at the surface of ocean" and "at deeper levels." The energy available due to the difference in the temperature of water at the surface of the ocean and at deeper levels is called ocean thermal energy (OTE). Ocean thermal energy is one of the forms in which solar energy manifests itself in oceans. The ocean thermal energy can be converted into a "usable form" of energy like electricity.

Harnessing Ocean Thermal Energy (OTE): The devices used to harness ocean thermal energy are called Ocean Thermal Energy Conversion power plants (or OTEC power plants). A temperature difference of 20°C (or more) between the surface water of ocean and deeper water is needed for operating OTEC power plants. In one type of OTEC power plant, the warm surface water of ocean is used to boil a liquid like ammonia or a chlorofluorocarbon (CFC). The high pressure vapours of the liquid (formed by boiling) are then used to turn the turbine of a generator and produce electricity. The colder water from the deeper ocean is pumped up to cool the used up vapours and convert them again into a liquid. This process is repeated again and again.

Advantages of Ocean Thermal Energy:
i. A great advantage of the ocean thermal energy is that it can be used continuously 24 hours a day throughout the year.
ii. Another advantage is that ocean thermal energy is a renewable source of energy.
iii. Its use does not cause any pollution.