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

• Amount 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 Sources of Energy

1. Fossil Fuels

• Fuels developed from the fossils. Eg. Coal and 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.

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

• This is our Turbine for generating electricity. Actually the steam/fluid impart energy to rotor of turbine which can move shaft of the generator to produce electricity. A very large amount of fossil fuels are burnt in Thermal Power
Plant to heat up water to produce steam.

• Hydropower Plants Convert the Potential energy of falling water into Electricity since there are few water-falls which could be used a source of potential energy, hence this is the reason, a large number of DAMS are built all over the world.

• Around 25% of our country’s energy requirement is met by Hydro Power Plants.

Dams are constructed to

• Prevent flooding of river, provide water for irrigation and to generate hydroelectricity.

Disadvantages of construction of Big Dams

1. Submerging/Loss of large variety of Flora fauna and human settlements and roting of submerged vegetation release green house gas (CH4).

2. They pose dangers of earthquakes, landslides etc.

3. Biomass is Agricultural and animal wastes that can be used as a fuel. Eg. of Biomass – Firewood, cattle dung, sewage, dry leaves, stems and bagasse.

• Normally biomass has low calorific value and produce lot of smoke when they are burnt. Their efficiency as a good fuel has been increased tremendously with the application of technology. For Eg. cowdung becomes efficient and
cheap good fuel in a Bio-gas plant.

• Charcoal is better fuel than wood because it do not contain water and other volatile material which are present in wood.

• Wood Limited Charcoal Supply of O2 Charcoal burns without smoke, flames and has high calorific value.

• Bio-gas is an excellent fuel and contain 75% of Methane (CH ). It burns 4 without smoke, leaves no reciters like ash, with high heat capacity.

• Biogas is produced by anerobic decomposition of the slurry (cowdung + water mixture) by microbes. This process is applied in a Bio gas plant.

• The Biogas is stored in the gas tank from which they are drawn through pipes for use in a Bio-gas plant.

• Bio gas is used for lighting, cooking in the rural areas. While the slurry left behind is used as excellent manure, rich in nitrogen and phosphorous.

• The large scale use of Bio-wastes and sewage material provide a safe and efficient method of waste-disposal besides supplying energy and manure.


• 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.

• This energy is utilised to lift water from the well and to generate electricity in the wind mill.

• Actually the rotatory motion of the windmill is used to turn the rotor of the turbine which then generate electricity through Dynamo.

• The output of a single windmill is quite small so a number of windmills are erected over a large area – called wind energy farm.

• India Ranked Fifth in the world in harnessing wind energy for the production of electricity. It is estimated that nearly 45,000 MW of electric power can be generated if Indias wind potential is fully exploited.

• The minimum wind speed for wind mill to serve as a source of energy is 20 KMPH.

Advantages of Wind Energy

1. Eco friendly
2. Efficient source of renewable energy.
3. No recurring expenses for production of electricity

 Limitations of Wind Energy

1. Wind energy farms need large area of land
2. Difficulty in getting regular wind speed of 15-20 Km/H.
3. Initial cost of establishing wind energy farm is very high.
4. High level of maintenance of blades of wind mill.

Non Conventional Sources of Energy

1. Solar Energy

• The energy limitted by the sun in form of heat and light is called solar energy.

• Solar Constant = 1.4 kJ/ m²

• Outer edge of the earth receives solar energy equal to 1.4 kJ/m² which is known as solar constant.

• Solar energy devices

A large number of devices that utilize solar energy directly like :

(i) Solar Cooker

(ii) Solar furnaces

(iii) Solar cells

(iv) Solar water heaters

• Solar heating devices

• Use black painted surface because black surface absorbs more heat as compared to white or other surface.

• Use of glass plate because it allows shorter infrared radiations to pass through it but doesn’t allow the longer wavelength infrared radiations to through it, that results in increase in temperature.

(i) Solar Cooker

• Box type solar Cooker.

• 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.


1. Use energy which is available in plenty (Solar Energy)

2. Is pollution free.

3. More than one food can be cooked simultaneously


1. Cannot be used for frying or baking purpose.

2. Food cannot be cooked at night or on a cloudy day.

3. Direction of reflector of solar heating has to be changed from time to time to keep it facing the sun.

(ii) Solar Cells

• Solar cells are device that convert Solar energy into electricity.

• Develops a voltage of 0.5 – IV and can produce about 0.7W of electricity.

Advantages of Solar Cell

1. Require a little maintenance

2. Have no moving part.

3. No focussing device is required

4. Can be set up in remote areas.

5. Environment – friendly i.e. do not cause pollution.

Disadvantage of Solar Cells

1. It require high cost

2. Efficiency is low

3. Initial cost of installation is quite high.

Uses of Solar Cell

1. Used in calculators, watches etc.

2. Used in artificial satellites and space probes.

3. It is used in radio or wireless transmission system.

(iii) Solar Panel

• A large number of Solar Cells connected to each other in an arrangement is called solar panel.

Material used for making solar cells

• Silicon

• Silver is used for inter connection of cells.

Energy from the Sea or Ocean

• The energy from the sea is available in the following forms.

(i) Energy of sea waves

(ii) Tidal energy

(iii) Ocean Thermal Energy.

(i) Energy of Sea Waves

• The waves are generated by the strong winds that blows across the sea.

• The kinetic energy of this moving water rotates the turbine of a generator Limitation When strong winds stop blowing, the generator stops producing electricity.

(ii) Tidal Energy

• The tidal energy possessed by water during tides.

• The tides are caused due to gravitational force of attraction exerted by the moon on the water of the ocean.

• This form of 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.

(iii) Ocean Thermal Energy

• The power plants used to harness the ocean thermal energy is known as “Ocean Thermal Energy Conversion plant) (OTEC)

• Temperature difference between surface water and water at the depth of 2 km should be 20°C or more.

• The warm surface water is used to boil ammonia (liquid).

• The vapours of the liquid are used to run the turbine of generator.

• The cold water (from deeper layers) is pumped up to condense the vapour into liquid.

Geothermal Energy

1. Energy stored as heat inside the earth

2. The steam of underground water is taken out by sinking pipes through holes drilled in the earth’s crust. The steam under high pressure is used to rotate the turbines of the generator to produce electricity.

Nuclear Energy

• Nuclear energy is the energy which is stored in the nucleus of an atom.

• Nuclear energy is of two types

(i) Nuclear fission = nucleus of a heavy atom (Uranium) when bombarded with low energy neutron split (break down) into lighter nuclei and huge amount of energy is released

(ii) Nuclear Fusion = When two lighter nucleic join up to form heavy nucleus and tremendous amount of energy is released.

• Nuclear fission process is utilized in nuclear reactors to produce electricity.

• Major Nuclear power plants : Tarapur, Rana Pratap Sagar, Kalpakkam.

Advantages of Nuclear Energy

1. Large amount of energy is released.
2. In nuclear power plant, the nuclear fuel is inserted once to get energy over a long period of time.

Disadvantages of Nuclear Energy

1. High cost of installation.

2. Environmental contamination may occur due to imporper nuclear waste disposal.

Environmental Consequences

1. Energy sources should be used judiciously otherwise it would disturb the environment.

2. Use of clean fuels like CNG (compressed natural gas) because burning of fossil fuel causes green house effect.

3. Assembly of devices like solar cell (otherwise renewable source of energy) would have caused environmental damage.

⇒ How long will energy source last

• The sources of energy can be divided into two categories

(i) Renewable sources of energy e.g., wind, sun, bio-gas

(ii) Non renewable sources of energy eg. Coal, Petroleum, Natural Gas. Continuous use of non renewable source of energy is a matter of concern because ultimately the deposit of these sources will be completely finished on the other
hand renewable sources of energy will last forever eg sun as a source of energy will be available for a very long period of time.

Important Questions

1. What is good source of energy ?

Ans. A good source of energy should have the following qualities :

(i) It should be easily available.

(ii) It should do a large amount of work per unit volume/mass.

(iii) It should be easy to store and transport.

(iv) It should be economical.

(v) It should not cause environmental pollution.

2. What is a good fuel ?

Ans. A good fuel is that which releases more heat on burning, is easily available at an economic rate and does not produce smoke.

3. What are the disadvantages of fossil fuels ?

Ans. There are following disadvantages of fossil fuels like coal and petroleum :

(i) Burning of coal or petroleum causes air pollution.

(ii) Acidic oxides like oxides of carbon, nitrogen and sulphur are released on burning fossil fuels. These oxides lead to acid rains, which adversely affect out water and soil resources.

(iii) Carbon dioxide gas also causes greenhouse effect in atmosphere.

(iv) Fossil fuels are non-renewable sources of energy.

4. What are the limitations of the energy that can be obtained from the oceans ?

Ans. Energy from the oceans can be obtained in the form of tidal energy, wave energy and ocean thermal energy. But these energy sources suffer from the following limitations :

(i) There are very few locations where dams to utilize tidal energy can be built.

(ii) Cost of installation of power houses is extremely high and efficiency of plants is comparatively small.

(iii) Power plants built in oceans or at sea-shores will need high continuous maintenance as chance of corrosion are extremely high.

5. What is geothermal energy ?

Ans. Geothermal energy is the heat energy present inside earth in certain regions called hot spots. Due to geological changes, molten rocks formed in the deeper hot regions of earth’s crust are pushed upwards and are trapped in hot spots. When underground water comes in contact with the hot spot, steam is generated. This steam is routed through a pipe to a turbine and used to generate electricity.

6. What is fuel ?

Ans. Fuel is that which produces heat energy on burning e.g., firewood, charcoal, coal, petroleum, cooking gas etc.

7. Why is the solar cooker box covered with a plane glass plate ?

Ans. The glass plate allows solar radiation to pass through it but the heat radiation form inside are not allowed to escape by the glass plate. Thus, it helps in maintaining the temperature inside the cooker high.

8. Identify the part of a solar cooker that produces greenhouse effect.

Ans. The plan glass plate cover.

9. What is the chief source of wind energy ?

Ans. Sun is the chief source of energy. Wind is caused by unequal heating of land mass and water bodies by solar radiation.

10. How does construction of dams across the river get linked with the production of greenhouse gases ?

Ans. The vegetation of submerged areas of forests rots under anaerobic conditions and gives rise to methane, which is a greenhouse gas.

11. Write the percentage of methane in bio-gas ?

Ans. 75% of bio-gas is methane.

12. List two nutrients that the slurry left behind in a bio-gas plant contains.

Ans. It serves as an excellent manure because it is rich in nitrogen and phosphorus.

13. Define the process of nuclear fission ?

Ans. Splitting up of a nucleus of uranium, when bombarded by neutrons, into two lighter nuclei along with release of a large amount of energy is called nuclear fission.

14. Explain, why fossil fuels are classified as non-renewable sources of energy.

Ans. Fossil fuels were formed due to certain very slow changes occurring inside the earth under special circumstances spread over millions of years. Since these conditions are not prevailing now, it is not possible to replenish the amount of fossil fuels being consumed. Hence, these are classified as non-renewable sources of energy.

15. Name the process for obtaining charcoal from wood. What are the advantages and disadvantages of burning charcoal over wood ?

Ans. Charcoal is obtained by destructive distillation of wood.

Advantages : (i) It burns easily producing no smoke. (ii) It gives twice as much heat as is obtained by burning the same mass of wood.

Disadvantages : It is expensive. One kilogram of wood gives only 1/4 kg of charcoal by destructive distillation.

16. State any three limitations of harnessing nuclear energy.

Ans. Three main limitations of harnessing nuclear energy are as follows :

(i) The most important problem is the storage and disposal of spent or used fuel because it is highly radioactive. Improper nuclear waste storage and disposal may lead to environmental contamination.

(ii) There is a risk of accidental leakage of nuclear radiation from the nuclear reactor.

(iii) Cost of installation of a nuclear power plant is very high.

17. Burning of fossil fuels causes lots of air pollution. Generally these fossil fuels are used in our vehicles.

(i) As an aware citizen, list two ways to reduce air pollution caused due to vehicles.

(ii) Write two harmful effects other than air pollution, that are caused due to burning of fossil fuels.

Ans. (i) Two ways to reduce air pollution caused due to vehicles are as follows :

(a) The petrol used should be free of lead and sulphur impurities because oxides of these elements are toxic and cause much air pollution.

(b) Vehicles using diesel should be replaced by vehicles using CNG as fuel.

(ii) Two harmful effects, other than air pollution, caused due to burning of fossil fuels are as follows :

(a) Burning of fossil fuels produce oxides of nitrogen and sulphur. These gases are acidic in nature and lead to acid rain which adversely affects our water and soil resources.

(b) Burning of fossil fuels produce carbon dioxide, which is a greenhouse gas and leads to global warming causing a lot of environmental and ecological problems.

18. Name two semiconductor used in solar cell. What is solar cell panel ? State two main advantages of solar panel.

Ans. Silicon and gallium are two semiconductors commonly used in solar cell. A solar panel is an arrangement in which a large number of solar cells are combined so that. it can deliver enough electricity for practical use.

Solar panels have following advantages :

(i) They have no moving parts, require little maintenance and work quite satisfactorily without the use of any focussing device.

(ii) These can be set up in remote and inaccessible hamlets or very sparsely populated areas where laying of a power transmission line is not commercially viable.

19. How can one say that Sun is the main source of energy on the earth ?

Ans. Solar energy manifestats itself in many forms through many processes both biological and physical. plants utilise solar energy for photosynthesis. The solar energy trapped by land and water bodies brings about many physical changes resulting in wind, storm, ocean waves etc. Also fossil fuels e.g., coalm natural gas, petrol etc. have been formed due to very slow changes which occurred millions of years ago and were related to solar energy. Energy of bio-gas etc., is also derived from solar energy.

20. What is bio-mass ? Explain the principle and working of a bio-gas plant using a labelled diagram.

Ans. Animal dung, crop residue, vegetable waste and sewage constitute the bio-mass. A bio gas plant as shown in NCERT book . In a bio-gas plant slurry of animal dung or other types of bio-waste and water is made in the mixing tank and then fed into the digester. The digester is usually a closed underground tank. In digester, the action of micro-organisms decomposes the complex compounds of the bio-mass in the slurry. In the presence of water anaerobic micro-organisms degrade the bio–mass in the digester. An anaerobic micro-organisms do not require oxygen, the digester is designed like a sealed chamber. The compete degradation process takes few days, during which bio-gases are produced. Noe the bio-gas can be supplied to consumers through pipes. Periodically the slurry, that is left behind in the digester, is removes and it serves as an excellent manure.

21. Explain, how does a hydro power plant produce electricity.

Ans. A hydro power plant converts the potential energy of stored water into electricity.

To produce hydro power electricity, a high rise dam is constructed at a suitable place on the river to obstruct the flow of water and thereby collect water in a large sized reservoir. Due to rise of water level, the kinetic energy of flowing water is transformed into potential energy of stored water.

The water from the high level in the dam is carried through sluice gates and pipes to the turbine of electric generator, which is fitted at the bottom of the dam. Due to flowing water, turbine is rotated at a fast rate and electricity is produced.