NCERT Class X Science Class: Chapter – 14. Sources of Energy – Part-4 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 20:

Mention three advantages of a solar cell?


(i) Solar cells have no moving parts

(ii) Requires little maintenance

(iii) They can be set up in remote areas

Question 21:

What is biomass? What can be done to obtain bio-energy using biomass?



Bio energy is the energy which is stored in biological matter or “biomass” .

Working of a biogas plant:

Image Working of a Biogas Plant with a Labelled Diagram

Biogas is a mixture of methane, carbon dioxide, hydrogen and hydrogen sulphide. The major constituent of biogas is methane. Biogas is produced by the anaerobic degradation of animal wastes like cow-dung or plant wastes in the presence of water. The biogas plant has a dome-like structure built with bricks. A 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 micro-organisms that do not require oxygen decompose or break down complex compounds of the cow-dung slurry. It takes a few days for the decomposition process to be complete and generate gases. The biogas is stored in the gas tank above the digester from which they are drawn through pipes for use.

Question 22:

What are the limitations in obtaining energy from wind?


a) It is not available at all times.

b) It requires a very large area of land.

c) it is not possible to have a windmill everywhere as to run it, the minimum wind speed of 15 km/hr is required (only possible in open areas like the seaside) .

Long Answer Questions

Question 23:

Which is the process used to harness nuclear energy these days? Explain it briefly.


Nuclear Fission. Explanation must include all the stepsstarting from splitting of uranium nuclei Nuclear fission process is used to harness nuclear energy these days.

The fission of an atom of uranium, for example, produces 10 million times the energy produced by the combustion of an atom of carbon from coal.

In a nuclear fission, the difference in mass, , between the original nucleus and the product nuclei gets converted to energy E at a rate governed by the equation , where is velocity of light.

In a nuclear reactor designed for electric power generation, such nuclear ‘fuel’ can be part of a self-sustaining fission chain reaction that releases energy at a controlled rate. The released energy can be used to produce steam and further generate electricity.

Image a Boiling Water Reactor (BWR)

Question 24:

How can solar energy be harnessed? Mention any two limitationsin using solar energy. How are these limitations overcome?


Solar energy can be harnessed directly by using solar cooker, solar hater or solar cells.

In solar cooker the heat energy by sun is used to for cooking purposes. In solar heater, heat energy is used to heat the water. In solar cells it is converted to electricity.

Limitations in using solar energy are:

  • Solar energy is not available in night or when the sun is hidden under clouds.
  • The domestic use of solar cells for production of electricity is not possible due to high cost.

These limitations can be overcome by following ways:

  • First limitation can be overcome by converting solar energy into electrical energy and storing them for future use.
  • Second limitations can be overcome by taking certain steps by government to reduce price of solar cells.

Question 25:

Make a list of conventional and non-conventional sources of energy. Give a brief description of harnessing one non-conventional source of energy.


Conventional sources of energy : Coal, petroleum, natural gas, fissionable materials like uranium.

Non-conventional sources of energy : Wind, Hydro Power, the Sun, Ocean Tidal Energy, Interior of the Earth, Biogas, Plants, Vegetable waste etc.

Image Sources of Energy

Biomass : 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. Anaerobic micro-organisms, the presence of water, degrade the bio-mass in the digester. As anaerobic micro-organisms do not require oxygen, the digester is designed like a sealed chamber. The complete degradation process takes few days, during which bio gases are produced. Now the bio gas can be supplied to consumers through pipes. Periodically the slurry, that is left behind in the digester, is removed and it serves as an excellent manure.

Question 26:

Why is there a need for harnessing non-conventional sources of energy? How can energy be harnessed from the sea indifferent ways?


There is a need for harnessing non-conventional sources of energy because of the following reasons: (i) The demand for energy is increasing day by day to meet out the basic requirement of our changed life-styles, growing use of machines and industrialization in order to improve our living standards. (ii) The fossil fuels are non-renewable sources of energy and were formed over millions of years ago and there are only limited reserves of fossil fuels. The energy from the sea can be harnessed in the different forms that are as follows: (i) Tidal energy It 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. (ii) Wave energy A wide variety of devices have been developed to trap huge waves near the seashore for rotation of turbine and production of electricity. (iii) Ocean thermal energy The water at the surface of the sea or ocean is heated by the sun, while the water in deeper sections is relatively cold. This difference in temperature is exploited to obtain energy in ocean thermal energy conversion plants. These plants can operate, if the temperature difference between the surface water and water at depths upto 2 km is 293 K (or 20 °C) or more. The warm surface water is used to boil a volatile liquid like ammonia. The vapors of the liquid are then used to run the turbine of the generator. The cold water from the depths of the oceans is used to condense vapor again to liquid.

Question 27:

What are the environmental consequences of using fossil fuels? Suggest the steps to minimize the pollution caused by various sources of energy including non-conventional sources of energy.


Steps to minimize the pollution are: (i) The pollution caused by burning fossil fuels can be reduced by increasing the efficiency of the combustion process and using various techniques to produce the smokeless appliances. (ii) The air pollution caused by burning of coal or petroleum products can be reduced by afforestation. (iii) The planned and judicious use of energy can minimize the pollution, e. g. use of Liquid Petroleum Gas (LPG) as domestic fuel and Compressed Natural Gas (CNG) in transportation vehicles.

Question 28:

Energy from various sources is considered to have been derived from the sun. Do you agree? Justify your answer.


Yes, I agree with the statement that energy from various sources is considered to have been derived from the Sun. Sun is the ultimate source of energy. All forms of energy are directly or indirectly derived from the solar energy.

  • Wind energy: Due to uneven heating of the Earth՚s surface by the sun rays at two different adjoining places, a pressure difference is created and wind is produced.
  • Wave energy: Strong winds are generated due to the Sun and the waves are generated by strong winds blowing across the sea.
  • Ocean thermal energy: The difference between the water at the surface and the water at a depth in seas and oceans is due to the Sun.
  • Energy from fossil fuels: Fossil fuels such as coal and petroleum are formed when large plants and ancient creatures are buried deep inside the Earth՚s crust under the effect of a lot of pressure and heat.
  • Solar energy: The phenomenon of generation of electricity by light (photoelectric effect) is possible only in the presence of Sun.
  • Biomass: Plants convert the solar energy into chemical energy with the help of inorganic substances such as air and water. These plants are consumed by the animals which produce wastes from which energy is obtained.
  • Energy from flowing water: Water from sea and rivers evaporate and fall at different places of the Earth in the form of snow and rainfall. This water is collected in a dam and when allowed to flow rotates the turbine of the generator and electricity is produced.

Question 29:

What is biomass? Explain the principle and working of a biogas plant using a labelled schematic diagram.


Biomass: The material contained in the bodies of plants and animals is called biomass. It acts as a fuel. It includes wastes from tree and grass crops forestry agricultural and urban wastes. The excreta of living organisms and their bodies after also contribute to the biomass.

Biomass is produced by the anaerobic degradation of animal wastes like cow-dung (or plant waste) in the presence of water. This degradation is carried out by anaerobic micro-organisms called anaerobic bacteria in the presence of water but in the absence of oxygen.

A slurry of cow-dung and water is made in the mixing tank. This slurry flows into the digester having no oxygen. Anaerobic micro-organisms decompose or breakdown complex compounds of the cow-dung slurry. It takes a few days for the decomposition process to be completed and generated bio-gas. The biogas is stored in the gas tank above the digester from which they are drawn through pipes for use.

Image a Slurry of Cow-Dung and Water is Made in the Mixing T …