NCERT Solutions for class 10th Science Chapter 14 Sources of Energy


Question 1. What is a good source of energy?

Sol. If a source of energy is easily accessible, easy to store and transport, has a very good calorific value and available cheaply then it is a good source of energy. A good source of energy has the following characteristics:

  1. It could do a large amount of work per unit volume.
  2. It burns easily in air at a moderate rate.
  3. It should not leave behind any undesirable substances.
  4. It should have high calorific value.

Question 2. What is a good fuel?

Sol. A fuel is said to be good fuel if it produces a large amount of heat on burning without producing a lot of smoke, and is easily available.
A good fuel should have the following characteristics:

  1. It should have high calorific value.
  2. It should have low ignition temperature.
  3. It should not leave more ashes.
  4. It should be less harmful gases on burning.
  5. It should be economic and easy to handle.

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

Question 4. What are the disadvantages of fossil fuels?

Sol. The disadvantages of fossil fuels are as follows:

  1. They are formed under the earth’s surface by a long process which takes million of years to form.
  2. It is non-renewable source of energy.
  3. It causes air pollution.

Note: In addition to air pollution, they also produce carbon dioxides etc. leading to green house effect.

Question 5. Why are we looking at alternate sources of energy?

Sol. 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.

Question 6. How has the traditional use of wind and water energy been modified for our convenience?

Sol. Following are the traditional uses of wind and water energy that has been modified for our convenience:
(i) Wind mills which have been used traditionally used are now being converted to wind energy farms. These farms have large number of wind mills to increase energy throughout.
(ii) Use of water energy needs the construction of big dams. These dams are being constructed near to a big river and it is more expensive. To improve the efficiency of water energy, the technology is being developed to increase the efficiency of devices used to convert water energy into electrical energy.

Question 7.What kind of mirror-concave, convex or plane- would be best suited for use in a solar cooker? Why?
Sol. Concave mirror is best suited for use in a solar cooker because this mirror converges the sunlight into the solar cooker, which helps the food to cook fast.

Question 8. What are the limitations of the energy that can be obtained from the oceans?

Sol. The energy from the ocean can be obtained mainly in three forms:

  1. tidal energy
  2. wave energy
  3. ocean thermal energy

But these energy can not be a potential source of energy in future because of the following reasons:

  • (a) There are very few places around the world which are suitable for building tidal dams.
  • (b) The rise and fall of sea-water during high and low tides is not enough to generate electricity on a large scale.

Question 9. What is geothermal energy?

Sol. It is a form of heat energy which is available under the earth’s crust. Temperature, inside the earth’s crust in the deeper levels is high as compared to temperature near the surface of earth i.e. as depth increases temperature also increases. Due to this heat, rocks melt to form hot magma and heat starts flowing from deeper levels to upper levels due to which heat gets collected at some depth below the earth’s surface. Such places are known as hot spots. These hot spots are the main source of geothermal energy. Water in contact with these hot spots turns into steam which can be utilized to generate electricity.

Question 10.What are the advantages of nuclear energy?

Sol. The advantages of nuclear energy are as follows:

  1. It doesn’t produce gases like carbon dioxide and sulphur dioxide which contribute to pollution.
  2. It produces a large amount of useful energy from a very small amount of nuclear fuel.
  3. The nuclear fuel (like U-235), once kept in nuclear power plants, produce electricity for a long time.

Note: The energy produced is tremendous and much larger as compared to the same amount of coal or any other fuel

Question 11. Can any source of energy be pollution-free? Why or why not?

Sol. No, source of energy can not be said pollution free because every source of energy cause some type of pollution, e.g., the wastes of nuclear reaction (source of nuclear energy) are very dangerous to the environment, a solar cell is pollution free in actual operation but the assembly of the device might have caused some damage to the environment. So, in absolute sense, no source of energy can be called pollution free.

Question 12. Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel than CNG ? Why or why not?

Sol. Hydrogen is a cleaner fuel than CNG because burning of CNG gives out CO
and CO2 gases but burning of does not give any such harmful gases.

Question 13. Name two energy sources that you would consider to be renewable. Give reasons for your choices.

Sol. Solar energy and wind energy are two kinds of renewable energy whose sources are the Sun and the wind respectively because both of these sources are available everywhere and can be used to produce other forms of energy like electric energy.

Question 14. Give the names of two energy sources that you would consider to be exhaustible. Give reasons for your choices.

Sol. Coal and petroleum are two exhaustible sources of energy. Both are the reserve inside the earth which are in a limited amount and can exhaust after some days if it will be used at present rate.


Question 1. 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.

Sol. (b) A solar water heater cannot be used to get hot water on a cloudy day because heat radiations coming from the Sun do not reach the solar water heater.

Question 2. Which of the following is not an example of a biomass energy source?
(a) Wood (b) Gobar-gas
(c) Nuclear energy (d) Coal

Sol. (c) Biomass is a source of energy that is obtained from plant materials and animal wastes. Nuclear energy is released during nuclear fission and fusion. Therefore, nuclear energy is not an example of biomass energy source.

Question 3. 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) Biomass.

Sol. (c) Nuclear energy is released during nuclear fission and fusion. But geothermal, wind energy and biomass they are the sources of energy which are derived either directly or indirectly from the Sun’s energy.

Note: Nuclear energy is generated by nuclear fission or nuclear fusion. The other forms such as geothermal/biomass etc. are ultimately derived from Suns energy.

Question 4. Compare and contrast fossil fuels and the Sun as direct sources of energy.

Sol. Comparison:
(i) Fossil fuel and the Sun both are widely used sources of energy.
(ii)They both are the natural sources.


Note: It is estimated that the Sun will continue radiating energy at the present rate for approximately 5 billion years more.

Question 5. Compare and contrast biomass and hydroelectricity as sources of energy.

Sol. Comparison :
(i) Both are non-conventional sources of energy.
(ii) Working cost of both sources is very low.

Question 6. What are the limitations of extracting energy from –
(a) the wind? (b) waves? (c) tides?

Sol. Limitation of extracting energy from
(a) Wind: (i) Everywhere and any time the flowing air is not available.
(ii)To generate electricity the wind should have speed more than 15 km/h.
(b) Waves: (i) The wave is not available source of energy to generate electricity.
(ii)It is costly to set up device to trap wave energy.
(c) Tides: (i) The location for constructing dam for harnessing tide are limited.
(ii)Construction of dams is a very costly setup.

Question 7. 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?

Sol. (a) If the source of energy can be converted back to its original form after extracting energy, then it is said to be renewable source of energy, e.g., hydroelectricity.
If the source of energy cannot be converted back into its original form. It is said to be non-renewable source of energy, e.g., coal and petroleum.
(b) Sources of energy that will not exhaust in significant time are called inexhaustible sources, e.g., Sun, water, etc. Sources of energy that will exhaust in reasonable time are called exhaustible source, e.g., coal. Renewable sources of energy produce inexhaustible energy whereas non-renewable sources produce exhaustible energy.

Note: Use of non-renewable sources such as fossil fuels has environmental consequences. Also, they will be depleted some day. So, use of renewable energy source as biomass etc. should be preferred. They can be regenerated by planting the trees.

Question 8. What are the qualities of an ideal source of energy?

Sol. The following are the qualities of an ideal source of energy:

  1. It should give large amount of work per unit volume or mass of fuel.
  2. It should be easily accessible.
  3. It should be easy to store and transport.
  4. It should be economical.

Note: The ideal source will produce less waste and will have least environmental impact.

Question 9. What are the advantages and disadvantages of using a solar cooker? Are there places where solar cookers would have limited utility?

Sol. Advantages :

  1. It cooks food without causing any pollution.
  2. It is economical to use a solar cooker.
  3. It is easy to handle a solar cooker.
  4. The nutrients of food do not get destroyed.

Disadvantages :

  1. It cannot be used at night and in cloudy day.
  2. It takes more time to cook food.
  3. The direction of solar cooker is to be changed continuously towards the direction of Sun.
  4. Solar energy is not available at all time and at all the place.
  5. It cannot be used for baking and frying purpose.

At south pole, sunlight doesn’t reach. Therefore, a solar cooker cannot be utilised at this place.

Question 10. What are the environmental consequences of the increasing demand for energy? What steps would you suggest to reduce energy consumption?

Sol. Affluent life style and industrialization have increased the demand of energy.
Consequences are given below:

  1. The increase of demand can make excessive exploitation of sources of energy which disturbs the environmental balance.
  2. The increase of demand makes excessive use of conventional sources of energy. Thus, energy crisis may occur in future.

Following steps can be suggested to reduce the energy consumption:

  1. Misuse of energy should be avoided.
  2. Renewable sources of energy, such as solar energy, tidal energy, etc., should be used.
  3. Pollution free sources of energy should be used.
  4. Best possible technology should be made available. For example, smokeless chullas should be preferred to traditional chullas.
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