NCERT Science Class 10 Chapter 15 Sources of Energy CBSE Board Sample Problems Short Answer

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Write three reasons why construction of Tehri Dam on river Ganga and Sardar Sarovar project on river Narmada is opposed by local people and environmentalist?


  • Rehabilitation problems, loss of agricultural land.

  • Disturbance of ecological balance.

  • Forming of green-house gases due to decomposition of plant in water.


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.


  • Renewable sources of energy are those sources which can be regenerated again. Non-renewable sources of energy are those sources which would get depleted some day and cannot be regenerated.

  • Exhaustible sources of energy are those which will be exhausted some day and cannot be regenerated. Inexhaustible sources of energy are those which can be regenerated again due to some continuing or repetitive currents of energy and are inexhaustible.

  • Options in both cases are exactly same.


Name three forms in which energy from ocean ¡s made available for use. What are OTEC power plants? How do they operate?


  • Muscular energy to perform different activities like taking bath, climbing stairs and muscular energy comes from food we eat.

  • Fossil fuel energy used in bus or car to reach the school and this energy come from fossil fuels like petrol, diesel and CNG.

  • Chemical energy in the form of breakfast and it also comes from the food we eat.


What kind of mirror concave, convex or plane would be best suited for the use in a solar cooker. Why? What is the role of glass sheet used in a solar cooker? Also write two disadvantages of using a solar cooker?


  • Concave mirror will be best suited for the use in solar cooker because it concentrates all the sunlight to a point after reflection from the mirror and thus raises the temperature of that point.

  • Plane glass plate does not allow the infrared or heat radiation entered in the box to go outside the box thus heating the box and creating greenhouse effect.

  • The two disadvantages of using a solar cooker are:

    • Food cannot be cooked at night and on a cloudy day by using a solar cooker.

    • Food cannot be cooked quickly by using a solar cooker.


What are the limitations of extracting energy from?

a. The wind?

b. Waves?

c. Tides?


  • The wind: Wind energy is trapped using windmills. One of the limitations of the wind energy is that the windmills requires a speed more than 15 km/h to generate electricity. And the number of windmills will be more to cover a huge area.

  • Waves: In order to extract energy from the waves, very strong ocean waves are needed.

  • Tides: In order to extract energy from the tides, the sun, the moon and the earth should be in a straight alignment and the tides should be very strong.


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?


a) Renewable and non-renewable:

  • Renewable energy sources are those which replenish on their own and are easily available in nature. Like solar energy, tidal energy, wind energy, bio-mass.

  • Non-renewable energy sources are those which do not replenish on their own and have limited in availability in nature. Like fossil fuels which includes petroleum, coal and natural gas.

b) Exhaustible and inexhaustible:

  • Exhaustible source of energy are those which deplete after few hundred years. Like coal and petroleum.

  • Inexhaustible source of energy are those which do not deplete and are available in abundant quantity. Like solar and wind energy.


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


Following are the two sources of energy that are renewable:

  • Wind: Wind energy is obtained from the air which is blowing at a high speed. Wind energy is trapped using windmills so as to generate electricity. Blowing of air is dependent on uneven heating of the earth. Since the heating of the earth is forever, wind availability will also be forever.

  • Sun: The energy obtained from the sun is known as solar energy. It is produced by the fusion of hydrogen into helium, fusion of helium into other heavy metals and it continues. A large amount of hydrogen and helium is available ¡n sun which will never be exhausted. Hence solar energy is renewable source of energy.


What are the disadvantages of fossil fuels?


The disadvantages of fossil fuels are as follows:

  • Fossil fuels like coal and petroleum results in air pollution as there is release of huge amount of pollutants.

  • Gases such as carbon dioxide is released when a fossil fuel is burnt which causes global warming.

  • Soil fertility and potable water is affected by the oxides of carbon, nitrogen, sulphur, etc., that are released from fossil fuels.


Write two difference between renewable and nonrenewable sources of energy.


  • Renewable source of energy: The energy sources which are inexhaustible, Example: Solar energy, Wind energy, energy of flowing water.

  • Non Renewable Source of energy: The energy sources which are exhaustible,

    • Example: Coal, petroleum, natural gas


Explain why fossil fuels are exhaustible natural resources. Name any two fossil fuels.


  • Fossil fuels require millions of years to form from the dead vegetation and animals that get buried deep inside the Earth.

  • They require high temperature and pressure for their formation, which cannot be provided in the laboratory.

  • Thus, fossils are limited. Therefore, the use of fossil fuels at this rate will lead to their exhaustion.

Coal and petroleum


Discuss the characteristics of a good fuel


  • Easily available

  • Easy to transport

  • High calorific value

  • Does not produce any harmful gases

  • Ignition temperature is neither high nor low


(a) State one limitation of solar energy available from solar cells.

(b) What ¡s geothermal energy?

(C) List two advantages of using bio gas over cow-dung cakes.


  • Solar energy obtained from solar cells is not always available like during night time or during rainy season when the sun is hidden under clouds. Also the energy obtained is very costly, thus cannot be used to produce electricity for domestic purpose.

  • Geothermal energy is defined as the energy obtained from the hot spots under rock beds of earth.

  • Bio gas on burning does not yield smoke whereas cow-dung does.

  • Bio gas also gives us nutrient rich slurry that can be used for crop production.


Name three forms in which energy from the oceans is made available for use. What are OTEC power plants? How do they operate?


  • Three forms of energy from oceans are: Tidal energy, Oceanic wave energy, Ocean thermal energy

  • OTEC power plants are Ocean Thermal Energy Conversion power plants.

  • These plants are operated by boiling the liquids like ammonia or CFC’s, which are volatile substance, for which heat energy is taken from the warm water of ocean surface.

  • These vapours are then used to drive the turbine of the generator to generate electrical energy. There must be a temperature difference of 20 K


(a) What is biomass? Give example of biomass.

(b) Describe the various steps involved in obtaining bio gas.


  • Biomass is a renewable energy source from living plant and animal materials which can be used as fuel.

  • An example of biomass is plant material that produces electricity with steam, animal fossil fuel.

  • Following steps are involved in obtaining bio gas:

    • Mixing (slurry of cattle-dung and water).

    • Digesting (cattle-dung undergoing decomposition by anaerobic bacteria).

    • The process by which the biomass changes into bio gas in the absence of air due to an anaerobic microorganism is termed as an anaerobic decomposition.


Why there is a need to harness non - conventional sources of energy? Give two main reasons.


  • Our demand for energy is increasing to improve quality of life and growth of population.

  • Fossil fuels are non-renewable sources of energy.


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



  • All fossil fuels (coal, petroleum, LPG, natural gas) are the remains of the prehistoric plants and animals. The energy of the fossil fuels is, in fact, that solar energy which was trapped by natural processes a long time ago. (1/2)

  • Solar energy appears in the form of energy of water flowing in rivers as is obvious from the water cycle in nature. It is this energy which generates electricity in hydroelectric power plants. (1/2)

  • Bio-mass energy is also the solar energy absorbed by the plants which convert ¡t into chemical energy through photosynthesis. (1/2)

  • Humans and animal use bioenergy to carry out their life activities. (1/2)

  • Solar energy is one of the main factors responsible for the movement of air in the atmosphere (i.e. the formation of winds) leading to wind energy. (1/2)

  • Solar energy is used directly in solar cookers and other solar energy devices.

  • Solar energy is stored in the oceans in the form of ocean thermal Energy (OTE). (1/2)

  • We, therefore conclude that the sun is the ultimate source of energy.