Sources of Energy NCERT Exemplar Problems

Long Answer Questions

Question 23: Which is the process used to harness nuclear energy these days? Explain it briefly.

Answer: Nuclear fission is the process during which a nucleus breaks to form two nuclei. The process generates a huge amount of energy. This phenomenon is utilized in nuclear power plants. The heat energy, thus produced is utilised to make steam from water. The steam is channelised to run turbine. The turbine, in turn, generates electricity.

Question 24: How can solar energy be harnessed? Mention any two limitations in using solar energy. How are these limitations overcome?

Answer: Solar energy can be harnessed in various ways. Solar cooker is a simple tool to cook food using solar energy. Solar furnace is a device which is composed of concave mirror. Solar furnace can be made of huge size, depending upon need. Solar panels are composed of solar cells or photovoltaic cells. These cells convert solar energy into electrical energy which is stored in a battery, for later use.

To limitations of solar energy are as follows:

• Technology is new and hence solar panels are costly
• Solar energy can be harnessed only at select places and that too during daytime only

Ovecoming limitations: With suitable growth in technology, solar panels are becoming more economical by every passing year. Electricity from Solar energy can be stored in batteries for later use so that it can be used even during night.

Question 25: Make a list of conventional and non-conventional sources of energy. Give a brief description of harnessing one nonconventional source of energy.

Answer: List of Conventional Energy: Petrol, coal, windmill, hydel energy, firewood

Non-conventional Energy: Solar energy, Tidal Energy, Geothermal Energy, Ocean Thermal energy

Ocean Thermal Energy: The water at sea surface is hot during daytime, while the water at lower level is cold. The temperature differential in water levels can be utilized to generate energy. If the temperature differential is more than 20°C, then ocean thermal energy can be utilized from that place. For this, a volatile liquid, like ammonia, is boiled using the heat from the warm water at the surface. The steam of the volatile liquid is utilized to run the turbine to generate electricity. Colder water from the surface below is utilized to condense ammonia vapour which is then channelized to the surface to repeat the cycle.

Question 26: Why is there a need for harnessing non-conventional sources of energy? How can energy be harnessed from the sea in different ways?

Energy from Sea can be harnessed in following ways:

Tidal Energy: Due to the gravitational pull of the moon, tides happen near seashores. Water rushes up near the seashore during a high tide and goes down during a low tide. Dams are built near seashores to collect the water which comes during a high tide. When the water runs back to the ocean, the flow of water can be utilized to generate electricity.

Wave Energy: Waves can also be a good source of energy. Many devices are being designed and tested to produce wave energy. For example; a hollow tower is built near the seashore. When water gushes in the tube because of wave, it forces the air upwards. The kinetic energy of air in the tube is used to run a turbine. When the wave goes down, air from up goes down the tube which is also used in running the turbine.

Ocean Thermal Energy: Detail about this is given in answer to previous question.

Question 27: What are the environmental consequences of using fossil fuels? Suggest the steps to minimise the pollution caused by various sources of energy including non-conventional sources of energy.

Answer: Following are the environmental consequences of using fossil fuels:

• Air pollution
• Acid rain
• Global warning

Steps to minimize pollution:

• Catalytic converters should be fitted in chimneys and in automobile exhaust
• Public transport system should be well developed
• Bicycle tracks should be built in cities
• We should follow the principles of three Rs, i.e. Reduce, Reuse and Recycle.

Question 28: Energy from various sources is considered to have been derived from the sun. Do you agree? Justify your answer.

Answer: It is a correct statement that energy from various sources is considered to have been derived from the sun. Here are some examples.

• Plants harness solar energy to make food, and this food is directly or indirectly utilized by all other living beings.
• Coal and petroleum were formed from buried remains of plants and animals. This shows that energy stored in coal and petroleum has come from the sun.
• Wind movement is caused by differential heating of earth’s surface by sun. So, kinetic energy in wind is because of sun.
• Ocean currents are formed due to differential heating by sun. So energy from ocean is because of sun.

So, we can safely say that sun is the ultimate source of energy.

Question 29: What is biomass? Explain the principle and working of a biogas plant using a labelled schematic diagram.

Answer: Plant or animal material used as fuel is called biomass. Farm waste, such as stalks of harvested plants and dung of cattle, can be used to generate methane. The decomposition of biomass produces methane, which can be channelized for useful purposes.

Bio-gas Plant: Bio-gas plant can be very useful in solving the energy need of rural areas. A bio-gas plant is a dome-like structure which is usually built from bricks and concrete. In the mixing tank, the slurry is made from cow-dung and water. The slurry then goes to the digester, which is a closed chamber. Since oxygen is absent in the digester, the anaerobes carry on their work of decomposition. The process of decomposition produces biogas. Biogas has about 70% of methane and the rest is composed of other gases. The biogas is channelized through a pipe and can be utilized as kitchen fuel and also as fuel for getting light. The slurry, left behind, is removed. It is used as manure, once it dries.

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