NCERT Exemplar Solutions
Class 10 - Science - CHAPTER 14: Sources of Energy

Long Answer Questions

Nuclear fission is the process used to harness nuclear energy. In this process, a heavy nucleus such as uranium-235 is bombarded with slow neutrons.

The nucleus becomes unstable and splits into two lighter nuclei, releasing a large amount of heat energy along with more neutrons. These neutrons further cause fission of other nuclei, resulting in a chain reaction.

The heat produced during the reaction is used to convert water into steam. This steam rotates the turbine connected to a generator, thus producing electricity. Proper control rods are used to regulate the speed of the chain reaction and ensure safety. The spent nuclear fuel is carefully disposed of as it remains radioactive for a long time.

Solar energy can be harnessed using solar devices such as solar cookers, solar cells, and solar heaters. Solar cookers use reflectors to concentrate sunlight, while solar cells directly convert sunlight into electricity using photovoltaic effect.

Limitations:

1. Solar energy is available only during daytime and in sunny weather.
2. Solar devices require large installation areas and are costly.

Overcoming limitations:

1. Using improved solar cell technology that works even in diffused sunlight.
2. Developing efficient batteries to store solar energy for use during nighttime or cloudy days.

Conventional sources: Fossil fuels (coal, petroleum), natural gas, water (hydropower), wind energy.

Non-conventional sources: Nuclear energy, solar energy, biomass, geothermal energy, energy from oceans (tidal, wave, OTEC).

Harnessing solar energy:

Solar energy is harnessed using solar panels that contain several photovoltaic cells. These cells convert sunlight directly into electricity. Solar cookers and water heaters use the heat of the sun with the help of reflectors or insulated boxes. Solar energy is clean and renewable, making it an important non-conventional energy source.

Need for non-conventional energy:

1. Fossil fuels are depleting rapidly and cannot meet future demands.
2. Population growth and increased energy needs require cleaner and renewable alternatives.
3. Non-conventional energy sources reduce pollution and improve quality of life.

Energy from the sea:

1. Tidal energy: Generated by the rise and fall of sea levels. Turbines in tidal dams convert kinetic energy of moving water into electricity.
2. Wave energy: Sea waves strike floating devices that drive turbines to generate electricity.
3. Ocean Thermal Energy Conversion (OTEC): Uses temperature difference between warm surface water and cold deep water to operate a heat engine that produces electricity.

Environmental consequences of fossil fuels:

1. Air pollution due to emission of harmful gases such as carbon dioxide, nitrogen oxides, and sulphur dioxide.
2. Acid rain resulting from atmospheric pollutants.
3. Global warming caused by excessive release of greenhouse gases.
4. Environmental degradation and health hazards.

Steps to minimise pollution:

1. Use of smokeless fuels and improved technologies.
2. Using refined and cleaner fuels.
3. Promoting renewable energy sources such as solar, wind, and biomass.
4. Adopting energy-efficient appliances and reducing wastage of energy.

Yes, most energy sources are directly or indirectly derived from the sun. Solar energy is the primary source that drives natural processes.

Justification:

• Solar energy supports plant growth through photosynthesis, which forms the basis of biomass energy.
• Wind energy is produced due to unequal heating of the Earth’s surface by the sun.
• Hydropower depends on the water cycle, which is driven by solar heat causing evaporation and rainfall.
• Fossil fuels were formed from plants and animals that originally obtained energy from the sun.

Thus, the sun is the ultimate source of energy for most renewable and non-renewable energy forms.

Biomass refers to organic matter such as plant and animal waste that can be used as a source of energy.

Principle: Biomass undergoes anaerobic decomposition by microorganisms to produce biogas, which mainly contains methane.

Working of biogas plant:

1. Biomass such as cow dung is mixed with water to form slurry.
2. The slurry is fed into a digester, an airtight tank where anaerobic bacteria decompose the organic matter.
3. Biogas produced during decomposition collects at the top of the digester.
4. The gas is drawn through pipes and used as fuel for cooking and lighting.
5. The remaining slurry is removed periodically and used as high-quality manure.