CG Board Class 8 Science Chapter 12 Solutions: Sources of Energy (ऊर्जा के स्रोत) – 2026-27

Water stored at a high level in a dam reservoir possesses gravitational potential energy. When this water is released to fall from a height onto the blades of a hydro-turbine, its potential energy converts into kinetic energy, rotating the turbine. The spinning turbine turns an electric generator connected to it, transforming mechanical energy into electrical energy (hydroelectricity).

Tidal energy is the energy generated from the natural rise and fall of ocean water levels (tides) caused by the gravitational pull of the moon and sun. Dams built across the mouth of sea bays channel water into turbines during high tide and release it back during low tide. The moving water spins turbines connected to generators to produce electricity. In India, suitable tidal energy sites exist at the Gulf of Kutch in Gujarat and the eastern coast near West Bengal.

Nuclear energy is the enormous amount of energy liberated when the nucleus of a heavy radioactive atom, such as Uranium, is split into smaller nuclei. This splitting process is called nuclear fission. Nuclear reactors (such as those at Tarapur, Kalpakkam, Kota, and Narora) harness this energy to produce electricity.

In a thermoelectric (thermal power) plant, coal is burned as the fuel in boilers to heat water and convert it into high-pressure steam. The pressure of this steam turns turbines connected to electric generators to produce electricity. (In Chhattisgarh, a major thermoelectric plant is located in the Korba district).

Petroleum pumped out from deep underground wells is refined in oil refineries to produce useful products including

1. Petrol
2. Diesel
3. Kerosene (lamp oil)
4. Liquefied Petroleum Gas (LPG)
5. Lubricating oil, paraffin wax, and bitumen (asphalt).

Other Name: Liquefied Petroleum Gas (LPG**).

• Constituents: It mainly consists of Butane (C_4H₁₀) gas, along with smaller proportions of Propane (C_3H₈) and Ethane (C_2H₆).

(Note: A strong-smelling chemical, ethyl mercaptan, is added to LPG cylinders so leaks can be detected quickly for safety).

Biogas is produced when cow dung mixed with water decomposes in covered pits (digesters) in the absence of air. It is a mixture of

1. Methane (CH₄) – Major combustible component
2. Carbon dioxide (CO₂)
3. Traces of Hydrogen, Nitrogen, and Hydrogen sulfide gases.

A black surface is a very good absorber and a poor reflector of heat radiation. Painting the inside walls and metal containers of a solar cooker black allows them to absorb the maximum possible heat from focused sunlight, rapidly raising the interior temperature to cook food efficiently.

In a solar water heater, solar light/radiant energy is transformed into heat (thermal) energy, which warms the water circulating through the blackened copper pipes.

Solar cells, which convert solar energy directly into electrical energy, are made of semiconductor materials, primarily Silicon.

Copper is an excellent conductor of heat. Making the solar water heater pipes out of copper ensures that the heat absorbed from sunlight is transferred quickly and efficiently to the water flowing inside the pipes.

To conserve electrical energy at home, we can

1. Switch off lights, fans, tubelights, and electrical appliances immediately when leaving a room or when not in use.
2. Use energy-efficient LED bulbs and tubelights instead of old filament bulbs.
3. Maximize the use of natural sunlight during daytime instead of artificial lighting.
4. Keep electrical devices (refrigerators, coolers, air conditioners) well-maintained to avoid excess power loss.

Bio-diesel is obtained from the oil extracted from the seeds of a plant named ‘Ratanjoth’ (Jatropha curcas). (The Chhattisgarh government established the Chhattisgarh Biofuel Authority to promote Ratanjoth plantation on wasteland).

In the future, as fossil fuels deplete, we can obtain clean energy from non-polluting renewable sources such as

1. Solar energy using advanced photovoltaic solar panels and solar cells.
2. Wind energy using wind farms.
3. Ocean energy (Tidal energy and Wave energy).
4. Bio-fuels like Bio-diesel and Biogas.
5. Hydrogen fuel cells and Geothermal energy.

(b) Petroleum**

(c) Diesel**

(d) Silicon**

(c) Electrical energy**

(b) Bio diesel**

Four major sources of energy are

1. Sun (Solar Energy)
2. Wind (Wind Energy)
3. Water (Hydroelectric Energy)
4. Fossil fuels (Coal and Petroleum)

Benefits of Solar Energy:**

1. It is a renewable, everlasting, and unlimited source of energy.
2. Its use does not produce smoke or toxic gases, so it causes no environmental pollution.
3. It is freely available to everyone in nature.

• Two Examples of Use in Daily Life:

1. Solar Cooker: Used for cooking food using sunlight heat.
2. Solar Water Heater: Used for heating water for bathing and domestic purposes.

Production Process:**

Water stored behind high dams possesses potential energy. When released through penstock pipes from a height onto the blades of a turbine, kinetic energy rotates the turbine at high speed. The turbine rotates an attached electric generator, transforming mechanical energy into electrical energy (hydroelectricity).

• Hydroelectric Projects in Chhattisgarh:

1. Hasdeo Bango Hydroelectric Project: Located on Hasdeo river at Machadoli (Korba district).
2. Gangrel (Ravishankar Sagar) Hydroelectric Project: Located on Mahanadi river in Dhamtari district.
3. Sikaser Hydroelectric Project: Located on Pairi river in Gariaband district.

By refining raw petroleum in oil refineries, the following useful products are obtained

1. Petrol
2. Diesel
3. Kerosene (lamp oil)
4. Liquefied Petroleum Gas (LPG)
5. Lubricating oil
6. Paraffin wax
7. Bitumen (asphalt/tar)

Biogas:** Biogas is a combustible clean gaseous fuel obtained by the decomposition of animal dung (cow dung) and organic wastes in the absence of oxygen. It is primarily a mixture of methane (CH₄) and carbon dioxide (CO₂).

• Production Process:

Cow dung is dissolved in water to make a slurry and poured into a covered underground pit (biogas plant digester). In the absence of oxygen, anaerobic microorganisms decompose this organic slurry. The gas produced accumulates in the dome above the pit and is drawn out through pipes for cooking and lighting.

• Process Representation:

Cowdung + Water Slurry → [Anaerobic Digestion (No Air)] Methane (CH₄) + Carbon dioxide (CO₂) + Slurry Manure

Energy Sources in Surroundings:** Sunlight, Wind, Flowing water, Coal, Petrol, Diesel, LPG, Kerosene, Biogas, Wood.

• Categorization Table:

Liquefied Petroleum Gas (LPG) mainly contains Butane (C_4H₁₀) as its major constituent. In addition to butane, it also contains smaller quantities of Propane (C_3H₈) and Ethane (C_2H₆).

Choice: Based on the abundant sunlight and agrarian resources in Chhattisgarh, I would like to install a Solar Photovoltaic Power Station or a Biogas Power Station**.

• Explanation:

1. Solar Power Station: Chhattisgarh experiences bright sunny weather for nearly 300 days a year. Solar power stations generate clean electricity directly from sunlight without consuming fossil fuels or creating smoke and pollution.
2. Biogas Power Station: In rural and agricultural areas, cattle dung and crop wastes are available in large quantities. Converting this biomass into biogas produces clean cooking gas/electricity and yields nutrient-rich organic manure for fields.