Chapter 14 — Sources of Energy
Welcome to HSLC Guru! This complete English-medium guide to Class 10 Science Chapter 14 — Sources of Energy is designed for ASSEB (Assam State School Education Board) students preparing for the HSLC examination. Energy is the backbone of modern civilisation — it powers our homes, industries, transport and communication. In this chapter we shall study what makes a fuel a good source of energy, examine conventional sources such as fossil fuels and hydropower, and explore non-conventional sources like solar, wind, tidal, geothermal and nuclear energy. The notes below cover the full NCERT textbook Q&A, additional MCQs, fill in the blanks, true/false practice and a handy glossary.
Chapter Summary
What is a good source of energy? A good source of energy must produce a large amount of work or heat per unit volume or mass, must be easily accessible, easy to store and transport, economical and should cause minimum pollution. No single source meets all these conditions perfectly, so societies use a mix of sources. Sources of energy are broadly classified as conventional (used for centuries — fossil fuels, hydropower, biomass) and non-conventional (developed recently — solar, wind, tidal, nuclear, geothermal). They can also be classified as renewable (sun, wind, water, biomass) and non-renewable (coal, petroleum, natural gas, uranium).
Conventional sources of energy. Fossil fuels — coal, petroleum and natural gas — were formed from the remains of plants and animals buried under the earth millions of years ago. They have high calorific value but their combustion releases carbon dioxide, oxides of sulphur and nitrogen, causing air pollution, acid rain and the greenhouse effect. Thermal power plants burn coal or oil to convert water into steam which rotates turbines to generate electricity. Hydropower plants are built across rivers; water stored at a height drives turbines. They are clean and renewable but submerge large areas, displace people and disturb ecosystems. Biomass — wood, cow dung, agricultural waste — is the oldest fuel of mankind. A biogas plant produces a clean gas (about 75% methane) by anaerobic decomposition of slurry; the spent slurry is excellent manure. Charcoal burns without flame, with little smoke and has higher heat-generating efficiency than wood. Wind energy is harnessed in wind farms where the kinetic energy of wind rotates the blades of windmills coupled to generators; it requires wind speed above 15 km/h.
Non-conventional sources of energy. Solar energy reaches the earth as electromagnetic radiation. Solar devices include the solar cooker (an insulated box with a glass cover and reflector that traps heat by the greenhouse effect, reaching 100–140 °C), the solar water heater, and the solar cell which converts sunlight directly into electricity using semiconductors like silicon. Solar panels are used in artificial satellites, calculators, traffic signals and remote villages. Energy from the sea takes three forms — tidal energy harnessed by building a dam across a narrow opening to the sea; wave energy captured by devices that use the up-and-down motion of waves; and ocean thermal energy (OTEC) which exploits the temperature difference (at least 20 °C) between warm surface water and cold deep water to vapourise a working fluid like ammonia and run a turbine. Geothermal energy uses steam from hot underground rocks to drive turbines. Nuclear energy is released in nuclear fission, where a heavy nucleus like uranium-235 splits on absorbing a neutron, releasing tremendous energy used in nuclear reactors; nuclear fusion, the joining of light nuclei (as in the sun), releases even more energy but is yet to be controlled commercially.
Environmental consequences and sustainable choices. Every source of energy has some environmental impact — fossil fuels cause global warming, hydropower disturbs river ecology, nuclear power produces radioactive waste, even solar and wind devices need land and materials. The principles of reduce, reuse and recycle together with the development of cleaner technologies, public transport, energy-efficient appliances and shifting to renewables are essential to ensure that the coming generations also have access to adequate energy. As citizens we should switch off unused lights, use LED lamps, prefer pressure cookers and solar heaters, walk or cycle for short distances and plant trees to protect our common future.
Textbook Questions and Answers
One-Mark Questions
Q1. What is a good source of energy?
Answer: A good source of energy is one that produces a large amount of work or heat per unit volume or mass, is easily available, easy to store and transport, economical and causes minimum environmental pollution.
Q2. Name the three forms of fossil fuels.
Answer: Coal, petroleum and natural gas.
Q3. What is the main constituent of biogas?
Answer: Methane (about 75%).
Q4. Which gas is produced when coal is burnt in air?
Answer: Carbon dioxide (along with oxides of sulphur and nitrogen as pollutants).
Q5. What is the minimum wind speed required to operate a windmill efficiently?
Answer: About 15 km per hour.
Q6. Name the material used in solar cells.
Answer: Silicon (a semiconductor).
Q7. What is OTEC?
Answer: Ocean Thermal Energy Conversion — a method of generating electricity from the temperature difference between warm surface ocean water and cold deep water.
Q8. Name the element used as fuel in a nuclear reactor.
Answer: Uranium-235 (or plutonium-239).
Q9. Why is the colour of the inner walls of a solar cooker painted black?
Answer: Because black surfaces absorb more heat radiation than coloured or shiny surfaces.
Q10. What is the energy released in the sun due to?
Answer: Nuclear fusion of hydrogen nuclei into helium.
Two/Three-Mark Questions
Q1. Differentiate between renewable and non-renewable sources of energy with examples.
Answer: Renewable sources are inexhaustible — they are continuously replenished by nature. Examples: solar, wind, hydro, biomass, tidal. Non-renewable sources are limited and once used cannot be replaced within a human lifetime. Examples: coal, petroleum, natural gas, uranium.
Q2. Why is biogas considered an excellent fuel?
Answer: Biogas (i) burns without smoke, (ii) has a high calorific value due to about 75% methane, (iii) leaves no residue like ash, (iv) the spent slurry is rich manure, and (v) it is produced from waste material that is otherwise a health hazard.
Q3. What are the limitations of harnessing wind energy?
Answer: Wind energy farms can be set up only at places where wind blows for most of the year and at a minimum speed of 15 km/h. A large area of land and high initial setup cost is needed. Wind is not constant, so storage batteries are required as a back-up. Towers and blades require regular maintenance.
Q4. Explain the working of a solar cooker.
Answer: A solar cooker is a wooden or metal box insulated with thermocol or glass wool. The inner walls are painted black to absorb heat. A glass sheet covers the top, and a plane mirror reflector concentrates sunlight into the box. The glass sheet allows short-wave radiation in but does not let long-wave radiation out (greenhouse effect), raising the temperature inside to 100–140 °C and cooking the food.
Q5. Distinguish between nuclear fission and nuclear fusion.
Answer: In fission, a heavy nucleus like uranium-235 splits into two lighter nuclei when bombarded with a neutron, releasing energy and more neutrons (used in nuclear reactors). In fusion, two lighter nuclei combine at very high temperature to form a heavier nucleus, releasing much more energy (the source of the sun’s energy). Fusion needs millions of degrees of temperature; fission can be carried out at ordinary temperatures inside a reactor.
Q6. What are the advantages of using solar cells?
Answer: Solar cells (i) have no moving parts, (ii) require little maintenance, (iii) work satisfactorily without focusing devices, (iv) can be set up in remote and inaccessible areas where laying a power line is costly, and (v) cause no environmental pollution during operation.
Five/Six-Mark Questions
Q1. Describe the construction and working of a biogas plant with a labelled diagram.
Answer: A biogas plant consists of (i) a mixing tank where cattle dung and water are mixed to form slurry, (ii) a digester — a deep underground sealed tank made of bricks where slurry is decomposed by anaerobic micro-organisms over several days, (iii) a gas holder (dome) which collects the gas produced and where pressure pushes it out through a pipe, (iv) an outlet pipe carrying biogas to a storage tank or burner, and (v) an overflow tank for the spent slurry which is used as manure. Anaerobic bacteria break down complex organic compounds in the absence of oxygen, producing biogas which contains about 75% methane along with carbon dioxide, hydrogen and hydrogen sulphide. Biogas is used for cooking and lighting in rural areas.
Q2. Explain how electricity is generated in a hydropower plant. Mention any two advantages and two disadvantages.
Answer: A high-rise dam is built across a river to store large amounts of water at a height. Water is allowed to fall through pipes from a height onto the blades of turbines placed at the bottom. The kinetic energy of falling water rotates the turbine, which is connected to a generator that produces electricity. Advantages: (i) it is a renewable source — water is replenished by the rain cycle; (ii) it produces no air pollution. Disadvantages: (i) construction of dams submerges large areas of forest land and displaces people; (ii) it disturbs aquatic life and causes problems of methane emission from decomposing vegetation in the reservoir.
Q3. Describe three ways of obtaining energy from the sea.
Answer: (i) Tidal energy — the rise and fall of sea level due to gravitational pull of the moon (high tide and low tide) is harnessed by building a dam across a narrow opening to the sea. Water flowing in and out of the dam rotates a turbine to generate electricity. (ii) Wave energy — the kinetic energy of moving sea waves is captured by devices like floats or oscillating water columns that drive a turbine. It is useful where strong waves are available. (iii) Ocean Thermal Energy Conversion (OTEC) — the temperature difference of at least 20 °C between warm surface water and cold deep water is used to vapourise a working fluid like ammonia. The vapour rotates a turbine and is then condensed by cold water from the ocean depths. All three methods are renewable and pollution-free, though their commercial use is still limited by cost and technology.
Q4. Explain nuclear fission with a suitable equation. State the advantages and disadvantages of nuclear energy.
Answer: When a heavy atomic nucleus such as uranium-235 is bombarded with a slow neutron, it splits into two medium-sized nuclei (e.g., barium and krypton) along with the release of three neutrons and a tremendous amount of energy. The newly released neutrons hit other uranium nuclei and produce a controlled chain reaction in a nuclear reactor. Equation: U-235 + n → Ba-141 + Kr-92 + 3n + Energy. Advantages: (i) huge energy output from a small mass of fuel, (ii) does not produce greenhouse gases, (iii) reliable and continuous supply. Disadvantages: (i) radioactive waste is hazardous and difficult to dispose of, (ii) risk of accidents like Chernobyl and Fukushima, (iii) high installation cost, (iv) limited fuel reserves of uranium.
Q5. What environmental consequences arise from increasing demand for energy? Suggest four ways to use energy sources sustainably.
Answer: Increasing energy demand leads to: (i) burning of fossil fuels causing air pollution, acid rain and the greenhouse effect that warms the planet; (ii) deforestation for hydropower reservoirs and biomass; (iii) radioactive contamination from nuclear plants; (iv) depletion of non-renewable resources for the future generations. Sustainable measures: (1) Switch to renewable sources — solar, wind, biogas. (2) Use energy-efficient appliances such as LED lamps, BEE-rated fans and pressure cookers. (3) Adopt public transport, cycling and walking instead of personal vehicles. (4) Practise reduce, reuse and recycle, plant more trees and avoid wastage by switching off unused lights and appliances.
Additional Practice — Multiple Choice Questions
Q1. Which of the following is NOT a fossil fuel?
(a) Coal (b) Petroleum (c) Natural gas (d) Wood
Answer: (d) Wood.
Q2. The energy of the sun is due to —
(a) Combustion (b) Nuclear fusion (c) Nuclear fission (d) Chemical reaction
Answer: (b) Nuclear fusion.
Q3. Solar cells are made of —
(a) Iron (b) Silicon (c) Carbon (d) Aluminium
Answer: (b) Silicon.
Q4. The main component of biogas is —
(a) Carbon dioxide (b) Hydrogen (c) Methane (d) Nitrogen
Answer: (c) Methane.
Q5. Which of these is a renewable source of energy?
(a) Coal (b) Uranium (c) Petroleum (d) Wind
Answer: (d) Wind.
Q6. The minimum wind speed for a windmill to work is —
(a) 5 km/h (b) 10 km/h (c) 15 km/h (d) 25 km/h
Answer: (c) 15 km/h.
Q7. OTEC works on the temperature difference of at least —
(a) 5 °C (b) 10 °C (c) 20 °C (d) 50 °C
Answer: (c) 20 °C.
Q8. Inside a solar cooker, walls are painted black because black —
(a) Reflects heat (b) Absorbs heat (c) Looks attractive (d) Stops radiation
Answer: (b) Absorbs heat.
Q9. Geothermal energy is obtained from —
(a) The sun (b) Hot underground rocks (c) Sea waves (d) Wind
Answer: (b) Hot underground rocks.
Q10. The fuel used in nuclear power plants is —
(a) Coal (b) Diesel (c) Uranium (d) Hydrogen
Answer: (c) Uranium.
Fill in the Blanks
Q1. Charcoal is obtained by destructive distillation of __________.
Answer: wood.
Q2. The reaction in which two light nuclei combine to form a heavier one is called __________.
Answer: nuclear fusion.
Q3. A solar cooker can attain a temperature of __________ °C.
Answer: 100–140.
Q4. Hydropower converts the __________ energy of falling water into electrical energy.
Answer: kinetic.
Q5. The greenhouse effect is mainly caused by __________ gas.
Answer: carbon dioxide.
True or False
Q1. Coal is a renewable source of energy.
Answer: False — coal is non-renewable.
Q2. Solar cells convert sunlight directly into electricity.
Answer: True.
Q3. Biogas leaves a lot of ash on burning.
Answer: False — biogas burns cleanly without ash.
Q4. Tidal energy is harnessed by building a dam across a narrow opening to the sea.
Answer: True.
Q5. Nuclear fusion is currently the main reaction used in commercial nuclear power plants.
Answer: False — nuclear fission is used; fusion has not yet been controlled commercially.
Glossary
| Term | Meaning |
|---|---|
| Fossil fuels | Fuels formed from the buried remains of plants and animals over millions of years (coal, petroleum, natural gas). |
| Calorific value | The amount of heat produced by complete combustion of unit mass of a fuel; expressed in kJ/kg. |
| Biomass | Plant and animal matter such as wood, dung and crop residues used as fuel. |
| Biogas | A mixture of gases (mainly methane) produced by anaerobic decomposition of biomass. |
| Hydropower | Electricity generated by using the kinetic energy of falling or flowing water. |
| Solar cooker | An insulated box that uses solar energy and the greenhouse effect to cook food. |
| Solar cell | A device made of semiconductor material that converts sunlight directly into electricity. |
| Tidal energy | Energy obtained from the rise and fall of sea level due to gravitational pull of the moon. |
| Wave energy | Energy harnessed from the motion of sea waves at the surface. |
| OTEC | Ocean Thermal Energy Conversion — uses the temperature difference between surface and deep ocean water to generate electricity. |
| Geothermal energy | Energy obtained from the heat of hot rocks present inside the earth. |
| Nuclear fission | Splitting of a heavy atomic nucleus into two lighter nuclei with release of energy. |
| Nuclear fusion | Combining of two light nuclei to form a heavier nucleus with release of energy. |
| Renewable source | A source of energy that is replenished by nature and never runs out (sun, wind, water). |
| Non-renewable source | A source that exists in limited amount and cannot be replaced once used (coal, petroleum, uranium). |
| Greenhouse effect | Trapping of heat near the earth’s surface by gases like CO₂, leading to global warming. |
| Charcoal | A black solid fuel obtained by partial combustion of wood; burns without smoke. |
| Windmill | A device that converts the kinetic energy of wind into mechanical or electrical energy. |
Quick Revision Points
1. A good source of energy must have high calorific value, easy storage and transport, low cost, and minimum pollution.
2. Coal, petroleum and natural gas are fossil fuels — non-renewable and polluting, but with high energy density.
3. A thermal power plant uses heat of burning coal/oil to convert water into steam to rotate turbines.
4. Hydropower is renewable but requires construction of huge dams that submerge land and displace people.
5. Biogas is produced by anaerobic decomposition of cattle dung; the slurry left over is excellent manure.
6. Wind energy needs minimum wind speed of 15 km/h; large land area and high installation cost are limitations.
7. Solar cookers reach 100–140 °C and work on the greenhouse effect; solar cells are made of silicon.
8. Sea offers tidal, wave and OTEC energy; OTEC needs at least 20 °C temperature difference between surface and depth.
9. Geothermal energy comes from hot rocks inside the earth; nuclear power uses fission of U-235.
10. Sustainable use of energy means choosing renewables, conserving electricity, and adopting reduce-reuse-recycle.
That completes our English-medium notes on Class 10 Science Chapter 14 — Sources of Energy for ASSEB students. Revise the textbook questions, attempt all the additional MCQs, fill in the blanks and true/false items, and memorise the glossary terms for full confidence in your HSLC examination. For more chapter-wise notes, keep visiting HSLC Guru.