NCERT Textbook Questions & Answers + Important Extra Questions
Section A: In-Text Questions
What is a good source of energy?
Answer
A good source of energy should: (i) produce a large amount of work per unit volume or mass, (ii) be easily accessible and convenient to store and transport, (iii) be economical (affordable), (iv) cause minimal environmental pollution, and (v) be safe to use.
What is a good fuel?
Answer
A good fuel has a high calorific value (produces large heat per unit mass), burns without producing harmful smoke or gases, is readily available, affordable, easy to store and transport, and has a moderate ignition temperature (not too low for safety, not too high for difficulty in lighting).
If you could use any source of energy for heating your food, which one would you use and why?
Answer
Natural gas (LPG) or electricity would be ideal because: (i) they produce no smoke or ash, (ii) they have high calorific value, (iii) they are easily controllable (temperature can be adjusted), (iv) they are available through pipelines/wires directly to homes, and (v) they cause minimal indoor pollution compared to wood or coal.
What are the disadvantages of fossil fuels?
Answer
(i) They are non-renewable — once exhausted, they cannot be replenished for millions of years. (ii) Burning fossil fuels produces CO₂ (a greenhouse gas causing global warming), SO₂ and NOₓ (causing acid rain), and particulate matter (air pollution). (iii) Their combustion products are harmful to health. (iv) Mining and extraction damage the environment. (v) Oil spills during transport cause water pollution and harm marine life.
Why are we looking at alternate sources of energy?
Answer
(i) Fossil fuels are being depleted rapidly and may run out in the near future. (ii) The growing population and industrialisation demand more energy than fossil fuels alone can sustainably provide. (iii) Fossil fuel combustion causes severe environmental problems — global warming, air pollution, acid rain. (iv) Alternative sources like solar, wind, and hydropower are renewable and cleaner. (v) Energy security requires diversification of sources to reduce dependence on any single fuel.
How has the traditional use of wind and water energy been modified for our convenience?
Answer
Traditionally, wind energy was used for sailing boats, pumping water, and grinding grain using windmills. Now, wind energy is harnessed using modern wind turbines connected to generators to produce electricity on a large scale in wind farms. Similarly, water energy was traditionally used for running water mills. Now, it is used in hydroelectric power plants where the kinetic energy of falling water (from dams) rotates turbines connected to generators, producing large-scale electricity.
What kind of mirror — concave, convex or plane — would be best suited for use in a solar cooker? Why?
Answer
A concave mirror would be best suited. A concave mirror converges (focuses) the incoming parallel rays of sunlight to a single point (the focus), concentrating solar energy at that point and generating very high temperatures. This focused heat can be used effectively for cooking. Plane mirrors can only redirect sunlight without concentrating it, and convex mirrors diverge light, spreading it over a larger area.
What are the limitations of the energy that can be obtained from the oceans?
Answer
(i) Tidal energy: Very few sites have sufficiently large tidal ranges; construction of tidal barrages is extremely expensive; power output varies with tidal cycles. (ii) Wave energy: Strong waves are available only at certain coastal locations; technology is still being developed; storms can damage equipment. (iii) Ocean thermal energy (OTEC): Requires a temperature difference of at least 20°C between surface and deep water; efficiency is very low; high initial cost; limited to tropical regions. Overall, ocean energy technologies are expensive, location-specific, and not yet commercially viable on a large scale.
What is geothermal energy?
Answer
Geothermal energy is the heat energy stored deep inside the Earth. It originates from the decay of radioactive materials in the Earth's core and from the original heat of the Earth's formation. In some regions, this heat causes underground water to turn into steam, which may emerge naturally as hot springs or geysers. This steam can be piped to rotate turbines and generate electricity. Geothermal energy is renewable, available 24/7, and produces minimal pollution, but is limited to geologically active areas.
What are the advantages of nuclear energy?
Answer
(i) A very small amount of nuclear fuel produces an enormous amount of energy — 1 kg of uranium produces as much energy as 20,000 kg of coal. (ii) Nuclear power plants do not emit greenhouse gases during operation. (iii) Nuclear fuel reserves (uranium, thorium) can last longer than fossil fuels. (iv) Provides a reliable, continuous baseload power supply independent of weather conditions.
Section B: NCERT Exercise Questions
Q1. A solar water heater cannot be used to get hot water on
(a) a sunny day (b) a cloudy day (c) a hot day (d) a windy day
Ans. (b) A cloudy day. Solar water heaters depend on direct sunlight to heat water. On a cloudy day, sunlight is blocked by clouds, so the heater cannot receive enough solar radiation to heat the water effectively.
Which of the following is not an example of a biomass energy source?
Answer
(a) wood (b) gobar gas (c) nuclear energy (d) coal
Ans. (c) Nuclear energy. Wood and gobar gas (biogas) are directly derived from biological matter (biomass). Coal, although formed from ancient biomass, is classified as a fossil fuel. Nuclear energy comes from atomic reactions (fission/fusion), not biological matter.
Most of the sources of energy we use represent stored solar energy. Which of the following is not ultimately derived from the Sun's energy?
Answer
(a) geothermal energy (b) wind energy (c) nuclear energy (d) biomass
Ans. (a) Geothermal energy and (c) Nuclear energy. Geothermal energy comes from radioactive decay inside Earth. Nuclear energy comes from nuclear reactions. Wind, biomass, fossil fuels, hydropower, and ocean thermal energy are all ultimately derived from the Sun.
Compare and contrast fossil fuels and the Sun as direct sources of energy.
Answer
Fossil fuels: Non-renewable, finite reserves that take millions of years to form. Easy to store and transport. Available on demand (can be burnt anytime). Cause significant pollution (CO₂, SO₂, particulates). High energy density.
Solar energy: Renewable and inexhaustible. Cannot be easily stored (needs batteries or other storage). Available only during daytime and depends on weather. Causes no pollution during use. Low energy density (diffuse — spread over large area). Free and available everywhere.
Compare and contrast biomass and hydroelectricity as sources of energy.
Answer
Biomass: Derived from plant and animal waste. Renewable but requires agricultural/waste material. Can be used as fuel directly or converted to biogas. Available in rural areas. Produces some pollution (smoke, CO₂) when burned. Technology is simple.
Hydroelectricity: Derived from kinetic energy of flowing/falling water. Renewable and clean (no direct pollution). Requires construction of large dams — high initial cost, causes displacement, ecosystem damage. Available only near rivers. Provides large-scale, reliable electricity. No fuel cost once dam is built.
What are the limitations of extracting energy from (a) the wind, (b) waves, and (c) tides?
Answer
(a) Wind: Wind speed must be above 15 km/h consistently. Not available everywhere. Requires large open areas for wind farms. Initial setup cost is high. Output is intermittent and unpredictable. Cannot provide baseload power. Turbines cause noise and can harm birds.
(b) Waves: Very few coastal locations have strong enough waves. Technology is still developing. Equipment is vulnerable to storms and corrosion from seawater. Power output is variable and relatively low.
(c) Tides: Very few sites globally have large enough tidal ranges. Construction of tidal barrages across estuaries is extremely expensive. Can disrupt marine ecosystems. Power generation is intermittent (depends on tidal cycle).
On what basis would you classify energy sources as (a) renewable and non-renewable? (b) exhaustible and inexhaustible?
Answer
(a) Renewable sources are those that are replenished by nature continuously and will not run out — solar, wind, hydropower, biomass, geothermal, tidal. Non-renewable sources are those that exist in limited quantities and take millions of years to form — coal, petroleum, natural gas, nuclear fuel (uranium).
(b) This is essentially the same classification. Exhaustible sources can be used up (fossil fuels, nuclear fuel). Inexhaustible sources will last as long as the Sun exists (solar, wind, water, biomass, geothermal).
What are the qualities of an ideal source of energy?
Answer
An ideal energy source should: (i) give a large amount of energy per unit mass/volume (high calorific value), (ii) be easily accessible and widely available, (iii) be easy to store, transport, and use, (iv) be economical, (v) cause no environmental pollution, (vi) be safe to handle, and (vii) be renewable/sustainable for long-term use.
What are the advantages and disadvantages of using a solar cooker? Are there places where solar cookers would have limited utility?
Answer
Advantages: Uses free solar energy, no fuel cost, no pollution, no smoke (healthy cooking), low maintenance, preserves nutrients in food due to slow cooking.
Disadvantages: Cannot be used at night or on cloudy/rainy days, cooking is slow, cannot achieve very high temperatures (no frying or chapati-making in box type), large and not very portable.
Limited utility: Regions with frequent cloudy weather, high-latitude areas with limited sunlight (polar regions), densely forested areas with limited open space, and urban areas with tall buildings blocking sunlight.
What are the environmental consequences of the increasing demand for energy? What steps would you suggest to reduce energy consumption?
Answer
Environmental consequences: (i) Air pollution from burning fossil fuels (respiratory diseases, smog), (ii) Global warming and climate change from greenhouse gas emissions, (iii) Acid rain from SO₂ and NOₓ damaging crops, forests, and buildings, (iv) Oil spills polluting oceans, (v) Deforestation for biomass/firewood, (vi) Land and water pollution from coal mining and nuclear waste, (vii) Ecosystem disruption from large dams.
Steps to reduce consumption: (i) Use energy-efficient appliances (LED bulbs, 5-star rated devices), (ii) Switch off lights and fans when not in use, (iii) Use public transport, carpool, or cycle, (iv) Use solar water heaters and cookers, (v) Insulate homes properly, (vi) Plant trees to absorb CO₂, (vii) Shift to renewable energy sources.
Section C: Important Extra Questions
What is biomass? How is biogas produced from biomass?
Answer
Biomass refers to organic matter from plants and animals — wood, crop residues, animal dung, food waste, etc. Biogas is produced by the anaerobic decomposition (breakdown without oxygen) of biomass by bacteria in a biogas plant. Cow dung and water are mixed into a slurry and fed into a sealed digester tank. In the absence of oxygen, anaerobic bacteria break down the organic matter, producing biogas (mainly methane CH₄, with some CO₂, H₂S). The gas is collected and piped for cooking or lighting. The leftover slurry is nutrient-rich and used as fertiliser.
What is nuclear fission? How is it used to generate electricity?
Answer
Nuclear fission is the process of splitting a heavy atom (like uranium-235 or plutonium-239) into two smaller nuclei by bombarding it with a neutron, releasing an enormous amount of energy and additional neutrons. These neutrons can split more atoms, creating a sustained chain reaction. In a nuclear power plant, this chain reaction is controlled using control rods (which absorb excess neutrons). The heat produced converts water to steam, which drives turbines connected to generators, producing electricity.
Explain the working of a solar cell. What are its applications?
Answer
A solar cell (photovoltaic cell) converts sunlight directly into electricity using the photovoltaic effect. It is made of semiconductor materials (usually silicon). When sunlight falls on the cell, photons knock electrons loose from atoms, creating a flow of electric current. A single cell produces about 0.5-1 V; many cells are combined into solar panels for higher voltage and power. Applications: calculators, watches, street lights, water pumps, satellites, space stations, remote area electrification, rooftop power generation.
What is the difference between conventional and non-conventional sources of energy?
Answer
Conventional sources: Used for a long time — coal, petroleum, natural gas, firewood, hydropower. Most are non-renewable (except hydro and biomass). Well-established technology. Cause pollution. Centralised large plants.
Non-conventional sources: Recently developed — solar, wind, tidal, geothermal, ocean thermal, biogas. All are renewable. Technology still evolving. Cause minimal/no pollution. Can be decentralised (solar panels on rooftops).