CBSE Class 10 Science Sources of energy Notes

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Sources of Energy notes for CBSE Class 10 Science Chapter 14, covering the meaning of energy and its units, characteristics of good sources and good fuels, renewable and non-renewable sources, conventional and non-conventional sources, fossil fuels, thermal, hydro and biogas plants, wind energy, solar energy and solar cookers, ocean energy, geothermal energy, nuclear fission and fusion, and environmental consequences, with intext and exercise questions.

CHAPTER 14: SOURCES OF ENERGY

ENERGY

Whenever a body is capable of doing work, the body is said to possess energy. Thus energy is defined as the ability of a body to do work, and the amount of energy possessed by a body is equal to the amount of work it can do when its energy is released.

Energy comes in different forms, and one form can be converted to another. For example, if we drop a plate from a height, the potential energy of the plate is converted mostly to sound energy when it hits the ground.

Units of Energy

In the S.I. system, energy is measured in joules or calories, and in the C.G.S. system in ergs. The commercial unit of energy is the kilowatt-hour. The energy is said to be one kilowatt-hour when a body consumes one kilowatt of energy in one hour.

SOURCES OF ENERGY

A source of energy is that which is capable of providing enough useful energy at a steady rate over a long period of time.

A good source of energy should be:
i) Safe and convenient to use. For example, nuclear energy can be used only by highly trained engineers with the help of nuclear power plants, so it cannot be used for household purposes.
ii) Easy to transport. For example, coal, petrol, diesel and LPG have to be transported from the places of their production to the consumers.
iii) Easy to store. For example, huge storage tanks are required to store petrol, diesel and LPG.

Characteristics of an Ideal or Good Fuel

1. It should have a high calorific or heat value, so that it can produce maximum energy with low fuel consumption.
2. It should have a proper ignition temperature, so that it can burn easily.
3. It should not produce harmful gases during combustion.
4. It should be cheap and easily available in plenty for everyone.
5. It should be easy to handle, store and transport from one place to another.
6. It should not be valuable for any other purpose than as a fuel.
7. It should burn smoothly and should not leave much residue after combustion.

INTEXT QUESTIONS (PAGE NO. 243)

1. What is a good source of energy?
A good source of energy fulfils the following criteria:
1. It produces a lot of heat per unit mass.
2. It does a huge amount of work per unit mass.
3. It is easily accessible.
4. It is easy to store and transport.
5. It is economical.
6. It produces less smoke.

2. What is a good fuel?
A good fuel produces a huge amount of heat on burning, does not produce a lot of smoke, and is easily available.

3. If you could use any source of energy for heating your food, which one would you use and why?
Natural gas can be used for heating and cooking food because it is a clean source of energy. It does not produce a huge amount of smoke on burning. Although it is highly inflammable, it is easy to use and transport, and it produces a huge amount of heat on burning.

CLASSIFICATION OF SOURCES OF ENERGY

Sources of energy can be classified as:
(i) Renewable
(ii) Non-renewable

1. Renewable Sources of Energy

Renewable sources of energy are inexhaustible, that is, they can be replaced as we use them and can be used to produce energy again and again. They are available in an unlimited amount in nature and develop within a relatively short period of time.

Examples: (i) solar energy, (ii) wind energy, (iii) water energy (hydro-energy), (iv) geothermal energy, (v) ocean energy, (vi) biomass energy (firewood, animal dung, biodegradable waste from cities and crop residues constitute biomass).

Advantages of Renewable Sources of Energy

(i) These sources will last as long as the earth receives light from the sun.
(ii) These sources are freely available in nature.
(iii) These sources do not cause any pollution.

2. Non-Renewable Sources of Energy

Non-renewable sources of energy are exhaustible and cannot be replaced once they have been used. These sources have accumulated in nature over a very long period of millions of years.

Examples: (i) coal, (ii) oil and (iii) natural gas. All these fuels are called fossil fuels.

Disadvantages of Non-Renewable Sources of Energy

(i) Due to their extensive use, these sources are fast depleting.
(ii) It is difficult to discover and exploit new deposits of these sources.
(iii) These sources are a major cause of environmental pollution.

Conventional and Non-Conventional Sources of Energy

Sources of energy are also classified as:
(i) Conventional sources of energy
(ii) Non-conventional sources of energy

CONVENTIONAL SOURCES OF ENERGY

Fossil Fuels

Fossil fuels are the remains of prehistoric plants and animals which got buried deep inside the earth millions of years ago due to natural processes. The energy of fossil fuels is in fact solar energy which was trapped by natural processes a very long time ago. Coal, petroleum and natural gas are fossil fuels.

Formation of Fossil Fuels: During formation, an entire organism or its parts often get buried in sand or mud. These then decay and disintegrate, leaving no signs of their existence. In fact, the harder parts of organisms, after their death, settle down and are covered by sediments. They are subjected to extreme pressure and temperature inside the earth, which converts them into fossil fuels. This process is referred to as fossilisation.

Disadvantages of Fossil Fuels:
1. Fossil fuels are non-renewable sources of energy and cannot be renewed once used.
2. Burning of fossil fuels causes air pollution.
3. Fossil fuel reserves in the earth are limited and may get exhausted soon.

Thermal Power Plant

The term thermal power plant is used since fuel is burnt to produce heat energy, which is converted into electrical energy. Large amounts of fossil fuels are burnt every day in power stations to heat water to produce steam, which runs the turbine to generate electricity. The transmission of electricity is more efficient than transporting coal or petroleum over the same distance. Therefore, many thermal power plants are set up near coal or oil fields.

Hydro Power Plants

Hydro power plants convert the potential energy of falling water into electricity. Since there are very few waterfalls that could be used as a source of potential energy, hydro power plants are associated with dams.

To produce hydel electricity, high-rise dams are constructed on a river to obstruct the flow of water and collect it in large reservoirs. The water level rises, and in this process the kinetic energy of flowing water is transformed into potential energy. The water from the high level in the dam is carried through pipes to the turbine at the bottom of the dam. Since the reservoir is refilled each time it rains (hydro power is a renewable source), we do not have to worry about hydroelectric sources getting used up the way fossil fuels would one day.

Advantages of Generating Hydro Electricity:
1. It is readily and abundantly available everywhere, free of cost.
2. It is eco-friendly and does not produce any kind of environmental pollution.
3. It is a renewable source, as water itself is a renewable and inexhaustible resource.
4. It is a cheap source of energy, as it does not involve any costly investment.

Biomass

Biomass is defined as living matter or its residue, and it is a renewable source of energy. Biomass includes:
(i) all the new plant growth
(ii) agricultural and forest residues (like biogas, bark, saw dust, wood shavings, roots and animal droppings)
(iii) carbonaceous wastes (like sewage, garbage and night-soil)
(iv) biodegradable organic effluents from industries.

Biogas

Biogas is a mixture of gases produced by anaerobic degradation of biomass in the presence of water but in the absence of oxygen. It is a renewable source of energy because it is produced from vastly and continuously available organic wastes.

Advantages of Biogas:
i) A biogas plant is quite simple and can easily be built in rural areas. A small plant using dung from 3 to 4 heads of cattle is capable of supplying biogas for 6 hours daily for cooking purposes.
ii) Biogas is a clean fuel that burns without smoke and leaves no ash.
iii) The main constituent of biogas, methane, has a higher calorific value (about 55 kJ/g) than petrol (about 50 kJ/g).
iv) The spent slurry, being rich in nitrogen and phosphorus, is good manure.
v) By using biogas, firewood is saved and deforestation is reduced.

Composition of Biogas: Biogas is mainly composed of methane (up to 75 per cent), carbon dioxide (about 25 per cent) and traces of other gases such as nitrogen and hydrogen. Methane is a high calorific value fuel, whereas carbon dioxide is an inert gas.

Biogas is prepared in biogas plants, which are of two types: (i) fixed dome type and (ii) floating gas holder type.

Fixed Dome Type Biogas Plant

The main parts of a fixed dome type biogas plant are:
1. Digester: a well-shaped underground tank made of bricks. Its roof is dome-shaped, which acts as a storage tank for biogas.
2. Mixing tank: constructed at ground level, where cattle dung and water are mixed.
3. Inlet tank: constructed underground below the mixing tank.
4. Overflow tank: constructed slightly below the level of the mixing tank.
5. Outlet tank: constructed below the overflow tank.

Working of Biogas Plant

Cattle dung and water are mixed in equal proportion in the mixing tank to form slurry. This slurry is fed into the digester through the inlet tank. When the digester is filled to about two-thirds of its capacity, the dome is left free for the collection of biogas. The slurry undergoes anaerobic fermentation, and biogas is produced after 50 to 60 days. As biogas collects in the dome, it exerts pressure, due to which the spent slurry goes to the overflow tank through the outlet tank. Fresh slurry is fed into the digester, and a continuous supply of biogas is obtained. The spent slurry is used as manure.

Wind Energy

When large masses of air move from one place to another, it is referred to as wind. During this process, kinetic energy is associated with it, which is referred to as wind energy.

Principle of Utilisation of Wind Energy: Wind energy is converted into electrical energy with the aid of a windmill. A windmill is a large fan with big blades, which rotate due to the force exerted by moving wind on them. The blades keep rotating as long as the wind blows and can be used to drive a large number of machines like water pumps and flour mills. Nowadays, a windmill is mostly used to generate electric current, and wind power stations are established all over the world to convert wind energy directly into electrical energy.

Uses of Wind Energy:
1. It is used to drive windmills, water-lifting pumps and flour mills.
2. It is used to propel sailboats.
3. It is used to fly engineless aeroplanes or gliders in the air.
4. It is used to generate electricity for purposes like lighting and heating.

Advantages of Generating Wind Energy:
1. It is readily and abundantly available at every place on earth, free of cost.
2. It is eco-friendly and does not produce any kind of environmental pollution.
3. It is a renewable source, as air itself is a renewable and inexhaustible resource.
4. It is a cheap source of energy, as it does not involve any costly investment.

INTEXT QUESTIONS (PAGE NO. 248)

1. What are the disadvantages of fossil fuels?
(a) Burning of coal and petroleum produces a lot of pollutants, causing air pollution.
(b) Fossil fuels release oxides of carbon, nitrogen and sulphur, which cause acid rain. Acid rain affects soil fertility and potable water.
(c) Burning of fossil fuels produces gases such as carbon dioxide, which cause global warming.

2. Why are we looking at alternate sources of energy?
Fossil fuels, which have been traditionally used by human beings as an energy source, are non-renewable. They are limited and cannot replenish on their own, and they are being consumed at a large rate. If this rate of consumption continues, fossil fuels will be exhausted from the earth. Therefore we have to conserve energy sources and look for alternate sources of energy.

How has the traditional use of wind and water energy been modified for our convenience?
Traditionally, waterfalls were used as a source of potential energy, which was converted to electricity with the help of turbines. Since waterfalls are few in number, water dams have been constructed in large numbers. Nowadays, hydro-dams harness the potential energy of stored water. In water dams, water falls from a height on the turbine, which produces electricity.
Earlier, windmills were used to harness wind energy to do mechanical work, such as lifting water from a well. Today, windmills are used to generate electricity. The kinetic energy of wind is harnessed and converted into electricity, as the rotating blades turn the turbine of an electric generator.

ALTERNATIVE OR NON-CONVENTIONAL SOURCES OF ENERGY

Non-conventional sources of energy are those which are not used as extensively as conventional ones and meet our energy requirements only on a limited scale. Solar energy, ocean energy (tidal energy, wave energy, ocean thermal energy, OTE), geothermal energy and nuclear energy belong to this category. These sources have been tapped with the aid of advances in technology to meet our growing energy needs and are also called alternative sources of energy.

SOLAR ENERGY

The energy produced by the sun in the form of heat and light is called solar energy.

Principles of Utilisation of Solar Energy:
(i) For appliances requiring a moderate temperature, the incident sun rays are reflected by a plane mirror onto a black container, which absorbs the solar energy and gets heated.
(ii) For appliances requiring a high temperature, the incident sun rays are reflected and concentrated by a large concave reflector, which focuses all the sun rays at a single point called the focus. Any object kept at the focus gets strongly heated.

Harnessing or Utilisation of Solar Energy: The sun is the ultimate source of energy and has a remarkable capacity to produce energy in the form of heat and light. The energy produced by the sun in one day is about 50,000 times more than the energy consumed in the whole world in one year. However, solar energy has certain limitations, which do not allow its large-scale use. Solar energy can be used in two ways:
1) Direct utilisation: solar energy is used directly, either by collecting it as heat energy or by converting it into electricity.
2) Indirect utilisation: solar energy is used indirectly, by converting it into chemical energy such as biomass, or by utilising the energy obtained from wind, sea waves, tides, etc.

Solar Heating Devices: A device that gets heated by absorbing solar energy in the form of heat and light is called a solar heating device. Solar cookers, solar water heaters, solar furnaces and solar cells are examples.

Solar Cooker: A solar cooker utilises solar energy for cooking food. It consists of an insulated wooden box (B) painted black from the inner side. The lid of the box has a plane mirror reflector (R) and a glass sheet (G). The food to be cooked is placed in a metal container (C) painted black from the outer side and kept in the box. The container is covered with the glass sheet. The box is kept in direct sunlight, and its reflector is adjusted so that a strong beam of sunlight falls over it.

[Figure: Solar cooker drawn as an insulated box (B) with a plane mirror reflector (R) hinged on its lid, a glass sheet (G) over the box, and a black-painted metal container (C) holding the food inside the box.]

Working of Solar Cooker: When the solar cooker is kept in direct sunlight, the reflector (R) reflects both visible and infrared rays of sunlight onto the top of the box as a strong beam. The black surface of the box and the vessel absorb it. When the inner black surface becomes quite hot, it radiates heat energy as infrared rays, but the glass sheet (G) does not allow these rays to pass out of the box. These infrared rays are trapped inside the box, which raises its internal temperature to about 100°C. This temperature cooks the food in the metal container.

Limitations of Solar Cooker:
1. It cannot be used during the night.
2. It cannot be used on a cloudy day.
3. The direction of the reflector has to be adjusted according to the position of the sun.
4. It cannot be used for making chappatis.
5. It cannot be used for frying.

Solar Cell: A solar cell is a device that converts solar energy (light energy) directly into electricity. It is made of semiconducting material such as silicon, germanium, selenium or gallium. A modern solar cell is made from wafers of semiconducting materials containing impurities in such a way that a potential difference is generated when light falls on them. A 4 cm² solar cell produces a potential difference of about 0.4 to 0.5 volts and a current of about 60 milliamperes. To generate a large amount of current, a number of solar cells are arranged together in a definite pattern in a solar panel. The energy generated in a solar panel is stored in a battery connected to it and can be used for various purposes.

Uses of Solar Cells: Solar cells are used for producing electricity for lighting houses and streets; for running electronic appliances such as televisions, radios, watches, calculators and toys; for offshore oil drilling platforms; and for generating electricity in artificial satellites, rockets and space vehicles.

ENERGY FROM THE SEA

The oceans cover almost 71 per cent of the surface of the earth. The enormous amount of water in them acts as a big collector of solar heat energy and also stores a large amount of it, because of its high specific heat. Thus ocean water can be used as a renewable source of energy. The main forms of ocean energy are:
i) Ocean thermal energy: the energy available due to the temperature difference between the deeper levels and the surface of an ocean.
ii) Ocean tidal energy: the rise of ocean water due to the attraction of the moon is called high tide, and its fall is called low tide. The enormous movement of water due to high and low tides provides a large amount of energy. This tidal energy can be utilised by constructing a tidal barrage or dam.
iii) Sea wave energy: the energy obtained from high-speed sea waves. These waves have a lot of kinetic energy, which can be used to drive dynamos that convert kinetic energy into electrical energy.
iv) Energy from nuclear deuterium of oceans: ocean water contains an unlimited amount of the heavy hydrogen isotope deuterium, which has one proton and one neutron in its nucleus. Scientists are working to produce energy by controlled nuclear fusion of deuterium. The process is still at the experimental stage.
v) Energy from salinity gradient in seas: the difference in the concentration of salts in the water of two or more seas is called salinity gradient. It is now used to obtain energy with suitable techniques.
vi) Energy from sea vegetation or biomass: sea weeds are a direct source of energy, because they provide an endless supply of methane fuel.

Limitations of Energy from Oceans:
i) Tidal energy: very few suitable sites are available for constructing dams, and power generation is intermittent and not very large.
ii) Wave energy: power output is variable, and the available technologies are very expensive.
iii) Ocean thermal energy: conversion efficiency is low (3 to 4 per cent), and a large capital investment is required.

GEOTHERMAL ENERGY

Geothermal energy is the heat of the earth, the naturally occurring thermal energy found within rock formations and the fluids held within them. It is one of the few sources of energy that do not come directly or indirectly from solar energy.

Underground hot water in contact with hot spots changes into steam. As the steam is trapped between the rocks, it gets compressed to high pressure. At some places, hot water and steam gush out from the earth's surface through large cracks between the rocks and form natural geysers. The geothermal energy carried by natural geysers is used for generating electricity.

Merits of Geothermal Energy:
1. It is the most versatile and least polluting renewable source of energy.
2. It can be harnessed for 24 hours a day throughout the year.
3. It is relatively inexpensive.
4. Its power generation level is higher than that of solar and wind energy.
5. It can be used for power generation as well as direct heating. In the USA, water is pumped from underground hot water deposits and used to heat houses.

Limitations of Geothermal Energy:
1. Geothermal hot spots are scattered and are usually some distance away from the areas that need energy.
2. Overall power production has a lower efficiency (about 15 per cent) than fossil fuels (35 to 40 per cent).
3. Although geothermal energy as a whole is inexhaustible, a single geothermal source has a limited life span of about 10 years.
4. Noise pollution is caused by drilling operations at geothermal sites.

NUCLEAR ENERGY

A reaction in which the nucleus of an atom undergoes a change to form a new atom and releases an enormous amount of energy is called nuclear energy. There are two distinct ways of obtaining nuclear energy: (a) nuclear fission and (b) nuclear fusion.

Nuclear Fission Reaction: This type of nuclear reaction was first reported by Otto Hahn in 1938. When an unstable heavy nucleus is bombarded with slow-speed thermal neutrons, it splits into two smaller, stable nuclei, liberating an enormous amount of heat and light energy.

Nuclear Fusion Reaction: This type of nuclear reaction was first reported by Hans Bethe in 1939. The word 'fusion' means 'to combine together'. Nuclear fusion means combining two or more nuclei to form a single nucleus. It is a process in which two lighter nuclei fuse together to form a stable heavier nucleus, with the simultaneous release of a very large amount of energy. The energy produced in a fusion reaction is much higher than that produced in a fission reaction.

Advantages of Nuclear Energy:
1. It produces a large amount of useful energy from a very small amount of nuclear fuel, such as uranium-235.
2. Once nuclear fuel is loaded into the reactor, the power plant can keep producing electricity for two to three years at a stretch, without refuelling.
3. It does not produce gases like carbon dioxide, which contribute to the greenhouse effect, or sulphur dioxide, which causes acid rain.

Disadvantages of Nuclear Energy:
1. The major hazard of nuclear power generation is the storage and disposal of spent fuel, as the uranium continues to decay into harmful subatomic particles (radiation).
2. Improper storage and disposal of nuclear waste result in environmental contamination.
3. There is a risk of accidental leakage of nuclear radiation.
4. The high cost of installing a nuclear power plant, the high risk of environmental contamination and the limited availability of uranium make large-scale use of nuclear energy prohibitive.

INTEXT QUESTIONS (PAGE NO. 253)

1. What kind of mirror (concave, convex or plain) would be best suited for use in a solar cooker? Why?
A concave mirror is best suited. It reflects and focuses sunlight at a point, and the temperature at that point rises, so the food placed there is cooked and heated.

2. What are the limitations of the energy that can be obtained from the oceans?
(i) Tidal energy depends on the relative positions of the earth, moon and sun.
(ii) High dams are required to convert tidal energy into electricity.
(iii) Very strong waves are required to obtain electricity from wave energy.
(iv) To harness ocean thermal energy efficiently, the difference in temperature between hot surface water and cold deep water must be 20°C or more.

3. What is geothermal energy?
Geothermal power plants use the heat of the earth to generate electricity; this heat is known as geothermal energy. Geological changes push molten rock from the core towards the crust, forming hot spots. Steam is generated when underground water comes in contact with these hot spots, forming hot springs. This trapped steam is used to generate electricity in geothermal power plants.

4. What are the advantages of nuclear energy?
(a) A large amount of energy is produced per unit mass.
(b) It does not produce smoke and is a clean energy.
(c) Fission of one atom of uranium produces about 10 million times the energy released by burning one atom of carbon.
(d) Fusion of four hydrogen atoms produces a huge amount of energy, approximately equal to 27 MeV.

ENVIRONMENTAL CONSEQUENCES

1. Combustion of fossil fuels produces acid rain, which damages plants (crops), soil and aquatic life.
2. Burning fossil fuels increases the amount of the greenhouse gas carbon dioxide in the atmosphere.
3. Deforestation for firewood causes soil erosion and destroys wildlife.
4. The construction of hydro-power plants disturbs the ecological balance.
5. Nuclear power plants increase radioactivity in the environment.

INTEXT QUESTIONS (PAGE NO. 253)

1. Can any source of energy be pollution-free? Why or why not?
No source of energy is completely pollution-free. Solar cells are considered pollution-free, but their manufacture causes environmental damage indirectly. Nuclear fusion produces no waste itself, but it needs a temperature of about 10⁷ K, which is provided by fission reactions, and the wastes from fission are very hazardous.

2. Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel than CNG? Why or why not?
Yes. CNG contains hydrocarbons, so it contains carbon, which is a pollutant. Hydrogen is waste-free when it burns, so it is cleaner than CNG.

INTEXT QUESTIONS (PAGE NO. 254)

1. Name two energy sources that you would consider to be renewable. Give reasons for your choices.
(a) Sun: solar energy is produced by the fusion of hydrogen into helium and so on. The sun has a large amount of hydrogen and helium and will shine for about 5 billion years more, so solar energy is renewable.
(b) Wind: wind is caused by uneven heating of the earth. Since this heating will continue, wind energy will also be available for a long time.

2. Give the names of two energy sources that you would consider to be exhaustible. Give reasons for your choices.
(a) Coal: it is formed from the dead remains of plants and animals buried under the earth's crust for millions of years. Coal cannot replenish within a short time, so it is non-renewable or exhaustible.
(b) Wood: it is obtained from forests. Deforestation at a fast rate has reduced the number of forests, and it takes hundreds of years to grow a forest. Therefore wood is exhaustible.

EXERCISE QUESTIONS (PAGE NO. 254)

1. 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
Answer: (b) a cloudy day. A solar water heater needs bright and intense sunlight. On a cloudy day, sunlight is reflected back by the clouds and cannot reach the ground in enough strength.

2. Which of the following is not an example of a bio-mass energy source? (a) wood (b) gobar gas (c) nuclear energy (d) coal
Answer: (c) nuclear energy. Biomass comes from plant materials and animal wastes. Nuclear energy is released by fission or fusion of nuclei, not from biomass. Wood and gobar gas are biomass, and coal is a fossil fuel formed from buried plant and animal remains.

3. 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? (a) Geothermal energy (b) Wind energy (c) Nuclear energy (d) Bio-mass
Answer: (c) nuclear energy. Nuclear reactions do not depend on sunlight. Geothermal heat comes from the earth's interior, heated partly by absorbed solar heat. Wind is caused by the uneven heating of the earth by the sun, and biomass is formed from plants and animal wastes that depend on sunlight.

4. Compare and contrast fossil fuels and the Sun as direct sources of energy.
Fossil fuels such as coal and petroleum are obtained from beneath the earth's crust and are directly available for use. They are limited, non-renewable, take millions of years to form, and are costly. The Sun is a renewable and direct source of energy. It has been shining for several years and will continue for about five billion years. Solar energy is free and available in unlimited amount to all.

5. Compare and contrast bio-mass and hydro electricity as sources of energy.
Both are renewable sources. Biomass is derived from dead plants and animal wastes and is replenished naturally through natural processes; wood and gobar gas are examples. Hydro-electricity is obtained from the potential energy of water stored at a height. It is produced again and again and is obtained through mechanical processes.

6. What are the limitations of extracting energy from (a) the wind? (b) waves? (c) tides?
(a) Wind: a windmill needs wind of more than 15 km/h to generate electricity, and a large number of windmills are required, which cover a huge area.
(b) Waves: very strong ocean waves are required to extract energy from waves.
(c) Tides: very high tides are required, and the occurrence of tides depends on the relative positions of the sun, moon and earth.

7. On what basis would you classify energy sources as (a) renewable and non-renewable? (b) exhaustible and inexhaustible? Are the options given in (a) and (b) the same?
(a) A renewable source replenishes in nature, such as sun, wind, moving water and biomass. A non-renewable source does not replenish in nature, such as coal, petroleum and natural gas.
(b) Exhaustible sources will deplete and exhaust after a few hundred years, such as coal and petroleum. Inexhaustible sources will not exhaust in future and are unlimited. Biomass is given as one example.
Yes. The options given in (a) and (b) are the same.

8. What are the qualities of an ideal source of energy?
An ideal source of energy must be: (i) economical, (ii) easily accessible, (iii) smoke or pollution free, (iv) easy to store and transport, and (v) able to produce a huge amount of heat and energy on burning.

9. What are the advantages and disadvantages of using a solar cooker? Are there places where solar cookers would have limited utility?
Advantages: a solar cooker uses the sun's energy, which is inexhaustible and clean. It is free and available in unlimited amount, so operating it is not expensive.
Disadvantages: a solar cooker is quite expensive, it does not work without sunlight, and it is useless on a cloudy day.
Places where the days are very short, or which are covered with clouds throughout the year, have limited utility for solar cookers.

10. What are the environmental consequences of the increasing demand for energy? What steps would you suggest to reduce energy consumption?
Industrialisation increases the demand for energy. Fossil fuels are easily accessible and meet this demand, but their increased use harms the environment. Too much exploitation of fossil fuels increases greenhouse gases in the atmosphere, causing global warming and a rise in sea level.
Complete reduction of fossil fuel use is not possible, but some steps can help: use electrical appliances wisely, do not waste electricity, avoid unnecessary use of water, and adopt a large-scale public mass transit system. These small steps help to reduce the consumption of natural resources and conserve them.