NCERT Questions Answers (Physics) Solved for Class 10 Science Chapter 14 - Sources Of Energy

NCERT Book, Page 243

Q.1. What is a good source of energy ? Ans. A good source of energy is one : (i) which would give a large amount of energy per unit mass (or per unit volume) (ii) which is cheap and easily available (iii) which is easy to store and transport (iv) which is safe to handle and use, and (v) which does not cause environmental pollution.

Q.2. What is a good fuel ? Ans. A good fuel is one : (i) which has a high calorific value, (ii) which has a proper ignition temperature, (iii) which burns without giving out any smoke or harmful gases, (iv) which burns smoothly and does not leave behind much ash after burning, and (v) which is cheap, easily available, easy to handle, safe to transport and convenient to store.

Q.3. If you could use any source of energy for heating your food, which one would you use and why ? Ans. I would use LPG (cooking gas) for heating food because : (i) LPG has a high calorific value. It gives a lot of heat (per unit mass) on burning. (ii) LPG burns with a smokeless flame and hence does not cause any air pollution. (iii) LPG does not produce any poisonous gases on burning (iv) LPG does not leave behind any ash after burning

NCERT Book, Page 248

Q.1. What are the disadvantages of fossil fuels ? Ans. Coal, petroleum and natural gas are fossil fuels. The burning of fossil fuels has the following disadvantages : (i) Fossil fuels are non-renewable sources of energy. Once exhausted, fossil fuels will not be available to us in the near future.

(ii) The burning of fossil fuels causes a lot of air pollution. This is described below :

(a) The burning of fossil fuels produces acidic gases such as sulphur dioxide and nitrogen oxides which cause acid rain. The acid rain damages plants (crops, etc.) , reduces fertility of soil by making it acidic, poses a danger to aquatic life (like fish) by making the water of lakes and rivers acidic, and damages buildings by corroding them slowly. (b) The burning of fossil fuels produces large amount of carbon dioxide gas which goes into air. The presence of increasing amounts of carbon dioxide gas in air is causing increased greenhouse effect leading to excessive heating of the earth. This global warming is harmful for all the life on earth. © The burning of fossil fuels (especially coal) produces smoke which pollutes the air. The burning of coal leaves behind a lot of ash. It also puts tiny particles of ash (called fly-ash) into the air causing air pollution.

Q.2. Why are we looking at alternative sources of energy ? Ans. We are looking at alternative sources of energy mainly due to two reasons : (i) because the fossil fuels and nuclear fuels present in the earth are limited which may not last for long, and (ii) because of the undesirable effects of pollution both from the burning of fossil fuels and from the radioactive wastes of the nuclear power plants.

Q.3. How has the traditional use of wind and water energy been modified for our convenience ? Ans. (i) The traditional use of wind energy has been modified by the improvements in technology to generate electricity through wind-powered generators. (ii) The traditional use of energy of flowing water has been modified by improvements in technology to generate electricity through flowing water-powered generators (by establishing hydropower plants).

NCERT Book, Page 253

Q.1. What kind of mirror–concave, convex or plane–would be best suited for use in a solar cooker ? Why ? Ans. A concave mirror reflector would be best suited for use in a solar cooker. This is because a concave reflector converges a large amount of sun‘s heat rays to a small area at its focus due to which a high temperature is produced in its focus area (which is suitable for baking and frying). Such a high temperature cannot be achieved in a solar cooker by using a plane mirror reflector. Convex mirror reflector, being a diverging mirror, cannot be used at all in a solar cooker.

Q.2. What are the limitations of energy that can be obtained from the oceans ? Ans. The energy from the oceans can be obtained mainly in three forms : Tidal energy, Wave energy and Ocean thermal energy. Some of the important limitations of these forms of energy are as follows : (i) There are very few places around the world where barrages (or dams) can be built to harness tidal energy. Moreover, the rise and fall of water during high and low tides is not enough to generate electricity on a large scale. (ii) The wave energy can be harnessed only at those places where the sea-waves are very strong. The efficiency of power plants based on wave energy is very low. Moreover, the power plants built in oceans or at seashores have high cost of installation, corrode easily and need a lot of maintenance. (iii) A temperature difference of 20°C (or more) between the surface water of ocean and deeper water is necessary to harness ocean thermal energy. The efficiency of OTEC power plants which work by utilising ocean thermal energy, is very low. Moreover, it is very expensive to establish OTEC power plants.

Q.3. What is geothermal energy ? Ans. Geothermal energy is the heat energy from hot rocks present inside the earth. This heat comes from the fission of radioactive materials which are naturally present in these rocks. Geothermal energy is available only at some places in the world. Geothermal energy can be used to produce electricity.

Q.4. What are the advantages of nuclear energy ? Ans. The advantages of nuclear energy are that : (i) it produces a large amount of useful energy from a very small amount of a nuclear fuel (like uranium-235). (ii) once the nuclear fuel (like uranium-235) is loaded into the reactor, the nuclear power plant can go on producing electricity for two to three years at a stretch. There is no need for putting in nuclear fuel again and again. (iii) it does not produce gases like carbon dioxide which contribute to greenhouse effect or sulphur dioxide which causes acid rain.

NCERT Book, Page 253

Q.1. Can any source of energy be pollution free ? Why or why not ? Ans. In reality, no source of energy can be said to be pollution free. The use of each and every source of energy disturbs the environment in one way or the other. For example, though the use of a wind generator, solar cooker and solar cells for obtaining energy is pollution free but the processes involved in making the materials for these energy devices must have caused some pollution and damaged the environment in one way or the other.

Q.2. Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel than CNG ? Why or why not ? Ans. Hydrogen is a cleaner fuel than CNG. This is because the burning of hydrogen produces only water, which is totally harmless. On the other hand, burning of CNG produces carbon dioxide gas and water. This carbon dioxide can produce greenhouse effect in the atmosphere and lead to the excessive heating of the environment in the long run.

NCERT Book, Page 254

Q.1. Name two energy sources that you would consider to be renewable. Give reasons for your choices. Ans. Hydroenergy (Energy from flowing water) and Biomass energy (Energy from biofuels such as wood) are the renewable sources of energy. (i) Hydroenergy is a renewable source of energy because it is supplied by the water cycle in nature (powered by sun’s energy) and it will never get exhausted. (ii) Biomass energy contained in wood is a renewable source of energy because if trees are cut from the forest for obtaining wood, then more trees will grow in the forest in due course of time.

Q.2. Give the names of two energy sources that you would consider to be exhaustible. Give reasons for your choices. Ans. Exhaustible sources of energy means non-renewable sources of energy. Coal and petroleum are the two exhaustible sources of energy (or non-renewable sources of energy). This is due to the following reasons : Coal and petroleum are fossil fuels which were formed in the earth very, very slowly. The coal and petroleum which are present in the earth today have taken millions of years to form and get accumulated. So, if all the coal and petroleum present in earth get exhausted, they cannot be produced quickly in nature. We will not get any coal or petroleum in the near future.

NCERT Book, Pages 254-255

Q.1. A solar water heater cannot be used to get hot water on :

Ans. © nuclear energy

Q.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 ?

Q.4. Compare and contrast fossil fuels and the sun as direct sources of energy. Ans. (i) The sun is a renewable source of energy but fossil fuels are a non-renewable source of energy. (ii) The sun’s energy does not cause any pollution but burning of fossil fuels causes a lot of pollution. (iii) The sun’s energy is available in a diffused form (scattered form) but fossil fuels provide energy in concentrated form. (iv) A special device (like solar cooker or solar cell) is always needed to utilise sun’s energy but this is not so in the case of fossil fuels. (v) The sun’s energy is available only during the day time when the sun shines but energy of fossil fuels can be used all the time.

Q.5. Compare and contrast bio-mass and hydroelectricity as sources of energy. Ans. (i) Biomass is a renewable source of energy and hydroelectricity is also a renewable source of energy. (ii) The use of biomass by burning causes air pollution but the use of hydroelectricity does not cause any pollution. (iii) Biomass gives heat energy which can be used for cooking and heating only. On the other hand, hydroelectricity can run all types of electrical appliances. (iv) Biomass energy can be obtained without using any special device but hydroelectricity can be produced only by establishing hydro-power plants.

Q.6. What are the limitations of extracting energy from : (a) wind ? (b) waves ? © tides ? Ans. (a) Limitations of extracting energy from wind : (i) Wind generators to produce electricity can be established only at those places where wind blows for the greater part of the year. (ii) The wind speed should be higher than 15 km/h to maintain the required speed of wind turbines for generating electricity. (iii) There should be some back-up facilities like storage cells to take care of energy needs during the period there is no wind. (iv) A large area of land is required for establishing wind energy farms (v) The initial cost of establishing wind energy farms is quite high.

(b) Limitations of extracting energy from waves (or sea-waves) : (i) The wave energy can be harnessed at only those places where the sea-waves are very strong. (ii) The efficiency of power plants based on wave energy is very low. (iii) The power plants built in oceans or at sea-shores have high cost of installation, corrode easily and need a lot of maintenance.

© Limitations of extracting energy from tides : (i) There are very few sites around the world which are suitable for building tidal barrages (or tidal dams). (ii) The rise and fall of sea-water during high and low tides is not enough to generate electricity on a large scale.

Q.7. On what basis would you classify energy sources as : (a) renewable and non-renewable ? (b) inexhaustible and exhaustible ? Are the options given in (a) and (b) the same ?

Ans. The options given in (a) and (b) are the same. This is because renewable sources of energy are also known as inexhaustible sources of energy whereas non-renewable sources of energy are also called exhaustible sources of energy.

(i) Those sources of energy which are being produced in nature continuously and can be used again and again for ever, are called renewable sources of energy (or inexhaustible sources of energy). These sources of energy will never get exhausted. For example, wood is a renewable source of energy (or inexhaustible source of energy), because if some trees are cut down from forests for obtaining firewood, then more trees will grow in the forest on their own or can be replanted by man. This will ensure continuous supply of wood for obtaining heat energy.

(ii) Those sources of energy which have accumulated in the earth over very, very long time of millions of years and cannot be made quickly when used up completely, are called non-renewable sources of energy (or exhaustible sources of energy). These sources of energy will get exhausted sooner or later. For example, coal is a non-renewable source of energy (or exhaustible source of energy) because coal has accumulated in the earth over a very, very long time, and if all the coal gets used up completely, it cannot be produced quickly in nature.

Q.8. What are the qualities of an ideal source of energy ? Ans. An ideal source of energy has the following qualities : (i) It gives a large amount of energy per unit mass (or per unit volume). (ii) It does not cause any environmental pollution. (iii) It is easy to store and safe to transport (iv) It is safe to handle and use (v) It is cheap and easily available.

Q.9. (a) What are the advantages and disadvantages of using a solar cooker ? (b) Are there any places where solar cookers would have limited utility ? Ans. (a)

The important advantages of a solar cooker for cooking food are the following : (i) The use of solar cooker for cooking food saves precious fuels like coal, kerosene and LPG. (ii) The use of solar cooker does not produce smoke due to which it does not pollute air. (iii) When food is cooked in solar cooker, its nutrients do not get destroyed. This is because in a solar cooker, food is cooked at a comparatively lower temperature. (iv) In a solar cooker, up to four food items can be cooked at the same time.

Some of the important limitations (or disadvantages) of a solar cooker are given below : (i) The solar cooker cannot be used to cook food during night time (because sunshine is not available at that time). (ii) If the day-sky is covered with clouds, even then solar cooker cannot be used to cook food. (iii) The direction of reflector of solar cooker has to be changed from time to time to keep it facing the sun. (iv) The box-type solar cooker cannot be used for baking (making chapattis, etc.) or for frying.

(b) The solar cookers have limited utility at those places which usually remain cloudy and have long winters. An example of such places are hilly areas.

Q.10. (a) What are the environmental consequences of the increasing demand for energy ? (b) What steps would you suggest to reduce energy consumption ? Ans. (a) Some of the environmental consequences of the increasing demand for energy are the following : (i) The combustion of fossil fuels is producing acid rain and damaging plants (crops), soil, aquatic life and buildings. (ii) The burning of fossil fuels is increasing the amount of greenhouse gas carbon dioxide in the atmosphere leading to global warming. (iii) The cutting down of forest trees (deforestation) for obtaining firewood is causing soil erosion and destroying wildlife. (iv) The construction of hydropower plants is disturbing ecological balance. (v) Nuclear power plants are increasing radioactivity in the environment.

(b) Some of the steps which can be taken to reduce energy consumption are as follows : (i) Switch off lights, fans, TV and other such electrical appliances when not needed, to save electricity. (ii) Use energy efficient electrical appliances such as compact fluorescent lamps (CFLs) and tube-lights to save electricity. (iii) Good quality stoves should be used to burn fuels like kerosene and LPG so as to obtain maximum heat. (iv) Pressure cookers should be used for cooking food to save fuel. (v) Solar cookers should be used to cook food wherever possible and solar water heaters should be used to get hot water. (vi) Bicycles should be used for short distances instead of scooters, motorcycles and cars so as to save petrol.