Page 243
Question 1. What is a good source of energy?
Answer 1 A good source of energy should have the following qualities:
(i) It should be easily available.
(ii) It does not produce pollution.
(iii) It should be renewable and environment friendly.
(iv) It should produce large amount of heat per unit volume/mass.
(v) It should be easy to store and transport.
(vi) It should be economical.
Question 2. What is a good fuel?
Answer 2 A good fuel is one which is cheap, easily available, easy to handle, transport, has proper ignition temperature and high calorific value and produces a large amount of heat on burning, does not produce a lot of smoke and is easily available.
Question 3. If you could use any source of energy for heating your food, which one would you use and why?
Answer 3 We shall use liquefied petroleum gas (LPG)/ compressed natural gas (CNG) or electricity for heating our food because these are efficient ways of supplying energy.
Thermal efficiency of the energy source is large and there is less pollution and the source can be used easily.
Page 248
Question 1. What are the disadvantages of fossil fuels?
Answer 1 There are following disadvantages of fossil fuels like coal and petroleum:
(i) Burning of coal or petroleum causes air pollution.
(ii) Acidic oxides like oxides of carbon, nitrogen and sulphur are released on burning fossil fuels. These oxides lead to acid rains, which affect our water and soil resources.
(iii) Carbon dioxide gas also causes greenhouse effect in atmosphere.
(iv) Fossil fuels are non-renewable sources of energy.
Question 2. Why are we looking at alternate sources of energy?
Answer 2 Fossil fuels, which are traditionally used by human beings everywhere as an energy sources, are nonrenewable sources of energy.These sources of energy are limited and will disappear after some time. They are being consumed at a large rate. If this rate of consumption continues , then the fossil fuels would be exhausted from the Earth.Therefore, we should look for alternate sources of energy.
Question 3. How has the traditional use of wind and water energy been modified for our convenience?
Answer 3 Water energy: Traditionally, waterfalls were used as a source of potential energy in which water is allowed to fall on the blades of the turbine where the rotatory motion of the blades of the turbine is converted into electrical energy. But waterfalls are few in number, therefore, water dams have been constructed. Now-a-days, hydro-electric power stations are used in order to harness potential energy of stored water.
Wind energy: Earlier, the windmills were used to harness wind energy to do mechanical work such as lifting or drawing water from a well. But now a days windmills are used to generate electricity. In windmills, the rotatory motion of the blades turns the blades of turbine to generate electricity.
Page 253-A
Question 1. What kind of mirror – concave, convex or plain – would be best suited for use in a solar cooker? Why?
Answer 1 A solar cooker uses heat of the sunlight to cook the food. A concave mirror would be best suited for use in a solar cooker because it reflects and focuses the sunlight at a point.The temperature at that point increases which heats and cooks the food placed at that particular area.
(i) These forms of energy are available in plenty in our natural environment in the form of continuous currents of energy.
(ii) These energy sources will not be depleted because their supply is large and extraction of usable energy from these sources is negligible.
Question 2. What are the limitations of the energy that can be obtained from the oceans?
Answer 2 Energy from the oceans can be obtained in the form of tidal energy, wave energy and ocean thermal energy. But there are several limitations to harness these energies. Those limitations are:
(i) There are very few locations where dams to utilise tidal energy can be built.
(ii) Very strong waves are required, which are not possible all the time.
(iii) Tidal energy depends on the relative positioning of the Earth, the Moon and the Sun.
(iv) To harness ocean thermal energy, the difference in the temperature of surface hot water and the cold water at depth must be 20°C or more.
(v) Power plants built in oceans or at sea-shores will need high continuous maintenance as chances of corrosion is extremely high.
Question 3. What is geothermal energy?
Answer 3 Geo-thermal energy is the heat energy present inside the Earth in certain regions called hot spots. Due to geological changes, molten rocks formed in the deeper hot regions of the Earth’s crust are pushed upwards and are trapped in hot spots.When underground water comes in contact with the hot spots, steam is generated. This steam is routed through a pipe to a turbine and used to generate electricity.
Question 4. What are the advantages of nuclear energy?
Answer 4 The advantages of nuclear energy are as follows:
(i) Large amount of energy is produced per unit mass.
(ii) It is also cheaper and has a lower fuel cost than other sources of energy. The cost of uranium, which is used as a fuel in generating electricity, is quite low.
(iii) It is a reliable source of power because the nuclear reactors used today have a pretty long life (they work for decades).
(iv) No Greenhouse Gases.
Page 253-B
Question 1. Can any source of energy be pollution-free? Why or why not?
Answer 1 No source of energy can be pollution free. It is considered that solar cells are pollution-free. However, even their making cause environmental damage indirectly. Also, in the case of nuclear energy, there is no waste produced after the fusion reactions. However, it is not totally pollution free. The wastes released from fission reactions are very hazardous. Hence, no source of energy is pollution free.
Question 2. Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel than CNG? Why or why not?
Answer 2 Hydrogen can be considered a cleaner fuel than compressed natural gas (CNG). CNG contains hydrocarbons. Therefore, it has carbon contents. Carbon is a form of pollutant present in CNG.
But hydrogen is waste-free because its burning produces water vapour which is non-polluting. However, due to explosive nature of hydrogen, its storage and transportation is difficult.
Page 254
Question 1. Name two energy sources that you would consider to be renewable. Give reasons for your choices.
Answer 1 Solar energy, wind energy, ocean energy, etc., are renewable sources of energy due to the following reasons:
(i) These forms of energy are available in plenty in our natural environment in the form of continuous currents of energy.
(ii) These energy sources will not be depleted because their supply is large and extraction of usable energy from these sources is negligible.
Question 2. Give the names of two energy sources that you would consider to be exhaustible. Give reasons for your choices.
Answer 2 Coal/petroleum and wood are two exhaustible sources of energy.
Coal/petroleum: These fuels were formed over millions of years ago and there are only limited reserves. If we continue to use them as at present, these reserves will be exhausted very soon.
Wood: It is obtained from forests. Forests are decreasing at very fast rate due to deforestation. If deforestation is continued at this rate, then there would be no wood left on the Earth. Hence, wood is an exhaustible source of energy.
Page 254-255
Question 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 1 (b) a cloudy day
A solar water heater uses solar energy to heat water. It requires day light (intense sunlight) to function properly. On a cloudy day, the sunlight reflects back in the sky from the clouds and is unable to reach the ground. Therefore, solar energy is not available for the solar heater to work properly. Hence, solar water heater does not function on a cloudy day.
Question 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 2 (c) nuclear energy
Biomass is a source of energy which is obtained from biodegradable natural things (plant materials and animal wastes). Nuclear energy is released during nuclear reactions like fission or fusion producing huge amount of energy. Hence, nuclear energy is not an example of biomass energy source. Wood is a plant material, gobar gas is formed from animal dung and coal is a fossil fuel obtained from the buried remains of plants and animals. Hence, these are bio-mass products.
Question 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 3 (c) nuclear energy
Nuclear energy is released during nuclear fission or fusion. These reactions produce tremendous amount of energy. The energy released during these reactions comes from the mass of elements (mass converts into energy). There is no roll of sunlight on these reactions. Hence, nuclear energy is not ultimately derived from Sun’s energy. Geothermal energy, wind energy and biomass are all ultimately derived from solar energy.
Question 4. Compare and contrast fossil fuels and the Sun as direct sources of energy.
Answer 4 Coal/petroleum and wood are two exhaustible sources of energy.
Coal/petroleum: These fuels were formed over millions of years ago and there are only limited reserves. If we continue to use them as at present, these reserves will be exhausted very soon.
Wood: It is obtained from forests. Forests are decreasing at very fast rate due to deforestation. If deforestation is continued at this rate, then there would be no wood left on the Earth. Hence, wood is an exhaustible source of energy.
Question 5. Compare and contrast bio-mass and hydro electricity as sources of energy.
Answer 5 Bio-mass and hydro electricity both are renewable sources of energy.
Bio-mass is derived from dead plants and animal wastes. Hence it is naturally replenished. It is the result of natural process. Wood, Gobar gas are some example of gobar gas.
Hydro- electricity is obtained from the potential energy stored in water at a height. Energy from it can be produced again and again.It is harnessed from water and obtained from mechanical processes.
Question 6. What are the limitations of extracting energy from—
(a) the wind? (b) waves? (c) tides?
Answer 6 Limitations of extracting energy from various sources are following:
(a) Wind energy is harnessed by windmills. A windmill requires wind of speed more than 15 km/h to generate electricity from wind energy. Also, large numbers of windmills are required to get feasible output which covers a large area.
(b) Very strong ocean waves are required in order to extract energy from waves. But these are not available all the time.
(c) Very high tides are required in order to extract energy from tides. Tidal energy is harnessed by constructing a dam across a narrow opening to the sea. But tides depend on the relative positions of the Sun, the Moon and the Earth.
Question 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) The source of energy that is regenerated in nature is known as renewable source of energy. Sun, wind, moving water and biomass are some of the examples of renewable sources of energy. The source of energy that does not regenerated in nature is known as non-renewable source of energy. Coal, petroleum and natural gas are some of the examples of non-renewable sources of energy.
(b) Exhaustible sources are those sources of energy, which will deplete and exhaust after a few hundred years. Coal and petroleum are the exhaustible sources of energy. Inexhaustible resources of energy are those sources, which will not exhaust in future. These are unlimited. Biomass is one of the inexhaustible sources of energy.
So, the options given in (a) and (b) are the same.
Question 8. What are the qualities of an ideal source of energy?
Answer 8 An ideal source of energy must be:
(i) Economical
(ii) Easily accessible
(iii) Smoke/pollution free
(iv) Easy to store and transport
(v) Able to produce huge amount of heat and energy on burning
(vi) Have high calorific value
(vii) Easy to carry and store
(viii) Have high calorific value
Question 9. What are the advantages and disadvantages of using a solar cooker? Are there places where solar cookers would have limited utility?
Answer 9 Advantage: Solar cooker uses solar energy to heat and cook food. It is inexhaustible, clean and renewable source of energy. It is available in unlimited amount for free. Therefore, operating a solar cooker is not expensive.
Disadvantage: Manufacturing cost of a solar cooker is very high. It works only in sunlight. If there is no sunlight then it will not work. Hence, on cloudy day, it becomes useless.
Solar cookers may have less applicable in places where days are shorter than nights, that is places towards the North and South poles. Also in winters, when the length of the days is short, these cookers might have limited application.
Question 10. What are the environmental consequences of the increasing demand for energy? What steps would you suggest to reduce energy consumption?
Answer 10 The demand of energy is increased due to the industrialisation all over the world. Fossil fuels are easily accessible sources of energy that fulfils this demand. The increased use of fossil fuels has a harsh effect on the environment.
The burning of fossil fuels causes air pollution and releases large amount of carbon dioxide which results in global warming. By the industrial wastes in environment it causes air water and soil pollution.
So, we should use these resources in optimum quantity. We should not waste the energy. We should use alternate source of energy such as solar energy.
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