5 MORE ABOUT ENERGY I. Tick ( ) the most appropriate answer. 1. The device which directly converts solar energy to electric energy is: (a) solar cooker (b) solar furnace (c) solar cell (d) solar water heater 2. The energy which directly changes into electric energy in solar cell is: (a) heat energy only (b) light energy only (c) heat and light energy (d) none of these 3. The kind of energy present in fossil fuels is: (a) heat energy (b) mechanical energy (c) chemical energy (d) geothermal energy 4. The ultimate source of chemical energy in fossil fuels is: (a) potential energy of water (b) kinetic energy of water (c) solar energy (d) magnetic energy 5. The renewable source of energy amongst the following is: (a) wind energy (b) hydel energy (c) solar energy (d) all the above 6. The non-renewable source of energy amongst the following is: (a) wind energy (b) hydel energy (c) solar energy (d) fossil fuels 7. The splitting of heavy nucleus of an element with the releases of energy is called: (a) combustion (b) fusion (c) fission (d) photosynthesis 8. The gas which is obtained from biomass: (a) LPG (b) CNG (c) Biogas (d) hydrogen 9. Bhakhra-Nangal hydel project is situated in the state of: (a) Gujarat (b) Orissa (c) Himachal Pradesh (d) Karnataka 10. The energy possessed by moving water is: (a) potential energy (b) kinetic energy (c) geothermal energy (d) hydroelectric energy Ans. 1. (c) 2. (b) 3. (c) 4. (c) 5. (d) 6. (d) 7. (c) 8. (c) 9. (c) 10. (b) 1
II. Complete the following sentences. 1. is the waste materials produced by living beings or their dead remains. 2. The water stored in the dams on river systems has. 3. Solar energy is responsible for wind energy and energy. 4. Petroleum is a fuel, formed from the remains of dead animals and plants. 5. Solar cooker does not cause any environmental. 6. Solar cell is a device which directly converts into electric energy. 7. At wind farms, the wind energy is harnessed in the form of. 8. The pollutant in air responsible for acid rain is. 9. Energy is the capacity to do. 10. The prime source of energy is. 11. The first fuel to be used was. 12. Coke is a fuel. 13. The gaseous fuel used for cooking are. 14. The of steam is used to drive turbines in thermo electric stations. Ans. 1. Biomass 2. Potential energy 3. Chemical 4. Fossil 5. Pollution 6. Solar energy 7. Kinetic energy 8. Sulphur dioxide 9. Work 10. Sun 11. Wood 12. Fossil 13. LPG and biogass 14. Heat energy III. The statements given below are incorrect, write the correct statements. 1. Wood is a non-renewable source of energy. Ans. Wood is a renewable source of energy. 2. In photovoltaic cells chemical energy is converted into electrical energy. Ans. In photovoltaic cells solar energy is converted into electrical energy. 3. In wind farms the potential energy of wind changes to electric energy. Ans. In wind farms the kinetic energy of wind changes to electrical energy. 4. Kerosene oil is a renewable source of energy. Ans. Kerosene oil is a non- renewable source of energy. 5. Nuclear energy is highly dangerous. Ans. Nuclear wastes are highly dangerous. 6. Hot spots are the places on the Earth where sun shines more brightly. 2
Ans. Hot spots are the places inside the earth where geothermal energy is present. 7. Solar cooker preserves all nutrients in food, as food cooks at comparatively high temperature. Ans. Solar cooker preserves all nutrients in food as food cooks at comparatively low temperature. 8. As the water stored in dams flows out, its kinetic energy changes into potential energy. Ans. As the water stored in dams flows out, its potential energy changes into kinetic energy. 9. The minimum height required for stored water is 43 m, so that its P.E. could be converted into electric energy. Ans. The minimum height required for stored water is 34 m, so that its P.E. could be converted into electrical energy. 10. The biomass ferments to form methane in the presence of air. Ans. The biomass ferments to form methane in the absence of air. 11. Smoke and dust particles are not the cause of air pollution. Ans. Smoke and dust particles are the cause of air pollution. 12. Pollens floating in the air do not cause any air pollution Ans. Pollens floating in the air cause air pollution. IV. Find the odd one out. Give reasons for your choice. 1. Flowing water, tidal waves, coal, hard blowing wind. Ans. Coal is a non-renewable source of energy while flowing water, tidal waves and hard blowing wind are renewable sources of energy. 2. Liquefied petrol gas, biogas, compressed natural gas, hydrogen gas. Ans. Hydrogen gas is not a fuel while liquefied petrol gas, biogas and compressed natural gas are fuels. 3. Farm residues, animal dung, wood, coal. Ans. Coal is a fossil fuel while farm residues, animal dung and wood are not fossil fuels. V. Match the different forms of energy with their sources. Column A Column B 1. Water power (a) obtained when the nucleus of an atom splits 3
2. Nuclear energy (b) obtained from sun 3. Wind energy (c) obtained from water falls 4. Solar energy (d) obtained from moving air Ans. 1. (c), 2. (a), 3. (d), 4. (b) VI. Give reasons for the following. 1. Electricity is the most important form of energy in use today. Ans. Electricity is the most important form of energy used today because, it is non-polluting, can be easily transmitted over long distances in a small time and needs least maintenance expenses. 2. Biogas is preferable to wood or coal. Ans. Biogas is preferable to wood or coal because, it is a convenient and clean fuel which does not produce smoke or other harmful gases while wood or coal produces harmful gases and cause air pollution. VII. Answer the following questions. 1. What do you understand by the term renewable source of energy? Support your answer by at least four examples. Ans. The natural sources of energy, which are continually replenished through a number of natural processes and will never run out are renewable sources of energy. Examples of renewable sources of energy are: (i) Sun (ii) Water (iii) Wind (iv) Wood 2. What are non-renewable sources of energy? Support your answer by two examples. Ans. The natural sources of energy, which once used up cannot be replaced over million of years are known as non-renewable sources of energy. Coal and petroleum are two examples of non-renewable sources of energy. 3. How does solar energy changed into chemical energy of biomass? Ans. Solar energy is changed into chemical energy by green plants by the process of photosynthesis. 4. Answer the following questions regarding the box type solar cooker. (a) Why is its interior painted black? (b) Why is it covered with a double plane glass lid? 4
(c) Why is a plane mirror reflector used? (d) What is the optimum temperature attained by this cooker? (e) State its any two advantages. (f) State its any two limitations. Ans. (a) Since black surfaces are good absorbers of heat energy, so interior of a solar cooker is painted black. (b) Solar cooker is covered with a double plane glass lid to allow more solar energy to enter the box but does not allow it to flow out in the form of radiant heat. (c) A plane mirror reflector is used to reflect solar energy into the box. (d) The optimum temperature attained by this cooker is 100 C to 140 C. (e) The two advantages of solar cooker are (i) the cost of cooking food is zero as solar energy comes free of cast. (ii) it can cook from one to four dishes simultaneously. (f) The two limitations of box type solar cooker are: (i) it cannot cook food at night or when the sky is cloudy during the day time. (ii) it cannot be used for making chapatis, frying or baking. 5. Briefly describe the construction of a photovoltaic cell. State any two uses of the cell. Ans. Solar cell or a photovoltaic cell is essentially a very thin sandwich of n-type and p-type wafer thin semiconductors of silicon, in which is placed a U-shaped metal grid. The lower end of the sandwich is fixed to a metal base which has a metal lead. 5
The grid has another lead which can be connected to another solar cell in series. The upper surface of semi conducting sandwich is coated with anti-reflection coating, a black coating. Uses of photovoltaic cell: (i) It is used for providing electric energy to the satellites and space probes. (ii) It is used to provide electricity to offshore oil rigs and light houses. 6. (a) State three common uses of wind energy. (b) How is wind energy harnessed in the form of electric energy? Ans. (a) Three common uses of wind energy are: (i) It is used to draw underground water as in the case of windmills. (ii) It is used to grind grains as in the case of windmills. (iii) It is used to generate electricity as in the case of electric wind machines. (b) Electric energy is produced from wind energy by using windmills. The windmills are conventional in their propeller design which employs two or three tapered blades. The shaft of the windmill is connected to the shaft of armature of the generator when the fast moving wind rotates the blades of windmill, its shaft rotates the armature inside the generator. The rotating armature produces electric current. Thus, the kinetic energy of wind is converted into electrical energy. 7. State two advantages and two disadvantages of hydel power. Ans. Two advantages of hydel power are: (a) The water required for generating hydel power is available free of cost. 6
(b) Hydel power stations are pollution free. Two disadvantages of hydel power are: (i) For running a hydel power station, a vast amount of water is required. (ii) The minimum height required to convert stored P.E. of water into electrical energy is 34 m between the turbine and free surface of water in reservoir. It involves a massive investment on the construction of dams. 8. How is nuclear energy released by the fission process? How is this energy converted into electrical energy? Ans. Most of the nuclear energy is obtained by nuclear fission. The nucleus of the atoms of uranium or plutonium are split in especially designed machines called nuclear reactors by slow moving neutrons, when a large amount of heat energy is produced. This heat energy is utilized in raising steam. The steam in turn is used to run a turbine, coupled with electric generator. Thus when the blades of a turbine rotate, the heat energy of steam is converted into electric energy. 9. State two limitations of nuclear power? Ans. (a) The waste products from nuclear power plants are very dangerous to living beings. (b) The waste products give out radiations which can cause diseases like cancer, leukemia, etc. 10. Answer following questions regarding the biogas plants. (a) Name the raw materials used in these plants. (b) What type of chemical reaction takes place within the plant? Define the chemical reaction. (c) What is slurry? (d) State two conditions due to which the slurry change into biogas. (e) Why do we prefer biogas, to burning dry biomass? (f) What is the approximate composition of biogas? Ans. (a) Animal dung and water are raw materials used in biogas plants. (b) The type of reaction takes place within the biogas plants is anaerobic fermentation. The fermentation of animal dung which takes place in the absence 7
of air due to presence of anaerobic bacteria present in animal dung is called anaerobic fermentation. (c) A mixture of animal dung and water is known as slurry. (d) Slurry changes into biogas in absence of air and in presence of water. (e) Biogas does not cause any pollution while burning dry biomass causes a lot of air pollution. (f) Biogas consists of methane (65%), carbon dioxide, hydrogen and traces of hydrogen sulphide along with water vapour. 11. What is polluted air? (a) How is air polluted by the activities of man? (b) Name five major pollutants in air. State the harm they can cause to living beings. Ans. When the air contains harmful or undesirable substances generated by the activities of man in concentrations that interferes with human health, safety or comfort, or injurious to plants and animals, it is said to be polluted air. (a) Most of the pollution is caused by activities of man: (i) Burning of coal in thermal power plants for the generation of electricity, reduce throw out huge amount of smoke and ash in the atmosphere. (ii) Vehicles introduce a large amount of smoke, carbon monoxide, lead, nitrogen oxides and particulate matter. (iii) Burning of wood or coal or kerosene oil in not properly designed chulha or stove causes air pollution. (iv) Burning of refuse, especially plastic bags, causes a lot of air pollution. (v) Cigarette smoking is the most direct and important source of air pollution. (vi) Pesticides sprayed in fields also cause air pollution. (b) The five major pollutants in air are dust and smoke particles, carbon monoxide, sulphur dioxide, oxides of nitrogen and chlorofluorocarbons. 8
The harms caused by various pollutants of air are: (i) Dust and smoke particles: Dust and smoke particles deposit themselves in the lungs. They can cause serious respiratory problems. (ii) Carbon monoxide: It reacts with the red blood cells to form carboxy haemoglobin and reduce their capacity to carry oxygen. (iii) Sulphur dioxide: If it is inhaled, it forms sulphuric acid. This acid can burn mucous membrane of lungs which can cause serious problem. It also causes acid rain which can damage crops, trees and marble buildings like Taj Mahal. (iv) Oxides of Nitrogen: They get dissolves in water vapour present in the air to form nitric acid and cause acid rain. (v) Chlorofluorocarbons: They are responsible for ozone layer depletion. Ozone layer prevents the harmful ultraviolet radiations to reach on earth. These radiations cause many diseases like skin cancer in living beings. 12. State one way of reducing air pollution in various activities at home. Ans. In the houses we should use improved chulha if we use wood, coal kerosene oil as a fuel, we must use improved stoves, so that the production of smoke and carbon monoxide is minimised. 13. State one way of reducing pollution of air by big industrial units. Ans. In big industrial units like thermal power plants, cement plants, steel plants, etc, we must provide electric precipitators so that the amount of smoke and ash entering the atmosphere is minimised. Sulphur dioxide and nitrogen dioxide gas can be removed by passing the gases through water, before getting them into the atmosphere. 14. State two ways of reducing air pollution caused by automobiles. Ans. (a) We must use efficient internal combustion engines in our vehicles, so that petrol or diesel burns completely without emitting any smoke. (b) The harmful gases present in the exhaust should be removed by using catalytic converter. 9
15. What are fossil fuels? Ans. The remains of organisms that lived on millions of years ago have changed to form coal and oil. These coal, oil and natural gas are called fossil fuels. 16. What type of energy does moving air has? Ans. Moving air has kinetic energy. 17. Where do you find atomic power stations in India? Ans. Atomic power stations in India are: (a) Kakrapara, near Surat in Gujarat (b) Rawat bhata, near Kota in Rajasthan (c) Tarapur, dist Thane, Maharashtra (d) Kalpakkam, near Madras in Tamil Nadu (e) Narora, dist Bulandshahar, U.P. (f) Kaiga, dist Uttar Kannda, Karnataka 18. Name the device that converts solar energy into electricity. Ans. Solar cell or a photovoltaic cell converts solar energy into electricity. 19. Name four types of energy which can be directly related to solar energy. Ans. (a) The solar energy is trapped in the plants in the form of chemical energy. (b) Chemical energy in the food is changed to heat energy and muscular energy in our bodies. (c) It is the solar energy that changes into the mechanical energy of water. (d) The chemical energy in the fossil fuels is basically the trapped solar energy. 20. What are the two limitations in harnessing solar energy? Ans. (a) Solar energy cannot be harnessed during night. (b) Solar energy cannot be harnessed on a cloudy day. 21. Name three traditional uses of solar energy? Ans. Traditional uses of solar energy are: 1. It is used for drying wet clothes. 2. It is used for drying grains before storage. 3. It is used for food preservation. 10
22. What are semiconductors? What are the elements used in making semiconductors in solar cell? Ans. The materials whose conductivity is less than conductors but more than insulators are called semiconductors. Germanium (Ge) and Silicon (Si) are the elements used in making semiconductors in solar cells. 23. What are p type and n type semiconductors? Ans. The semiconductors which are doped with boron and have an affinity to attract electrons are called p type semiconductors. The semiconductors which are doped with phosphorus and have surplus of electrons are called n type semiconductors. They have an excess of electron and hence can donate electrons. 24. What are solar cells and solar panels? What are the main uses of solar cells? Ans. A device which directly converts solar energy into electric energy is called solar cell. The group of solar cells connected in a specific pattern to produce desired potential difference and magnitude of current is called solar panel. Uses of solar cells: 1. Solar cells provide electric energy to the satellites and other space projects. 2. Solar cells are used in providing electricity for the street lights and operating community centre like T.V. and radios where direct supply of electricity is neither possible nor commercially viable. 3. Solar cells are used to provide electricity to offshore oil rigs and light houses. 4. Solar cells are used for providing electricity to electronic calculator and wrist watches. 25. Give three differences between box type solar cooker and spherical reflector type solar cooker. Ans. Difference between box type solar cooker and spherical reflector type solar cooker: Box type solar cooker: 11
1. It does not concentrate solar energy at one point as plane reflector is used. 2. The rise in temperature is in the order of 100 C to 140 C. 3. It cooks only those kinds of food items which require low heat. Spherical reflector type solar cooker: 1. It concentrates energy at one point as concave reflector is used. 2. The rise in temperature is above 500 C. 3. It cooks those food materials which require strong heating. 26. State and explain five energy changes taking place in hydro- electric power station. Ans. Energy changes taking place in hydro electric power stations are: 1. As the water flows into the reservoir from the catchment area, the kinetic energy of flowing water changes into potential energy. 2. As the water is released through control valve, the potential energy of water is converted into kinetic energy. 3. As the water flowing at high speed strikes the blades of turbine, its kinetic energy changes into rotational kinetic energy of the turbine. 4. As the turbine is coupled to the shaft of the generator, the rotational kinetic energy of the turbine changes into rotational kinetic energy of the armature. 5. As the coil of armature rotates in magnetic field, the rotational kinetic energy of the armature changes into electric energy in the coils. 27. Explain how tidal energy is harnessed into electrical energy? Ans. The tides have enormous amount of mechanical energy (potential energy + kinetic energy). This energy can be used to generate electricity. The tidal power plants are constructed near the narrow bays, where the water level rises up and then falls down appreciably during the tides. During high tides, sea water is allowed to pass through the dam walls by opening gates. When the level of water tends to fall during low tide, the gates are closed. Thus, the sea water collects behind the dam has potential energy. This stored water is allowed to fall on the blades of turbine to an electric generator. The kinetic energy of the outgoing water changes into electrical energy. 12