CBSE Class 10 Science Sources of energy Sure Shot Questions Set 01

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ASSIGNMENT QUESTIONS FOR PRACTICE
SOURCES OF ENERGY

VERY SHORT ANSWER TYPE QUESTIONS (1 MARK)

1. Define energy. [1 Mark]

Answer: Energy is the capacity to do work.

Teacher's Note:
a) The SI unit of energy is the joule (J).

2. What is good source of energy? [1 Mark]

Answer: A good source of energy is one that gives a large amount of useful energy per unit mass or volume, and is easily available, convenient to store and transport, and economical.

Teacher's Note:
a) Cost and safety are as important as the amount of energy.

3. What is good fuel? [1 Mark]

Answer: A good fuel is one that burns easily at a suitable temperature, gives a high amount of heat, and leaves behind little or no residue.

Teacher's Note:
a) A good fuel should also not produce harmful gases on burning.

4. Name the physical quantity associated with the capacity to perform work. [1 Mark]

Answer: Energy.

Teacher's Note:
a) Work and energy are measured in the same unit, the joule.

5. Write the two forms of mechanical energy. [1 Mark]

Answer: Kinetic energy and potential energy.

Teacher's Note:
a) Mechanical energy is the energy possessed by a body due to its motion or position.

6. Define kinetic energy of a body. [1 Mark]

Answer: Kinetic energy is the energy possessed by a body due to its motion.

Teacher's Note:
a) A body at rest has zero kinetic energy.

7. Define potential energy of a body. [1 Mark]

Answer: Potential energy is the energy possessed by a body due to its position or state.

Teacher's Note:
a) Gravitational potential energy depends on the height of the body.

8. Write the formula for kinetic and potential energies. [1 Mark]

Answer: Kinetic energy = ½ mv², and potential energy = mgh.

Teacher's Note:
a) Here m is mass, v is velocity, g is acceleration due to gravity and h is height.

9. What is the common name for radiant energy? [1 Mark]

Answer: Light energy.

Teacher's Note:
a) Radiant energy travels as electromagnetic waves and needs no medium.

10. State four different forms of energy? [1 Mark]

Answer: Heat energy, light energy, chemical energy and electrical energy.

Teacher's Note:
a) Sound, mechanical and nuclear energy are other forms of energy.

11. Do kinetic energy and potential energy have the same units? [1 Mark]

Answer: Yes, both are measured in joules (J).

Teacher's Note:
a) Because both are forms of energy, they share the same SI unit.

12. Which form of energy is associated with the deflating of a balloon? [1 Mark]

Answer: Potential energy (the elastic potential energy stored in the stretched rubber).

Teacher's Note:
a) The stored energy is released as the air escapes and the balloon moves.

13. Name a source of radiant energy. [1 Mark]

Answer: The Sun.

Teacher's Note:
a) The Sun is the main natural source of radiant energy for the Earth.

14. Name the natural and man made sources of radiant energy. [1 Mark]

Answer: Natural source: the Sun. Man-made source: an electric bulb.

Teacher's Note:
a) Fire and candles are also sources of light energy.

15. What is the chemical energy? Is it a form of potential energy? [1 Mark]

Answer: Chemical energy is the energy stored in the chemical bonds of substances. Yes, it is a form of potential energy.

Teacher's Note:
a) Fuels, food and batteries store chemical energy.

16. What do you mean by a muscular energy? [1 Mark]

Answer: Muscular energy is the energy produced in the muscles of living beings, obtained from the food they eat.

Teacher's Note:
a) Muscular energy is a form of chemical energy.

17. Name the form of energy that gives us warmth. [1 Mark]

Answer: Heat energy.

Teacher's Note:
a) Heat flows from a hotter body to a colder body.

18. What type of energy that gives us while climbing a staircase? [1 Mark]

Answer: Gravitational potential energy is gained by the body while climbing.

Teacher's Note:
a) The energy for climbing comes from the muscular energy of the body.

19. A torch cell converts one form of energy into another form. Name these two forms of energy. [1 Mark]

Answer: Chemical energy is converted into electrical energy.

Teacher's Note:
a) The electrical energy then gives light and heat in the bulb.

20. Which form of energy is stored in a compressed spring? [1 Mark]

Answer: Elastic potential energy.

Teacher's Note:
a) The spring regains its shape and releases this energy as kinetic energy.

21. Give an example where potential energy is acquired by a body due to a change in its shape. [1 Mark]

Answer: A stretched rubber band (or a bent bow) stores potential energy because its shape has changed.

Teacher's Note:
a) Any elastic material stores energy when it is stretched, bent or compressed.

22. Why do you feel exhausted after doing laborious work? [1 Mark]

Answer: During laborious work, the muscles use up the chemical energy stored in the body from food, so the stored energy gets used up and you feel tired.

Teacher's Note:
a) Replenishing the body with food restores the stored chemical energy.

23. Name the form of energy that man has been using over the ages. How does he get this energy? [1 Mark]

Answer: Chemical energy. Man gets it by burning fuels such as wood.

Teacher's Note:
a) Burning converts the chemical energy of the fuel into heat and light.

24. What is wind? Does it possess energy? [1 Mark]

Answer: Wind is moving air. Yes, it possesses kinetic energy.

Teacher's Note:
a) Faster-moving air carries more kinetic energy.

25. What is the source of energy in the sun? [1 Mark]

Answer: Nuclear fusion of hydrogen nuclei into helium nuclei.

Teacher's Note:
a) Fusion releases a large amount of energy because some mass is converted into energy.

26. Does wind possess kinetic or potential energy? [1 Mark]

Answer: Kinetic energy.

Teacher's Note:
a) Moving air is the reason wind has kinetic energy.

27. What is a windmill? [1 Mark]

Answer: A windmill is a machine that converts the kinetic energy of wind into mechanical energy.

Teacher's Note:
a) The rotating blades turn a shaft that does useful work.

28. Mention two uses of windmills. [1 Mark]

Answer: Pumping water and grinding grain.

Teacher's Note:
a) Windmills are also used to generate electricity in wind farms.

29. Can we produce electricity by using a cluster of windmills? [1 Mark]

Answer: Yes, a cluster of windmills, called a wind farm, can produce electricity.

Teacher's Note:
a) The rotating blades drive generators that produce electricity.

30. What kind of energy transformation takes place when wind moves a sailboat? [1 Mark]

Answer: Kinetic energy of the wind is transformed into kinetic energy of the sailboat.

Teacher's Note:
a) The sail acts as a surface on which the wind exerts a push.

31. Name one place in India where a wind energy power station has been established. [1 Mark]

Answer: Kanyakumari in Tamil Nadu.

Teacher's Note:
a) Other important wind energy sites include Jaisalmer in Rajasthan and Gujarat.

32. Does moving water possess energy? [1 Mark]

Answer: Yes, moving water possesses kinetic energy.

Teacher's Note:
a) Stored water at a height also possesses potential energy.

33. What is a turbine? [1 Mark]

Answer: A turbine is a machine with blades that rotates when a moving fluid such as water, wind or steam strikes it, converting kinetic energy into mechanical energy.

Teacher's Note:
a) Turbines are the main part of hydro and thermal power plants.

34. What is hydro-electricity generation? [1 Mark]

Answer: Hydro-electricity generation is the production of electricity from the energy of flowing or falling water.

Teacher's Note:
a) Water turns a turbine, which drives a generator.

35. What is a water wheel? [1 Mark]

Answer: A water wheel is a wheel with paddles or buckets that turns when flowing water strikes it, converting the kinetic energy of water into work.

Teacher's Note:
a) Water wheels were used for grinding grain and running mills.

36. Name the device that converts mechanical energy in the turbine into electrical energy. [1 Mark]

Answer: Generator.

Teacher's Note:
a) A generator works on the principle of electromagnetic induction.

37. What is a dam? [1 Mark]

Answer: A dam is a barrier built across a river to store water and create a difference in height.

Teacher's Note:
a) The height difference gives the stored water potential energy.

38. What kind of energy is possessed by water stored in dams? [1 Mark]

Answer: Potential energy.

Teacher's Note:
a) As the water falls, its potential energy changes into kinetic energy.

39. Name the device that converts the kinetic energy of flowing water into mechanical energy. [1 Mark]

Answer: Turbine (water turbine).

Teacher's Note:
a) A water wheel does the same job on a smaller scale.

40. In a water wheel, which form of energy is transformed into work? [1 Mark]

Answer: Kinetic energy of flowing water.

Teacher's Note:
a) The water's energy turns the wheel and does mechanical work.

41. What is the name given to moving air? [1 Mark]

Answer: Wind.

Teacher's Note:
a) Wind is caused by differences in air pressure produced by uneven heating of the Earth.

42. Why are the inner walls and bottom of the box-type solar cookers painted black? [1 Mark]

Answer: Black surfaces absorb more heat from sunlight, so the inside of the cooker gets hotter.

Teacher's Note:
a) Light surfaces reflect much of the sunlight and absorb less heat.

43. What is the use of glass sheet cover in solar heating device? [1 Mark]

Answer: The glass sheet lets sunlight in and traps the heat inside the box, so it cannot escape easily.

Teacher's Note:
a) This trapping of heat is called the greenhouse effect.

44. What is the use of a reflector in the solar heating device? [1 Mark]

Answer: A reflector reflects extra sunlight into the box, so more heat is received.

Teacher's Note:
a) A reflector increases the amount of sunlight falling on the cooker.

45. Name the two types of commonly used reflectors in a solar heating device. [1 Mark]

Answer: Plane mirror reflectors and concave (spherical) reflectors.

Teacher's Note:
a) A concave reflector focuses the sunlight at a point and gives higher temperatures.

46. What is the maximum temperature attainable in case of box type solar cooker? [1 Mark]

Answer: About 100°C to 140°C.

Teacher's Note:
a) This is enough for slow cooking but not for frying.

47. Can we use the box type solar cooker for a quick heating? [1 Mark]

Answer: No, the box type solar cooker is used for slow heating.

Teacher's Note:
a) Quick heating needs a higher concentration of sunlight.

48. Out of box-type and spherical reflector type solar cookers, which (i) produces a higher temperature (ii) produces slow heating? [1 Mark]

Answer: (i) Spherical reflector type solar cooker. (ii) Box type solar cooker.

Teacher's Note:
a) The spherical reflector focuses sunlight onto a small area.

49. What is meant by a solar furnace? [1 Mark]

Answer: A solar furnace is a device that uses a large number of concave mirrors to focus sunlight and attain very high temperatures.

Teacher's Note:
a) Solar furnaces are used for research and for melting materials.

50. What is the order of temperature that can be attained by a solar furnace? [1 Mark]

Answer: Of the order of 3000°C or more.

Teacher's Note:
a) This temperature is much higher than that of a box-type solar cooker.

51. Name the device which converts solar energy directly into electricity. [1 Mark]

Answer: Solar cell (photovoltaic cell).

Teacher's Note:
a) A solar cell does not need any moving part to produce electricity.

52. When was first practical solar cell made? [1 Mark]

Answer: In 1954.

Teacher's Note:
a) The first practical silicon solar cell was developed at Bell Laboratories.

53. Which material is used for connecting the solar cells in a solar panel? [1 Mark]

Answer: Copper strips (tinned copper), sometimes silver.

Teacher's Note:
a) Metal strips carry the current from one cell to the next.

54. Name the type of semiconductor used in a modern solar cell. [1 Mark]

Answer: Silicon.

Teacher's Note:
a) Silicon is doped to form p-type and n-type layers.

55. What do you mean by a solar cell panel? [1 Mark]

Answer: A solar cell panel is an arrangement of many solar cells connected together to produce more electrical power.

Teacher's Note:
a) A single solar cell gives only a small voltage.

56. State two uses of solar cells. [1 Mark]

Answer: Powering calculators and satellites (also street lights and water pumps).

Teacher's Note:
a) Solar cells are useful in remote areas where there is no electricity supply.

57. Name the type of cell used for operating electronic watches and calculators. [1 Mark]

Answer: Solar cell.

Teacher's Note:
a) These devices need only a small amount of power, which a solar cell can provide.

58. Does wind energy produce environmental pollution? [1 Mark]

Answer: No, wind energy does not produce air pollution or greenhouse gases.

Teacher's Note:
a) Wind turbines can cause noise and affect birds, which are minor concerns.

59. What do wind-energy maps indicate? [1 Mark]

Answer: Wind-energy maps indicate the wind speed and wind power potential of different regions.

Teacher's Note:
a) These maps help in choosing sites for wind farms.

60. Name any two wind-energy regions of India. [1 Mark]

Answer: Tamil Nadu (Kanyakumari) and Gujarat (Kutch).

Teacher's Note:
a) Both regions have strong and steady winds for much of the year.

61. What is the first step in utilizing wind energy on a commercial scale? [1 Mark]

Answer: Surveying and mapping the wind speeds of suitable sites.

Teacher's Note:
a) Only sites with strong and steady winds are suitable for a wind farm.

62. What do you mean by hydro energy? [1 Mark]

Answer: Hydro energy is the energy obtained from flowing or falling water.

Teacher's Note:
a) Hydro energy is used mainly to generate electricity.

63. What is geothermal energy? [1 Mark]

Answer: Geothermal energy is the heat energy obtained from the hot rocks and molten magma inside the Earth.

Teacher's Note:
a) Hot water and steam from underground are used to drive turbines.

64. Is hydro energy a direct or an indirect source of energy? [1 Mark]

Answer: Indirect source of energy, because water flows due to the energy of the Sun.

Teacher's Note:
a) The Sun evaporates water, which falls as rain and flows in rivers.

65. Is water energy a non-renewable source of electric energy? [1 Mark]

Answer: No, water energy is a renewable source of electric energy.

Teacher's Note:
a) Water is replenished continuously by the water cycle.

66. Name the property of water which enables the oceans to act as a storehouse of solar heat energy. [1 Mark]

Answer: High specific heat capacity of water.

Teacher's Note:
a) Water absorbs a large amount of heat with only a small rise in temperature.

67. Which kind of mirror, concave, convex or plane, would be best suited for use in a solar cooker? Why? [1 Mark]

Answer: A concave mirror, because it converges the sun's rays at a point and produces high temperature.

Teacher's Note:
a) A convex mirror spreads the light out, so it is not suitable.

68. What is the full form of OTE? [1 Mark]

Answer: Ocean Thermal Energy.

Teacher's Note:
a) OTE is also called Ocean Thermal Energy Conversion (OTEC).

69. What do you mean by ocean thermal energy? [1 Mark]

Answer: Ocean thermal energy is the energy obtained from the temperature difference between warm surface water and cold deep water of the ocean.

Teacher's Note:
a) This temperature difference is used to run a heat engine.

70. What do you mean by tidal energy? [1 Mark]

Answer: Tidal energy is the energy obtained from the rise and fall of sea water caused by the gravitational pull of the Moon and the Sun.

Teacher's Note:
a) Tidal energy is used to drive turbines during high tides.

71. Give one important way of trapping heat inside the solar cooker? [1 Mark]

Answer: Covering the box with a glass sheet, which traps heat (greenhouse effect).

Teacher's Note:
a) Insulating the box with a poor conductor also reduces heat loss.

72. Name the most important green house gas in the earth's atmosphere. [1 Mark]

Answer: Carbon dioxide (CO₂).

Teacher's Note:
a) Burning fossil fuels is the main source of extra carbon dioxide.

73. What should be the temperature difference between surface water and water at depth in an ocean, to tap ocean thermal energy? [1 Mark]

Answer: At least 20°C.

Teacher's Note:
a) A difference of about 20°C is needed for the system to work efficiently.

74. A students constructed a model of box-type solar cooker. He used a transparent plastic sheet to cover the open face of the box. He found that this cooker does not function well. What modification should he make to enhance its efficiency? Give reason? [1 Mark]

Answer: He should use a glass sheet instead of the plastic sheet, because glass traps heat inside the box more effectively.

Teacher's Note:
a) Glass does not allow the trapped heat to escape as easily as plastic.

75. Can we use a box-type solar cooker for baking and frying? [1 Mark]

Answer: No, the box-type solar cooker does not reach the high temperature needed for baking and frying.

Teacher's Note:
a) It is suitable only for slow cooking of food such as rice and pulses.

76. What are the substances used for producing heat known as? [1 Mark]

Answer: Fuels.

Teacher's Note:
a) A fuel releases heat energy when it burns.

77. Name three important sources of modern fuels. [1 Mark]

Answer: Coal, petroleum (crude oil) and natural gas.

Teacher's Note:
a) Modern fuels are mostly fossil fuels formed from ancient remains of living things.

78. What are propellants? [1 Mark]

Answer: Propellants are fuels used to produce thrust in rockets, such as liquid hydrogen and liquid oxygen.

Teacher's Note:
a) A propellant burns to produce hot gases that push the rocket forward.

79. What is the value of solar constant? [1 Mark]

Answer: About 1400 W/m² (1.4 kW/m²).

Teacher's Note:
a) This is the energy from the Sun received per second on each square metre outside the atmosphere.

80. Name two gaseous fuels. [1 Mark]

Answer: Natural gas and LPG (also biogas and coal gas).

Teacher's Note:
a) Gaseous fuels are easy to transport through pipes and burn cleanly.

81. What are processed fuels? Give one example. [1 Mark]

Answer: Processed fuels are fuels prepared from natural raw fuels by processing them, for example petrol.

Teacher's Note:
a) Charcoal, coke and kerosene are other examples of processed fuels.

82. What is biomass? [1 Mark]

Answer: Biomass is organic matter from plants and animals, such as wood, dung and crop waste, which is used as a fuel.

Teacher's Note:
a) Biomass stores energy from the Sun through photosynthesis.

83. Name the ultimate source of fossil fuel. [1 Mark]

Answer: The Sun.

Teacher's Note:
a) Plants captured the Sun's energy, and their remains formed fossil fuels over millions of years.

84. What is the composition of coal? [1 Mark]

Answer: Coal is mainly carbon, with small amounts of hydrogen, oxygen, nitrogen, sulphur and ash.

Teacher's Note:
a) The percentage of carbon decides the quality of coal.

85. Name the places in India where coal is mainly found. [1 Mark]

Answer: Jharkhand, Odisha, West Bengal and Chhattisgarh (Jharia and Raniganj are major coalfields).

Teacher's Note:
a) Jharia in Jharkhand is one of the largest coalfields in India.

86. Name the coal which has a high carbon content. [1 Mark]

Answer: Anthracite.

Teacher's Note:
a) Anthracite contains about 90 per cent carbon and gives the most heat.

87. Why are sources of energy required inspite of law of conservation of energy? [1 Mark]

Answer: Although energy is conserved, it changes into forms that are not useful, such as heat spread in the surroundings. So we need sources that provide energy in a usable form.

Teacher's Note:
a) Usable energy is the reason sources matter, not the total amount.

88. Name two acidic oxides released on burning fossil fuels. [1 Mark]

Answer: Sulphur dioxide (SO₂) and carbon dioxide (CO₂).

Teacher's Note:
a) Sulphur dioxide causes acid rain.

89. What is LPG? [1 Mark]

Answer: LPG is Liquefied Petroleum Gas, a mixture of butane and propane kept in liquid form under pressure.

Teacher's Note:
a) LPG is used as a cooking fuel in homes.

90. What is the composition of petroleum gas? Give one of its uses. [1 Mark]

Answer: Petroleum gas is mainly butane and propane. It is used as a domestic fuel for cooking.

Teacher's Note:
a) It burns with a clean blue flame and leaves little residue.

91. Why is ethyl hydrogen sulphide added in the LPG cylinders? [1 Mark]

Answer: A foul-smelling compound, ethyl mercaptan (ethanethiol), is added so that any leakage of the gas can be detected by its smell.

Teacher's Note:
a) LPG itself is odourless, so the added compound gives a warning.

92. Give two uses of natural gas. [1 Mark]

Answer: It is used as a domestic and industrial fuel (cooking and heating), and as a fuel for vehicles in the form of CNG.

Teacher's Note:
a) Natural gas is also used to make fertilisers and chemicals.

93. Name two gas pipelines in India which have been installed to supply natural gas to the industries and homes. [1 Mark]

Answer: Hazira-Bijapur-Jagdishpur (HBJ) pipeline and Dahej-Vijaipur pipeline.

Teacher's Note:
a) Both pipelines carry natural gas from western India to industries and homes.

94. Name the acidic oxide released by burning fossil fuel which causes green house effect. [1 Mark]

Answer: Carbon dioxide (CO₂).

Teacher's Note:
a) Carbon dioxide traps heat in the atmosphere and warms the Earth.

95. Which gas is released by anaerobic decomposition of submerged vegetation in dams? [1 Mark]

Answer: Methane (marsh gas).

Teacher's Note:
a) Anaerobic decomposition takes place in the absence of air.

96. Define calorific value of fuel. [1 Mark]

Answer: Calorific value is the amount of heat energy produced when a unit mass of a fuel is burnt completely.

Teacher's Note:
a) It is measured in joules per kilogram (J/kg).

97. "The calorific value of cooking gas (LPG) is 50kJ/g". What does it mean? [1 Mark]

Answer: It means that when 1 gram of LPG is burnt completely, it produces 50 kJ of heat energy.

Teacher's Note:
a) A higher calorific value means more heat from the same mass of fuel.

98. Define ignition temperature of a fuel. [1 Mark]

Answer: The lowest temperature at which a fuel catches fire and starts burning.

Teacher's Note:
a) Fuels with a low ignition temperature are easy to light.

99. What do you understand by nuclear wastes? [1 Mark]

Answer: Nuclear wastes are the radioactive materials left over after nuclear reactions, which remain harmful for a long time.

Teacher's Note:
a) Nuclear waste must be stored safely to protect living beings.

100. Out of alpha, beta and gamma radiations, name the radiations that are most dangerous for human beings? [1 Mark]

Answer: Gamma radiation.

Teacher's Note:
a) Gamma rays have the greatest penetrating power and can damage cells deep inside the body.

101. What is the name given to a slow moving neutron? [1 Mark]

Answer: Thermal neutron.

Teacher's Note:
a) Slow neutrons are more effective in causing fission of uranium-235.

102. Which type of bomb was used in the second world war in 1945? [1 Mark]

Answer: Atom bomb (nuclear fission bomb).

Teacher's Note:
a) Atom bombs were dropped on Hiroshima and Nagasaki.

103. In case of an atom bomb, is it controlled or uncontrolled chain reaction which takes placed during explosion? [1 Mark]

Answer: Uncontrolled chain reaction.

Teacher's Note:
a) In an uncontrolled chain reaction, the energy is released in a very short time.

104. Name any two non-conventional sources of energy. [1 Mark]

Answer: Solar energy and wind energy.

Teacher's Note:
a) Non-conventional sources are also called alternative or new sources.

105. What are renewable sources of energy? [1 Mark]

Answer: Renewable sources are sources that are replenished naturally in a short time, such as sunlight, wind and water.

Teacher's Note:
a) Renewable sources do not run out when used.

106. What are non-renewable sources of energy? [1 Mark]

Answer: Non-renewable sources are sources that take millions of years to form and cannot be replenished quickly, such as coal, petroleum and natural gas.

Teacher's Note:
a) These sources will eventually be used up.

107. What is the chemical name of marsh gas? [1 Mark]

Answer: Methane (CH₄).

Teacher's Note:
a) Methane is the main constituent of natural gas and biogas.

108. Name the device in which nuclear fusion reaction is controlled and maintained. [1 Mark]

Answer: Fusion reactor (for example, a tokamak).

Teacher's Note:
a) Very high temperatures are needed to make fusion happen.

109. What is a controlled chain reaction? [1 Mark]

Answer: A controlled chain reaction is a self-sustaining chain reaction in which the number of neutrons is kept steady, so energy is released at a steady rate.

Teacher's Note:
a) Control rods are used to keep the reaction steady in a reactor.

110. Which isotope of uranium is used in nuclear fission in nuclear reators? [1 Mark]

Answer: Uranium-235 (U-235).

Teacher's Note:
a) Natural uranium contains only about 0.7 per cent of U-235.

111. What is nuclear reactor? [1 Mark]

Answer: A nuclear reactor is a device in which a controlled nuclear fission chain reaction takes place to produce heat and electricity.

Teacher's Note:
a) The heat produced is used to make steam that runs turbines.

112. Name the fuel used in atomic reactors. [1 Mark]

Answer: Enriched uranium (uranium-235).

Teacher's Note:
a) Plutonium is also used as a reactor fuel.

113. What is the importance of cadmium rods in nuclear reactor? [1 Mark]

Answer: Cadmium rods absorb neutrons and control the rate of the chain reaction, so that the reactor does not overheat.

Teacher's Note:
a) These rods are called control rods.

114. What is meant by the critically of a reaction? [1 Mark]

Answer: A reaction is critical when the number of neutrons produced in fission equals the number absorbed or lost, so the chain reaction is steady.

Teacher's Note:
a) A reaction below critical dies out, and one above critical grows rapidly.

115. Name the process that forms the principle of a nuclear (i) atom bomb (ii) hydrogen bomb. [1 Mark]

Answer: (i) Nuclear fission. (ii) Nuclear fusion.

Teacher's Note:
a) The atom bomb splits heavy nuclei, while the hydrogen bomb joins light nuclei.

116. Which of the two processes is carried out at a higher temperature: nuclear fission or nuclear fusion? [1 Mark]

Answer: Nuclear fusion.

Teacher's Note:
a) Fusion needs temperatures of millions of degrees to overcome the repulsion between nuclei.

117. Name the main constituent of biogas? [1 Mark]

Answer: Methane.

Teacher's Note:
a) Methane makes up about 50 to 75 per cent of biogas.

118. What do you understand by dry distillation of wood? [1 Mark]

Answer: Dry distillation of wood is heating wood strongly in the absence of air to obtain charcoal, wood tar, wood gas and other products.

Teacher's Note:
a) Charcoal is the solid product left behind.

119. What are fossil fuels? [1 Mark]

Answer: Fossil fuels are coal, petroleum and natural gas, formed from the remains of plants and animals buried millions of years ago.

Teacher's Note:
a) Fossil fuels are non-renewable sources of energy.

120. Why are fossil fuels called 'depletable' sources of energy? [1 Mark]

Answer: They take millions of years to form, so once they are used up they cannot be replaced in a short time.

Teacher's Note:
a) Their reserves are limited and decrease with every use.

121. Describe in brief how petroleum is formed inside the earth. [1 Mark]

Answer: Remains of tiny marine plants and animals were buried under layers of sediment. Over millions of years, high temperature and pressure in the absence of air converted them into petroleum and natural gas.

Teacher's Note:
a) Petroleum is found in porous rocks trapped under non-porous rocks.

122. Explain why the fuels containing oxygen have a low calorific value? [1 Mark]

Answer: Oxygen is already combined with carbon and hydrogen in such fuels, so less energy is released on burning.

Teacher's Note:
a) Wood and alcohol contain oxygen and have lower calorific values than petrol.

123. A tremendous amount of energy is released during a nuclear fission reaction. Why? [1 Mark]

Answer: A small amount of mass is converted into a very large amount of energy, as given by Einstein's equation E = mc².

Teacher's Note:
a) The speed of light squared is a very large number, which explains the large energy.

124. What would happen if all the hydrogen present in the sun is converted into helium? [1 Mark]

Answer: The fusion of hydrogen would stop, so the Sun would no longer produce energy in its present way and its structure would change.

Teacher's Note:
a) The Sun will later change into a different kind of star.

125. It is said that the energy produced from nuclear fusion would create fewer pollution problems than the energy produced by nuclear fission. Explain. Why? [1 Mark]

Answer: Fusion mainly produces helium, which is not radioactive, while fission produces long-lived radioactive waste.

Teacher's Note:
a) Fusion also needs much less fuel per unit of energy produced.

SHORT ANSWER TYPE - I QUESTIONS (2 MARKS)

1. What are primary and secondary fuels? Give two examples of each. [2 Marks]

Answer:
1. Primary fuels are fuels obtained directly from nature, such as wood and coal.
2. Secondary fuels are obtained by processing primary fuels, such as charcoal and petrol.

Teacher's Note:
a) Primary fuels are sometimes called natural fuels.
b) Coke and LPG are also examples of secondary fuels.

2. Why is charcoal considered a better fuel than wood? [2 Marks]

Answer:
1. Charcoal has a higher calorific value than wood, so it gives more heat for the same mass.
2. Charcoal burns without smoke and leaves very little residue.

Teacher's Note:
a) Wood contains moisture and volatile matter, which reduce its heating power.
b) Charcoal burns with a steady, smokeless flame.

3. Coal is said to be formed from the wood of trees. Why then coal considered to be a non-renewable source of energy whereas wood is renewable source of energy? [2 Marks]

Answer:
1. Coal takes millions of years to form under heat and pressure, so it cannot be replaced within a human lifetime.
2. Wood can be regrown by planting trees within a few years, so it is renewable.

Teacher's Note:
a) The formation time is the key difference between the two sources.
b) Wood is renewable only if trees are replanted faster than they are cut.

4. Name all the products obtained by the fractional distillation of petroleum. [2 Marks]

Answer: Refinery gases (LPG), petrol, kerosene, diesel, lubricating oil, paraffin wax, bitumen (asphalt) and petroleum coke.

Teacher's Note:
a) The products separate according to their boiling points.
b) Lighter fractions such as LPG come off at the top of the fractionating tower.

5. What is LPG? Give one advantage and one disadvantage of LPG. [2 Marks]

Answer:
1. LPG is Liquefied Petroleum Gas, a mixture of butane and propane.
2. Advantage: it burns cleanly with a high calorific value. Disadvantage: it is highly inflammable and can cause explosion if it leaks.

Teacher's Note:
a) Safe handling of cylinders and regulators prevents accidents.
b) LPG is stored as a liquid under pressure in cylinders.

6. Why is LPG considered a good fuel? [2 Marks]

Answer:
1. LPG has a high calorific value, so it produces a large amount of heat.
2. It burns without smoke, ash or residue, and it is easy to store and transport in cylinders.

Teacher's Note:
a) A clean fuel reduces the health problems caused by smoke in kitchens.

7. Why is LPG considered a better fuel than coal? [2 Marks]

Answer:
1. LPG produces more heat per unit mass than coal and burns completely.
2. It gives no smoke, ash or sulphur dioxide, so it is cleaner and more convenient to use.

Teacher's Note:
a) Coal leaves behind ash that must be removed.
b) LPG can be controlled easily with a knob on the stove.

8. Define calorific value of a fuel. Which has a high calorific value: wood or charcoal? [2 Marks]

Answer:
1. Calorific value is the amount of heat produced when a unit mass of fuel is burnt completely.
2. Charcoal has a higher calorific value than wood.

Teacher's Note:
a) Calorific value is measured in kJ/kg.
b) Charcoal has less moisture and volatile matter, so it burns hotter.

9. Explain why a match stick can light a splinter of wood but not a log of wood? [2 Marks]

Answer:
1. A splinter has a small mass, so the heat from the match stick raises its temperature to the ignition temperature quickly.
2. A log has a large mass and loses heat quickly to its surroundings, so the match cannot raise it to its ignition temperature.

Teacher's Note:
a) A large body needs much more heat to reach the ignition temperature.
b) A thin splinter also has a large surface area to catch fire.

10. Explain why one can boil water in a paper cup without burning it? [2 Marks]

Answer:
1. Water absorbs the heat from the flame and its temperature stays at 100°C while it boils.
2. The paper stays below its ignition temperature, so it does not burn until all the water has evaporated.

Teacher's Note:
a) The paper must be kept wet by the water inside.
b) A paper cup should not be left over the flame after the water has boiled away.

11. What do you mean by transformation of energy? Give some examples. [2 Marks]

Answer:
1. Transformation of energy is the change of energy from one form into another.
2. Examples: an electric bulb changes electrical energy into light and heat, and a battery changes chemical energy into electrical energy.

Teacher's Note:
a) A green plant changes light energy into chemical energy.
b) The total energy remains the same during transformation.

12. What do you mean by conservation of energy? Briefly explain the statement. [2 Marks]

Answer:
1. The law of conservation of energy states that energy can neither be created nor destroyed; it can only be changed from one form into another.
2. The total energy of an isolated system always remains constant.

Teacher's Note:
a) Energy transformation often produces heat that spreads out.
b) The law applies to every process in nature.

13. How can energy crisis be averted? [2 Marks]

Answer:
1. Use renewable sources such as solar, wind and water energy.
2. Save energy by using efficient appliances, switching off unused lights and using public transport.

Teacher's Note:
a) Reducing waste is as important as finding new sources.
b) Efficient appliances give the same service with less energy.

14. Write four functions performed by the sun's energy. [2 Marks]

Answer:
1. It heats the Earth and provides light.
2. It drives the water cycle and the winds.
3. It supports photosynthesis in plants.
4. It forms fossil fuels and biomass over long periods.

Teacher's Note:
a) Without the Sun, life on Earth would not be possible.
b) Wind energy and hydro energy both come indirectly from the Sun.

15. Give two applications of nuclear fission. [2 Marks]

Answer:
1. Generation of electricity in nuclear power plants.
2. Making atom bombs, and producing radioisotopes for medical use.

Teacher's Note:
a) Nuclear power plants use controlled fission.
b) Radioisotopes are used in diagnosis and treatment.

16. What constitutes nuclear reactions? How is the problem of disposal of nuclear waste being tackled at present? [2 Marks]

Answer:
1. Nuclear reactions are reactions in which the nuclei of atoms split (fission) or join (fusion), releasing large amounts of energy.
2. Nuclear waste is currently treated by converting it into a glass-like solid and sealing it in steel containers, which are stored deep underground.

Teacher's Note:
a) Nuclear waste remains radioactive for many years.
b) Careful storage keeps the radiation away from people and water.

17. State two differences between nuclear reactions and chemical reactions. [2 Marks]

Answer:
1. Nuclear reactions involve changes in the nucleus, while chemical reactions involve only the outer electrons of atoms.
2. Nuclear reactions release a very large amount of energy, while chemical reactions release a comparatively small amount.

Teacher's Note:
a) Chemical reactions do not change one element into another.
b) Nuclear reactions are not affected by temperature or pressure.

18. Mention four uses of wind energy. [2 Marks]

Answer:
1. Pumping water for irrigation and drinking.
2. Grinding grain in flour mills.
3. Generating electricity in wind farms.
4. Propelling sailing ships and boats.

Teacher's Note:
a) Windmills are the traditional devices for these uses.
b) Wind power is most useful where the wind blows steadily.

19. Write two limitations of wind energy. [2 Marks]

Answer:
1. Wind is not constant and cannot be controlled, so the power supply is not steady.
2. Wind farms need large areas of land and have a high initial cost.

Teacher's Note:
a) Wind energy cannot be stored easily without batteries.
b) Wind turbines can also produce noise.

20. Write two advantages of wind energy. [2 Marks]

Answer:
1. Wind energy is renewable and free of cost once the plant is installed.
2. It does not produce air pollution or greenhouse gases.

Teacher's Note:
a) Running costs of a wind farm are low.
b) The land below turbines can still be used for farming.

21. What is the difference between a thermal power plant and a hydro power plant? [2 Marks]

Answer:
1. A thermal power plant burns fuel to produce steam that turns the turbine, and it causes pollution.
2. A hydro power plant uses the energy of falling water to turn the turbine, and it causes no air pollution.

Teacher's Note:
a) Thermal plants depend on a fuel supply, while hydro plants do not.
b) Hydro plants need a dam and a suitable site.

22. Give two advantages of liquid and gaseous fuels over solid fuels. [2 Marks]

Answer:
1. Liquid and gaseous fuels can be transported easily through pipes.
2. They burn without ash or smoke and their flow can be controlled easily.

Teacher's Note:
a) Solid fuels leave ash that must be removed.
b) Easy control gives better efficiency.

23. Give two advantages of smokeless chulhas over traditional chulhas. [2 Marks]

Answer:
1. Smokeless chulhas release less smoke, which protects the health of the family.
2. They use less fuel and save wood, which reduces deforestation.

Teacher's Note:
a) Less smoke means less eye and lung irritation.
b) They also cook food faster.

24. Give four examples of renewable sources of energy. [2 Marks]

Answer:
1. Solar energy.
2. Wind energy.
3. Hydro energy.
4. Biomass energy.

Teacher's Note:
a) Geothermal and tidal energy are also renewable.
b) These sources are replenished naturally.

25. Give four examples of non-renewable sources of energy. [2 Marks]

Answer:
1. Coal.
2. Petroleum.
3. Natural gas.
4. Nuclear fuels such as uranium.

Teacher's Note:
a) Fossil fuels form the first three examples.
b) Nuclear fuels are finite because uranium is mined from the Earth.

SHORT ANSWER TYPE - II QUESTIONS (3 MARKS)

1. Define combustion. What are the various conditions required for combustion to take place? [3 Marks]

Answer:
1. Combustion is a chemical process in which a substance reacts with oxygen, releasing heat and light.
2. A combustible substance must be present.
3. Oxygen (air) must be supplied, and the temperature must reach the ignition temperature of the fuel.

Teacher's Note:
a) Removing any one condition stops the fire.
b) Water cools the fuel below its ignition temperature.
c) A fire extinguisher works by cutting off the supply of oxygen.

2. What are the various characteristics of an ideal fuel? [3 Marks]

Answer:
1. It should have a high calorific value and a moderate ignition temperature.
2. It should burn without leaving smoke, ash or harmful residue.
3. It should be cheap, easily available, easy to store and transport.

Teacher's Note:
a) An ideal fuel should be safe to store.
b) Easy control of the burning is also desirable.
c) Harmful gases from burning should be minimal.

3. Give examples to explain the transformation of potential energy into kinetic energy and vice versa. [3 Marks]

Answer:
1. Potential energy into kinetic energy: a stone dropped from a height falls faster and gains kinetic energy as its potential energy decreases.
2. Kinetic energy into potential energy: a ball thrown upwards slows down and gains potential energy as its height increases.
3. Water stored in a dam has potential energy, which becomes kinetic energy as it falls and turns the turbine.

Teacher's Note:
a) The total mechanical energy stays the same if air resistance is ignored.
b) A pendulum swinging back and forth shows the same conversion.
c) Falling water is the basis of hydro-electric power.

4. Mention any three harmful effects of nuclear radiation on our body. [3 Marks]

Answer:
1. Radiation damages the cells and tissues of the body, causing burns.
2. It can cause cancer, such as leukaemia.
3. It can cause genetic mutations, hair loss and radiation sickness.

Teacher's Note:
a) The effect depends on the dose and the time of exposure.
b) Protective shielding reduces exposure.
c) Damage may appear years after the exposure.

5. What is solar constant? What is the value of solar constant? If the energy received by 5m2 area in 10 minutes is 4200kJ, calculate the value of solar constant. [3 Marks]

Answer:
1. Solar constant is the amount of solar energy received per second on a unit area held perpendicular to the Sun's rays, at the top of the Earth's atmosphere.
2. Its value is about 1400 W/m².
3. Energy received per second = 4200 × 10³ J / (10 × 60 s) = 7000 J/s.
4. Solar constant = 7000 J/s / 5 m² = 1400 W/m².
5. Hence, the solar constant is 1400 W/m².

Teacher's Note:
a) Convert 10 minutes into 600 seconds before dividing.
b) Divide by the area to get the energy per square metre per second.

6. What is a solar cell? Draw the labeled diagram of a solar cell. [3 Marks]

Answer:
1. A solar cell is a device that converts sunlight directly into electrical energy.
2. It is made of a thin p-type and n-type silicon layer forming a junction, with metal contacts on the top and bottom.
3. When sunlight falls on the cell, electrons are released at the junction and flow through the external circuit as current.

[Figure: Labelled diagram of a solar cell showing sunlight falling on the top, a thin metal grid contact on the top surface, an n-type silicon layer, a p-type silicon layer forming the junction, and a metal back contact at the bottom.]

Teacher's Note:
a) The top grid lets light reach the silicon.
b) The p-n junction separates the electrons and holes.
c) A single cell gives a small voltage, so many cells are joined in a panel.

7. What is biogas? Name the major component of biogas. [3 Marks]

Answer:
1. Biogas is a mixture of gases produced by anaerobic decomposition of animal dung and plant waste.
2. Its major component is methane.
3. Biogas also contains carbon dioxide and small amounts of hydrogen sulphide and other gases.

Teacher's Note:
a) Anaerobic decomposition takes place in the absence of air.
b) Methane burns with a clean blue flame.
c) The left-over slurry makes good manure.

8. What is the origin of biomass energy? Briefly explain it. [3 Marks]

Answer:
1. Green plants capture the energy of sunlight during photosynthesis and store it as chemical energy.
2. Animals obtain this energy by eating plants, so the stored energy passes along the food chain.
3. When biomass such as wood or dung is burnt, this stored chemical energy is released as heat.

Teacher's Note:
a) Biomass energy is therefore an indirect form of solar energy.
b) Burning biomass in the open can cause smoke pollution.
c) Biomass can be replenished by growing new plants.

9. What are environmental consequences of increasing demand for energy? What steps would you suggest to reduce energy consumption? [3 Marks]

Answer:
1. Increasing demand for energy causes air pollution, global warming and acid rain from burning fossil fuels.
2. Forests are cut down for fuel and fossil fuel reserves are depleted.
3. Steps to reduce consumption: use energy-efficient appliances, use public transport, and switch off lights and fans when not needed.

Teacher's Note:
a) Using LED bulbs can cut lighting energy use considerably.
b) Car pooling reduces fuel use.
c) Renewable energy reduces dependence on fossil fuels.

10. State two difference between nuclear fission and nuclear fusion. Also give examples. How is the sun able to liberate such a large amount of energy without getting cooled? [3 Marks]

Answer:
1. Fission is the splitting of a heavy nucleus, such as uranium-235, into lighter nuclei. Fusion is the joining of light nuclei, such as hydrogen, into a heavier nucleus.
2. Fission is used in nuclear power plants, while fusion occurs in the Sun and in hydrogen bombs.
3. The Sun releases energy steadily by fusion in its hot core. The energy produced there is carried outwards, so the Sun does not cool down quickly.

Teacher's Note:
a) Fusion needs very high temperature, while fission can start at ordinary temperatures with a neutron.
b) The Sun's huge mass keeps the core hot and dense.

LONG ANSWER TYPE QUESTIONS (5 MARKS)

1. What is hydroelectric power? Draw a schematic diagram of a hydroelectric power plant. [5 Marks]

Answer:
1. Hydroelectric power is electricity produced from the potential energy of water stored at a height, which becomes kinetic energy as it falls.
2. A dam stores water in a reservoir. Water is led through a pipe called a penstock to a turbine placed at the bottom.
3. The falling water turns the turbine, and the turbine rotates a generator.
4. The generator converts mechanical energy into electrical energy, which is sent to the transmission lines.

[Figure: Schematic diagram of a hydroelectric power plant showing a dam with a reservoir of stored water, a penstock (pipe) carrying water down to a turbine, a generator coupled to the turbine, and transmission lines carrying the electricity out.]

Teacher's Note:
a) The height of the water is the most important factor for the power produced.
b) Hydro power is clean and renewable.
c) Dams also help in flood control and irrigation.
d) The schematic must show the flow of water from reservoir to turbine.

2. What is geothermal energy? What is the source of heat contained in geothermal energy? Explain how geothermal energy is used to generate electricity. State two advantage and disadvantages of geothermal energy. [5 Marks]

Answer:
1. Geothermal energy is the heat energy stored in the hot rocks and molten magma inside the Earth.
2. The heat comes from the radioactive decay of elements in the Earth's interior and from the original heat of the Earth's formation.
3. Water is pumped into hot rocks, where it turns into steam. The steam rotates a turbine connected to a generator.
4. Advantages: it is renewable and does not depend on weather, and it produces little pollution. Disadvantages: it is available only in certain regions, and it may release harmful gases such as hydrogen sulphide.

Teacher's Note:
a) Iceland and New Zealand use geothermal energy on a large scale.
b) Geothermal plants are usually built near volcanic or hot spring regions.
c) The cost of drilling deep wells is high.
d) Two advantages and two disadvantages must be written as the question asks.

3. What is biogas? Describe the construction and working of a biogas plant with the help of a labeled diagram. Write any two uses of biogas. [5 Marks]

Answer:
1. Biogas is a mixture of gases, mainly methane, produced by anaerobic decomposition of animal dung and plant waste.
2. Construction: a biogas plant has a masonry digester tank built underground, a dome-shaped gas holder on top, an inlet for dung slurry and an outlet for spent slurry.
3. Working: dung mixed with water is fed into the digester. Anaerobic bacteria decompose the slurry and release biogas, which collects in the gas holder.
4. Uses: biogas is used for cooking and lighting, and for running engines to produce electricity. The spent slurry is used as manure.

[Figure: Labelled diagram of a biogas plant showing the inlet for dung slurry, the underground digester tank, the dome-shaped gas holder, the gas outlet pipe to the stove, and the outlet for spent slurry.]

Teacher's Note:
a) The temperature inside the digester should be suitable for bacterial activity.
b) Methane burns with a clean blue flame.
c) The spent slurry is rich in nitrogen and phosphorus.
d) Label every part of the diagram.

4. What is solar energy? What are the various devices based on solar energy? Briefly describe the construction and working of a solar cooker. [5 Marks]

Answer:
1. Solar energy is the radiant energy received from the Sun.
2. Devices based on solar energy include solar cookers, solar water heaters, solar cells, solar furnaces and solar ponds.
3. Construction of a box-type solar cooker: an insulated box with a black painted base and walls, covered with a glass sheet, and a reflector placed outside to direct sunlight into the box.
4. Working: sunlight passes through the glass and is absorbed by the black surfaces. The heat is trapped inside by the glass, which produces the greenhouse effect, and the food cooks slowly at about 100 to 140°C.

Teacher's Note:
a) The glass cover is what traps the heat.
b) Black surfaces absorb more heat than light surfaces.
c) The reflector adds more sunlight and speeds up the cooking.
d) Solar cookers do not produce air pollution.

5. What are solar cells? How are they designed? Does solar energy cause any environmental pollution? State important uses of solar cells. [5 Marks]

Answer:
1. Solar cells are devices that convert sunlight directly into electricity.
2. Design: a thin layer of n-type silicon is placed over a p-type silicon layer to form a p-n junction. Metal contacts are fixed on the top and bottom, and an anti-reflective coating protects the top surface.
3. Solar energy does not cause pollution during its use. The manufacture of solar cells uses some chemicals, but the operation is clean.
4. Uses: powering calculators, watches, satellites, street lights, water pumps and homes in remote areas.

Teacher's Note:
a) The junction creates an electric field that drives the electrons.
b) Many cells are connected to form a solar panel.
c) Solar cells need no fuel and have no moving parts.
d) The answer should state both the design and at least three uses.

6. Which property of water in oceans enables it to act a storehouse of solar energy? Name some important forms of energy which could be harnessed from the oceans. What is meant by OTE? Name three important potential sites in India which have been identified harnessing tidal energy. [5 Marks]

Answer:
1. Water has a high specific heat capacity, so the oceans absorb a large amount of solar heat with only a small rise in temperature.
2. Energy that can be harnessed from the oceans includes ocean thermal energy, tidal energy and wave energy.
3. OTE is Ocean Thermal Energy, which is the energy obtained from the temperature difference between warm surface water and cold deep water.
4. Three potential sites in India for tidal energy are the Gulf of Khambhat, the Gulf of Kutch (both in Gujarat) and the Sundarbans (West Bengal).

Teacher's Note:
a) Warm surface water and cold deep water are needed for ocean thermal energy.
b) Tidal energy depends on the difference between high and low tide.
c) Sites with a large tidal range are best suited for tidal power.
d) Name the three sites exactly as asked.

7. Give the principle, construction and operation of a Box type solar cooker. [5 Marks]

Answer:
1. Principle: a box-type solar cooker works on the greenhouse effect. Glass lets sunlight in but traps the heat, and black surfaces absorb the heat.
2. Construction: an insulated wooden box, with its inside and base painted black, covered by one or two glass sheets, and fitted with a plane mirror reflector.
3. Operation: the cooker is placed facing the Sun. The reflector directs sunlight into the box, the black surfaces absorb it, and the food cooks slowly.
4. Maximum temperature attained is about 100°C to 140°C, which is suitable for slow cooking.

Teacher's Note:
a) Insulation stops heat from escaping through the walls.
b) The cooker must be turned to face the Sun for best results.
c) Glass covers are placed at a slant so that sunlight enters at right angles.

8. States three advantages and three limitations of a box type solar cooker. What are the main differences between box-type solar cooker and spherical reflector type solar cooker? [5 Marks]

Answer:
1. Advantages: it uses free solar energy, it causes no pollution, and it is simple and safe to use.
2. Limitations: it cooks slowly, it cannot be used at night or on cloudy days, and it cannot reach the high temperature needed for frying or baking.
3. Differences: a box-type cooker uses flat reflectors and gives a temperature of about 100°C to 140°C, while a spherical reflector type uses a concave mirror that focuses sunlight on a small area and gives higher temperature and faster cooking.

Teacher's Note:
a) The spherical reflector cooker needs more frequent adjustment to follow the Sun.
b) Box cookers are cheaper and easier to build.
c) The question asks for three advantages and three limitations, so list exactly three of each.

9. What is solar energy? How do form a solar panel cell? What are the uses of solar cells? [5 Marks]

Answer:
1. Solar energy is the radiant energy from the Sun, which reaches the Earth as light and heat.
2. A solar panel is formed by connecting many solar cells in series and parallel with metal strips, placing them between a glass sheet on top and a backing sheet, and sealing the whole assembly in a frame.
3. The panel receives sunlight and produces direct current electricity.
4. Uses: powering calculators and watches, satellites and spacecraft, street lights, water pumps and homes in remote areas.

Teacher's Note:
a) A single solar cell gives only a small voltage.
b) Connecting cells in series increases the voltage.
c) Solar panels need little maintenance.
d) The uses should include at least three examples.

10. What is a nuclear reactor? What is the fuel used in a nuclear reactor? With the help of a labeled diagram, describe the working of a nuclear power plant. [5 Marks]

Answer:
1. A nuclear reactor is a device in which a controlled nuclear fission chain reaction takes place to produce heat.
2. The fuel is enriched uranium (uranium-235).
3. Working: fission in the fuel rods releases heat. A coolant such as water carries the heat to a steam generator, which produces steam.
4. The steam drives a turbine connected to a generator, which produces electricity. Control rods of cadmium or boron absorb excess neutrons to control the reaction.

[Figure: Labelled diagram of a nuclear power plant showing the reactor core with fuel rods, control rods, coolant circulating to a steam generator, a turbine, a generator producing electricity, and a condenser cooling the steam.]

Teacher's Note:
a) The moderator slows down the neutrons to sustain the reaction.
b) Shielding protects workers from radiation.
c) Label every part of the diagram correctly.