Ques 1: What is the minimum wind velocity required for obtaining useful energy with a windmill?
Ans: The minimum wind velocity required for obtaining useful energy is 15 km/h.
Ques 2: Explain how the energy of flowing water is related to solar energy.
Ans: The energy of flowing water is an indirect source of solar energy. The heat of solar energy evaporates water from the oceans and the surface of earth. The water vapour thus formed rise high in the atmosphere, get cooled and fall back to the earth in the form of rain and snow. This rain water and the water formed by the melting of snow then flows rapidly in the rivers and provides us energy.
Ques 3: Mention any two advantages and disadvantages of producing hydroelectricity by building dams on rivers.
Ans: Advantages
(i) The use of water energy does not cause any pollution.
(ii) It is a cheap and renewable source of electricity, which will never get exhausted.
Disadvantages
(i) The construction of dams on rivers results in to a variety of ecological changes in the downstream area of the river.
(ii) The soil in the downstream area becomes poor in quality because of the lack of annual floods to deposit nutrient rich slit on the banks of the river.
Ques 4: Name the process that converts solar energy into chemical energy in nature.
Ans: Photosynthesis.
Ques 5: Mention any two situations where muscular energy of animals is utilized to do mechanical work.
Ans: (i) To burn the dry bio-mass, like cattle dung, directly to produce heat.
(ii) To convert the bio-mass into more useful fuels and then use these fuels for heating purposes.
Ques 6: Why is burning firewood in traditional chulhas considered disadvantageous?
Ans: (i) Most of the heat produced by burning the fuels in traditional chulhas is lost to the surroundings, only 8% of the heat is utilized in cooking food, etc.
(ii) It produces lot of smoke due to the incomplete combustion of wood, etc.
Ques 7: Name the main constituent of biogas.
Ans: Methane is the main constituent of biogas.
Ques 8: Give two examples of fossil fuels.
Ans: Coal and Petroleum.
Ques 9: Name any three varieties of coal found in nature.
Ans: Anthracite, Bituminous and Lignite.
Ques 10: How is petroleum gas obtained? Name the gas that is its main constituent.
Ans: Petroleum gas is obtained as a by-product in oil refineries from the fractional distillation of petroleum. It is also produced by the cracking of petrol. The main constituent of petroleum gas is butane.
Ques 11: Name any two components obtained by fractional distillation of petroleum that are not used as a fuel.
Ans: Asphalt and lubricating oil.
Ques 12: Write the full name for LPG and CNG.
Ans: LPG – Liquefied Petroleum Gas
CNG – Compressed Natural Gas.
Ques 13: Name three characteristics of a fuel that determines its quality.
Ans: The three characteristics of a fuel that determines its quality are:
(i) It should burn easily.
(ii) It should cause least pollution in air.
(iii) It should be easy to store and should not produce any hazards during transportation.
Ques 14: The heat produced by burning of cow dung cake of mass 210 g raised the temperature of 50 g of water by 2° C. Assuming that all the heat was utilized for heating water whose specific heat capacity is 4.2 J/g ° C, calculate the calorific value of the dung cake.
Ans: Mass of cow dung = 210 g
Mass of water, m = 50 g
Specific heat of water, s = 4.2 J/g/° C
Rise in temperature, t = 2 ° C
Heat produced Q = m × s × t = 50 × 4.2 × 2 = 420 Joules
Calorific value of cow dung = 420/210 = 2 J/g.
Ques 15: Explain why is it difficult to burn a wet piece of wood
Ans: It is difficult to burn a wet piece of wood because water present in it keeps the temperature of wood below its ignition temperature.
Ques 16: Pouring dry sand over fire helps to contain it. Explain why?
Ans: When sand is poured over fire, the supply of air is cut off and the fire stops. When the supporter of combustion is cut off, then the combustion also stops. Hence we say that pouring dry sand over fire helps to contain it.
Ques 17: What is the ultimate source of almost all our sources of energy?
Ans: The energy that the earth receives from the sun, in the form of heat and light, is the ultimate source of almost all our source of energy.
Ques 18: Define net energy or useful energy with an example.
Ans: The difference between the output energy and the input energy is usually referred to as the net energy or useful energy. For example, when we use fuel, say coal or wood or kerosene, we have to provide some initial heat to ignite it, which in this case, is the input. The difference between the heat given off by the burning fuel i.e. the output and the input, will be the useful energy in this case.
Ques 19: How does technology help in the transformation of energy?
Ans: Technology provides the means to transform the energy of different sources into either a fuel or electricity
Ques 20: How is bio gas produced?
Ans: Bio-gas is produced by the anaerobic degradation of bio-mass in the presence of water but in the absence of air.
Ques 21: Why is sun called the ultimate source of fossil fuels?
Ans: Fossil fuels are formed by plants and animals in the course of millions of years. Fossil fuels are the energy rich compounds of carbon which were originally made by plants with the help of sun’s energy (by photosynthesis) when they (plants) were alive millions of years ago.
Ques 22: Why can a match-stick light a splinter of wood but not a log of wood?
Ans: The ignition temperature of a splinter of wood is much lower than that of a log of wood. Thus, a match-stick can produce sufficient heat to reach the ignition temperature of a splinter of wood and make it burn while a match-stick cannot produce sufficient heat to reach the ignition temperature of log of wood and thus cannot cause it burning.
Ques 23: What is combustion? Give the conditions necessary for combustion.
Ans: The process of burning of a substance in the presence of air (oxygen) to produce heat and light is known as combustion.
Conditions required for combustion are:
(i) presence of a combustible substance.
(ii) presence of a supporter of combustion (oxygen).
(iii) heating the combustible substance to its ignition temperature.
Ques 24: Give the uses of Natural gas.
Ans: Natural gas is used
(i) as a domestic and industrial fuel.
(ii) in thermal power plants to generate electricity.
(iii) as a fuel in vehicles like buses, cars, autos etc.
(iv) in the fertilizer industry as a source of hydrogen gas.
(v) in the tyre industry as a source of carbon
Ques 25: Wood is a renewable source of energy, but the use of wood as a fuel is not a wise decision. Explain.
Ans: Wood is a renewable source of energy. It is obtained by cutting trees. A newly planted sapling usually takes more than 15 years to grow and mature into a tree. Therefore replenishment of cut down trees takes a very long time. Cutting down of trees causes the depletion of forests leading to an imbalance in nature. Due to unsuitable air and the above reasons, usage of wood as a source of energy is not a wise decision.
Ques 26: What is meant by solar heating device?
Ans: A device, which utilises sun’s energy, is called a solar heating device.
Example: Solar Cooker, Solar Heater, etc.
Ques 27: Solar energy can be harnessed directly as well as indirectly. Give two examples of each type.
Ans: (i) Direct utilization of solar energy
The direct utilization of solar energy can be done either by collecting it as heat or by converting it directly into electricity.
Examples: Solar cooker and solar water heater uses this type of utilization of solar energy.
(ii) Indirect utilization of solar energy
The indirect utilization of solar energy can be done by converting it into chemical energy like the bio-mass of plants. The other indirect ways of utilizing the solar energy are: Harnessing the energy of the wind and utilization of energy of the sea-waves.
Ques 28: Explain why a solar cooker box is painted black from the inside?
Ans: A black coloured surface absorbs much more heat when compared to a white coloured surface. Hence in the designing of solar heating devices, black painted surfaces are used so that they may absorb the maximum amount of heat from the sunlight or sunshine. In a box-type solar cooker, the inner walls and the bottom of the cooker are painted black to maximize absorption of heat and to minimize heat loss due to reflection. A metal sheet, which is painted black, can be considered as the simplest solar heating device because on keeping in sunshine, it absorbs a lot of sunrays and becomes quite hot. However, after becoming hot, a black surface itself starts radiating the absorbed heat. So, after some time its temperature starts decreasing and tends to become equal to the temperature of its surroundings.
Ques 29: Explain why the property of glass in trapping heat is called greenhouse effect?
Ans: The property of glass is made use of in constructing green-houses used for protecting green plants in very cold climate. A greenhouse is a house-like structure made entirely of glass. The plants to be protected from the cold weather are placed inside this glass house. The glass roof and glass walls of the green-house allow the heat and light of the sun to pass through them and go inside it. They do not allow the inside heat to go out. In this way, heat gets trapped in the glass house due to which it becomes quite warm inside.
Ques 30: How are solar cells made?
Ans: Solar cells are made from semi-conductor materials like silicon and gallium. To make solar cells, the wafers of semiconductor materials containing impurities are arranged in such a way that when light falls on them, a potential difference is produced between the two regions of the semi-conductor wafers. This potential difference then produces electric current. The potential difference produced by a single solar cell of 4 square centimeter size is about 0.4 volt to 0.5 volt and generates a current of 60 milliamperes.
Ques 31: What causes the wind to blow?
Ans: Solar energy is responsible for the blowing of the wind. The sunrays fall on the whole earth. But the intensity of sunrays is much more stronger near the equator of the earth than in the polar regions. Due to more intense sun-heat, the air near the surface of the earth in equatorial regions becomes quite hot. This hot air, being lighter, rises upwards. The cooler air from the polar regions of the earth starts flowing towards the equatorial regions of the earth to fill the space vacated by the hot rising air. In this way, air flows from the higher pressure regions to the low pressure regions of the earth. This flow of air from one place to another constitutes wind.
Ques 32: Explain the transformation of energy taking place in the generation of hydroelectricity.
Ans: The water stored in a high dam has potential energy in it. When the dam water is allowed to fall down, the potential energy of stored water is converted into the kinetic energy of flowing water. When this fast moving water falls on the water turbine, its kinetic energy is transferred to the water turbine. By gaining the kinetic energy, the water turbine starts rotating rapidly and also rotates the armature of the generator fixed on its shaft. The armature then converts the kinetic energy into electrical energy.
Ques 33: Explain how tidal energy can be used for generating electricity.
Ans: The rise of ocean water due to the attraction of the moon is called ‘high tide’ whereas the fall of ocean water is called ‘low tide’. The tidal water in the oceans builds up and recedes twice a day. The enormous movement of water between the high tides and low tides provides a very large source of energy in the coastal areas of the world. The tidal energy can be harnessed by constructing a tidal barrage or tidal dam. During high tide,when the level of water in the sea is high, sea-water flows into the reservoir of the barrage and turns the turbines. The turbines then turn the generators to produce electricity. During the low tide, when the level of seawater is low, the seawater stored in the barrage reservoir is allowed to flow out into the sea. This flowing water also turns the turbines and generates electricity. Thus, as seawater flows in and out of the tidal barrage during high and low tides, it turns the turbines to generate electricity.
Ques 34: State two advantages and two disadvantages of geothermal energy.
Ans: Advantages
(i) It is economical to use because the cost of electricity produced by using geothermal energy is almost half of that produced from conventional energy sources.
(ii) It does not cause any pollution.
Disadvantages
(i) It is not available everywhere, it is available only in those areas where there are hot rocks near the earth’s surface.
(ii) Deep drilling in the earth to obtain geothermal energy is technically very difficult and expensive.
Ques 35: Why is it not good to burn animal dung directly as a fuel?
Ans: It is not good to burn animal dung directly as a fuel because
(i) Animal dung contains important elements like nitrogen and phosphorus which are required by the soil to support crops. So, burning of dung in the form of dung cakes, destroys the useful nutrients which can otherwise be use as a manure in agriculture.
(ii) They produce a lot of smoke on burning which causes air pollution.
(iii) They do not burn completely, they produce a lot of ash as residue.
(iv) They have a low calorific value.
