Q 1. Define fossil fuel.
Ans. Fossil fuels were formed millions of years ago when plants and animal remains got buried under the earth and were subjected to high temperature and pressure conditions. For example, Coal, Petroleum, etc.
Q 2. Mention any four areas where solar cells are being used as a source of energy.
Ans. Solar cells are used for street lighting, traffic signals, operating water pumps, and running radio and television sets in remote areas.
Q 3. Explain why only a part of the solar energy that strikes the upper regions of atmosphere, reaches the surface of the earth.
Ans. The ultra-violet rays and the gamma rays present in the solar energy is absorbed by the atmosphere as it comes down towards the surface of the earth, hence we say that only a part of solar energy reaches the surface of the earth.
Q 4. Exposure to which component of solar radiation could be a health hazard?
Ans. Ultra-violet rays and gamma rays are the components of solar radiation which could a health hazard.
Q 5. Define solar cells.
Ans. The solar cell is a device that converts solar energy into electricity.
Q 6. Write down the disadvantages of fossil fuels.
Ans. These fossil fuels are non-renewable sources of energy and cause environmental problems due to pollution.
Q 7. What are the role of a plane mirror and a glass sheet in a solar cooker?
Ans. The plane mirror reflects sunlight so that maximum sunlight can enter the box.
Glass sheet traps solar radiation by the greenhouse effect.
Q 8. What kind of mirror is used in concentrating type-solar cooker?
Ans. A concave mirror is used in concentrating type solar cooker so that maximum heat can be concentrated at a given point.
Q 9. Why can wind energy farms be established only at specific locations? Give reasons to support your answer.
Ans. Wind energy farms can be established over a big area of land, where sufficient wind is blowing always, because electricity generated by a single wind turbine is quite small, so in order to generate a large amount of electricity, a large number of wind turbines are erected over a big area of land.
Q 10. Name the process that produces a large amount of energy in the sun.
Ans. Nuclear fusion
Q 11. Name the materials used for making solar cells.
Ans. Silicon, Germanium, and Selenium
Q 12. Define Nuclear energy.
Ans. Nuclear energy: Energy released when some changes take place in the nucleus of the atom of a substance, is called Nuclear energy.
Q 13. State the advantages of obtaining biogas from animal dung and bio-wastes.
Ans.
(i) The biogas obtained from animal dung and bio-wastes can be used as a smokeless fuel.
(ii) It gives us clean fuel.
(iii) The spent dung can be used as manure.
Q 14. Define non-conventional sources of energy.
Ans. Non-conventional sources of energy are those which are not used as the conventional ones and meet our energy requirement only on a limited scale.
Examples:
- Tidal energy
- Geothermal energy.
Q 15. Write down the characteristics of good fuel.
Ans. Characteristics of a good fuel:
- Has high calorific value
- Produces less smoke
- Produces less residue after burning
- Easy availability
- Inexpensive
- Easy to store and transport
Q 16. What is a thermal power plant?
Ans. In a thermal power plant
- Coal, petroleum and natural gas is used to produce thermal electricity.
- Electricity transmission is very efficient.
- The steam produced by burning fossil fuels runs the turbine to produce electricity.
Q 17. Define conventional sources of energy.
Ans. Conventional sources of energy are those which are used extensively and meet a major portion of our energy requirement.
Examples:
- Fossil fuels
- Hydropower plants.
Q 18. Describe energy harnessed from wind and write down its advantages and disadvantages.
Ans. Wind energy:
- It can converted into mechanical and electrical energy.
- Kinetic energy of the wind is used for running windmills, which can be used to lift water, grind grains, etc.
Advantages:
- Eco friendly
- Renewable
Disadvantages:
- Wind speed not uniform always.
- Needs a large area to erect series of windmills.
- Big amount of investment is needed.
- Output is less as compared to investment.
Q 19. Write down the advantages and disadvantages of a nuclear power plant.
Ans. Advantages of Nuclear energy:
- Alternative source of energy due to depletion of fossil fuels.
- From a small amount of fuel, a large amount of energy is released.
Disadvantages of nuclear energy:
- Difficult to store and dispose nuclear waste which may cause environmental contamination.
- High cost of setting up a nuclear plant.
- Limited availability of raw material (uranium).
Q 20. How is charcoal produced and what is the advantage of charcoal in comparison to wood?
Ans. When wood is burnt in a limited supply of oxygen, the volatile materials present in it get removed, and charcoal is produced. Charcoal bums without flames and is comparatively less smoky.
Q 21. How can solar energy be harnessed? Mention any two limitations of using solar energy. Write down the advantages of solar cell
Ans.
Solar energy:
Solar radiations can be converted into electricity through solar cells (photovoltaic cells). Photovoltaic cells convert solar radiations directly into electricity through silicon solar cells. Solar cells are arranged on large flat sheets to form a solar panel.
Advantages of solar cells:
- Solar cell have no moving parts, require little maintenance and work quite satisfactorily.
- They can be setup in remote and inaccessible hamlets or very sparsely inhabited areas.
Limitations:
- Solar cells are expensive.
- Solar devices are only useful during day time and on a sunny day.
Q 22. Write two different ways of harnessing energy from the ocean.
Ans. Energy from the sea:
1. Tidal energy: Locations in India – Gulf of Kutch, Gujarat, and West Bengal
- Depends upon harnessing the rise and fall of sea level due to tidal action.
- Dams are constructed across a narrow part of sea the and a turbine converts tidal energy into electrical energy.
Disadvantage: Uniform tidal action is not seen.
2. Wave energy:
- Kinetic energy of the sea waves are used to rotate turbines.
- These turbines generate electrical energy.
Q 23. Write a short note on geothermal energy.
Ans.
- Energy harnessed from the heat of the Earth is called geothermal energy.
- Magma is formed when this heat melts the rocks. The molten rocks and hot gases are called magma.
- The magma gets collected at some depths below the earth’s surface. These places are called “hot spots’.
- When underground water comes in contact with these hot spots, it changes into steam, which can be used to generate electricity.
Advantages of geothermal energy:
- Renewable.
- Cost of production is not much.
Disadvantages of geothermal energy:
- Only few sites available for harnessing energy.
- Expensive to set up.
Q 24. What is biomass? Explain the principle and working of a biogas plant using a labelled schematic diagram. Write down its advantages.
Ans. Biomass is a source of conventionally used fuels that are used in our country, e.g., cow dung cakes, firewood, coal, charcoal.
Biogas: It is a mixture of gases produced during the decomposition of biomass in the absence of oxygen.
Methane is the major component of biogas. (Biogas contains 75% methane, carbon dioxide, hydrogen, and hydrogen sulfide).
Biogas plant: Animal dung, sewage, crop residues, vegetable wastes, poultry droppings, etc. are used to produce biogas in biogas plants.
Construction and working:
The plant has a dome-like structure built with bricks. A slurry of cow dung and water is made in the mixing tank from where it is fed into the digester. The digester is a sealed chamber in which there is no oxygen. Anaerobic microorganisms, that do not require oxygen, decompose or breakdown complex compound of cow dung slurry and produce methane, carbon dioxide, hydrogen and hydrogen sulfide.
Advantages of biogas
- It bums without smoke and leaves no residue.
- Its heating capacity is high.
- The slurry of biogas plant is used as an excellent manure, rich in nitrogen and phosphorus.
Q 25. Write down construction, advantages and limitations of a solar cooker.
Ans.
Solar cooker:
Construction:
Outer surface of the solar cooker is painted with black colour and a glass plate is used as cover.
Black colour absorbs more heat and the glass plate traps solar radiation by greenhouse effect. Mirror reflects the light.
Advantages of solar cookers
- Eco friendly
- Renewable
- Used in rural areas.
- Retains all the nutrients in food due to slow cooking.
Disadvantages of solar cooker
- Silicon cells are expensive.
- Solar radiations are not uniform over the Earth’s surface.
- Cannot be used at night or on cloudy days.
- Cannot be used to make chapattis for frying as these require a temperature of 140°C or more. (Maximum temperature of 100°C only can be achieved in a solar cooker.)
Q 26. What is hydroelectric power plant? Write down its advantages and disadvantages.
Ans. Hydroelectric power plant: A conventional renewable source of energy is obtained from water falling from a great height. Dams are constructed to collect water flowing in high altitude rivers. The stored water has a lot of potential energy. When water is allowed to fall from a height, potential energy changes to kinetic energy, which rotates the turbines to produce electricity.
Advantages:
- It is clean and non polluting source of energy.
- Hydropower is renewable source of energy.
Disadvantages:
- Highly expensive to construct.
- Dams cannot be constructed on all river sites.
- Large areas of human habitation and agricultural fields get submerged.
People face social and environmental problems.
Q 27. A. Reddy visited his grandfather’s village Koodankular with his younger sister. They saw people protesting against building a nuclear power plant there. Reddy’s sister asked the reason for the protests. Reddy said that it could be due to the risks involved in setting up the plant near people’s habitation.
(i) Write down the advantages and disadvantages of a nuclear power plant. ‘
(ii) Values shown by A. Reddy.
(iii) Which process is used to harness nuclear energy? Explain briefly.
Ans.
(i) Advantages of Nuclear energy:
- Alternative source of energy due to depletion of fossil fuels.
- From a small amount of fuel, a large amount of energy is released.
Disadvantages of Nuclear energy:
- Hazardous nature of nuclear waste and risk of nuclear waste leakage.
- High cost of setting up a nuclear plant.
- Displacement of people from their habitat.
(ii) A. Reddy is a very intelligent person.
(iii) Nuclear energy is produced by a process called nuclear fission. During this process, the nucleus of a heavy atom (such as uranium, plutonium or thorium) when bombarded with low-energy neutrons, can be split apart into lighter nuclei. The process releases a tremendous amount of heat energy. The released energy can be used to produce steam which is used to produce electricity.
Q 28. During summer vacations Bhaskar visited his uncle’s village. He saw a biogas plant installed by villagers for their basic energy needs.
(i) What is biogas and biogas plant?
(ii) What values are shown by villagers?
Ans.
(i) Biogas: It is a mixture of gases produced during the decomposition of biomass in the absence of oxygen. Methane is the major component of biogas.
Biogas plants: Animal dung, sewage, crop residues, vegetable wastes and, poultry droppings are used to produce biogas in biogas plants.
(ii) Villagers are intelligent and care for the environment.
Q 29. latin Aggarwal took admission in DTO (Delhi Technological University). He saw solar devices were installed in the university.
(i) Write down a short note on solar energy and solar cooker.
(ii) Write down the values shown by the university management.
Ans.
(i) (a) Solar energy:
- Solar radiations can be converted to electricity through solar cells (photovoltaic cells).
- Photovoltaic cells convert solar radiations directly into electricity through silicon solar cells.
- Solar cells arranged on a large flat sheet to form a solar panel.
(b) Solar cooker:
Solar cookers are painted black from outside and covered with a large glass plate to trap solar radiations by greenhouse effect.
(ii) University management is concerned about energy conservation.
Q 30. With the help of a diagram, explain the construction and working of a box-type solar cooker.
Ans.
The heat-absorbing property of black surfaces and the greenhouse effect is utilized in making solar cookers. There are various types of solar cookers out of which, the box type solar cooker is one. The box of the solar cooker is made from non-conducting materials like plastics or fiberglass, with its inner walls painted in black. The walls of the box are made thick with a lining of non-conducting material to prevent loss of heat. Usually, a plane mirror is hinged on the top of the box. The mirror is adjusted in such a manner that it reflects sunlight into the box. The main purpose of the mirror is to increase the effective area for the collection of solar energy. The box is covered with a glass sheet that facilitates the retention of heat inside due to the greenhouse effect. The temperature inside this type of solar cookers can go from 100 ºC to 140 ºC when they are kept in the sun for two to three hours. This type of cooker can be used to prepare food items that require slow heating, for example, for boiling rice, vegetables or dal. The solar cookers are provided with small containers with their outer surfaces painted in black. Two or three items could be prepared simultaneously by using these containers.
Q 31. Mention any two differences between a box type and a concentrator ty
Ans.
Box-type solar heater:
In this type of heater, a plane reflector is used which does not concentrate solar energy at a point.
It produces comparatively low temperatures.
Concentrator type solar heater:
In this type of heater, a concave reflector or a parabolic reflector is used which concentrates the solar energy in a small region called the focus.
It produces high temperatures.
Q 32. Why is it not possible to make use of solar cells to meet all our energy needs? State at least two reasons to support your answer.
Ans. (i) The electricity generated by solar cells during the day is stored by using it to change storage batteries. These storage batteries give us direct current, but to operate the various devices, alternating current is required.
(ii) The usage of solar cells becomes expensive since the direct current given by the storage batteries has to be changed into alternating current using suitable appliances.
Q 33. It is difficult to use hydrogen as a source of energy, although its calorific value is quite high. Explain.
Ans. (i) When hydrogen gas is lighted, it burns with an explosion which is very unsafe.
(ii) It is very expensive to produce hydrogen gas by the electrolysis of water because it requires a lot of electricity.
(iii) It is very difficult to store and transport hydrogen gas as it is explosively inflammable.
Hence it is difficult to use hydrogen as a source of energy, though it has a high calorific value.
Q 34. State the forms in which energy stored in the oceans manifests itself. Which one of these is utilized in OTEC systems?
Ans. The energy from oceans is obtained in three forms:
(i) Tidal energy
(ii) Ocean waves energy and
(iii) Ocean thermal energy
Ocean thermal energy is used in the OTEC system.
Q 35. With the help of a diagram, explain the process of destructive distillation of wood.
Ans.
Let us take two glass tubes. In one tube we put some small pieces of softwood and in another tube, we put some water. These two tubes are then arranged as shown in the figure. When we heat the tube with a burner, we can find that gas goes out through the open end of the delivery tube. If we alight a match stick near this gas, it starts burning to show that it is a combustible gas. This combustible gas obtained from wood is called wood gas and it is used as a fuel. In another tube, we can find that there are drops of deep black liquid at the bottom of the tube, underwater. This deep black liquid is called tar. The black residue left behind the other tube is called charcoal. When this tube is cooled we can remove the charcoal from it.
Q 36. Why is charcoal considered a better fuel than wood? What are the disadvantages of converting wood into charcoal?
Ans. Charcoal is a better fuel than wood because of the following reasons:
(i) Charcoal has a better calorific value than wood, i.e., charcoal produces more heat on burning than an equal mass of wood.
(ii) Charcoal does not produce smoke while burning whereas wood produces a lot of smoke on burning and pollutes the air.
(iii) Charcoal is a compact fuel that is easy to handle and convenient to use.
Disadvantages of converting wood into charcoal:
Cutting down trees in the forests has created a shortage of wood to prepare sufficient charcoal. As a result of this, charcoal has now become expensive fuel. Hence the use of charcoal as a fuel is discouraged so as to prevent deforestation and hence to conserve the ecosystem from the ill effects of deforestation.
Q 37. Describe the steps involved in obtaining biogas and explain what is meant by anaerobic decomposition.
Ans. Two types of biogas plants are being used in our country; they are
1. The fixed-dome type
2. The floating gas-holder type
The main raw material used in these plants is animal dung (cow-dung, the dung of horse, elephant, goat, etc). Plant wastes like vegetable skins, fruit pulp, and human excreta may be added to the animal dung.
Biogas is produced by the anaerobic decomposition of animal wastes like animal dung in the presence of water. The decomposition which takes place in the absence of oxygen is called anaerobic decomposition. This decomposition is carried out by anaerobic micro-organisms called anaerobic bacteria in the presence of water but in the absence of oxygen. Animal dung and plant wastes contain a lot of carbon compounds like carbohydrates, proteins, and fats. The anaerobic bacteria decompose these carbon compounds to form methane gas, which is the main constituent of biogas. Some other gases like carbon dioxide, hydrogen, and hydrogen sulfide are also formed.
Q 38. Why are fossils fuels classified as non-renewable sources of energy? What steps should be taken to conserve these sources?
Ans.
Fossil fuels are non-renewable sources of energy because if they are exhausted, it cannot be regenerated in a short time. This is because of the fact that the fossil fuels which we use today, took millions of years to be formed. Fossil fuels are being fast depleted. To overcome this depletion, we have to conserve energy. We can follow the following simple rules to conserve energy:
(i) Switch off lights and fans when not in use.
(ii) Use solar cookers, heaters, dryers wherever possible.
(iii) Use tube lights that consume less energy instead of incandescent lamp wherever possible.
(vi) Use efficient home appliances and fuel-efficient stoves.
Q 39. The heat produced on complete combustion of 10 g of fuel could raise the temperature of 2 kg of water from 20° C to 70° C. Calculate the calorific value of the fuel if the specific heat capacity of water was 4.2 J/g ° C. Assume that the heat taken by the container is negligible.
Ans.
Mass of water = 2 kg = 2000 g
Specific heat of water = 4.2 J/g ° C
Rise in temperature, t = 70 ° C – 20 ° C = 50° C
We know that heat produced Q = m × s × t
= 2000 × 4.2 × 50
= 420000 joules
This heat of 420000 joules has been produced by burning 10 g of fuel.
Let us calculate the heat produced by burning 1 g of fuel, hence
10 g of fuel produce heat = 420000 joules
and 1 g of fuel produce heat = 420000 /10 = 42000 joule.
Thus, the calorific value of the fuel is 42000 joules per gram.
To convert it into kilojoules per gram, we have to divide by 1000, hence
Calorific value of fuel = 42000/1000 kJ/g = 42 kJ/g.
Q 40. How much energy will 1 kg mass of wood yield on complete combustion if its calorific value were 15 kJ/g?
Ans.
Calorific value of wood = 15 kJ/g = 15000 J/g
Mass of wood = 1 kg = 1000 g
Therefore heat produced by 1 kg of wood = 15000 × 1000 = 15 × 105 J.
Q 41. Suppose the average solar energy incident on the green canopy (leaves) of a tree is 108 J per day. The calorific value of the wood obtained from the tree could convert 1% of incident solar energy as wood, how many days will it take to produce 10 kg of wood?
Ans. Calorific value of wood = 15KJ/g
Conversely, the same energy (15KJ) is required to form 1g of wood.
The energy required to form 10Kg of wood = 10 × 1000 × 15 = 15 × 104 KJ of energy
The average solar energy received per day = 108J day-1
But 1% of incident energy that is useful for the formation of wood = (108) / 100 = 106 J day-1
No of days required to form 10kg of wood = (15 x 104 x 103) / 106 = 15 × 10 = 150 days.
Q 42. 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.
Q 43. Explain why a solar cooker box is painted black from the inside?
Ans. A black-colored surface absorbs much more heat when compared to a white-colored 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 the 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 the 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.
Q 44. 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, seawater 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.
Q 45. Draw a neat diagram of the box-type solar cooker. Label it and explain the function of each part.
Ans. A solar cooker is like a hot box, in which we can cook our food without any cooking gas or kerosene, electricity, coal, or wood. We do not need to spend even a single paisa on fuel. This cooker works with solar energy, which is available for free. In a solar cooker, we can boil, bake, and roast, and in a cooker for domestic use, we can cook food for four to five people. Bigger size solar cookers are also available for cooking food for up to 15 people.
Insulating Box
A solar cooker consists of a well-insulated box for storing energy. To increase the absorption of solar radiation, the inside of the box is painted back.
Glass Lid
The insulated box has a glazing glass surface for collecting energy. The solar cooker absorbs the Sun’s radiant energy through this glass lid.
Reflector
The insulated box has a reflector, which is an ordinary mirror used to reflect sunlight into the box. This reflector helps to track the movement of the sun, focusing sunlight and therefore concentrating the heat energy inside the box.
Black Pots
Solar absorption is also increased by using dark-colored pots for cooking. The vessels that are painted in black are placed inside the box with the food to be cooked.
Q 46. Give the purpose of mirror and glass sheet in the solar cooker.
Ans. In the solar cooker glass sheet acts as a transparent medium which allows light into the medium. The mirror reflects the light which emerges from the cooker.
Q 47. What are the advantages of using a solar water heater? Draw a labeled schematic diagram for a solar water heater.
Ans.
A Solar Water Heater (SWH) is a device that uses solar energy to heat water. Solar Water Heater have several advantages over conventional water heating systems. For consumers, they save electrical energy, save interior space (because they are usually located on rooftops,) and eliminate the risk of accidents in bathrooms due to electrical water heating equipment, They require little or no care and attention while providing hot water for about 300 days in a year in most parts of India. For society at large, they reduce the need for fossil fuels for electrical generation and for fuels such as firewood, coal, furnace oil, etc, that are used in domestic, commercial and industrial boilers. Thereby, they also reduce degradation of the environment
Q 48. Write two limitations of wind mill.
Ans. An estimated 1% to 3% of energy from the Sun that hits the earth is converted into wind energy. This is about 50 to 100 times more energy than is converted into biomass by all the plants on Earth through photosynthesis. Most of this wind energy can be found at high altitudes where continuous wind speeds of over 160 km/h (100 mph) occur. Eventually, the wind energy is converted through friction into diffuse heat throughout the Earth’s surface and atmosphere.
The origin of wind is simple. The Earth is unevenly heated by the sun resulting in the poles receiving less energy from the sun than the equator does. Also, the dry land heats up (and cools down) more quickly than the seas do. The differential heating powers a global atmospheric convection system reaching from the Earth’s surface to the stratosphere which acts as a virtual ceiling.
Q 49. Explain wind power, the principle behind the windmill, and its uses.
Ans. Wind power is the conversion of wind energy into more useful forms, usually electricity using wind turbines. In 2005, the worldwide capacity of wind-powered generators was 58,982 megawatts; although it currently produces less than 1% of worldwide electricity use, it accounts for approximately 25% of electricity use in Denmark, 9% in Spain, and 6% in Germany. Globally, wind power generation more than quadrupled between 1999 and 2005.
Most modern wind power is generated in the form of electricity by converting the rotation of turbine blades into electrical current by means of an electrical generator. In windmills (a much older technology) wind energy is used to turn mechanical machinery to do physical work, like crushing grain or pumping water.
Wind power is used in large-scale wind farms for national electrical grids as well as in small individual turbines for providing electricity to rural residences or grid-isolated locations. Wind energy is ample, renewable, widely distributed, clean, and mitigates the greenhouse effect if used to replace fossil-fuel-derived electricity.
Q 50. (a) What is a solar water heater?
(b) Mention one advantage and one disadvantage of harnessing solar energy.
Ans.
(a) A Solar Water Heater (SWH) is a device that uses solar energy to heat water.
(b) Solar Water Heater has several advantages over conventional water heating systems. For consumers, they save electrical energy, save interior space (because they are usually located on rooftops) and eliminate the risk of accidents in bathrooms due to electrical water heating equipment, They require little or no care and attention while providing hot water for about 300 days in a year in most parts of India. For society at large, they reduce the need for fossil fuels for electrical generation and for fuels such as firewood, coal, furnace oil, etc, that are used in domestic, commercial, and industrial boilers. Thereby, they also reduce the degradation of the environment.