Energy is the capacity of a body for doing work. Energy stored in a body or system is equivalent to total work done by the body till whole of its energy has been completely exhausted. Most of our energy requirement is fulfilled from the fuels & electricity. Solar energy is also available to us in the form of a variety of fuels that have been stored in the earth’s crust. Energy can be converted from one form to another.
13.1 SOURCES OF ENERGY :
We have a wide range of sources of energy such as the sun, the wind, the earth geothermal), flowing water, coal, gasoline, diesel, natural gas, biogas, etc. at our disposal. We utilize this energy to perform a wide range of activities, i.e. industrial, commercial, household etc.
13.1 (a) Types of Sources of Energy :
There are two types of sources of energy :
(i) Renewable sources of energy (or non-conventional sources of energy):
The sources of energy which are in constant supply to us by nature and are inexhaustible are known as renewable sources of energy.
Example : The sun (solar energy), oceans, tidal energy, wind energy, running water energy, wood, geothermal energy etc.
(ii) Non- renewable sources of energy (or conventional sources of energy):
The sources which can’t used again and again and are exhaustible are known as non-renewable sources of energy.
Example : Coal, natural gas, petroleum, fossil fuels etc.
13.1 (b) Characteristics of Sources of Energy :
For a good source of energy, following conditions must be fulfilled by it :
(i) It should provide large amount of useful energy.
(ii) It must be easily storable in small space.
(iii) It must be easily transportable.
(iv) It must provide the energy in regular manner.
(v) It should be convenient to use.
13.2 SOLAR ENERGY :
The sun is the primary source of energy for all living beings on the earth. It provides all of us heat and light. The energy generated by the sun is the result of reaction called nuclear fusion, occurring continuously in the interior part of the sun. Hans Bethe, a physicist proposed that the enormous release of energy from the sun is due to the fusion (combination) of four hydrogen atoms to yield a single helium atom
Solar energy in trapped by plains, plateaus, mountains, rivers, lakes, oceans and ponds. Plants use solar energy to manufacture food by photosynthesis and also solar energy is the source of wind storms, rain, snow fall & ocean waves.
It is the perennial (forever) source of energy.
This perennial source of energy has some features :
(i) It comes to earth surface in a very diffused form.
(ii) Upper atmosphere gets 1.3 KJ of energy per second per square meter of this only 47% i.e. approximately 0.64 KJ energy reaches per second per square meter on earth’s surface.
(iii) Moreover this amount of energy is not available uniformly and keeps changing everyday at a place and place to place during a day.
13.2 (a) Composition of Solar Energy (Sunlight):
(I) Ultra violet rays :
The invisible rays whose wavelength is shorter than that of the visible violet light are called ultra violet rays. We can detect these rays by using a photographic film or a fluorescent paper because they darken the photographic film just like ordinary light rays. They are used to kill bacteria in food stuff and drinking water. Too much ultra-violet radiation is dangerous for our health due to its ionising effect and can cause skin cancer.
(ii) Visible light :
The visible rays whose wavelength range from 400 nm (in violet) to 700 nm (in red) are called visible light. The visible apart of the sunlight consists of seven different wavelet, each wavelength corresponding to a different colour. Thus the visible part of the sunlight consists of seven different colours.
(iii) Infra – red rays :
The invisible rays whose wavelength is longer than that of the visible red light are called infra-red rays. They can heat the object on which they fall. About one-third of the solar energy consists of infra-red rays. They ca be detected by its heating effect, by using a thermometer. Every hot object emit infra-red rays. They are used to get relief from body aches.
13.2 (b) Uses of Solar Energy :
(i) Solar cooker absorb solar energy and cook food,
(ii) Solar water hearers are used for heating water.
(iii) Solar cell convert solar energy into electricity to run watches, calculators and in spaceships for various
experiments.
(iv) Solar energy is absorbed by green plants to make their food by photosynthesis.
(v) Solar energy is used for drying clothes and food grains.
(vi) Solar energy is used for making salt from sea water.
13.2 (c) Advantages of Solar Energy :
(i) It is inexhaustible and renewable as it producing continuously in the core of sun by nuclear fusion of H-
atoms.
(ii) Its quantity is unlimited and is available in all parts of the world in abundance.
(iii) It does not cause any pollution.
(iv) It can be put to practical appliances.
13.2 (d) Limitations of Solar Energy :
(i) It is not available at night.
(ii) It is not available uniformly in all parts of world.
(iii) Solar energy received by the earth is quite diffused and in scattered form and hence only a part of it is
utilized.
(iv) It is not available at constant rate due to clouds, fog, mist, haze, winds etc.
13.2 (d) Direct and Indirect Harnessing (or Collection) of Solar Energy :
(i) Direction utilization of solar energy can be done by collecting the heat radiation on reflecting these by plane mirrors on to black boxes containing uncooked food (in solar cooker) and for heating water in solar heaters. These rays can be converted into electrical energy as in solar cells.
(ii) Indirect utilization of solar energy can be done by first converting solar energy into chemical energy as in biomass of plants. Heat energy of sun can be utilized in sea waves (ocean thermal energy) and into energy of winds etc.
13.2 (f) Solar Heating Devices :
These are the devices which can collect and store heat obtained from solar energy. These are used for heating and cooking purposes. Solar heating devices are designed in such a way so that these can make maximum utilization of solar heat radiations. It is done by adopting following procedure :
(i) Concentration of solar energy of using reflectors :
For moderate heating sun rays are reflected by using plane mirrors, as in solar cookers and solar water heaters. For high temperature, sun’s energy is concentrated using concave mirrors as reflectors.
(ii) Black paint :
Since black bodies are good absorbers as well as good radiators of heat, hence black paint is used to absorb and store heat radiations in large quantity by using large surface area.
(iii) Glass – sheet cover :
Glass sheet cover is used to protect the hot infra red rays of solar energy from escaping the body of black box. It allow the IR radiations (of shorter wavelength) to enter the box of solar heating device and do not allow IR radiations (of longer wavelength) to escape from the solar heating device. Hence more heat is retained by solder heating device for long time.
(A) Box type solar cooker :
Solar cooker is a device used for cooking food with the help of solar energy. A box is made of a non – conducting material like plastic or fiber glass and painted black from inside is used for making the solar cooker.
The cooker is placed in the sunlight and the position of the reflector is adjusted in such a ways that a strong beam of light falls over the cooker top. These rays pass through the transparent glass sheet, therefore the box and the containers absorb maximum amount of infra red radiations from the sunlight falling on it. As a result the temperature inside the box rises to about 1000C to 1300C.
(B) Spherical reflector type solar cooker :
In this type of solar cooker, there is an insulated metal box, painted black from inside. A spherical reflector is used here (in place of plane mirror) Because a very high temperature is required, the spherical reflector is either a concave reflector or a parabolic reflector. The sunlight falling on the surface of spherical reflector get concentrate to one point. This will produce a lot of heat at that point & temperature in that region will become very high, therefore in this type, an utensil is placed at this point. Since a high temperature can be produced, so it can be used for making chapattis and for frying purposes.
| Difference between box – type & spherical reflector type solar cooker | |||
| Box – type solar cooker | Spherical reflector type solar cooker | ||
| 1 | A plane mirror reflector is used. | 1 | Spherical reflector is used |
| 2 | In a box type solar cooker, comparatively low temperature is produced | 2 | Quite high temperature is produced in the spherical reflector type solar cooker. |
| 3 | It cannot be used for making chappaties | 3 | It can be used for frying and making chappaties |
| 4 | Used for cooking food requiring slow heating. | 4 | Used for cooking food requiring strong heating. |
(C) Solar cells :
Solar cells is a device which converts solar energy directly into electricity. Energy radiated from the sun also contains light energy. So solar cells are also known as photo voltaic cells. The process of generating electricity directly from sunlight is referred to as a photo voltaic effect. The photo voltaic effect occurs when solar radiation strikes certain sensitive material directly and results in the flow of electrons. It was found that when 0.6% of the solar energy falling on the selenium layer got converted into electricity. With the advancement in the field of semiconductors, the solar cells made from these semiconductors can convert 10 to 15% of solar energy into electricity.
(a) Semi-conductors :
Semi-conductors are those substances which have very low electrical conductivity. Under ordinary conditions, semi-conductor materials conduct only a small amount of electric current. But if certain impurities are added to semi-conductor materials then their electrical conductivity increases considerably. Semi-conductors are neither good conductor of electricity nor they are completely insulators. The process of adding impurity is called doping/ The material (semiconductors) doped with boron has an affinity to attract electrons and is termed as p-type (acceptor) semiconductor. The phosphorous doped silicon material, which has a surplus of electrons, is termed as n-type (donor) semiconductor. When solar energy falls on semi-conductor material, even then their electrical conductivity increases.
(b) Conduction of solar cell :
It is constructed usually from silicon & gallium. Its conductivity increases when light falls on them. Therefore in a solar cell, the pieces (usually wafers) of semi conducting materials containing impurities are so arranged that when light falls on them then potential difference is produced between two regions of the semi conductor. It has been observed that a solar cell of about 4 cm2 may produce potential difference of about 0.4V to 0.5V and generate and current of 60 mA. A large number of solar cell joined together in a definite pattern can provide much higher power for many use. The group of solar cell is called solar cell panel.
(c) Solar cell panel :
When solar cells are arranged side by side, connecting each other in such a way that total potential difference and the total capacity to provide electric current is much increased the arrangement is called solar cell panel. The electric power required for the working of artificial satellites in outer space, street lighting in remote areas and running of irrigation water pumps in far-off areas obtained with the help of solar cell panels. In a solar cell panel hundreds of solar cells are joined together, the electricity produced by this solar panel is stored in battery.
This battery runs an electric motor and finally the motor drives the water pump, which pump out the underground water. The various solar cells in a solar cell panel are joined together by using connecting wires made of silver. This is because silver metal is the best conductor of electricity having very low resistance. The use of silver for connecting solar cells makes the solar cell panel more expensive but it increase the efficiency. This is because if connecting wires of other metals ware used in solar cell panel, then a substantial apart of the electricity generated by it could be lost in overcoming the resistance of such connecting wires.
(d) Uses of solar cell :
(i) Solar cells are used for providing electricity in artificial satellite and space probes.
(ii) In India, solar cells are behind used for street lighting, for traffic, signals, for operating water pump and for running radio and television sets i remote areas.
(iii) Solar cells are used for providing electricity to “lighthouses” situated in the sea and to the off – shore oil drilling ring platforms.
(iv) Solar cells are used for operating electronic watches & calculators.
13.3 WIND ENERGY :
Moving air is called wind. As the moving objects possess K.E. and as such they are capable of doing mechanical work by virtue of its motion. Wind also possess the ability of performing mechanical work because it is air in motion. So wind energy is the K.E. associated with large mass of air by virtue of its motion.
Solar energy is responsible for the blowing of air.
This can be explained as follows : The sunrays fall on the whole earth but eh intensity of sun-rays is much more stronger near equator of the earth than in the polar regions. Due to more intense sun-light, the air near the surface of earth in equatorial regions becomes quite hot. This hot air, being lighter, rises upwards. The cooler air form the polar regions of the earth start flowing towards the equatorial regions of the earth to fill the space vacant by the hot rising air. In this way air flows from the higher pressure regions to the lower pressure regions of the earth. This flow of air from one place to another constitutes wind.
13.3 (a) Advantages of Wind Energy :
(i) use of wind energy is not a source of pollution.
(ii) Wind energy is available free of cost.
(iii) The source of wind energy i.e. air is an inexhaustible and reversible source.
13.3 (b) Disadvantages of Wind Energy :
(i) One of the most important limitation of wind energy is that it may not be available at all times.
(ii) The windmill and sail-boats remain unoperational and no useful work can be done unless there is a
plenty of fast blowing wind.
(iii) The speed of the wind at a place varies with time and season.
(iv) The K.E. of the wind can be utilized only at the site.
(v) There is no guarantee that we will get wind energy when required, since there is no place in the world
where wind blows all the time.
(vi) The wind is not predictable.
13.3 (c) Practical Devices Making use of Wing Energy :
(i) Wind mill
A windmill is a machine, which works with the energy of blowing air or wind. It consists of large blades to catch the wind. When the wind strikes against these blades, They start rotating. The motion can then be passed on the other connected parts & is used to do useful work. A windmill consists of a system of big blades (or vanes) capable of rotating about a horizontal axis. The system of vanes is mounted on the top of a high tower. The system of blades in connected to one end of the rod called shaft. The other ends of the shaft is connected to a pump rod in case of water pump. This end is bend in form of inverted V and is connected to the free and of the pump rod of the water pump. When the wind blows, it rotates the bladed of the windmill. The shaft turning about its axis rotates the crank. The rotates the blades of the windmill. The shaft turning about its axis rotates the crank. The rotation of the crank moves the piston rod of the water pump up and down & draws water from the well.
(A) Windmill to operate flourmill :
It is similar to one used to grind grains by suitable arrangements of toothed wheel & shafts. The other end of the shaft is connected to a toothed wheel. Grinding arrangement of flour mill has a fixed mill stone A and another heavy will stone B. B is capable or rotating by a shaft rod (W2) having a toothed wheel (W2). The wheel (W1) is coupled with the wheel (W2) such that the rotation of wheel (W1) about a horizontal axis rotates the wheel (W2) about a vertical axis. The wheel (W1) rotates as the shaft (W1) connected to blades rotates due to rotation of blades of windmill. Thus the K.E. of wind by virtue of this motion rotates the windmill which in turn operates a flourmill and is able to grind grains.
(B) Wind generator :
A modern generator, which is used to generate electricity by using wind energy is wind generator. When the fast moving wind strikes the blades of wind turbine, then the wind turbine starts rotating continuously. The coil of a small electric generator is attached to the shaft of wind turbine. So when the wind starts rotating and generates electricity. The electricity generated by a single wind turbine is quite small. So, in order to generate a large amount of electricity, a large amount of wind turbines are erected over a big area of land. Such a set-up of having a large number of wind turbines working at a place to generated electrical energy on a large scale is called a wind energy farm.
13.4 HYDRO ENERGY (HYDEL POWER) :
Flowing water having a lot of kinetic energy can be utilized as hydro energy Hydro energy can be stored by storing water in high altitude dams. Hence it is a renewable source of energy. Solar energy is the source of hydro energy. Hydro energy is used to grind crops, to irrigate fields and to transport logs of wood from remote areas. Energy trapped in flowing water is used to generate electricity on a large scale at hydroelectric power stations. Dams are needed to be constructed to utilize the kinetic energy of flowing water. The water stored in dam is made to fall from a height through pipes and made to run over the blades of huge turbines at the bottom. This moves the turbine which in turn rotate the coils of an electrical generator to produce power or electricity.
Hykro electric power plant
13.4 (a) Advantage of Hydel Power :
(i) The process of hydel power does not cause any environment pollution.
(ii) The moving water needed for the purpose is available free of cost.
(iii) Water energy is a renewable source of electrical energy which will never get exhausted.
(iv) The construction of dams on rivers helps in controlling floods and in irrigation.
13.4 (b) Limitations of Hydel Power :
(i) Moving or flowing water is not available at all places. It is available near the sites of flowing rivers
(ii) To generated a reasonable amount of electricity from water, fast moving water should be available in
large quantities.
(iii) The construction of dam on a river disturbs the ecological balance in downstream area of the river.
(iv) The soil in the downstream area may become poor in quality because there were no annual floods to
deposit nutrient rich silt on the bank of the river. Therefore there may be ecological problems.
13.5 TIDAL ENERGY :
The rise of ocean water due to attraction of moon is called ‘high-tide’ whereas the fall of ocean water is called ‘low-tide’/ The enormous movement of water between the high tide and low tide provides a very large source of energy in the coastal areas of the world.
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 generator to produce electricity. And during the flow tide when the level of sea-water is low, the sea water stored in a barrage reservoir is allowed to flow out into the sea. This following water also turn the turbine and generate electricity.
The tidal energy is not likely to be potential source of energy in future because of following reasons :
(i) There are very few sites around the world, which is suitable for building tidal barrages (or tidal dams).
(ii) The rise & fall of sea water during high and low tides is not enough to generate electricity on a large scale.
13.6 SEA WAVES ENERGY :
Due to the blowing of wind on the surface of ocean, very fast sea-waves move on its surface. Due to their high speed, sea waves have a lot of kinetic energy in them. The energy of moving sea-waves can be used to generate electricity. This can be done as follows :
(i) One idea is to set up floating generators in the sea. These would move up and down with the sea-waves. This movement would drive the generators to produce electricity.
(ii) Another idea is to let the sea-waves move up and down inside large tubes. At the waves more up, the air in the tubes is compressed air can then be used to turn a turbine of a generator to produced electricity.
13.7 OCEAN THERMAL ENERGY :
The energy available due to the difference in the temperature of water at the surface of the ocean and at a deeper levels is called ocean thermal energy. The drives used to harness ocean thermal energy are called ocean thermal energy conversion power plants. A temperature difference of 200C between the surface water of ocean and deeper water is needed for operating OTEC power plants. In one type of OTEC power plant, the warm surface water of ocean is used to boil a liquid like ammonia or a chlorofluorocarbon (CFC). the high-pressure vapors of the liquid are then used to run the turbine of a generator and produce electricity. The colder water from the deeper ocean in pumped up to cool the used up vapours & convert than again into a liquid. This process is repeated again and again.
Advantage of ocean thermal energy :
(i) It can be used continuously 24 hours a day throughout the year.
(ii) It is a renewable source of energy and its use does not cause any pollution.
13.8 GEOTHERMAL ENERGY :
Geothermal energy is the heat energy of hot rocks present inside the earth. This heat can be used as a
source of energy to produce electricity. Geothermal energy is one of the few sources of energy that do not come directly or indirectly from solar energy. The places where very hot rocks occur at same depth below the surface of earth are called ‘hot spots’ and are sources of geothermal energy.
The geothermal energy is harnessed as follows :
(i) The extremely hot rocks present blow the surface of earth, heat the underground water and turn in into steam. As more and more steam is formed between the rocks, it gets compressed to high pressures. A hole is drilled into the earth and the hot rocks comes up through the pipe at high pressure. This high-pressure steam rum the turbine of a generator to produce electricity.
(ii) Large rocks are present in the underground rocks, which allow steam and hot water to go up. The steam & hot water gushing out of the ground are a kind of natural geyser. This steam is then used to turn turbines and generated electricity and the hot water is used to cook food.
13.9 BIO ENERGY :
The energy obtained from the biomass of plants and animals is called bio energy. Biomass is a renewable source of energy because it is obtained from plants (or animals) which are produced again and again. The waste material of living things and dead parts of living things is called biomass. Biomass included cattle dung, wood, sewage, agricultural wastes or crop residues like bagasse (Bagasse is the remaining part of the sugar cane or ganna, from which juice has been extracted). Biomass is carbon compounds and it is oldest source of heat energy for domestic purposes. There are two ways of using biomass as a fuel. One is to burn the dry biomass like cattle-dung or wood directly to produce heat. Another method is to convert the biomass into more useful fuels and then use these fuels for heating purposes.
13.9 (a) Biogas :
The decomposition which takes place in the absence of oxygen is called anaerobic degradation. Anaerobic degradation is carried out by anaerobic bacteria. Biogas is a mixture of methane, carbon dioxide, hydrogen and hydrogen suphide. The major constituents of biogas is methane. Biogas is produced by the anaerobic degradation of animal wasters like animal dung in the presence of water.
13.9 (b) Biogas Plant :
Biogas is prepared by anaerobic degradation of animal wastes like cow dung in biogas plant. The two types of biogas plant are :
(i) Floating gas holder type biogas plant.
(ii) Fixed – dome type bio-gas plant.
The raw material used for producing biogas is both the plants is the same, it is a mixture of cattle during and water.
(i) Floating gas holder type bio-gas plant :
This biogas plant consist of a well shaped underground tank called digester, which si made up to bricks. A drum shaped gas holder made of steel floats in the inverted position over the dung slurry in the digester tank. Since the gas holder floats over the dung-slurry, so this biogas plant is called floating gas-holder type biogas plant. The gas holder can move up and down, and its movement is controlled by the control pipe. There is a gas outlet at the top of the gas holder tank having a valve. A partition wall divides the digester tank in two parts: on the left of the digester tank is an inlet pipe made of cement (the inlet in connected to a mixing tank), on the right side of the digester tank is an outlet pipe, also made of cement, which is connected to an over-flow tank. The inlet pipe is for feeding the fresh dung slurry into the digester tank whereas the outlet pipe is for removing the spent dung after the extraction of biogas. Cattle dung and water are mixed in equal proportions in the mixing tank to prepare the slurry. This slurry of dung and water is fed into the digester tank through the inlet pipe. The dung undergoes anaerobic degradation in the presence of water with the gradual evolution of biogas. As more and more bio-gas collects in the gas-holder, the pressure of gas in it increases. As the spent dung slurry goes out, more fresh dung slurry is added to the digester tank on daily basis.
Floationg gas holder type biogas plant
(ii) Fixed dome type biogas plant :
It consists of a well-shaped underground tank called digester, which is made up of bricks & has a dome-shaped rood also made of cement and bricks. The dome of the digester tank acts as a gas holder or storage tank for the biogas. In this case the gas holder is fixed. There is a gas outlet at the top of dome having a valve on the left side of the digester tank in a sloping inlet chamber and on the right side is a rectangular outlet chamber. The inlet chamber is connected to a mixing tank white the outlet chamber is connected to the overflow tank. This gas plant is unique at the digester tank & the gas-holder are combined in one unit and no separated steel gas-holder is required. Cattle dung & water are mixed in equal proportions in the mixing tank to prepare the slurry. The dome being left free for the collection of biogas. The cattle dung undergoes degradation by anaerobic bacteria in the presence of water with gradual evolution of biogas. This biogas starts collecting in the dome. As more & more biogas collected in the dome, it exerts pressure on the slurry in the digester tank and force the spent slurry to go into overflow tank through the outlet chamber from the overflow tank, the spend slurry in removed gradually.
We use cattle dung in the biogas plant, but human excreta can also be added along with cattle dung in the biogas plants. Biogas can also be obtained by the action of bacteria on domestic sewage in the absence of air. The biogas obtained from the degradation of domestic sewage is also called sewage-gas.
Fixed Dome Type Biogas Plant
13.10 FUEL :
A fuel is a chemical which releases energy when heated with oxygen. The energy may release in form of heat or light.
Eg : Wood, gas, petrol, kerosene, diesel, coal and animal waste.
Note : Fuels are combustible substances.
13.10 (a) Types of Fuels :
There are three types of fuels solid flues, liquid fuels & gaseous fuels.
(i) Solid fuels :
The various kinds of solid fuels are wood, charcoal, coke, coal, paraffin and tallow. Wood was the first solid fuel to be used by humans. Paraffin and tallow are used to make candles.
(ii) Liquid fuels :
Petrol, kerosene, diesel and methanol are some common liquid fuels. Most liquid fuels are obtained from petroleum. They leave no solid residue when burnt and can be stored easily.
(iii) Gaseous fuels :
Natural gas, coal gas producer gas, water gas and liquefied petroleum gas are some examples of gaseous fuels.
13.10 (b) Fossil Fuels :
Fossil fuels are the remains of the prehistoric animals or plants, burried under the earth, millions of years ago.
Eg. Coal, petroleum and natural gas.
Fossil fuels are formed in the absence of oxygen. The chemical effects pressure, heat and bacteria, convert the burried remains of plants & animals into fossil fuels like coal, petroleum and natural gas.
It was the sunlight of long ago that made plants grow, which were then converted into fossil fuels. Fossil fuels are energy rich compounds of carbon, which are originally made by the plants with the help of sun’s energy.
DAILY PRACTIVE PROBLEMS # 13
OBJECTIVE DPP – 13.1
1. Solar energy can be used to produce :
(A) electrical energy (B) heat energy (C) mechanical energy (D) all of the above
2. The energy processed by wind is :
(A) kinetic energy (B) potential energy (C) thermal energy (D) sound energy
3. The fuel used in a stream engine is :
(A) water (B) carbon monoxide (C) carbon dioxide (D) none of these
4. A solar cell convert heat and light energy into :
(A) heat energy (B) sound energy (C) electrical energy (D) nuclear energy.
5. Biogas is a mixture of :
(A) CO + H2 + CH4 (B) CO + H2
(C) CO2 + CH4 + H2 + H2S (D) CO2 + N2
6. The sources of energy which are being produced continuously in nature and are inexhaustible are called :
(A) conventional sources (B) non-conventional sources
(C) non-renewable sources (D) none of these
7. Choose the source of energy which is different from others :
(A) Wood (B) Falling water (C) Wind (D) Petroleum
8. The vast amount of sea weeds present in oceans may provide an endless source of :
(A) nuclear energy (B) ocean thermal energy (C) methane (D) none of these
9. Energy available from the oceans is :
(A) OTE (B) sea-waves energy (C) tidal energy (D) all of the above
10. Radiations used to get relief from bodyaches are :
(A) I.R.radiations (B) U.V. radiations (C) Visible radiations (D) None of these
11. Wood is a :
(A) primary fuel (B) liquid fuel (C) processed fuel (D) secondary fuel
12. Which of the following is as example of fossil fuel ?
(A) Coal gas (B) Coke (C) Natural gas (D) Producer gas
SUBJECTIVE DPP – 13.2
1. Name any two radiations emitted by the sun that the are not visible to human eye.
2. Name the devices in which solar energy in harnessed directly.
3. Describe the principle of solar cell.
4. A concentrator-type solar water heater has a surface area of 5m2 and it can reflect 80% of the incident solar radiation falling on it, while it absorbs the rest. Calculate the energy concentrated by the heater at its focus in a time period of 3hrs, if solar energy incident on its iat the rate of 0.4 KJ/m2s.
5. A concentrator type solar heater having a surface area of 4 m2 can absorb only 80% of the solar radiations incident on it, while the rest is reflected. Calculate the energy concentrated by the heater at its focus in 3 hrs, if the rate of solar energy incident on it is 0.5 KJ/m2s.