Monday, November 28, 2011


1.            What is solar energy?
                The energy received from the sun in the form of heat and light is called solar energy.
2.            Mention the different forms of energy derived from solar energy?
                The different forms of energy are wind energy, water energy, energy in food, energy of fossil fuels etc.
3.            What is the main constituent of the sun? Mention the basis on which this was established?
             a) The main constituent of sun is hydrogen.
            b) This was established based on spectrum analysis
4.            Name the nuclear reaction that satisfactorily explains the source of solar energy?
                Nuclear fusion satisfactorily explains the source of solar energy.
5.            Name the two types of nuclear fusion reactions considered to be the source of solar energy?
                The two types are:P – P chain or proton-proton chain wherein four hydrogen nuclei fuse to form a helium nucleus liberating huge amount of energy.Carbon cycle in which carbon acts as a catalyst to favor nuclear action.               The two together accounts for sun’s energy
6.            Give an account of the arrangements in nature to transform solar energy into other forms useful to us.
·         Solar energy transforms itself into wind energy, ocean energy, fossil fuel energy, ocean energy etc.
·         Wind energy: Solar energy heats the air near the earth’s surface and the hot air becomes lighter, moves up and the cooler flows to the lower region causing wind.
·         Water energy: Solar energy heats up water in the water bodies, the water evaporates and forms clouds and bring rain. The rainwater flowing down has kinetic energy and helps to generate electricity.
7.Explain briefly how wind energy is due to solar energy?
·         The box is kept outside in the sunlight and the reflector is adjusted such that a strong beam of sunlight is reflected on the box cover.
·         The sunrays reflected by the reflector, pass through the cover and get absorbed by the black surface of the box and food material inside the container.
·         As a result, the temperature inside the box increases and the food gets cooked.
16. Write a note on solar cooker?
·         Solar cooker is a device used to cook food by directly harnessing solar energy.
·         It works on the principle of thermal conversion and the property of black body to absorb heat.
·         It is pollution free and preserves the nutrients present in the food.
·         It is made of a wooden or an insulating box painted inside with black and covered with a glass sheet that is transparent to infrared radiations of short wavelength and opaque to infrared radiations of longer wavelengths.
·         It cannot be used effectively during night times and on a cloudy day.
18.          Write a note on solar water heater?
·         Solar water heater, as the name suggests is a device used to heat water by directly harnessing solar energy.
·         The water gets heated in the copper tube painted black from outside.
·         Hot water remains in the upper portion of the tank and cold water from the reservoir replaces the hot water and the hot water gets stored in a small cylindrical tank meant for the purpose.
·         The use of solar water heater is pollution free and hence environment friendly, but cannot be used on a cloudy or rainy day.
19.          Write a note on solar cell?
The device that converts solar energy into electrical energy is called a solar device. It consists of a small piece of light absorbing material like silicon Solar energy falling on the material releases electrons from the silicon atoms and produces electricity. It produces a voltage of 0.5V and a power of 0.7W.
Higher voltage and power can be obtained by connecting a series of cells in series. It is used in radio and TV communication, artificial satellites and research centers in remote areas.
1.            Mention any two of the characteristics of solar energy?
                The characteristics of solar energy are:
·         It is an inexhaustible source of energy.
·         It is the ultimate source of almost all forms of energy.
2. Why was radioactive decay as the source of sun’s energy rejected?
Solar energy is very much more than the energy that can be released by radioactive decay and hence the  hypothesis was rejected.
3. Who were the first to identify nuclear reaction as the source of solar energy? What was their observation?
a)      Hans Beethe and Cvan Wiezsacker were the first to identify nuclear reaction as the source of solar energy.
b)      They found that hydrogen nuclei fuse to form helium nuclei liberating huge amount of energy as solar energy.
4.            Sun is the ultimate source of almost all forms of energy. Discuss with two examples?
a)      The radiation from the sun provides us heat and light and thus solar energy is converted into heat energy and light energy.
b)      The fossil fuels are formed when living things die and get embedded in the earth’s crust and undergo chemical change. Life forms are due to solar energy and thus fossil fuels are due to solar energy.
c)       The food that we eat is from the plants. Plants need sunlight to prepare their food. Hence solar energy is required for the growth of plants and for the food we eat. Hence, solar energy is the ultimate source of all forms of energy.
7.            The inner surface of a box type solar cooker is painted black. Why?
a)      Black surfaces absorb about 98% of the heat incident on them, while white surfaces do not.
b)      Accordingly, to maximize absorption of heat and minimize heat losses, the inner surfaces of the solar cooker are painted black.
8.            Why are reflectors used in solar heating devices?
Reflectors are used to   Increase the area over which the solar energy is collected.      Focus the sunrays to the required area.
9.            Explain briefly, how the glass sheet used in the solar cooker helps to cook food and increase its efficiency?
a)      The glass sheet used has the following properties:
·         It allows infrared radiation of shorter wave length at high temperature to pass through it.
·         It prevents infrared radiation of longer wavelength at lower temperature to pass through it.
b)      Thus, infrared radiations of shorter wavelength incident on the glass sheet allow them to pass through. When they enter the cooker they heat the body inside and the temperature is reduced resulting in radiations of longer wavelength which are not allowed to go out.
c)       Thus, there is no loss of heat, but the efficiency is increased.
11.          What is a solar cell? On what principle does it work?
a)      Solar cell is a solar electric device by which solar energy is directly converted electrical energy.
b)      It works on the principle of photovoltaic conversion
12.          How are solar cells made?
a)      Solar cells are made using thin wafers of semiconductor material containing impurities.
b)      They are arranged in such a way that when light falls on them, a potential difference is developed between the two regions, which produces electric current.
13.          Explain how the design of a solar cooker ensures minimum loss of heat from its inside?
a)      The thick glass plate used to cover the box ensures minimum loss of heat from inside.
b)      This glass plate has the following properties.
It allows infrared radiations the short wavelengths into the box containing the food to be cooked.When it enters the box, the heat energy is used up by the food for cooking and hence the temperature around it reduces. This results in infrared radiations of longer wavelength, which are not allowed to pass out through the glass plate.
Hence minimum energy is lost from inside
14.          In a solar water heater, copper pipes are bent as a coil. Why?
                Bending copper pipes helps to increase the area of absorption of solar energy and hence are bent.
15.          How are higher voltage and power obtained from solar cells?
                Higher voltage and power are obtained by connecting a number of solar cells in series
16.          What is a solar panel?
                The arrangement of a number of solar cells connected in series is called a solar panel
17.          Mention any four uses of solar cells?
·         Solar cells are used in:  Artificial satellites   Radio and wireless transmission   TV relay stations    Traffic lights.
·         Solar energy heats the air near the earth’s surface.
·         The air becomes warm and lighter
·         It moves up and the cooler air flows to the lower region
·         This causes wind and results in wind energy.
18..  Name the scientists who explained the nuclear reactions as the source of solar energy?
                Hans Bethe and C. Von Weizsacker in 1939
 19..  Name the two types of fusion reactions considered to be the source of Solar energy?
                 (a) Proton – Proton Chain reaction: In which four hydrogen nuclei fuse to form one helium
      (b)Carbon cycle – Here the carbon atom acts as a nuclear catalyst. 
20.  Explain briefly how Solar energy maintains every form of life on the earth
     Basically Solar energy is converted into chemical energy by the plants during photosynthesis. 
    This chemical energy is used by the plants.  A part of this is also flow from plants to herbivorous animals, Man,
    Carnivorous  animals and microbes directly or indirectly depend on plants and  herbivorous animals for their food. 
   This is  how every form   of life on the earth is maintained by solar energy.
21.  Which form of energy is trapped in the fossil fuels?                               Chemical energy
22.  Fossil fuels are non – renewable sources of energy. why?
                Fossil fuels are non-renewable sources of energy, because once we use them they become exhaustible and not
        replenished by the nature immediately, millions of years are required for the formation of fossil fuels.
23.  Name the fossil fuels?          Coal, Petroleum and Natural Gas.
24.  Name the renewable sources of energy.
                Solar energy, Wind energy, Water energy, Atomic energy etc…
25.  Name the component of Solar radiation that mainly carries heat?   Infrared radiation.
26.  Mention the principle on the basis of which solar devices are classified into 2 types?
                (a) In one type of device the solar energy is collected in the form of heat.
                                Ex: Solar cooker, Solar heater, Solar cars etc….
                (b) In the other type Solar energy is converted into electricity.                   Ex: Solar cells
27.  Name the type of mirror present in solar cookers?                  Plane mirror reflector.
28.  Name the type of mirror present in solar water heaters?     Parabolic mirror reflectors (concave mirrors)
29.  What is the function of copper pipe as a coil, in solar water heaters?
                Copper pipes  helps to increase the area for absorption of Solar energy.


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