Wednesday, August 8, 2012


Q.1. How many groups of nuclei are generated in the fission of 235 U? what are their characteristics? Explain briefly the terms  (i) prompt (ii) delayed, and  (iii) spontaneous fission.
Ans.  Two groups of nuclei are generated in the fission of 235 U
(i)One of these groups is a heavy group of nuclei with mass numbers ranging from 130 to 149,e.g., Barium and Lanthanum.
(ii)The other groups are a lighter group of nuclei with mass numbers ranging from 85 to104, e.g., Krypton and Molybdenum.
(iii)In prompt fission, the heavy nucleus disintegrates the moment it is bombarded.
(iv)In delayed fission, a projectile (e.g., neutron) enters the target nucleus and causes an instability which results in the fission of the target nucleus after a short while.
(v) Spontaneous fission is just out of the nature of the element.
Q.2. Write the four processes that can take place, after neutrons are emitted in a fission reaction. Which of these processes will cause the fission to continue? How is the energy of released neutrons lowered from 2 Me V to 0.025 Me V?
Ans. The four process that can take place after neutrons are emitted in a fission reaction are :
·         At least one such neutron hits another nucleus to cause its fission releasing more neutrons.
·         The neutrons may be captured by other nuclei without causing their fission.
·         The neutrons may be captured by other nuclei (present either in the fissionable material or in    
          the  surrounding container without causing fission.
·         The neutrons may not interact with other nuclei and may escape the system.
·         The energy of released neutrons is lowered from 2Me V to0.025Me V by passing them 
           through a moderator.
Q.3. (i) write the value of solar constant of sun.  
(ii) what type of mirror is used in Box Type Solar cooker?  
(iii) why it is difficult to use hydrogen as a source of energy?
 (iv)what is the maximum temperature attained in a concave reflector type solar cooker?  
(v)Fossil fuels are classified as non-renewable source of energy. Explain why.
Ans. (i) 1.4kW/m2
(ii) Plane mirror.
 (iii) It cannot be stored safely as (i) it burns with explosion and
(ii) has low ignition temperature. (iv) about 200oC.
 (v)fossil fuels cannot be replenished. It takes millions of years for their formation.
Q.4. (a) Why is solar cooker box covered with a plane glass plate?  (b) Why is the energy of water flowing in a river considered to be an indirect form of solar energy? (c) Write one advantage of nuclear fission reaction.
Ans. (a) the plane glass plate traps heat radiation and thus produces a kind of greenhouse effect. Consequently, the temperature inside the cooker become high (100oC to140oC).
(b) Solar energy changes into potential energy of water vapour rising in the atmosphere during evaporation of water from rivers, seas, oceans and other water masses. The water vapour from clouds and also cover mountains with snow. When the clouds bring rain and the snow on the mountains melts, their potential energy changes into kinetic energy of water flowing in the rivers and streams.
(c) Amount of energy produced in nuclear fission is enormous, one kg of U-235, on fission produces as much energy as 2500 tons of coal produces on combustion.
Q.5. (a) State  one limitation of solar energy available from solar cells.  
(b) What is the minimum wind velocity required to obtain useful energy with a wind will  ?
(c) Define the term ‘nuclear fission’.
Ans. (a) the electricity (dc) produced by solar cells panels is stored by charging dc batteries which can be used only to operate dc devices. To operate any ac device, this dc has to be first converted into ac by using inverters.
(b) 15km/h (c) Nuclear fission- In Nuclear fission, the heavy nucleus distingerates, the
Moment it is bombarded with law energy neutrons, it splits up into two Righer nuclei and tremendous amount of energy is released.
Extra score Guess questions
Q.1. Name two gases, other than carbon-dioxide that are given out during burning of fossilfuels and contribute towards acid rain formation?
Ans. Sulphur dioxide and Nitric oxide.
Q.2. Why are many thermal power plants set up near coal or oil fields?
Ans. Coal or petroleum is used to heat water to generate steam required for running for turbines in thermal power plants. Thus, if thermal power plants are set up near coal or oil fields, the cost of transportation of coal or petroleum is greatly reduced.
Q.3. Justify in one sentence that hydropower (hydel electricity) is a renewable source of energy.
Ans. Hydropower is generated from water flowing out of the dam which gets filled up again when it rains (or the snow melts) through the rivers which supply water to the dam.
Q.4. Out of two solar cookers, one was covered by a plane glass slab and the other was left open. Which of the two solar cookers will be more efficient and why?
Ans. The solar cooker covered by plane glass slab will be more efficient than the one which is left open. This is due to the reason that the plane glass slab enables the cooker to trap heat inside it. On this account, the temperature inside the cooker rises. In fact, the plane glass slab placed on the cooker produces a kind of greenhouse effect.
Q.5. A student constructed a box type solar cooker. He found that it did not work efficiently. What could this be due to? Give any four possible mistakes in the construction and operation of the cooker. What maximum temperature can ordinarily be reached inside a solar cooker?
Ans.  The four possible mistakes in the construction and operation of the cooker are:
·         The interior of solar cooker might not have been blackened,
·         Plastic cover might have been used instead of glass cover,
·         The cooker might not have been properly insulated,
·         The cooker utensils might not have been blackened.
Q.6. A student has set up a solar cooker in a box by using a black painted aluminum sheet, a black cooking vessel, some glass wool, a glass sheet and a mirror plate. What is the role of each item used in the solar cooker?
Ans. (i) Black painted aluminum sheet absorbs heat radiation
(ii) Glass wool, which is a bad conductor of heat, prevents the loss of heat.
(iii) Glass sheet traps the heat radiation in the box.
(iv)Mirror plate reflects the heat radiation onto the glass sheet.
Q.7. Solar energy can be harnessed directly as well as indirectly. Give two examples of each type.
Ans. Solar energy is used directly as heat or by converting it directly into electricity. Examples: solar cooker (or solar water heater) and solar cell panels. Solar energy is used indirectly by converting it into chemical energy (like biomass energy), harnessing the energy of wind, sea waves, tides and ocean thermal energy. Example: Biogas plants and windmills.
Q.8. What is anaerobic degradation?
Ans. Anaerobic degradation is the process of decomposition of animal and plants wastes by anaerobic bacteria in the presence of water but in the absence of air to form methane (the main constituent of biogas), carbon dioxide, hydrogen and hydrogen sulphide. Anaerobic bacteria (or anaerobe) is a minute organism that can live without free oxygen.
Q.9. what is a solar power plant? Name one such plant in our Country.
Ans. Sunlight is concentrated on black painted pipes (filled with water) with the help of concave reflectors. Due to solar heat, water starts boiling and then changes into steam. The steam so produced rotates a steam turbine which drives a generator to produce electricity. Asolar thermal power plant of 22k W capacity is in operation at Solajipally village in Andhra Pradesh. A 50kW solar thermal power plant is being installed at the Twal Pahari Premises of the Solar Energy Centre, Gurgaon (Haryana).
Q.10. How can wood be made a renewable source of energy ?
Ans. Wood can be made a renewable source of energy only if trees are planted in a planned manner.
Q.11. What causes the wind to blow?
Ans. Equatorial regions on the Earth receive more solar radiation than other parts of the Earth. As such, the air at equatorial regions is hotter and rises upwards into the atmosphere. The cooler air from other regions (especially polar regions) starts blowing to the equatorial regions to fill the space vacated by hot air. It is this moving air which is called wind.
Q.12.To which category of fuel does hydrogen belongs to?
Ans. Hydrogen ,which is available in plenty in water on earth, is classified as a renewable sources of energy.
Q.13. Why is Nuclear fuel classified as renewable source of energy?
Ans. Nuclear fuel is required only in a very small quantity to generate electric power and as such is inexhaustible.
Q.14. Why the Geothermal energy is classified as a renewable sources of energy?
Ans. Geothermal energy will be available for a long time.
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