SHORT QUESTIONS WITH ANSWERS (2 MARKS)
Q.1. What is solar cell ? Name two materials mostly used for making solar cells.
Ans. A solar cell is a device which converts solar energy directly into electric energy. Silicon and germanium are the two most commonly used materials for making solar cells.
Q.2. What type of mixture are separated by fractional distillation? Name the fraction of crude oil whose boiling point is more than 350oC. Where is it mostly used as a fuel?
Ans. The various components of a mixture should have different boiling points in case these are to be separated by fractional distillation. Fuel oil is the fraction of crude oil (petroleum) whose boiling point is above 350oC. It is used as a fuel in various industries to heat boilers because it does not leave any residue after burning.
Q.3.How were fossil fuels formed ? How these fossils fuels were made?
Ans. Fossil fuels were formed due to high pressure, heat and the action of anaerobic bacteria of fossils for million of years. These fuels were made form the fossils of plants and animals buried deep in the Earth.
Q.4. Define the term ‘fuel’. What are primary and secondary fuels? To which class of fuel do the following belong: Coke , Wood, Petroleum, LPG.
Ans. A material which produces heat on combustion is called a fuel.
Primary fuels. Fuels which are used directly to produce heat are called primary fuels.
Secondary fuels. Fuels which are manufactured by chemical process using primary fuels are called secondary fuels. Wood and Petroleum are primary fuels and Coke and LPG are secondary fuels.
Q.5. 48 kJ of energy is produced per minute in a nuclear reactor. Calculate the number of fissions which would be taking place in the reactor per second, if the energy released per fission is 3.2 x 10 -11J
Ans. Total energy produced/minute =48kJ =48 x 103 J
Energy produced/second = 48x103/60 j = 0.8 x 103 J
Energy released/fission= 3.2 x 10-11J.
Number of fissions/second = [Energy produced/second] / [the energy released per fission]
=0.8 x 103 J / 3.2 x 10 -11 2.5 x 1013
Q.6. Give one example of a nuclear fusion reaction. Describe one method for making such reactions possible
Ans. One example of a nuclear fusion reaction is when two deuterons fuse together to form He-3 nucleus and a neutron with the emission of a large amount of energy
21H + 2H1 --------à 3He2 +1no
This reaction is possible at extremely high temperatures (=107 K) and pressures (=107 Pa).2A1
Q.7. Which type of nuclear process is currently used in nuclear electricity generators? Give one example each for the substances used in this context as (i) coolants (ii) moderators and (iii) nuclear fuel?
Ans. In nuclear electricity generators, the process used currently involves the fission of U-235 nuclei by bombarding them with thermal neutrons. Water is one of the coolants; graphite is one of the moderators and enriched uranium is one of the nuclear fuels.
Q.8.For producing electricity, the energy from flowing water is preferred to energy obtained by burning coke. State two reasons for it.
Ans. (i) production of electricity from water is pollution free.
(ii) water energy is a renewable source of energy. Coke, on the other hand, is a non- renewable source of energy and causes pollution on burning.
Q.9. Define a nuclear fusion reaction .Describe the conditions for the occurrence of a nuclear fusion reaction. 21H + 2H1 --------à 3He2 +1no
Ans. A nuclear fusion reaction involves combining of two or more lighter nuclei to form a heavier nucleus with the liberation of a large amount of energy.
The conditions for the occurrence of a nuclear fusion reaction are:
Temperature of fusing nuclei should be raised to 107 K at pressure of millions of pascals. Or
The fusing nuclei be accelerated to high speeds using particle accelerators.
Q.10. Name the places of out country where fields of natural gas are found. Why is it called a clean fuel? Give two reasons.
Ans. Tripura, Jaisalme, off-shore area of Bombay, Krishna-Godavari Delta.
Natural gas is a clean fuel as: (1)It does not produce harmful gases on burning,(ii) It burns without smoke and does not leave any residue after burning.
Q.11. What is the cause of release of unusually large energies in nuclear fission reactions? How is the energy per fission calculated?
Ans. In nuclear fission reactions, a creation mass disappears. This is called mass defect (Δm). It appears in the form of energy according to Einstein equation,E=( Δm)c2,where c is velocity of light in vacuum. To calculate energy per fission, we sum up the masses on left hand side of the fission reaction; and also on the right hand side of the fission reaction. The difference in masses on the two sides of the reaction gives us mass defect (Δm). Usually, this mass defect is in atomic mass unit (u).
As 1 u = 931 Me V, we can calculate energy released/fission in MeV) by multiplying (Δm) by 931.
Q.12. The use of dry wood as domestic fuel is not considered as good. State two reasons for it.
Ans . (i) Wood produces lot of smoke on burning.
(ii)It leaves lot of residue after burning.
Q.13. In one fission of uranium, 3x10-11 J of energy is made available. Calculate the total number of fissions necessary per second to generate power of 15 kW.
Ans. Energy released/fission =3x10-11 J.
Total energy required/s = 15 kW = 15000W =15000 J/s.
Number of fissions/s = [Total Energy/Second] ÷ [Energy/fission] = 15000 ÷ 3x10-11 J = 5 x 1014
Q.14.How is charcoal obtained from wood? Why is charcoal considered better fuel than wood?
Ans. Charcoal is obtained from wood by destructive distillation of wood. When wood is strongly heated in the absence of air, the residue left behind is called charcoal.
Charcoal is better fuel than coal due to the following reasons:
(1)No harmful gases are produced on burning charcoal (ii) No residue is left after it has burnt itself out.
(iii) It has a higher calorific value.
Q.15. Describe how hydro energy can be converted into electrical energy. Write any two limitations by hydro energy.
Hydro power plants :-
In hydro power plants water from rivers are stored by constructing dams. The water from the dam flows down through pipes and rotates the turbines of generators to produce electricity.
Two limitations by hydro energy. i) The initial cost is high. ii) It causes displacement of people from large areas of land.
Q16.what is geothermal energy? What are its advantages?
The deeper regions of the earth’s crust is very hot. This heat melts rocks and forms magma. The magma moves up and collects below at some places called Hot spots. The underground water in contact with hot spot gets heated into steam at high pressure. By drilling holes into hot spots the steam coming out can be used to rotate turbines of generators to produce electricity.