1.Define nuclear fission. Give an example.
Ans: Nuclear fission is the process of splitting of a heavy nucleus into two lighter nuclei of nearly comparable masses with release of energy.
Example: Fission of U-235 by neutrons.
2. Mention few important characteristics of nuclear fission.
Ans: (i) Upon capturing a neutron, a heavy nucleus splits into two or more nuclei.
(ii) Two or more neutrons are produced by fission of each nucleus.
(iii) Enormous amount of energy is produced as a result of conversion of small mass into energy.
(iv) All the fission products are radioactive giving off beta and gamma radiations.
3. How a nuclear fission becomes a chain reaction?
Ans: When the number of secondary neutrons available in each fission reaction (multiplication factor) is controlled to one, the fission reaction becomes a chain reaction.
4. State critical mass.
Ans: Critical mass of a fissionable material is the minimum mass of fissionable material required to sustain a chain reaction. The critical mass varies for each reaction. For U-235 fission reaction it is about 10kg.
5. What is sub-critical mass and super-critical mass of U235?
Super-critical mass is a mass greater than its critical mass. In this case, the secondary neutrons emitted increases and hence the rate of fission chain reaction also increases rapidly which leads to explosion as in atom bomb.
6. What are the uses of nuclear energy ?
(i) generating electricity
(ii) the treatment of diseases like cancer
(iii) the improvement of agriculture and industry.
7. What is nuclear energy or fission energy?
Ans: The fission of U235 nucleus by slow neutrons released enormous amount of energy. The fission of U235 is an exo-ergic reaction. The energy released is called nuclear energy or fission energy.
10. What is the function of moderator in nuclear reactions?
Ans: The main purpose to use moderator is to slow down the fast fission neutrons.
Fig. Moderator slows down a fast neutron
The most commonly used moderator is ordinary water. Graphite, heavy water and beryllium are also used as moderators. Neutrons slow down by losing energy due to collision with atoms or molecules of the moderator.
11. What is the significance of control rods in light weight nuclear power plant?
Ans: They control the rate of fission U-235. These rods absorb the excess neutrons and thus remove them. With the removal of excess neutrons, the fission reaction proceeds at steady speed. Cadmium or boron rods are used as control rods.
12. Give a method for the disposal of nuclear waste.
Ans: The products of fission Ba -140 and Kr-93 are themselves radioactive. They emit dangerous radiation for several hundred years. The waste is packed in concreted barrels which are buried deep in the earth or dumped in to the sea.
13. What is wind energy?
Ans: Air in motion is called Wind. The energy derived from the force of the wind is called wind energy.
Ans. When a beam of sunlight stricks the upper p-type semiconductor layer, electrons in the valence band absorbs light energy and get promoted to the conductor band. Conducting electrons easily cross the p-n junction into the n-type semiconductor. Thus a potential difference between two layer is created.
Thus, when terminals attached to the p and n-layers are connected to an external circuit, electrons flow from n-type semiconductor to the p-type semiconductor, there by generating electric current.
15. Give any two applications of solar cells.
Ans. Solar cells are used to provide,
(i) electricity to electric toys, watches, calculators and portable radios.
(ii) electrical energy for distribution to industrial, commercial and residential purposes.
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