In semiconductors, thermal collisions are responsible for taking a valence electorn to the conduction band.Why does the number of conduction electrons not go on increasing with time as thermal collisions continuously take place?

Electric conduction in a semiconductor takes place due to

Assertion (A): Metals have high thermal conductivity. Reason (R): Due to thermal excitation of many electrons from the valence band to the conductance band, metals have high thermal conductivity.

A doped semiconductor has impurity levels 30meV bleow the conduction band,(a)Is the material m-type or p-type? (b)In a thermal collision,and amount kT of energy is given to the extra electron loosely bound to the impurity ion and this electron is just able to jump into the conduction band. Calculate the temperature T.

When a semiconducting meterial is doped with an impurity,new acceptor levels are created .In a particular thermal collision,a valence electrons recives an energy equal to 2kT and just reaches one of the acceptor levels. Assuming that the energy of the electron was at the top edge of hte valence band and that the temperature T is equal to 300K, find the energy of the acceptor levels above the valence band.

A neutron moving with a speed u strikes a hydrogen atom in ground state in ground toward it with the same speed Find the minimum speed of the neutron for which ineleastic (completely or perially) collision may take place .The mass of neutron = mass of hydrogen = 1.67 xx 10^(-27)kg

The left block in figure moves at a speed v towards the right block placed in equilibrium. All collisions to take place are elastic and the surfaces are frictionless. Show that motion of the two blocks are periodic. Find the time period of these periodic motions. Neglect the widths of the blocks.

A town has a population of 1 million. The average electric power needed per person is 300W. A reactor is to be designed to supply power to this town. The efficiency with which thermal power is converted into electric power is aimed at 25%.(a) Assuming 200 MeV of thermal energy to come form each fission event on an average, find the number of events on an place every day. (b) Assuming the fission to take place largely through U^235 , at what rate will the amount of U^235 decrease ? Express uour answer in kg per day. (c) Assuming that uranium enriched to 3% in U^235 will be used, how much uranium is needed per month (30 days)?

Answer the following questions . a. The earth's magnetic field varies from point is space. Does it also change with time ? It so , on what time scate does it change appreciably ? b. The earth's core is known to contain iron. Yet geologists do not regard this as a source of the earth's magnetism why ? c. The charged currents in the outer conducting regions of the earth's core are thought to be responsible for earth's magnetism . What might be the ' battery ' (i.e., the source of energy) to sustain these currents ? d. The earth may have even reversed the direction of its field several times during its history of 4 to 5 billion years. How can geologists know about the earth's field in such distant past ? e. The earth's field departs from its dipole substantially at large distances (greater than about 30,000 km) what agencies may be responsible for this distortion ? f. Interstellar space has an extremely weak magnetic field of the order of 10^(-12) T, Can such a weak field be of any significant consequence ? Explain. [Note : Exercise 2 is meant mainly to arouse your curiosity . Answers to some question above are tentative or unknown . Brief answers wherever possible are given at the end . For details , you should consult a good text on geomagnetism.]