Question

A conductor behave as a superconductor

A. Above critical temperature
B. At critical temperature
C. At ${100^0}C$
D. At boiling point of that metal

Answer 1

Hint: Whenever we speak about conductivity of a substance let it be an insulator or conductor or semiconductor we consider energy levels of charge carriers. There are two bands one is valence band and the other is conduction band. Super conductor is a conductor with zero resistance.

Complete step by step answer:
The energy variation between lower valence band and upper conduction band is called the forbidden energy gap of a semiconductor. In between this energy gap in case of intrinsic semiconductor no electron exists. Now with increase in temperature thermal energy of an electron in the valence band increases and reaches the conduction band and this charge carrier conducts electricity. But in case of conductors charge carriers which conduct electricity exist already.
When temperature of the conductor is increased both atoms and free electrons gain thermal energy and start vibrating vigorously with higher amplitudes.

Forbidden energy gaps in semiconductors would be nearly 1ev.

In case of conductors both conduction and valence bands get overlapped
Due to increase in temperature number of collisions increases and rate at which collision occurs also increases
When this rate gets increased it becomes obstruction to flow of electrons in that conductor. This slows down electron flow which means current flow is decreased because current is nothing but rate of flow of free charge carriers. Current flow reduced means resistance is increased
Superconductor is a perfect conductor with zero resistance. Since increase in temperature increases the resistance of the conductor, at critical temperature i.e at zero kelvin conductors behave as superconductors.
Hence option B will be the correct answer.

Note:
In case of semiconductors also the rate of collisions increases due to temperature rise and relaxation time (time consumed between two successive collisions) gets decreased. But this factor will be overcome by the huge amount of creation of conducting charge carriers and hence its resistance decreases with increase in temperature. So change in carrier density plays a key role in varying resistance of semiconductor rather than a conductor.