Question

At what temperature ${(^ \circ }C)$ , does the resistance in the case of mercury (Hg) drop to zero?
(A) $3.2K$
(B) $4.2K$
(C) $5.2K$
(D) $6.2K$

Answer 1

Hint
For answering this question, we need to consider the superconductivity of mercury, which is the property of a material to lose its electrical resistance. The temperature at which this effect begins, the resistance will drop to zero.

Complete step by step answer
A super-conductor is a material whose electrical resistance vanishes, when brought down a certain temperature. In the case of ordinary conductors, that we usually talk about (for example, copper, iron etc), the resistance decreases continuously as their temperature is brought close to the absolute zero. We know that the absolute zero can’t be reached for any material. Therefore, the resistance of ordinary conductors cannot become zero.
But this is not the case with the superconductors. The superconductors have a characteristic temperature, below which the resistance suddenly drops to zero. So, they can conduct current even with a very low electrical energy source.
Mercury is a superconducting material. Its resistance drops abruptly to zero, when its temperature is lowered below $4.2K$ .
Hence, the correct answer is option (B).

Note
The effect of superconductivity was discovered by H, Kammerlingh Onnes in 1911. The superconductors have a wide range of applications in technology as listed below:
-They are used in energy saving systems, due to their ability to conduct even with a low power source.
-They are used in magnetic propulsion systems for the launching of rockets.
-They are used in the power transmission cables, due to the constant current which flows through them.
-They are also used in generators and motors.
-Magnetic resonance imaging.
-Fault current limiters.