Question

Superconductivity in a material can be destroyed by

1. increasing the temperature above a certain limit

2. applying a magnetic field above a certain limit

3. passing a current above a certain limit

4. decreasing the temperature to a point below the critical temperature

Which of the above are correct?

1. 1 and 2 only
2. 2 and 3 only
3. 1, 2 and 3 only
4. 1, 2, 3 and 4

Answer 1

Correct Answer - Option 3 : 1, 2 and 3 only

Effect of magnetic field on superconductors:

• By applying a magnetic field of sufficient strength, the superconductivity of a material can be destroyed.
• The minimum magnetic field strength required to destroy the superconductivity of a substance, below T_C is called a critical magnetic field (H_C) at that temperature. H_C varies with temperature.
• For material, the critical magnetic field increases with a decrease in temperature below T_C.
• At T_C, no magnetic field is required to change the material from superconducting to a normal state.

The maximum magnetic field is required to destroy superconductivity at 0 K. The critical magnetic field at 0 K is H₀. The critical magnetic field (H_C) at any temperature ‘T’ below T_C can be represented as:

\({H_C} = {H_0}\left[ {1 - {{\left( {\frac{T}{{{T_C}}}} \right)}^2}} \right]\)

Where,

Hc is a critical magnetic field at T°K

H0 is a critical magnetic field at 0°K

Tc is a critical temperature

Critical currents:

Suppose a material carries electric current in the superconducting state, this current produces a magnetic field. If this magnetic field exceeds the critical magnetic field (H_C) at that temperature T (< T_C), then normal resistance will be included in the material and it will be in the normal state.

Hence, it is not possible to pass large currents through a superconductor. The maximum current that can be passed through a superconductor in a superconducting state is called critical current, represented by I_C.