Question

“Superconductor is a perfect diamagnetic”, Explain.

Answer 1

  A superconducting material kept in a magnetic field expels the magnetic flux out its body when cooled below the critical temperature and exhibits perfect diamagnetism. This is called MEISSNER EFFECT.

⦁ It is found that as the temperature of the specimen is lowered to T_c, the magnetic flux is suddenly and completely expelled from it. The flux expulsion continues for T < T_c. The
effect is reversible. 

⦁ When the temperature is raised from below T_c. The flux density penetrates the specimen again at T = T_c and the material turns to the normal state. 

⦁ For the normal state the magnetic induction inside the specimen is given by:
B = μ_o (H+M) = μ_O(1+χ)H ………………………………………..(1)
Here H is the applied magnetic field , m is the magnetization produced within the specimen , is χ the susceptibility of the material and μ_o is the permeability of free space. 
⦁ At T < T_c  as seen above
B = 0
Hence equation (1) reduced to,

M = -H

And thus `χ = M/H = -1 `

⦁ The specimen is therefore a perfect diamagnetic. The diamagnetism produces strong
repulsion to the external magnets.
⦁ This effect is used to identify a superconductor , in levitation effect and suspension effect.