A ring radius `a = 50 mm` made of thin wire of radius `b = 1.0 mm` was located in a unifrom magnetic field with induction `B = 0.50 mT` so that the ring place was perpenficular to the vector `B`. Then the ring was cooled down to superconducting state, and the magnetic field was swichted off. Find the ring current after that. Note that the inductiance of a thin ring along which the surface current flows is equal to `L = mu_(0)a (In (8a)/(b) - 2)`.

Explanation

The flux linked to the ring can not change on transition to the superconduction state, for reasons, similar to that given above. Thus a current `I` must be induced in the ring. Where,
`I = (Phi)/(L) = (pi a^(2) B)/(mu_(0) a(In (8a)/(b) - 2)) = (pi a B)/(mu_(0) (In (8a)/(b) - 2))`