Problem 3.323

A superconducting round ring of radius $a$ and inductance $L$ was located in a uniform magnetic field of induction $B .$ The ring plane was parallel to the vector $\mathbf{B}$, and the current in the ring was equal to zero. Then the ring was turned through $90^{\circ}$ so that its plane became perpendicular to the field. Find: (a) the current induced in the ring after the turn; (b) the work performed during the turn.