A supereconducting round ring of radius a and inductance `L` was located in a uniform in a uniform magnetic field of induction `B` . The ring plane was parallel to the vector `B` and the current in the ring was equal to zero, then the ring was turned through `90^(@)` 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.

Explanation

`phi_(1)=B.pia^(2)`
`phi_(2)=0`
`Deltaphi=Li`
`pia^(2)B=Li` implies `i=(pia^(2)B)/(L)`
`W=(1)/(2)Li^(2)=(1)/(2)L((pia^(2)B)/(L))^(2)`
`=(pia^(2)B)^(2)/(2L)`