Question

A flat coil of area a and n turns is placed at the centre of a ring of radius and resistance r. The two are co-planar when current in the coil increases from zero to i, the total charge circulating in the ring is

Answer 1

Answer:

B at the centre of the coil is μnI/2a, where a is the radius of the coil. Flux through the bigger ring is B*A. The problem I am facing is that the radius of the ring is given, not that of the coil.