Question

Find the mutual inductance of two concentric coils of radii `a_(1)` and `a_(2)(a_(1) lt lt a_(2))` if the planes of the coils are perpendicular.

Answer 1

Let a current `i` flow in the coil of radius `a_(1)`. The magnetic field at the centre of this coil will now be parallel to the plane of smaller coil and hence no flux will passs through it, hence M=0