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 same.

Explanation

Let a current `i` flows in coil of resius `a_(2)`.
Magnetic field at the centre of the coil `= (mu_(0)i)/(2a_(2))`
flux in smaller coil `phi = (mu_(0)i)/(2a_(2)) pia_(1)^(2)`
or `Mi = (mu_(0)i)/(2a_(2)) pia_(1)^(2)` or `M = (mu_(0)pia_(1)^(2))/(2a_(2))`