A coil of n number of turns is wound tigtly in the form of a spiral with inner and outer radii `a` and `b` respectively. When `a` current of strength `I` is passed through the coil, the magnetic field at its centre is

Options

A. `(2mu_0nI)/b`
B. `(mu_0nI)/(2(b-a)) log_e frac ba`
C. `(mu_0nI)/(b-a) log_e frac ab`
D. `(mu_0nI)/(2(b-a))`

Answer

B