A coil having N turns is wound tightly in the form of a spiral with inner and outer radii a and b respectively. When a current I passes through the coil, the magnetic field at the centre is a. μ0NI/b b. 2μ0NI/a c. μ0NI/2(b-a)lnb/a d. μ0iⁿ/2(b-a)lnb/a
