Question

 A flat circular coil which has 𝑁 turns each of diameter d carries a current 𝐼. If 𝜇0 is the permeability of free space, what is the expression of magnetic field 𝐵 at the center of the coil? 

Answer 1

The magnetic field  𝐵 at the center of the coil is

$B=\frac{\mu_0NI}{2R}$

Here $d=2R$ is the diameter.

$B=\frac{\mu_0NI}{d}$