Question

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

Answer 1

Consider an element of thickness dr at a distance r from the center. The number of turns in the element will be dN = \((\frac{N}{b-a}) dr\) 

Magnetic field due to this element at the center of the coil will be dB = \(\frac{\mu_0(dN)I}{2r}\)