Question

A toroidal solenoid has 3000 turns and a radius of 2cm with a current of 5A flowing through it. Determine the magnitude of the magnetic field. 

Answer 1

$B=\frac{\mu_0 nI}{2\pi r}=\frac{2\cdot 10^{-7}\cdot 3000\cdot 5}{0.02}=0.15~T.$