Question

  A toroid of inner radius 15cm and an outer radius 18cm carries an electric current of 5A. How many times must the wire be wound about it such that the toroid will yield a magnetic field of 8x10^-3 T at its center?

Answer 1

$B=\mu _0 nI\frac Rr,\implies$

$n=\frac{Br}{\mu_0 IR},$

$n=\frac{8\cdot 10^{-3}\cdot 15}{4\pi \cdot 10^{-7}\cdot 5\cdot 18}=1061.$