I have a difficult probelm to solve: There is a dougnut shape (toroidal coil) with N turns of wire wrapped around it. Current I flows in the wire. THe cross section of the 'doughnut' is square with height h. I am meant to use ampere's law to prove that the magnetic field at a radius r from the centre (half way through the coil) is given by: B = uNI/2pi*r (u = permittivity) SO far I have that ampere's law says that the integral around a closed path of the Magnetic field is equal to the permittivity times the current enclosed. How does this translate into the above formula? I am probably just missing something really obvious.