Can someone help me with this please? Consider a toroidal sample of magnetic material wound, uniformly, with coils of wire that carry a current I. If the total number of coils is N and the relative permeability of the material is μr, calculate the magnetic field B, inside the toroid at radius r. The problem is expressing the magnetic field in terms of the relative permeability. In my notes, I have H = B/μ0μr, but I can't use that can I? I mean, I can't substitute B = μ0μrH into the Ampere's law integral, right?