1. The problem statement, all variables and given/known data A bar of soft iron of cross sectional area A and relative permeability μ is bent into a ring of radius R. The bar is wound with n turns of a wire carrying a current I. Deduce a value for the magnetic field in the iron, explaining your assumptions. (b) A narrow gap of width w is sawn into the ring. Find the magnetic field in the gap. 2. Relevant equations 3. The attempt at a solution For part a. I used Ampere's Law with H to get H = nI/(2*pi*r) B = μμ0nI/(2*pi*r) But part b? I thought of trying to do some sort of superposition. Perhaps working out M in the original problem, and then doing the field due to the original problem plus the field due to a small chunk with opposite magnetization. Is this at all on the right lines?