Lag1989 problem number 17.48

On an iron toroidal core with an average radius R there is a winding with a total number of turns N. A slot of small width b is made in the core. With a current of force I in the winding, the magnetic induction in the gap B. Neglecting the dispersion of the magnetic flux at the edges of the gap, determine the magnetic permeability μ of iron under these conditions.