# Magnetic Permeability & Reluctance on old exam question

This is a past exam question from one of our lectures, and we have an issue with (i), I believe I need to use the equation $$\rho=\frac{RA}{l}$$, but I am not sure - could someone enlighten me on the issue?

A mild steel ring of magnetic permeability 380, having a cross sectional area of $$500mm^2$$ and a mean circumference of $$400mm$$, has a coil of $$200$$ turns wound uniformly around it.

Given that the magnetic permeability of free space is $$400 nH/m$$ determine:

• (i) The reluctance of the ring.

• (ii) The current required to produce a flux of $$800\mu$$ Wb in the ring.

• The equation above has nothing to do with the problem below. In case the problem would be in SI units, I might be able to solve it... :/ – Pygmalion May 7 '12 at 17:03
• That figure for magnetic permeability of free space doesn't seem to be right. – Mark Beadles May 7 '12 at 19:19
• I was puzzled by N=200 in the numerator and I found it was NOT necessary. Please check codecogs.com/library/physics/magnetism/magnetic-reluctance.php for the explanation. – user34961 Nov 29 '13 at 13:54

As Pygmalion says, that equation is not related to the question. $\rho=R\frac{A}{l}$ is electrical resistivity, not magnetic.
$$\mathcal{R}=\frac{\mathcal{F}}{\Phi}=\frac{l}{\mu_0\mu_rA}=\frac{(200\cdot400mm)}{(4\pi\times10^{-7}Hm^{-1})(380)(500mm^2)}=\text{answer}$$