# Ferromagnetism, electrodynamics and field theory

1. Dec 20, 2007

### Babcock McGee

Hi all,

I'm a bit confused about ferromagnetism (and I've come to realize that I'm not the only one)! I'm currently studying electrodynamics and field theory in general to solidify my understanding of such, but permanent magnets and ferromagnetic materials seem to be often ignored in the literature. I'm comfortable with charged body scenarios in the presence of magnetic fields, and the creation of magnetic fields by moving charges, etc., but I'm unable to make predictions or construct field equations involving permanent magnets, and also completely unable to understand how a magnet seems to do work by picking up an iron nail.

Before this gets too verbose, here's the bottom line: Could someone refer me to reading on the fields resulting from permanent magnets, and interaction of ferromagnetic materials with magnetic fields? I'm really interested in this from an engineering perspective, so while reductive explanations (e.g. magnetic domains as microscopic current loops) are interesting, I'm really looking for theory which will facilitate design and prediction at the macro level, ideally a set of equations.

Thanks.

p.s. The main book I've settled on is Dynamic Electromagnetics by Paul Diament. Any opinions?

2. Dec 22, 2007

### marcusl

Sorry, I'm not familiar with the engineering literature. I would say that you should study the theory (M, B, mu, chi) to do a proper job of understanding applications. It won't take you long if you've mastered the fields of moving charges. As for physics texts, take a look at Reitz and Milford, they cover the material in one chapter that is straightforward and quite applied (for a physics text...).

The problem with magnetic fields and ferromagnetic materials is that analytic methods fail for all but the simplest cases. This is the domain of finite element and similar codes, which are used extensively by anyone working in magnetics (whether it be microscopic read/write heads or huge iron shields for magnetic resonance imaging). There is a gigantic literature on numerical methods and applications.

As for analytic calculations, Smythe, Static and Dynamic Electricity, calculates the field from a magnet with a gap and also the lifting power of a horseshoe magnet under simplified conditions. These should get you started.