# Ferromagnetism help

1. Jun 7, 2012

### dorker

I'm trying to work out how fast I can switch an electromagnet's polarity, assuming I know the properties of the core's material. The magnetization dynamics are described by the Landau-Lifgarbagez-Gilbert equation (dM/dt = -ℽMxHeff + λMx(MxHeff), which is quite a chore to solve, seeing as it uses the effective field Heff, which also has M as one of its variables.

But this book I'm reading, Physics of Ferromagnetism by Sochin Chikazumi, simply uses the applied H field instead of effective H, which makes the equation tremendously easier to solve. The thing is, it does so with no explanation. Is this a valid approach, and what assumptions does it take?

On a side question, the same book describes the relationship between the magnetization vector M and flux density B as B = M + µ0H, whereas wikipedia (can't post link, but the magnetization article ) says it's B = µ0(H + M). How does that work, are the µ's different or something?

Last edited: Jun 7, 2012
2. Jun 9, 2012

### marcusl

Switching polarity of a large electromagnet is almost always limited by the winding. The time constant, of course, is L/R. Trying to force too rapid a change (dI/dt too large) will create an emf that exceeds the breakdown voltage of the windings. The core will most likely follow the coil without slowing you down.

3. Jun 10, 2012

Thanks!