# Calculating Magnetic field of permanent magnet

1. Sep 6, 2005

### Hobnob

Hi: I'm having real trouble finding resources for the problem of accurately calculating (and drawing the field lines for) a magnetic field around a permanent bar magnet, or between two permanent magnets (I assume this is the same problem, as fields are superposable). I've found lots of sites talking about Maxwell's equations, but nothing about how to relate this to permanent magnets, rather than fields induced by currents. For that matter, I'm a little lost even with Maxwell's equations, as my field calculus is *very* rusty and was never that good to start with... I can't quite see how to move between the path/surface integrals and the actual field at a particular point.

Can anyone point to some good resources for someone who needs a bit of hand-holding but is ultimately up to the task?

All this is ultimately to be part of a motor/generator simulator, so a final question is: can I manage by just pre-calculating the magnetic field and superposing it on the field from the wire, or am I barking up completely the wrong tree?

Thanks

2. Sep 6, 2005

### shyboy

in first approximation the field well outside the magnet is a usual dipole field.
The next step is quite troubling, because you need to take into the account the effect of one part magnet on the magnetization of another part of the magnet. The problem is that you may have hysteresis und saturation. I have now idea how take the hysterisis into the account, but saturation is fine, as soon as you are using a computer.
The problem may be formulated like this:
let's imagine we have a small dipole with a magnetic moment $$M$$. Then, if we put another dipole near it, their magnetic field will be a superposition of two dipoles. But their dipole moments will be affected by the external magnetic field from another dipole. The additional induced magnetic moment will be proportional to the external magnetic field and the dipole's supsceptibility, which may be nonlinear function of the external magnetic field.