# What things cause drag when moving through a ->uniform<- magnetic field?

1. Nov 23, 2012

### cephron

I understand a little about eddy currents and synchrotron radiation; I believe I am correct in saying that they are examples of energy loss (essentially drag) when moving though a gradient of magnetic flux.

But are there any materials/effects/things that cause drag when moving through a uniform magnetic field?

In other words: say we have an object moving through an infinite, uniform magnetic field in a vacuum; what properties or components does the object need to have in order to eventually come to rest relative to the magnetic field? Are there several possibilities? Would it matter whether it was moving parallel/perpendicular to the field lines?

Thanks for any input!

Last edited: Nov 23, 2012
2. Nov 23, 2012

### Enthalpy

Yes, a magnetic field uniform at a wire can induce currents and forces... But I'm cheating.

Think of a cylindrical radial field. Like in a loudspeaker, but this time take a short coil and a long magnetic pole. Never mind if the induction is weak, this question is not about technology.

The coil will still produce a force if a current flows. Movement still creates a voltage. If short-circuited, the moving coil brakes through "eddy current" - one may call it differently.

The induction can be as uniform as you want, but I strongly doubt one could let it extend indefinitely and obtain an eddy current in a part allowed to move indefinitely. In the present example certainly not. It would need alternating poles, just as a rotating motor without a collector needs AC current.

If you take again the loudspeaker's voice coil, the flux does vary with the coil's position. Not the induction at the wire, but the flux through the coil, where the coil has no material, since ever more of the flux goes radially through the air gap and is diverted from the core.

Bizarre, isn't it? The force at the wire depends on what happens elsewhere... One reason is that we've thought using B and H, while the vector potential A [you know, B = rot(A)] would give a more sensible answer; but A has the drawback of not being measured statically, hence is more abstract. Electromagnetism is in essence about evolution, even if we could measure a force at the immobile voice coil.

Also interesting: the magnetic field, even if uniform, has a speed linked with the object that produces it. If measured at a different speed, it is supplemented by an electric field which is just E=V×B, plus relativistic corrections if needed. But as an observed electric field needs not be zero, the observer couldn't tell "this magnetic field moves or not", keeping reference frames equally good - more so if including the relativistic corrections.