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Re: ORIGINAL simple magnets question



On Mon, 28 Jun 1999, Leigh Palmer wrote:

I keep going on about the spinning magnet-disk because it seems to be the
simplest example which illustrates this "motional field" stuff.

It is, indeed, a simple example which illustrates (if only theoretically)
that "motional emf" when it is said to pertain to the motion of magnetic
fields (as opposed to the motion of magnets) is misleading and, often,
incorrect. Realizing the ideal setup of a spinning disk magnet is probably
not worth the effort, but if you produce the DC radial electric field you
expect I and others will be be thrilled.

It's easy to produce this field in rotationally-symmetric moving
conductors, such as in a homopolar generator. Rotate the metal disk, or
rotate the external circuit, and you get a big current. But nobody
believes that a rotating magnet alone in space will create such a field!
The magnet has to rotate darned fast before it can create a measurable
force on a test charge, to say nothing about deflecting the e-beam coming
from a hot cathode in a vacuum tube. I think we'd need to take some
ultra-cold ions and let them fall under gravity past a spinning
disk-magnet. Collect them in an ion-trap, cool them to microkelvins, then
pulse the trap fields so they fall out of the bottom. See if a spinnig
magnet acts differently than a non-spinning magnet.

There, I've just given you a quote you can use in your Nobel acceptance
speech. Now go to it!

Heh.


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