Re: Magnets

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

Colby wrote:
> I can't see where it stops being a magent and starts
> being an electromagnet.

What a nice question.  Careful there, asking questions like
that is how Einstein (and not a few others) got started.

One answer is that it starts being an electromagnet as soon
as it moves, or as soon as it's field changes.  But that
would be wrong, or at least misleading.

Stand stationary next to a magnet and you see just a magnetic
field.  Move past that magnet at some speed (as slow as you like)
and you still see a magnetic field, but with a bit of electric
field mixed in with it.  Not only that, but as you approach
the 'stationary' magnet, the field builds up to a peak and then
dies away again as you carry on past.  That looks exactly like
a pulse of electromagnetic radiation, and indeed it is.  

So I'm saying that a stationary magnet is radiating energy?
Well in this case yes, you and the magnet exchange a little
energy and momentum as you go past one another.  Your course
through space is slightly altered by the encounter, and the
magnet is given a small velocity.  That comes about because
your 'detection' of the stationary magnet's field inevitably
takes some energy and momentum from that field, and hence
the magnet.

Thus, even if you don't alternate the poles of your battery,
in this sense the magnet can still radiate an EM wave. 
Maintaining a constant polarity, you can also radiate
circularly polarised EM waves by simply spinning the magnet
so that north and south are swapping ends.  In both cases,
the amplitude of the radiation seen by a stationary observer
is proportional to the frequency of alternation.
 
The distinction between electric and magnetic field is intimately
related to the distinction between space and time, so we must
expect (and find) that relative motion of source and observer
drastically affect the way that the field appears.  Modern
physics offers an astoundingly beautiful and accurate description
of the details of all this, which is well worth the study.

Terry wrote:
> "Static" electric charges have no magnetic field.

Exchange the words electric and magnetic and replace the
magnetic loop with an electric dipole.  Then the above 
comments still apply.  I'm afraid 'static' is only a relative
term, for both motion and field.  To make sense of all of this
you have to dig beneath the observer-dependent appearance
of the field and deal directly with the entity.  When you do so,
you find that the EM field, and 'charge' itself, are both inevitable
consequences (and their physics determined by) certain simple
geometric symmetries of nature.
--
Paul Nicholson
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