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Re: simple magnets question

I think with William Beaty's most recent posting it has become clear
that his question has switched from being a "simple" magnet question
to being a complicated magnet question. I think the underlying
question is a question of relativity... and relativistic
electrodynamics is not simple.

If two electrons are "at rest" then the only force between them is the
electrostatic force. If the same two electrons at the same spacing are
both "in motion," they have a magnetic interaction as well as an
electrostatic interaction. But... wait a minute... there is no such
thing as "at rest" or "in motion."

If I am moving with the electrons, they are at rest from my viewpoint
and I must measure only an electrostatic interaction. If we (the
electrons and I) are moving with respect to you, then you must measure
both magnetic and electrostatic interactions between the same two
electrons for which I measure only an electrostatic interaction.
Hence, you and I observe different physics because of our relative
motion.

I think William is questioning if this type of situation is correct...
that is, he is describing two situations (involving relative motion)
that intuitively seem equivalent, but they seem to yield different
results. Is this okay? The answer is yes, this is normal for problems
involving relativity. Although high relative velocity is often
required before relativistic effects become noticeable, the magnetic
field surrounding "moving" charges is an example for which we have a
"relativistic problem" without the need for real high relative
velocities. However, with the proper relativistic treatment, the
apparent problem goes away. But the appropriate treatment is not real
simple.


Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817



-----Original Message-----
From: William Beaty [SMTP:billb@ESKIMO.COM]
Sent: Thursday, June 24, 1999 3:34 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: simple magnets question

On Wed, 23 Jun 1999, Leigh Palmer wrote:

Yes, so the question becomes: "does an axially-spinning disk magnet
create
a motional e-field which can attract/repel a stationary charged
particle?"

I guess the answer is, simply, no, it does not. If the field and its
source are axially symmetric then spinning the source about its axis
will affect nothing outside. As John Mallinckrodt emphasizes, one
should never think of a field (electric or magnetic) as "moving".

I'm still confused about this. I think I'm having trouble
communicating
why this is so. I'm talking about situations where the intensity of
fields does not change, yet electrons are still affected. I'm ignoring
any situations where the intensity of the b-field is changing. A
simple,
non-rotating example: if an electron is flying across the *uniform*
field
between the cyclotron pole-pieces, then that electron does not
encounter
changing field intensity. Yet it is deflected sideways.

In the cyclotron, if the relative motion between an electron and the
b-field is a meaningless concept, and if a "stationary" b-field is the
same as a "moving" b-field, then I have a problem: why is an electron
deflected sideways if the electron moves *differently* than the
cyclotron,
but an electron is *not* deflected sideways if the electron and the
cyclotron move together? Let me put it this way:

Suppose an entire cyclotron is moving uniformly with respect to my
frame. I should see a b-field between the pole-pieces, but because
of
the relative motion, I should also see a transverse e-field. If I
put
an electron between the cyclotron's pole-pieces, and if the electron
is
moving but is NOT moving with respect to the cyclotron, then from my
viewpoint the electron is strangely unaffected by the transverse
e-field, and the electron moves in a straight line. (This makes some
sense, because in the cyclotron's frame, the electron is sitting
stationary between the pole pieces.) Now for the important part. If
instead I place an unmoving electron between the pole-pieces of the
moving cyclotron, then I see the electron get accelerated sideways.
I
have now observed that the electron responds differently depending on
the relative motion between it and the cyclotron, EVEN THOUGH THE
B-FIELD BETWEEN THE POLE-PIECES IS UNIFORM AND THE ELECTRON DOESN'T
ENCOUNTER CHANGING FIELD STRENGTHS. I conclude that, for uniform
b-fields, the motion of the field is *not* a meaningless concept.
Either that, or the uniform motion of the POLE-PIECES is important to
the electron, and the electron will only be deflected if it moves
across
the pole-pieces or if the pole-pieces move past the electron.

I find that it is frequently illuminating in such questions to
consider the interactions directly in terms of the sources. Given
the fact that there exist no monopoles, the only sources possible
are the elementary electric charges which make it up. If the
disc magnet is spun, the only source that can change is the current
source. If the magnet is electrically neutral and nearly homogeneous
then the resulting currents will also vanish, and with them any
magnetic fields which might have otherwise resulted.

Yet if this reasoning is applied to the uniform field inside a
cyclotron,
we would incorrectly predict that a moving electron would experience
the
same forces as a stationary electron. After all, the electron doesn't
encounter any non-uniform spots in the field.

The central issue: if the pole-pieces of a uniformly moving cyclotron
appear to generate a transverse e-field, then I conclude that the
relative
motion of the field (or of the pole-pieces) is important, and I predict
that a cyclotron which rotates on axis will appear to generate a radial
e-field. Is my reasoning clear?

((((((((((((((((((((( ( ( ( ( (O) ) ) ) )
)))))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST
website
billb@eskimo.com http://www.amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird
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