Re: mag. force on wire?

[William Beaty <billb@ESKIMO.COM>]

On Tue, 2 Nov 1999, Leon Leonardo wrote:

> Greetings,
>
> I'm about ready to cover magnetic forces on a current
> carrying wire when I realized how little I understand
> what's really happening. Any help would be appreciated
> concerning the following:
>
> Just how is it that the force on moving charges
> "contained" in a wire is transferred to the wire
> itself? I've problems with the two explanations that
> seem pretty standard. That is, by collisions (with
> what, exactly, and what happens at the boundary?); and
> by electrostatic separation of charges (what happens
> if both positive and negative charges are equally
> responsible for charge flow?)

This has always befuddled me as well.

A possible clarification:  Think about the effects of one wire upon
another, where both contain a current.  For visual models, it's better to
include Newton's 3rd by including two physical objects.   If we have a
static force-pair between a wire and a b-field, we have a problem because
the b-field is not a physical object.  If the b-field is associated with
another wire or with a permanent magnet, the concepts aren't as exotic.

Hmmm.  If the EM forces try to pull an electron away from the surface of a
wire, an image-charge will form in the metal, and an attractino force will
appear between the electron and the metal.  It takes "work" to fight the
"work-function" voltage while trying to extract charges from a metal.
Doesn't this explain how a charged object can apply an attraction force to
a nearby neutral conductive object?  Perhaps this surface-charge effect
can also explain why magnets apply forces to current-carrying conductors.



Now, suppose I have a bar magnet that's a few million KM long, and it is
suddenly approached by a current-carrying metal ring (which also is a few
million KM across).  Or if you prefer a desktop experiment, imagine that
the speed of light is less than 1 cm/sec, then relativistically wave your
bar magnet near an electromagnet and see what happens.  The force-delays
and pulses should be interesting.  If I could visualize them, I'd have a
handle on the way that mechanical forces are communicated by EM fields
between objects.  Please send me a virtual-reality system with gloves
attached to force-feedback robot arms, and a supercomputer to simulate a
workspace having variety of coils and permanent magnets where c is very
slow.


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