Re: Question 07/02 CURRENT IN A WIRE

[Bernard Cleyet <anngeorg@PACBELL.NET>]

Eisberg and Lerner (Physics, 1981) devote 10 (yes ten) pp. to this "problem".  First with two
wires and then two beams (in a vacuum).  He quickly obtains the result for the beams:  F / unit
length = [1 / (2 Pi * permitivity)] * [(charge / unit length)^2 / separation] * [1- v^2 / C^2].
He does this first in the lab. frame and then for an observer traveling with the beams.  Note:
he actually (uniquely in my, admittedly meager reading) assumes the F of repulsion must be the
same for both observers, and, thereby, derives the Lorentz contraction.  I quote,  "Starting
from the experimental phenomena of magnetism, we have derived the Lorentz contraction.
Alternatively, it is possible to assume the Lorentz contraction and derive from it the
properties of magnetism.  Thus we can say that [italics] magnetism is an essentially
relativistic effect [itl. end], in spite of the fact that it is observable as quite small
speeds."

".... [italics] it is the Lorentz force -- and not the electric or the magnetic force
individually -- that is fundamental."

bc who thinks many texts > 1981 do this analysis, and therefore, if true,  wonders why the long
thread

Michael Burns-Kaurin wrote:

> Looking at the situation of two parallel charged beams in the lab frame
> (the particles are moving):
> we would see a magnetic attraction between the beams, but we would also see
> electrostatic repulsion.  Both will depend on the charge density of the
> beams and on the inverse of the distance between them, but the magnetic
> attraction will be smaller by a factor of (v/c)^2, if I did the math right.
> So, the lab frame sees a net repulsion.
>
> In the particle frame:
> There is only an electrostatic repulsion.  It will have a magnitude
> depending on the charge density in this frame, which is less than the
> charge density in the lab frame (length contraction).  Again, net
> repulsion.
>
> Michael Burns-Kaurin
> Spelman College
>
>                       Hugh Haskell
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>                       Sent by:                 Subject:  Re: Question 07/02 CURRENT IN A WIRE
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>                       11/19/2002 03:09
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> At 20:52 +0100 11/19/02, Mark Sylvester wrote:
> > But Hugh, just last week I was demonstrating to my class the mutual
> > attraction of two conductors carrying parallel currents!
> No problem there. The magnetic fields are real, and there is plenty
> of relative motion to account for it. But imagine two beams of
> identically charged particles in a vacuum, traveling parallel to each
> other (and in the same direction) and with the same velocity. Other
> than some random thermal motion which should cancel out, there is no
> relative motion of the charges in one beam with respect to the
> charges in the other beam, so can there be a magnetic interaction
> between these beams? I can't see how, but I am open to arguments that
> refute my claim.
>
> Hugh
> --
>
> Hugh Haskell
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> <mailto:hhaskell@mindspring.com>
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> This posting is the position of the writer, not that of SUNY-BSC, NAU or
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>
> This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.