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displacement current



Last words (maybe):

After taking a deep breath and thinking some more regarding the anti
"displacement current" faction I will try to summarize. My posts were and
are my thinking out loud concerning how I justify to myself the common and
well used terminology "displacement current". I think David and I are
almost in agreement that this is semantic problem; maybe better to say a
problem and disagreement regarding terminology.

My opinion is that it is reasonable terminology; we need to call the
"displacement current density" term that appears in Ampere's law something;
and I find referring to it as "flux of time changing electric displacement
density" to be awkward and worse than the term "displacement current". I
may be open to suggested alternatives; but one does run into the pedagogical
delima of whether or not to use the quasi-standard terminology or new
terminology. Since, whatever we call it, is a source of magnetic field and
conduction current is a source of magnetic field; then why not call it a
current because of this similarity(David has offered some reasons)?

Does anybody have any suggestions of what to call it?

He wrote: (in reply to another's posting)
A moving electron *is* different than a displacement current
between capacitor plates.

I don't disagree with this statement; and I hope no one took anything I said
to imply otherwise. Calling it a current is a generalization of the term
"current" and as such does not imply isomorphism between the original use of
the word and it generalized use. Of course, it should imply some sort of a
relationship.

David also wrote:
The physical effects produced by 'displacement current', although
admittedly real, are not quite "in the same fashion as conduction
current". The only similarity is that both of them, written as 3-space
vectors, seem to formally act as sources for a non-longitudinal magnetic
field.

I don't disagree with this, except I would delete the words "only" and "seem
to formally act" in the last sentence. This similarity *is* the crucial
relationship that makes use of the term "current" reasonable. I would argue
that it not only "seemingly" acts as a source of this non-longitudinal
magnetic field, but sometimes, in fact, *is* a contributing source of the
field. (where, what we are talking about are 3-space vector fields.)

Otherwise, I don't know how to interpret the classic example of the axially
symmetric situation of a long straight wire, with current I, and a "close
to ideal" axially symmetric parallel plate capacitor in the middle of the
wire. Namely, the application of Ampere's law (integral form) for a closed
loop around the wire, far from the capacitor, but the surface, for which the
Amperean loop is the boundary going through the capacitor in such a manner
that the no physical wire or conducting material crosses the surface. In
order to obtain the correct magnetic field you have to treat the
"displacement current" in exactly the same way you treat conduction current.

I think it was Maxwell who first called it a current.

I am reasonably sure that is correct; but I didn't want to name drop.
___________________________________
Joel Rauber

Einstein also named 'relativistic mass'. I don't think it's a good idea to
use that
terminology either.

Here we are in agreement, as I eschew the use of the term "relativisitic
mass" as well.