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Re: funny capacitor (EUREKA ?)



A few comments relating to Ludwik's comments,
I use delta_V to represent potential differences. Some of the
things I say about potential differences are not true about
absolute potentials (which I represent by V).

That what confused me. The standard notation is symbol V
(or the corresponding Greek symbol used by Feynman) for
a difference of potential. I would probably not be confused
if another symbol, such as A or B, was used for the "absolute
potential".

Any speck of dust can be a reference to which you would
attach the black lead of a voltmeter; I am more comfortable
with a grounded chassis or with "infinity". The "infinity"
is not the best term, I prefer to say "a conductive enclosure."
The far-away enclosure has an advantage that its shape and
size are not significant. That is what the word infinity stand
for, in my mind.


I often mention in my classes that the standard two conductor formula for a
capacitor (Q=C*V) would be better written Q = C * (Delta V) as that is what
the V in the formula represents, the voltage across a capacitor which is a
*difference* of potentials!

I am trying to explain my confusion, not to argue
about a powerful new concept of gauge. I would like to know what
other teachers think about that concept. I was not familiar with it;
would this concept be useful in an introductory physics course?


The gauge freedom of electrical potentials is not particularly new, and I'm
frankly a little surprised that you are not familiar with it. I believe
that you are familiar but just haven't use the word "Gauge" to associate
with the common fact that you are free to choose you reference point, which
is all gauge freedom means in the context of electro-static potential
discussions. Indeed, when you teach first semester physics and talk about
gravitational potential energy and use formulas like U = m*g*h where you
define h as the height above the gravitational reference, that you get to
choose, you are excercising gauge freedom and therefore must be familiar
with the idea.

Joel Rauber