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Re: Capacitor charge, were is it?





On Fri, 25 Oct 1996, Tesla List wrote:

> From: huffman <huffman-at-fnal.gov>
> 
> I'm having trouble with the idea of charge being stored in the dielectric.
> This may not be totally Tesla related but I would like some comments,
> stones, etc.

I'm a day late on a reply here, but I don't yet see any responses to this,
so hopefully I won't be junking up the list with a reply that will already
have been answered by someone else. The problem is that some folks use the
term "charge" with respect to a dielectric, when they really mean to use
"polarization". I suspect that some people confuse the two, but that others
mix them up intentionally because not everyone here has had the opportunity
to take an electromagnetic fields course, and referring to charge can help
everyone picture what they're talking about. What's usually being described
is energy being stored in the dielectric. Often it will be said that there
is charge in the dielectric, when actually the energy is stored by 
polarizing the dielectric. This is the electric field analogy to the 
magnetic field act of magnetizing a peice of iron or nickel. There are even
electric field analogs of a magnet, called electerets, which retain their
polarization analagous to the way a magnet retains its magnetization. The
electeret contains no excess charge, just as the magnet contains no 
magnetic monopoles, but the fields produced by each look as though there
are charges or monopoles there. The charges already in the molecules or
atoms of the dielectric get aligned, rotated, or slightly bent under the
influence of an electric field, and this takes (and stores) energy.

> If this is true we could not have a capacitor with a charge that has no
> dielectric (vacuum).
> We can, however, and the charge must be held on the plates. The energy is
> stored in the electric field which can only be there if the plates have a
> different charge from each other. I recall a post of an experiment in which
> a capacitor is charged, then carefully dismantled. The two metal plates
> were handled, shorted together, then reassembled getting the charged
> capacitor which can be shorted out yielding a large spark.
> There must be something going on here that is not obvious. 8?/

What's happened is that the dielectric is of a type that retains its
polarization for a time, analagous to a piece of iron that's been 
temporarily magnetized. While it retains its polarization, it still contains
energy. You're getting some of it back when you short the plates. What you
should also find is that after the dielectric has had time to unpolarize, 
you should be able to short the plates together again and get another spark.
>From what's been described about polarization, can you figure out why?

Your problem with the two shorted capacitors is a good one, which (at least
in my day) was given to all introductory Electrical Engineering students as
a lesson in understanding when an equation is relevant and when it's not.
Or as some on the list might say, "You're dinkin' with too much theory".
I'll stay quiet on this one, to let others who haven't seen it yet agonize
as much over it as I did. It's a really worthwhile lesson to mull over.

As always, corrections and clarifications are welcome, especially in a post
like this one that tries to use analogy as a visualization tool.

Wes B.   YAPD (Yet Another Proud Dinker)