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Re: supercaps



In reference to the energy stored in a Leiden jar I said:

It is clear that the energy is somehow stored in the glass. Is it
chemical energy? What does it matter what it is called?

Is it any more than the fact that glass is an insulator and metal is a
conductor, so the charge of the charged capacitor will reside on the
surface of that insulator, not on the conductor?

My point, of course, was that energy can be stored in a dielectric in
ways that have nothing to do with what we ordinarily call chemical
changes. By removing the electrodes I simplify the system. It is an
electret at that point, with a very long relaxation time. I don't know
the submicroscopic details, but the conclusion that the energy is
stored in the glass is fairly conservative.

I think that there is an appreciable free charge present at the surface
of the glass, by the way. If one has experience with the face of a TV
set or computer monitor one should be familiar with the fact that small
sparks can be drawn from that glass surface after some operating time.

My picture of what is happening in the disassemblable Leiden jar is
that the bound charge on the polarized glass is neutralized by free
charge from the environment when the electrodes are removed. The result
is that the electric field within the glass is greatly reduced. Since
it is the electric field which would cause the polarization to relax,
it relaxes much more slowly with the free charges on its surface.

If my conjecture is correct, the disassemblable Leiden jar demo can be
improved over the way I do it now. At present I remove the inner
electrode with an insulating rod, an overhead projector pen or the
like. I then let the outer electrode fall away (it is a fairly tight
fit) by lifting the glass. In this process I can see that I do not
provide as much free charge as I might. Next time I do the demo I will
lift the center electrode out by hand, thus providing a source of free
charge. The outer electrode will be insulated from ground during this
process, of course. I will then pull the outer electrode off the jar
by hand rather than letting it fall. If my conjecture is correct I
ought to get a larger spark on reassembly than I get by the older
technique.

Leigh