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Re: supercaps



I'm unsure of what is meant by "chemical energy" in this or any other
context. This phrase, like "heat energy" is a trap. Energy is energy.

If the behaviour of the supercap on rebounding after discharge is
taken to represent this arcane sort of energy, then it must mean
equally that a capacitor with a glass dielectric stores chemical
energy, too. When I was a youth I used to fix radios and, later,
TVs for the neighbors. In those days kinescopes (which we called
picture tubes) used to suffer a slow death that could be remedied
for a short while with a device which boosted the filament voltage
(a "brightener"), but eventually one had to replace the tube.
I've replaced more than a dozen black-and-white kinescopes. Doing
so is a dangerous job because of the potential for catastrophe if
the tube is dropped.

Kinescopes are capacitors. They are effectively Leiden jars. They
are charged to a high voltage (10 kV or more in the old sets) and
they retain their charge for a long time. They must be discharged
before one works with them, and because they exhibit the same
rebound phenomenon that was observed with the supercap. A jumper
is best left in place while the set is worked on. When a picture
tube is changed there is always the chance that it will give the
person holding it a shock, and that is a particularly bad time to
be startled by a shock.

The rebound phenomenon with a glass dielectric capacitor is quite
dramatic. You can demonstrate it with a Leiden jar as well. In
fact I do it with a disassemblable Leiden jar*. I can charge the
jar and remove the aluminum electrodes. After touching them
together, I replace them and a healthy arc can be drawn off the
jar. After a minute or so I can draw another spark.

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

The capacitance of a capacitor is merely an engineering parameter
that describes the linear approximation to the behaviour of the
component. Resistance is a similar parameter. Ohm's law doesn't
hold rigorously for a real resistor. Even an ideal resistor with
a nonzero temperature coefficient of resistivity is evidently
nonlinear due to Joule heating. Inductances with iron cores are
(or used to be at any rate) sometimes designed as saturable
reactors (or "swinging chokes" as we used to call them) for which
the inductance is a strong function of the current. The
capacitance of an electrolytic is simply the parameter Ludwik
measured in his experiment. No electrolytic, supercap or otherwise,
behaves linearly.

Leigh

*These can be purchased from, I think, Sargent Welch. Mine is
better than the commercial version because it has more glass.