Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: supercaps





On Sun, 23 Feb 1997, Leigh Palmer wrote:

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.

Is this really the same 'rebound' phenomena found in supercaps? In the
dissectable Leyden jar what you describe is, of course true. But were
those aluminum electrodes really charged before you touched them together?
Did you test them for charge *after* dissecting the capacitor? Chances are
they had very little charge, it all being on the surface of the glass or
plastic liner between them. Therefore it's not surprising that when
reassembled, it produces a healthy spark. See my teaching scenario on
electrostatics on my web pages for discussion of this.

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?

The capacitance of a capacitor is merely an engineering parameter
that describes the linear approximation to the behaviour of the
component.

That's an excellent point to keep in mind.

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.

Someone else remarked that the supercap is akin to a chargeable cell.
The older technology of electrolytic capacitars are also, akin to a
chemical cell.

-- Donald

......................................................................
Dr. Donald E. Simanek Office: 717-893-2079
Prof. of Physics Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745 CIS: 73147,2166
Home page: http://www.lhup.edu/~dsimanek FAX: 717-893-2047
......................................................................