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Re: Capacitor energy experiment



Early electrolytics were constructed of Al foil and an electrolytic that
cause an Al2O3 film (anodize) when the cap. is formed by charging. (AC
will result in a nonpolar cap.) Since the plates are not of the same
material (polarized cap.) and they are connected by a conducting fluid,
I expect that they make a chemical cell. Not surprising they are a
"battery". I don't know about contemporary caps. [I made them using
baking soda. If one uses an Al plate and a carbon rod in NaH2Co3? soln.
the result is a high current rectifier.

Some of the large storage caps (> 1 kV) are marked keep shorted when not
in use. I think the term is relaxation (of the dielectric). I've never
heard of this for electrolytics, but it doesn't surprise me. They leak
so much that it's not an ever present danger as with high Q high V
caps. I think the effect is electrostrictition, When charged the
dielectric stretches (like an electret). If the discharge is quicker
than the time for the dielectric to return to its original state, on
relaxation it induces a charge on the plates. My intuition suggests
that if this is the modus operandi the "recharged" cap should be with
reversed polarity. I have been shocked by this! That's why I don't
have a memorism about the effect! I'll "go to the net" after posting.

It's not clear how LK will separate the charging from the discharge in
the last (audio freq.) experiment. Maybe he will have double heating?
Or could separate them by using diode(s)?

bc who may have read LK's too rapidly





Ludwik Kowalski wrote:

About a week ago I suggested the following
student-oriented problem:
* * * * * * * * * * * * * * * * * * * * * * * *
"By how many degrees will the water temperature
change if a C=10 F capacitor is charged from a
3 volts battery and then discharged through a
resistor R immersed in 50 grams of water? Assume
the procedure is repeated twenty times. The mass
of the resistor and the mass of the styrofoam cup
are negligible. Heat received by connecting wires
can be ignored because their resistance is much
smaller than R.

(Answer dT=4.3 degrees C. A teacher may also add
a question for a short essay. Something like this:)

The answer does not depend on R. Does it mean that
a resistor of one hundred kilo-ohms can be used in
the experiment? Justify your answer.

(Due to a very long discharging time, at least
3*R*C, heat escaping through styrofoam would not
be negligible. Furthermore, too large R could cause
self-discharging. A sizable fraction of the discharge
current would then bypass R resulting in heating of
the capacitor instead of water.)"
* * * * * * * * * * * * * * * * * * * * * * * *

I think that we should be prepared to face the unexpected
(in a real experiment performed with an electrolytic capacitor).
A year or two ago we became aware that some electrolytic
capacitors exhibit strange battery-like properties. For
example, a discharged capacitor tends to partially recharge
itself spontaneously. Any comments? Any references?

Unless I am convinced (via preliminary experiments) that a
battery-like effect is negligible I would use a different method
to validate the 0.5*C*V^2. I would use an air capacitor, for
example, 500 pF connected to a generator of rectangular
pulses. Pulses would be sufficiently long to totally charge
and discharge C in each cycle. Working at an acoustical
frequency I would be be able to perform tens of thousands
of discharges (through R in water) in a very short time. The
"amount of heat" delivered in each pulse would by much
smaller than from an electrolytic C but the number of pulses
would be sufficiently large to compensate for this limitation.
Ludwik Kowalski