Charge stored in Dielectric? Not really - MISCONCEPTION

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: dhmccauley-at-spacecatlighting-dot-com 

 > Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com>
 >
 >
 > A classic capacitor physics experiment.  Charge a Leyden jar.  Remove the
 > plates completely and short them both together and to ground.  Reassemble
 > the jar and it delivers it's normal spark.  Charge is stored in the
 > dielectric.  The effect of the plates are completely eliminated in this
 > experiment.
 >
 > Dr. Resonance

Actually, I've been doing a lot of research into this since I last saw this
effect demonstrated at the Rochester Teslathon and have found this to be a
misconception.
I've also tried of few of my own experiments to verify this and to see what
was really going on.

In the demonstration I saw, a capacitor comprised of a thin dielectric
separated by two flat aluminum plates was charged via a small flyback supply
to about 10-20kV.  The charging
supply was removed and then the capacitor carefully disassembled.  The
dielectric was removed and passed around the room for all to touch, etc...
and then the capacitor was
re-assembled.  Upon re-assembling the capacitor, you could once again draw a
big arc of it proving that the charge of the capacitor was still there and
it is concluded that
the dielectric stores the charge all along.  To further prove this, the
dielectric could be rolled up, mailed across the country, and assembled in a
separate set of plates and once together,
again an arc could be reproduced again proving that the charge is indeed
charged in the dielectric.

However, this is not correct.

What is actually happening is quite complex.  When the two plates of the
capacitor are moved apart, the total capacitance of the plate capacitor
drops and as a result, the potential
difference between the plates increases to enormous levels.  Because of the
physical arrangement of the two plates, this potential cannot increase
infinitely and instead it forms
corona along the outer metal edges of the plates and leaks the excess charge
into the surrounding air.  This corona (which can be heard during this
experiment) then allows
opposite electrical charges to be "sprayed" or "painted" onto both sides of
the internal dielectric material.  So when you dissassemble this capacitor,
this corona effect transfers a
large percentage of the separated charges from the aluminum plates onto the
dielectric surfaces.  The energy is still there, but its stored as a field
in the dielectric material.

To further disprove this misconception, i did the two experiments which were
recommended to me by an old timer friend of mine.
With a similar set-up, I did the following experiments.

1.  I repeated the standard experiment.  I had two plates, measuring about
6" x 6" and a thin dielectric.  I charged this plate capacitor up to about
15kVDC using a small
EMCO DC-DC High Voltage Power supply.  I completely dissassembled the
capacitor, removed the dielectric, re-assembled the capacitor, and managed
to pull a
nice discharge from this capacitor.

2.  Now, to prove that the charge isn't stored in the dielectric and is
actually created by the corona effect on the separation of the metal plates,
I did the repeated the above
experiment in a basin of mineral oil.  When performed under oil, the corona
would be minimal and no "spraying" effect would be present between the
plates and the dielectric.
I repeated the experiment, charged the capacitor plates to 15kVDC,
dissassembled them, removed the dielectric, and then re-assembled them.  I
tried discharging this
capacitor now, and there was absolutely nothing.  I repeated this a few
times and still nothing.

3.  On the second version of the above experiment, I charged the plates in
air and dis-assembled them in air.  I then put the capacitor together under
oil, and voila, as I guessed,
I could pull a nice discharge from the capacitor.  On the reverse, I charged
the capacitor under oil, dissassembled it under oil, and then reassembled
the capacitor in air.  Again,
as I guessed, there was no discharge.

4.  On a final experiment, if the energy was indeed stored in the dielectric
as the Common Misconception goes, then it should hold true no matter what
the capacitor voltage
was.  So I repeated the experiments at a variety of low voltage (100VDC,
200VDC . . . to about 500VDC)  I charged the capacitors up.  Checked the
voltage with a multimeter
to verify charge, and then dissassembled the capacitors. (Again, this was
done in air)  The capacitors were put together and the multimeter was used
again to check for any voltage.
Again, there was no voltage present in that newly assembled capacitor.
Thinking maybe the internal impedance of the multimeter was discharging the
capacitor before it could
take a reading, i switched to an electrostatic meter.  Again, the experiment
was repeated, and nothing.

So in light of all this, you can clearly see that the charge really isn't
stored in the dielectric.  It just so happens that when performing the
dissectable capacitor experiment at
high voltages that other effects are occuring causing the dielectric to be
charged up by fringing corona fields created by the increased voltage
potential as the capacitor plates
are being separated.

If anyone still doubts this, I challenge you to perform the experiment under
oil.  If the charge is *indeed* stored in the dielectric, you will be able
to reproduce this
dissectable capacitor experiment under oil.


Dan