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Re: dissectible capacitor



Ah, another flamewar topic!


On Tue, 30 May 2000, Carl E. Mungan wrote:

The claim in the video is that all of the charge is
stored on the glass and none on the plates.

The above is an "educator misconception", as opposed to a student
misconception. If this bogus explanation has found its way into the
videodisk demos, then the video can act as Typhoid Mary and infect
thousands of teachers. Soon all students will be certain that capacitor
plates don't store charge, only the dielectric stores the charge!

:)

On the other hand, the demonstration could be presented as an example of a
widely spread misconception. Use it intentionally to shatter student's
blind trust in reference books. This might open the way to their
developing critical thinking skills. It's harder to spoon-feed the kids
if they keep questioning the contents of the spoon, and I think this is a
good thing.

Can someone explain? If some part of my explanation was not clear,
I'd be happy to try to provide additional details if the video
provides them. Carl

If I understand it right, an unnoticed corona discharge during the
disassembly of the capacitor causes the metal plates to coat the
dielectric with ions. The edges of the plates act as "charge spray
needles", the same as the combs on a VandeGraaff.

Why would the edges of the plates start spraying charges into the air? As
the plates are separated, the capacitance drops to a low value. The
charge on the plates doesn't change, and by V = Q/C it causes the
potential difference to go through the roof. With the edges of the metal
parts being adjacent, an air-discharge is guaranteed. The edges of the
two plates spew out charge, but because they are separated by the
dielectric, the charged air plasters itself all over the surface of the
dielectric. The dielectric now behaves as a charged capacitor, with
surface ions being the "charged plates."

I suspect that many educators side with the reference books, and reject
any information that contradicts them. Therefore here are some suggested
experiments.

__________
_| |
/ \ |
\_/ |
Adjustable |_|_| Dissectable
Gap _ | | Cap
/ \ | |
\_/ |___|
|_________|

1. Assemble and charge the Dissectable Cap., and set the spark-gap
separation to just above the point where a discharge will occur. Now
pull the capacitor cups apart, and a spark will appear. Repeat this
with larger and larger gap distances. See how large a potential you
can create via "capacitive multiplication." Perhaps try insulating the
edges of the Dissectable Cap. with "corona dope" or with layers of nail
polish. See if this lets you create sparks at an even larger gap
setting.

This shows that the voltage goes crazy as the plates are separated. If
the initial voltage across the plates was near the breakdown voltage of
the air, then separating the plates MUST cause charge-leakage via corona.

Hey, maybe a camera could pick up the glow from the edges of the plates in
a pitch dark room, especially if the plates were coated with flourescent
ink. Lots of UV light in N2/O2 discharges. Or perhaps an AM radio will
pick up the EM hiss or squeal from the current impulses which occur as the
corona ignites. Corona typically creates a squealing sound because a
"reaction oscillator" is formed (similar to the oscillator formed by an
NE-2 bulb, resistor, capacitor, and 200VDC power supply).


2. Perform the usual Dissectable Cap. demo, but do it under a tank of
oil. This will (probably ) eliminate all corona, and prevent the
sharp edges of the plates from "painting" opposite charges upon the
surfaces of the dielectric. I've not tried this, but I predict that
the demo will fail, and no spark will appear after the capacitor is
reassembled.

3. Combine (1) and (2). See how long a spark you can make by dissecting
the charged capacitor under oil (with the spark gap in the air, of
course.)

If these experiments can sway the minds of educators, perhaps students
will get a kick out of them too. Use "real science" to defeat incorrect
textbook dogma.


((((((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird science
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