Re: Dissectable Capacitor

[brian whatcott <inet@INTELLISYS.NET>]

At 18:11 3/3/99 -0500, you wrote:
> On Wed, 3 Mar 1999, John P Lewis wrote:
>
> > I recently purchased a Cenco dissectable capacitor which consists of an
> > outer aluminum can (no top), a plastic (beaker) which fits nicely inside,
> > and an inner aluminum can which has a conducting post attached.
> ....
> > The owner's manual suggests that the CHARGE is stored in the dielectric
> > between the conductors but I don't understand how this could be.
...
> Donald E. Simanek


(A not quite relevant contribution)

It must be a week or two since there was a mild discussion about a  model
of the charge distribution arising from a charge applied to one capacitor
plate, then a charge being applied across the plates.

 Though dissecting a capacitor to provide an explanation seems like a
natural mode to physicists, I was uncomfortable.
   I wanted to use another style of model - the lumped parameter model.

So I played with an array of three capacitors arranged in a triangle.

First charge a loose cap acting as a charge carrier and apply it to one cap.
The one that represents the stray capacitance to ground of a plate.
 Then apply another charge from the transfer cap to the cap representing
the 'physical' cap.

Measure two PDs.
 That was the idea, at least.
I thought that a DVM and some 0.1 microfarad caps would do the trick - but
the digital voltmeter's input resistance is a modest 10 megohms, so even 2
microfarad caps discharge too fast.  So now, i am lining up 470 microfarad
caps charged from a 9 volt battery. Their time constants promise reasonable
reading
stability.
 It surprised me that I could not easily intuit the voltages to expect.
   Now at least, I can see why placing a charge onto one plate of an
isolated capacitor leaves the other plate at very close to the same
potential: simply a matter of the ratio of stray capacitance to actual
capacitor value...

Brian
brian whatcott <inet@intellisys.net>
Altus OK