Re: Disectable Capacitor

["Donald E. Simanek" <dsimanek@EAGLE.LHUP.EDU>]

On Wed, 3 Mar 1999, John P Lewis wrote:

> I recently purchased a Cenco disectable 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.

This is a great demo. You'll find a pretty fair discussion of it on my web
page in the documents on "Teaching Scenarios."

The cans touch the plastic dielectric at many places, and the strong field
between the cans forces electrons from those contact points to the
dielectric on one side of the dielectric, and from the dielectric to the
can on the other side. They stay there when the cans are disassembled. The
mobile charges are not *in* the dielectric but *on* it, and the dielectric
is polarized as well.

> If there is some sort of memory of the internal field set up in the =
> dielectric, why does it "remember" while I take it apart but not "remember"=
>  after discharge.

The charges are strongly held by the dielectric. The capacitor *is* the
dielectric. The metal cans just facilitate charging it. Imagine the same
thing being done with a parallel plate capacitor with dielectric, and then
removing the plates. The dielectric stays "charged".

A more interesting question is to explain the details of how the charge
gets from the dielectric to the spark gap when you discharge the
capacitor. I'll leave that to some of the electricity gurus on this list.
I've been posting too much lately. Why does the act of putting a
conductive jumper across the plates cause the mobile charges to move the
greater distance through the jumper?

        -- Donald

.....................................................................
Donald E. Simanek
dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek
.....................................................................