Re: capacitance formula

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

Original poster: "Peter Lawrence by way of Terry Fritz <teslalist-at-qwest-dot-net>" <Peter.Lawrence-at-Sun.COM>

Robert,
        I came to my conclusion by thinking about the physics of the situation.

Imagine a one-layer capacitor with one thick sheet of dialectric, now think
of building the dialectric from two sheets half as thick. You get the same
resulting capacitance in the end. Now imagine placing an aluminum sheet
between the two half-thick dialectric sheets - result is same overall
capacitance, but you've got what can also be considered as two capacitors
in series. Now imagine that one of the half-sheets is replaced with oil,
again you've got two capacitors in series.

I think this logic is correct, so if you claim "NO: That is not correct."
you'll have to give a supporting argument based on physics.

You're answer "it is somewhere between the capacitance that would result
in all plastic verses all oil" is consistent with the mathematical
formula C = 1/(1/c1 + 1/c2), so I'm not sure how your observational
evidence contradicts what I and others are saying.

One thing I am not claiming is that if the space between the conductive
plates is filled with materials of two different dialectric constants
that are not essentially flat (plastic sheet and oil are both essentially
flat, but powered or granular material suspended in oil or glass fibers
in an epoxy or polyester matrix for example are not) then the simple formula
still holds, in fact it probably fails in this case. In this case the lines
of the electrostatic field are not straight and parallel, and the two
different dialectrics can no longer be (conceptually) separated into
two independent capacitors.

-Pete Lawrence.


 >
 >Original poster: "June Heidlebaugh by way of Terry Fritz 
<teslalist-at-qwest-dot-net>"
<rheidlebaugh-at-desertgate-dot-com>
 >
 >NO: That is not correct. I can not give you an answer that is definite. The
 >value is somewhere between the max capacitance of the dielectric of a given
 >spacing and the min capacitance of the other dielectric of the same distance
 >of separation.I have the same problem with PE sheets and oil capacitors. so
 >I say use the limits and measure the results. In my case the difference is
 >not very great. If I was using something like paper and oil the difference
 >would much greater, but the results would still have to be measured to know.
 >Robert  H
 >----- Original Message -----
 >From: Tesla list <tesla-at-pupman-dot-com>
 >To: <tesla-at-pupman-dot-com>
 >Sent: Tuesday, June 03, 2003 7:55 PM
 >Subject: capacitance formula
 >
 >
 > > Original poster: "Peter Lawrence by way of Terry Fritz
 ><teslalist-at-qwest-dot-net>" <Peter.Lawrence-at-Sun.COM>
 > >
 > >
 > > I've been wondering what the capacitance between two conductive plates
 > > separated by some distance that is filled with both a sheet of plastic
 > > and some oil, where the plastic and oil have different dialectric
 > > constants, and the thickness of each is different.
 > >
 > > I've come to the conclusion that it will be the same as if it were two
 > > capacitors in series, one purely the plastic, the other purely the oil,
 > > each with their own individual thicknesses. Then use the series
 >capacitance
 > > formula C = 1/ (1/c1 + 1/c2).
 > >
 > > Is this correct?
 > >
 > > -Pete Lawrence.
 > >
 > >
 >
 >