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Re: capacitance formula
Original poster: "Peter Lawrence by way of Terry Fritz <teslalist-at-qwest-dot-net>" <Peter.Lawrence-at-Sun.COM>
Robert,
OK lets work this example out graphically, in reverse order
1. start with two capacitors in series, each using a different dialectric
material and different spacing
|
|
---------
d1, s1
---------
|
|
---------
d2, s2
---------
|
|
total capacitance is 1/(1/c1 + 1/c2), the "standard series formula"
2. now move the capacitors closer together, eliminating the wire between them
|
|
---------
d1, s1
---------
d2, s2
---------
|
|
again, total cap is 1/(1/c1 + 1/c2), the standard formula
3. now remove the middle conductor:
|
|
---------
d1, s1
d2, s2
---------
|
|
still the capacitance must remain the same, and we have a single space
filled with two different amounts of two different dialectric materials,
and the original formula 1/(1/c1 + 1/c2) must still hold.
I needed this formula because I'm building a variable capacitor from scratch,
and the dialectric will be partly plastic sheet, and partly air or oil (you
cannot squeeze all the space out between plates or they will not slide and the
thing will not be "variable"!).
hope it makes sense this time, I'll not beat this dead horse again...
-Pete.
>Original poster: "robert & june heidlebaugh by way of Terry Fritz
<teslalist-at-qwest-dot-net>" <rheidlebaugh-at-desertgate-dot-com>
>
>PL: let me show the flaw , In par 1 you separate the two sheets with a plate
>of Al. Now to show the problem .Capacitance is directly proportional to the
>area and inverse to the spacing. YES you now have two capacitors, BUT
>spacing is now 1/2 and capacitance is double in each half to give C total
>the same. Ie . now 200 pf and 200pf gives Ct= 100 same, NOW change the
>dialectric constant of one series capacitor and you now have 200 pf in
>series with 210 pf and the total is not the same it is 102pf for Ct. Your
>illistration dose not take account of the fact that you changed the spacing.
>In a real capacitor with mixed dialectric the effect is parallel not series
>because the spacing is not changed only the total dialectric constant even
>if powdered mica is added to oil the effect is parallel not series.
>-- Robert H
>
>
> > From: "Tesla list" <tesla-at-pupman-dot-com>
> > Date: Wed, 04 Jun 2003 15:01:53 -0600
> > To: tesla-at-pupman-dot-com
> > Subject: Re: capacitance formula
> > Resent-From: tesla-at-pupman-dot-com
> > Resent-Date: Wed, 4 Jun 2003 15:13:11 -0600
> >
> > 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.
> >>>
> >>>
> >>
> >>
> >
> >
>
>