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Re: dielectric constant of water



At 09:41 PM 8/22/2003 -0400, you wrote:
>///what would you expect to read in your setup with a strong
> salt solution in the bag? or a thick metal plate in the bag?
>
>Brian Whatcott Altus OK


With a thick metal plate in the bag, I would expect the capacitance
to increase. I see that you're suggesting that salty water would act
like a thick metal plate producing, in effect, two capacitors in
series whose dielectric is the thin plastic bag. Being thin, the two
caps have a very small plate separation which produces a large
capacitance, but having the two in series cuts that large capacitance
in two. Is that right? -- Wolfgang
--
Wolfgang Rueckner


That was what I had in mind. I wrote up on this list an
informal test of water's k using kitchen foil and kitchen
wipe as a separator, and flooding the wipe with distilled
water.
But that was some years ago.
I tried rigging it again this evening, but this time used
tap water, and a different meter and found the resistance
soon became too lowfor reliable capacitance readings.
So I shall need to wait for
a fresh supply of distilled water to try again.


Brian Whatcott Altus OK Eureka!

O.K. - I follow. But it seems to me that an important difference
between conducting water and a thick metal plate is that the metal
plate is an equipotential surface and the water is not. In analyzing
a thick metal plate in the gap of (but not touching) two capacitor
plates and saying it's equivalent to two caps in series relies on the
fact that the metal plate is an equipotential surface. As I see it,
the thick metal plate collapses, in effect, the potential gradient
that was in the space it now occupies and thereby reduces
(potential-wise) the separation between the two capacitor plates and
therefore the overall capacitance goes up. But that argument doesn't
hold for conducting water. Or is it that the actual result of having
water inside a plastic bag inside the capacitor plates is a
complicated combination of having both a conductor and dielectric
inside the cap? -- Wolfgang