Re: Why does electrostatic attraction in water decrease?

[Pentcho Valev <pvalev@BAS.BG>]

"John S. Denker" wrote:

> Pentcho Valev wrote:
> > Please see fig. 6-7 on p. 112 in Panofsky.
>
> I don't have easy access to that.
>
> >  A capacitor
> > with vertical plates is only partially immersed into a pull of liquid,
> > and the liquid inside the capacitor has risen high above the surface of
> > the pull. Now punch a hole in one of the capacitor plates, above the
> > surface of the pull but below the surface of the liquid inside the
> > capacitor. Will the liquid leak out through the hole? I would be very
> > surprised if it does not.
>
> I would be very surprised if it did.
>
>  -- We could run a water-wheel using the water that leaks out of
> the hole, and have a nice perpetual-motion machine.

Intuition confronts the second law. One punches a hole in the wall of an open
vessel filled with water and expects the water to come out, but the water does
not come out because the the second law says no.

>  -- To say the same thing in other words, there will be a force
> on the liquid due to the field-gradient in the hole.
>
> What means "surface of the pull"?  Pulling is not a substance,
> and it doesn't have a surface.

Sorry. Read "pool".

> > I am afraid the discussion goes directly to the second law of
> > thermodynamics
>
> I see no reason to think the 2nd law is involved.

Didn't you say "perpetual motion machine" above?

> In particular, repeat the experiment with liquid helium
> instead of water.  Cool it to a few millikelvin.  Dielectric
> effects will be essentially independent of temperature over
> a wide range.
>
> > a topic I hate because it always generates personal
> > attacks but never science.
>
> Never?
>
> I'm disappointed to learn that.  I always thought thermodynamics
> was a branch of science.

Mythology people are afraid of. "Perpetuum mobile of the second kind" implies
that those who attempt a verification of the second law are just as mad as
those who try to extract work out of nothing. In fact, the second law is not
even defined properly. Consider, for instance, the following Kelvin's
versions:

K1: No process is possible in which the sole result is absorption of heat from
a reservoir and its complete conversion into work.

K2. No processes is possible in which a system absorbs heat from a reservoir,
completely converts it into work and returns to its initial state.

K1 and K2 sound almost identical but in fact say entirely different things.
But I am still reluctant to discuss this - there will be no positive result in
the end and the original electrostatic problem will remain unresolved too.



> > Is
> > it justified to identify the dielectric constant that one places in the
> > Coulomb law, for two charges immersed in a liquid dielectric, with the
> > dielectric constant participating in the expression for the voltage of a
> > capacitor with a box filled with the same liquid dielectric between the
> > plates?
>
> I can't parse that sentence.  The word "with" occurs too many times.

Sorry. You have the dielectric constant (= 80) in the Coulomb law for two
charges in water, and you have the same constant (= 80) in the expression for
the voltage of a capacitor with water between the plates. In the former case,
according to Panofsky, the decrease in force is due to liquid pressure between
the charges, whereas in the latter the effect is due to polarization. So the
question "Why is the same quantity used?" is reasonable, although someone may
prove that using the same quantity is correct.

Pentcho