Re: volume polarization vs. surface charge

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

On Thu, 4 Mar 1999, John S. Denker wrote:

> At 01:14 AM 3/5/99 -0500, Bob wrote:
> > Donald has made the crucial, clarifying point; let me just expand a little:
>
> This "crucial" point is not entirely correct.  There is a physics mistake,
> compounded by confusion over terminology and viewpoints.
> [supporting discussion follows]

I won't take a stand on this interesting discussion of terminology and
viewpoints. But it is irrelevant to the "sticking" point in the discussion
of the dissectable capacitor *demonstration*. It was "What role does the
dielectric constant of the insulator play in the dissectable capacitor
*demo*?

The surface charge due to polarization of a dielectric is, whether you
consider it "real" or "virtual", not capable of constituting the charge
which makes the spark when the capacitor discharges. It isn't
macroscopically mobile.

On another post, by David Wall... It's not surprising that a cardboard
insulating cup or bucket between the metal ones works quite well. I pulled
a table if dielectric constants from a handy textbook:

Dielectric Constants

Material        k

Vacuum          1.0000
Air             1.0006
Parafin         2.2
Rubber, hard    2.8
Vinyl (plastic) 2.8 - 4.5
Paper           3 - 7
Quartz          4.3
Glass           4 - 7
Porcelain       6 - 8
Mica            7
Ethyl alcohol   24
Water           80

In my view, the larger dielectric constant is desirable in this experiment
only because it increases the amount of mobile charge which goes to the
capacitor plates for a given 'charging' voltage. This is the amount of
charge which can be released to cause the spark. Hence, larger k, "hotter"
spark.

You see that paper has a value of k roughly equivalent to glass. We found,
after someone broke the glass cup of our "classic" (brass cup) demo
apparatus, that a cardboard or plastic cup worked as well, and niether had
to have close contact with the metal. As David pointed out, the corona
discharge between the metal and plastic is sufficient to insure transfer
of charge from metal to to the surface of the insulator.

Many older books, before the widespread availablity of plastics, suggest
wax paper between the plates of a homebuilt capacitor. Why not simply
paper which would seem to be better? I suspect because this kept the paper
from getting damp.

        -- Donald

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