Try this link. I think it is a good explaination though it does not
include the value of the permittivity of free space, 8.85 x 10-12
farad per meter (F/m).
solidstate/dielect.htm
Yes, the dielectric can increase the charge stored in a capacitor.
Actually, the dielectric always increases the charge stored in a
capacitor.
--- In Electronics_101@y..., Doug Hale <doughale@x> wrote:
Mark Kinsler wrote:
Vacuum has an inherent dielectric constant, known as the
permittivity of a
vacuum (or of 'free space'.) It also has an inherent inductance,
known as
the permeability of vacuum. These are values you can look up.
They are
both very close to what we find in air at atmospheric pressure.
A vacuum capacitor does indeed store charge in its vacuum, odd as
that may
seem. They typically have very small capacitance values, just as
we find in
air-dielectric capacitors. The air doesn't store the charge in an
air-dielectric capacitor, of course: you can blow the air out from
between
the plates and the charge will remain.
The air dielectric capacitor is the finest example of why the
charge is
in the plate, not the dielectric -
and how do the plates know that the dielectric is a vacuum and
therefor
"the vacuum stores the charge" or that the dielectric is air and
since
the air can be blown away, the plate needs to store the charge?
It is basic material physics - plates store charge - dielectrics
insulate the charges.
This leads to another way to explain things. If a capacitor is
charged,
one plate has a negative charge and the other plate has a positive
charge. If the dielectric stored the charge, how does the
dielectric
keep the negative charges away from the positive charges?
That IS the purpose of the dielectric - to insulate the charges
opposing
charges on the plates from one another.
Doug Hale
There are lots easier ways to determine the electrical qualities
of air at
low pressures than the one suggested. While a balloon experiment
would be
fun, I've done the same thing with a small vacuum pump, my trusty
ignition-coil high-voltage power supply and a suitably-rigged
jelly jar.
What you're looking for is something called the Paschen curve for
air. One
axis of the Paschen curve plot is the voltage and the other is the
pressure--though I think that there's a provision for the
electrode gap in
there somewhere (memory fails at times.) It turns out that this
curve is
fairly linear near atmospheric pressure. The voltage necessary
for arc
initiation is lowest at a partial vacuum. It rises for very high
vacuums
and for very high pressures.
M Kinsler
512 E Mulberry St. Lancaster, Ohio USA 740 687 6368
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