# Re: Variable Capacitance and dielectric quantity

Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>" <davep-at-quik-dot-com>

Hi Dave:
Tesla list wrote:

>>Hi Ed,

>>>>When considering a static capacitor, one with no frequency applied to the
>>>>charge, the closer the charged body is to the earth, the higher the
>>>>capacity.  This is obvious.  The further the charged body is from the
>>>>earth, the lower the capacitance will be.  However, Tesla was

>>>>measuring a dynamic charged body.

The defintion of capacitance makes no distinction between
'DC and AC ('dynamic').
Certain classes (electolytics, mostly) of caps only
WORK on DC, that is a detail.  And measurments at AC
are more complex, as lead inductance must be accounted
for.

>>>> The amount of charge a body will hold depends on the dielectric

>>>>between the charged bodies.

This is a unique, and new, definition of capacitance.
Consider a vacuum capcitor (one with vacuum for the
dielectric).
What Holds The Charge?

Suggested experiment (easily doable):
Construct a simple glass and foil cap (or even foil
and plastic wrap).  Use ONE layer of wrap.
Measure the capacitance.
Record.
Double up the warp, reassemble (*).
Measure the capacitance.
Record.
Compare.
Repeat for triple layers.

Alternately:
Look up the maths that derfine a capacitor.  See what
happens to the CAPACITANCE as the dielectric gets thicker.

Or do both.

Note:
(1) Spot the risk in my simple experiment:

It will take some skill to get the cap reassembled without
introducing 'stray' effects.  Glass is perhaps better than
plastic wrap.  Or electrodes in oil.  Or air (with spacers),
easier yet...
=============================================================
Meta:
Does this seem off in hyperspace?
I suggest it does help to think about the basics.
As simple a thing as a capacitor is (I built my first, a bit
under 50 years back (now you know)).  I have been learning
about them ever since.  I am STILL learning.

(it Does NOT help that SOME authors say that the charge
is 'in the dielectric'.  'tisn't.  Think:  The dielectric
is an insulator.  charge can neither enter nor leave.
Charge CAN BE REARRANGED.  Charge carriers in the dielectric
are TEMPORARILY SHIFTED.  The charge, the physcial eletrons,
(which is what charge is) is in the plates...)

best
dwp