Re: "Charged" capacitor mis-terminology

["Donald E. Simanek" <dsimanek@eagle.lhup.edu>]

On Wed, 18 Feb 1998, Mark Shapiro wrote:

> 	In many electronic applications you do indeed depend on the fact
> that the plates of a capacitor store EXCESS charge as well as energy.  For
> example, the resistor divider chain setting the voltage on the dynodes of
> a photomultiplier tube is bypassed with capacitors.  They are put there to
> make sure that there is a source of charge it the dynodes "ask" for more
> charge than the power supply can provide during the short period of time
> that it take a pulse of charge to transit the tube (a few tens of
> nanoseconds).

This may be the origin of the "store charge" interpretation. When one side
of a capacitor, or bank of capacitors, is held at ground potential, and
ground is considered an infinite charge source/sink, then the grounded
plate of the capacitor isn't thought of as having a charge. It is ignored
in one's thinking.  The excess charge on the other plate is thought of as
"the charge on the capacitor". Limiting one's thinking to these cases
allows you to ignore the details of the more general uses of capacitors. 

> 	Likewise electrolytic capacitors are used as "filters" in power
> supply circuits.  The "filter" capacitors don't "filter" anything.  They
> supply charge to the load during part of the a.c. cycle.

Here, I think, the word "filter" is used in the same sense as you filter
particulate matter from a liquid, or use a colored filter to selectively
absorb (remove from the beam) certain colors of light from white light. In
the electrical filter, one isn't filtering out a tangible "something" like
particles. In the power supply filter it is filtering out high frequencies
and allowing low frequencies to pass. Ideally it filters out all AC,
leaving only the DC level.
> 	Indeed, in some circuits capacitors do store a NET amount of
> charge.  This is done to shift the d.c. level from one side of the
> capacitor to the other.....

Again, this is the "one side is grounded, therefore ignore it" model.

Suppose, however, you consider a number of capacitors in series, as in a
Cockroft-Walton high voltage machine. Or consider a voltage multiplier
circuit. Indeed a student who has only thought of the "one side grounded"
model will have a bit of trouble thinking through the charging process of
a typical voltage double or tripler circuit consisting of capacitors and
diodes. Here's where an ill-formed concept can turn on you.

	-- Donald

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Dr. Donald E. Simanek                            Office: 717-893-2079
Professor of Physics                                FAX: 717-893-2048
Lock Haven University, Lock Haven, PA. 17745
dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek
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