Re: "Charged" capacitor mis-terminology

[William Beaty <billb@eskimo.com>]

On Wed, 18 Feb 1998, Mark Shapiro wrote:

> 	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.

I know what you mean by the above, but to a naiive student, haven't you
just said that charge flows from capacitor to load?  But this does not
happen, since simultaneously the charge flows from load to capacitor.  So
what does a "filter capacitor" really do? It's not like a charge-storage
device, it's more like a pump attached to a wind-up spring motor.  Pumps
do not store water, since water flows through pumps. 

During one part of the AC cycle, the power supply causes charge to flow
through the filter capacitor and causes energy to be stored there.  A sort
of "back pressure" also arises as the "spring" of e-field inside the
capacitor "winds up."  During the other part of the AC cycle, direction of
charge flow is reversed and the capacitor acts as a pump, expending energy
in causing a closed-loop flow of charge in the load circuit.

The total charge inside the capacitor stays constant (zero), but the
capacitor's P.D. acts upon the external circuit and causes
charge-conserving loops of current to arise.  Energy goes into the
capacitor and is later delivered to the load, but since charge flows
THROUGH the capacitor and THROUGH the load, it is not right to say that
charge moves from capacitor to load. 

See what profound changes must be made to all our explanations of simple
circuitry once we adopt the "capacitors do not store charge" rule? 


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