Re: "Charged" capacitor mis-terminology

[William Beaty <billb@eskimo.com>]

On Wed, 18 Feb 1998, Mark Shapiro wrote:

> 	Charge never flows through a capacitor (it can flow in the
> external circuit).....

A homer-simpsonesq  DOH!    ;)


But this bring up a good point.  I could say "there is a current through
the capacitor."   In the capacitor's dielectric this current is not a
charge flow, it is Maxwell's Displacement Current.

But if I think a bit more deeply, I see that Mark's above statement could
be an origin of the misconceptions regarding capacitors.  Because the
dielectric is an insulator, charge cannot flow THROUGH capacitors. 

If "displacement current" is allowed into our explanation, then we are
able to say that electric currents can only exist in loops.  When charge
flows in a single wire between two oppositely-charged isolated objects,
there must be a corresponding displacement current in the space
surrounding the objects, directed oppositely to the charge-flow in the
wire.

Water analogy:  in a water-filled chamber with a rubber membrane dividing
it in half, water can never flow between the two halves.  However, if I
force water into one side via a pipe, the membrane will deflect, and an
equal amount of water will exit from a pipe connected to the other side.
If we do not inspect the inside of this chamber, we can say that water
does flow THROUGH the chamber.  

If I do not inspect the inside of a capacitor, I can say that charge flows
THROUGH the capacitor.  For black-box explanations of capacitor behavior,
it is extremely valuable to realize that charge flows through (i.e.
electric current takes the form of a loop, equal charge goes into one
capacitor lead and out of the other.)  But when explaining the internal
details of capacitor operation, we must modify our concept to include an
idea which resembles displacement current in a dielectric. 

A bottom-up explanation of capacitors might miss this issue entirely, no? 
But a top-down analysis makes us confront the fact that a changing e-field
can mimic a charge-flow.  Capacitors are NOT electrostatic devices,
because no matter how small the rate of "charging," the displacement
current in the dielectric still takes the same value as the electron
current in the capacitor's connecting wires.

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