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Re: funny capacitor (EUREKA ?)



I wrote:

> Here is an easy counterexample: Consider a slowly-changing
>voltage applied to three capacitors in series. Starting from V=0
> everywhere, ramp up the voltage-source to 10 volts. ...


At 01:35 PM 3/18/01 -0500, Ludwik Kowalski wrote:

In my opinion this interesting, and worth discussing, problem
does not belong to electrostatics. "Slowly-changing" puts its
into the category of questions about "how the system evolves?"


This is getting pretty silly.

What is the point of having a capacitor if I can't run a current through it?

As I said before, the usual definition of "electrostatics" involves short
lengthscales lambda and long timescales tau such that lambda/tau is small
compared to the speed of light. The ratio lambda/tau does not have to be
zero, just small.

An electrical engineer will tell you that at DC, capacitors don't even
exist. Step 1 in analog circuit analysis is to check the DC operating
point, treating all capacitors as open circuits. (As a second step, you
can analyze the small-signal AC performance at this operating point.)

There is nothing in my counterexample that requires, or even suggests, a
departure from standard electrostatics.