[Phys-L] Fw: non-polarized capacitor

[Jeffrey Schnick <JSchnick@Anselm.Edu>]

Excellent.  From what you write below, I see you simply didn't know what I meant by "the central conductor."  I'm all set now.

________________________________________
From: John Denker <jsd@av8n.com>
Sent: Friday, February 26, 2021 1:59 PM
To: Jeffrey Schnick
Subject: Re: [Phys-L] non-polarized capacitor

On 2/26/21 10:51 AM, Jeffrey Schnick wrote:
> > The field lines go from plate to plate.  This keeps Kirchhoff
> > happy.

> I agree with the first sentence, not the second.

That puts a sharp focus on things.

> Let's establish a gaussian surface that encloses the central
> conductor and nothing else.  It encloses one plate from each
> capacitor and the wire connecting those two plates.

IMHO that's not fair.  At this level of analysis, appropriate at
the introductory level, and several levels beyond that, the capacitor
is treated as a black box.  It's not fair to reach in and take
only half of what's inside the box.

There's no advantage to doing that. You don't learn anything.
Indeed it's a step in the wrong direction in terms of understanding
what's going on.

A huge amount of engineering revolves around the idea of black
boxes.  We hide all the complexity inside the box, and worry
only about the behavior exposed at the interface.  That includes
software engineering as well as electronic engineering many other
sub-fields as well.

Kirchhoff's laws are simple.
A capacitor, considered as a black box, is simple.

You have defined the central conductor to include two plates,
i.e. two half-capacitors.  You're allowed to do that, but I
don't recommend it.  In contrast, I focus attention on the
central /node/ which is entirely external to the capacitors.
The focus is on the behavior exposed at the terminals of the
capacitor. What's going on inside the black box is not what
we should be worrying about at this level of analysis.

Physics analyzes what happens inside the black box, including
*both* plates, and then packages it up inside a well-behaved
box and hands it off to engineering.

Including half a capacitor without the other half is IMHO
neither good physics nor good engineering.