Re: funny capacitor

["John S. Denker" <jsd@MONMOUTH.COM>]

At 02:18 PM 3/8/01 -0500, Ludwik Kowalski wrote:
> Telling students that mother nature has many
> solutions [(of the Q(V) or V(Q) problems] is not
> very helpful;

The Q(V) problem is well-posed and has a unique solution.

The V(Q) problem has a solution-set with many different elements.

> it can confuse beginners.

I don't see how it helps students to tell them "all apples are red".  Sure,
it simplifies the concept of apple, but they've already seen lots of Granny
Smiths in the store, and they will instantly know you're fibbing.

> Let us use one gauge
> and focus on essentials. It is true that one
> may sometimes take advantage of another choice
> but in my opinion this is not worth emphasizing.

Ahhh, it depends on what one means by "emphasizing".

Every voltmeter I've ever seen has two leads, not one.  You have to hook
the black lead to *something*;  then you can use the red lead to make
measurements.  You have to measure voltage relative to *something*.  It's
not worth discussing more than a little bit.  Start your stopwatch, and
repeat after me:
        You have to measure voltage relative to something.
Elapsed time:  3 seconds.


Having said that, if you want to choose V4 as your reference, that's
fine.  But please don't pretend there is anything obvious or compulsory
about it.

> So let us agree that "V4=0 at infinity", as we do in
> less demanding problems.

I'll go along with choosing "V4=0 on the chassis" and many variants
thereof, but I'm reeeeally not happy with V4=0 at "infinity".

If object 4 really is at infinity, then there's no way the students can
hook the black lead of the voltmeter to it.  And if they can't do that,
they can't measure any voltages.  Remember:
        You have to measure voltage relative to something.

Again, this doesn't require any time or any emphasis.  Just say "chassis"
instead of "infinity" and everything's fine.