Re: non-potential voltage

[Bob Sciamanda <trebor@VELOCITY.NET>]

----- Original Message -----
From: "John Denker" <jsd@MONMOUTH.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, April 19, 2000 7:24 AM
Subject: Re: non-potential voltage


> On Mon, 17 Apr 2000 14:36:39 -0400, I wrote:
>
> > > . . .
> > > Kirchhoff's laws are tantamount to making two simplifying
approximations:
> > >   a) A capacitor is a two-terminal black box, and there are no
significant
> > > capacitances outside of capacitors;
>
>
> At 10:18 PM 4/18/00 -0700, Bob Sciamanda wrote:
>
> > It is not necessary to place this burden on Kirchhof's loop law.  It is
> > valid even for non ideal capacitors and inductors;  eg, it even works for
> > an open circuit - a highly fringing capacitor situation.
>
> When I spoke of K's laws, I spoke of laws plural (the voltage law *and*
the
> current law) for a reason.  As I see it, there are two ways to describe
> Bob's open circuit:
>    1) We blithely ignore the current that flows into the "highly fringing
> capacitor situation" (a current that violates K's current law), or . . .

Stating K's current law in the familiar form: "In steady state, the net
current into any point is zero",  this is simply a statement of charge
conservation and applies even to volume current distributions.  It is
inapplicable only during the transient startup when (mostly) surface charge
distributions are being established to drive the steady state current.

>    2) We model the current that flows into this "stray capacitance" by
> including in our analysis a "stray capacitor" that doesn't show up in the
> fabrication parts list, and has a value that cannot be determined from the
> usual sort of circuit diagram.  (This makes K's laws formally correct, but
> alas the real physics guarantees that any correct version will be very
very
> hard to apply in the real world of fringing fields.) . . .

Such stray capacitances are included in analyses of real situations,
employing K's laws.  The medical device engineer, for example, considers a
grounded patient to be capacitively coupled to the "hot" AC power lines and
must consider the resulting current.  The mfgr of electrically powered
consumer devices also must consider the "leakage" current through such stray
capacitive paths, using K's laws to the letter.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
www.velocity.net/~trebor