Re: [Phys-l] Kirchoff revisited

[John Denker <jsd@av8n.com>]

Rauber, Joel wrote:

> My thought is that the law has general applicability if I re-interpret
> it as the sum of electrical scalar potental differences around a loop.
>  
> I.e.  sum of DELTA Phi around a closed loop.

That's entirely kosher.  That's the "physics" way of expressing it.

> ( I note that in general this scalar potential is not sufficient to
> determine E, but still exists, as E is determined in general from both
> phi and time derivatives of the magnetic vector potential.)

That's quite an important note.  It becomes more and more important
as circuits get larger and/or frequencies get higher.

> However, (and this is a significant caveat),
>  
> While now a generally valid statement it is now of limited usefullness,
> because we note that this is not saying the 
>  
> Integral E DOT dl around the loop is zero for the general case and
> therefore can not be related to volt meter readings.

Relating it to voltmeter readings is easy.  The voltmeter forms part
of a loop:
                 ____________________
                |        |           |
                |  "B"   |     "A"   |
                |        |           |
               VM      source       load
                |        |           |
                |        |           |
                |________|___________|


It's true that applying the theorm to loop "A" doesn't tell you what
you need to know, but applying it to loop "B" does.

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How many times have I said DRAW THE CIRCUIT DIAGRAM?