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Subject: Re: output impedance
X-Newsreader: Microsoft Outlook Express 5.50.4920.2300
Date: Fri, 08 Nov 2002 17:37:47 GMT
NNTP-Posting-Date: Fri, 08 Nov 2002 12:37:47 EST
Organization: Cox Communications
"Reg Edwards" wrote in message
> > The impedance is in parallel with the current source, not in series.
> > the Thevenin-Norton theorem.
> I have had 60 or more years of understanding electric circuits and have
> NEVER NEVER needed to refer to a "theorem". I have never even heard or
> long forgotten most of them or what they are about. Nothing beyond Ohm's
> Law has ever been of any use.
> Anybody who drags up a theorem in support of some explanation or other
> understands very little of what he/she is waffling about and is merely
> plagiarising and parotting an old wives' tale. To mention a theorem is an
> admission of ignorance. Or an admission that he/she has a memory and has
> read something somewhere. Or who had a teacher who was none the wiser
> Painting by numbers? The answer to your question is Theorem 19. Or is it
> 33.5 ? End of answer.
> OK? Next question please.
Yikes! Reg, I beg to differ!
I've much less than 60 years experience, but I make use of a variety of
theorems on a regular basis. I remember the names of a few of them, and
would have to look up the names of others, but using the names of theorems
isn't intended to take the place of understanding. Using names can be a
convenient shorthand - it states the underlying mathematics without having
to restate the proof.
I actually used the reciprocity theorem in a design review a while back,
when we were discussing the properties of an RC circuit. I might be able to
derive the proof of the reciprocity theorem, but I suspect it would take me
quite a while. On the other hand, knowing the name of the theorem _and_ how
it applied meant that I didn't need to provide a proof - at least not at the
design review. It also meant that I could find a proof quickly, by looking
it up in one of my circuits texts.
I suppose I could have tried to explain conversions between equivalent
voltage and current sources, but it's something most textbooks explain
reasonably well, and I'm guessing that the original poster is a student, and
that he has a circuits textbook. I don't know how else it might be indexed -
in all my books it's called either Thevenin's theorem or the Thevenin-Norton
theorem. Even if he doesn't have a textbook, he probably has web access, and
could Google for Thevenin-Norton. I know seems a bit like that line every
student dreads reading: "It can be shown that...," which always seems to
precede some remarkably complicated mathematical result, but that's really
not the intention.
-- Mike --
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