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From: email@example.com (Roy McCammon)
Subject: Re: Deriving H for magnetic cores
Date: Wed, 18 Dec 2002 08:48:24 -0600
References: <3DFF8FA6.4593A00A@mmm.com.DELETETHIS> <3DFFE9DB.1BEFBBE3@earthlink.net>
X-Mailer: Mozilla 4.5 [en]C-CCK-MCD 3M/NCP 4.5 (WinNT; I)
Robert Baer wrote:
> Roy McCammon wrote:
> -------- SNIPped for brevity --------
> > The symmetry is ruined. The contours of constant H are
> > difficult
> > to determine or don't exist.
> ** Sorry; contours of constant H *always* exist. Just because one finds
> it difficult or next to impossible to calculate is no excuse to deny
> what happens in nature
Perhaps you missed that I conjoined "difficult" with
"don't exist" by using "or". I was afraid that
leaving the "or don't exist" might imply that
they do exist and I was not sure. I was attempting
to avoid both affirmation and denial of something
I was unsure of.
Anyway the truth depends on exactly what is meant by
"contour of constant H"
I'm not even sure what a "contour of constant H"
means in a general context. In the specific
context we were speaking of, H points different
directions at different points along the contour,
but we say it is constant in the sense that
Hz = constant = 0
Hr = constant = 0
Hphi = constant = non zero
but that only applies in the special case.
So what is the equivalent definition in a general case?
Here are some candidates.
1. A contour such that H (a vector) = constant.
Well that doesn't work even in the special case.
2. A contour such that | H | = constant. You probably
mean it in this sense. That seems plausible, (requiring
only continuity?), but that sense doesn't pertain to the
discussion because in the special case we needed the
fact that H . dL = constant on the contour of interest.
3. A contour such that H . dL = constant
OK, #3 is what I mean. I should have expressed the
idea more clearly. That type of contour exists in
the special case, but it is not obvious to me that it
exists in all cases. The special case is easy to
work because H . dL = constant. And that is why
I consider the special case to be not representative.
In fact, I consider it pathological.
Unfortunately, this is often the only case presented
in text books, and it leaves you with a feeling
that you know all there is to know about it, but
as soon as you start applying it to almost any
practical case, suddenly things don't work and you
think that somehow you misunderstood. Well, you
didn't misunderstand. The necessary information
simply was not presented.
Thank you for reading and or replying
If you are one in a million, there are 6000 people just like
Local optimization almost never yields global optimization.
Opinions expressed here are my own and may not represent those of my employer.
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