From: John Woodgate
Subject: Re: Deriving H for magnetic cores
Date: Thu, 19 Dec 2002 07:52:00 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Thu, 19 Dec 2002 11:58:58 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Roy McCammon wrote (in <3E010057.FD7DACF3@mmm.com.DELETETHIS>) about
'Deriving H for magnetic cores', on Wed, 18 Dec 2002:
>John Woodgate wrote:
>> I like to use the simplest models that are adequate.
>excellent suggestion, hardily seconded
>> In this case,
>> consider a very long, thin, straight conductor. Ampere, B-S and Laplace
>> all show you that the H contours are circles centred on the conductor,
>> and in SI units H = I/2[pi]r.
>The reason that you know that the H contours
>(perhaps in this case we might say "lines of flux")
>are circles is because of the axial symmetry of the
>wire and currents.
Indeed, that is the big advantage of a simple model. Simple does not
mean unrealistic or impracticable
> If the wire were square in
>cross section, the lines of flux would not be circles
>close to the wire.
And the model would no longer be simple. But you could still use
Ampere/B-S/Laplace to calculate the H contours - it would just take a
>Your formula works close to the (round) wire because
>close to the wire, the universe looks axially symmetric.
>And that is good enough for a lot of qualitative
That is quite unfair and you know it. H = I/2[pi]r is not 'qualitative',
nor is the argument that you cannot break the closed contours of
constant H by deforming the conductor.
>The problem is that it leaves you thinking that that
>is all there is too it and if that was all there is
>to it, friend Chris wouldn't be in such a conundrum.
You snipped all the text where I explained how to go from this simple
model to more complex ones, eventually the toroid.
What are you trying to achieve?
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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