From: John Woodgate
Subject: Re: Problem with magnetic boundary conditions
Date: Sat, 14 Dec 2002 03:42:45 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Sat, 14 Dec 2002 06:54:36 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Chris Carlen
wrote (in ) about 'Problem with magnetic boundary conditions',
on Sat, 14 Dec 2002:
>It would seem then that the inclusion of a spatial inhomogeneity of
>permeability into a calculation of magnetic field complicates things
If the geometry is simple, an assumption of 'no fringing' (all field
lines cross the boundaries perpendicular to the faces) allows quite
simple calculations. You can often use the non-gyrated 'Magnetic Ohm's
Law' (not the one that I used in the analogy), considering total flux
PHI, rather than induction B. Define the reluctance S of a homogenous
section of flux path in a given medium as l/(mu_0a), l = length, mu_0 =
*absolute* permeability, a = area of cross-section. Then, for a
composite magnetic circuit,
PHI = (Total H)/(S_1 + S_2 +...),
corresponding to I = V/(R_1 + R_2 +...).
>I suppose that's why I hear of finite element methods
>being applied to map fields in real world engineering problems dealing
>with designing magnets with specific (or at least known) field shapes?
Yes, if the geometry is not simple, or if you want to map the field
accurately, finite element analysis is necessary.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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