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# Re: [femm] Ampere's law

- To: femm@yahoogroups.com
- Subject: Re: [femm] Ampere's law
- From: David Meeker <dmeeker@ieee.org>
- Date: Thu, 17 Apr 2003 17:19:30 -0400
- In-reply-to: <b7n3vp+lmda@eGroups.com>
- References: <b7n3vp+lmda@eGroups.com>
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`tano1938 wrote:`

>     Now that I've modeled my my first design, i.e. FRT_OLD.fem in files
>     database, I've been using the result to reinforce some of the basic
>     laws in magnetics. In FRT_OLD.fem, I drew a closed contour through
>     the magnet, steel, air gap and back to the magnet. Then, using the
>     H.t integral, I expected to get an answer near zero, but got a huge
>     number of ampere turns instead. I'm obviously doing something wrong.
>     Can someone help?

`
The answer that you got is correct. Remember the model of permanent magnets that FEMM uses--a non-magnetic material wrapped by a current sheet. (See http://groups.yahoo.com/group/femm/message/1865 ) When you draw a closed contour through magnet, steel, air gap, and back through the magnet, you are enclosing the current sheet "flowing" at the edge of the magnet. The since the density of current on the equivalent current sheet is equal to Hc, result for enclosed current which one would expect is Hc*(length of magnet). In your case, Hc=50963 A/m. The length of the magnet in the magnetized direction is 1.6" (0.04064 m). The product of these two quantities is 2071 A. One one particular loop (linking the points (0.2,2.2), (1.4,2.2), (1.4,-0.1), (0.2,-0.1) into a closed loop), the H.t integral result that I get is 2069 A.`

    Dave.
    --
    David Meeker
    email: dmeeker@ieee.org
    www: http://femm.berlios.de/dmeeker

- References:
  - Ampere's law
    - From: "tano1938" <rgarritano@comcast.net>

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