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# Problem in force calculations

- To: femm@egroups.com
- Subject: Problem in force calculations
- From: "Dave Squires" <djsquires@plix.com>
- Date: Thu, 25 May 2000 22:23:10 -0000
- User-agent: eGroups-EW/0.82

    David,
    I have a problem with force calculations in a simple
    permanent magnet simulation. The setup is to have
    two permanent magnets set up in a opposed arrangement
    with + to + (N to N) and an intervening layer of
    soft magnetic material like supermalloy or pure iron.
    This material would be placed against one end of one
    magnet. The other magnet in repulsion is brought
    closer and closer to the iron. There should be a point
    where the force switches from positive to zero and then
    to negative. It will reverse direction from repulsion
    to attraction at some distance. I don't see this with
    FEMM no matter how close I get.
    Any idea why this would be the case?

    You can do this test on the bench with a couple of
    NdFeB magnets to prove it to yourself. Just use an
    equal thickness of magnets and iron or steel.
    So in this case FEMM does not seem to reflect reality.
    How come? Is there any way to fix it so it gets the
    right answer? Or am I doing something wrong?

    Thanks,
    Dave Squires

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