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Re: Magnets and induction

At 02:09 PM 7/10/01 +0200, Savinainen Antti wrote:
When a magnet is brought nearer to another magnet there is a change in
flux (assuming non-uniform magnetic field). A change in flux induces an
emf and eddy currents which create a magnetic field of their own.

OK.

According to Lenz's law magnetic fields created by these eddy currents act
to counter-balance the change.

I would say "tend" to act in the "direction" of counter-balancing the change.

In this example it means that there is a repelling force between magnets.

Yes.

This effect seems to be insignificant with respect to magnetic force due
to magnetic domains in the magnets.

Whether or not it is significant depends on
-- how rapidly the magnets are brought together,
-- the electrical conductivity of the magnet material
-- the thickness of the magnet material.

I would guesstimate that the skin depth for _hard iron_ that is
_permanently_ magnetized is about one centimeter at 60 Hz. So if you take
magnets that are thick compared to 1 cm and put them next to each other and
wiggled them at 60 Hz, I would expect significant effects from induced
currents. With smaller magnets and/or slower wiggling, I would expect
smaller effects.

BTW note that the skin depth for _soft iron_ that is _not_ permanently
magnetized would be less, because magnetic permeability enters into the
skin-depth formula. Permeability is basically the slope of the B versus H
curve. This is large _while_ the iron is being magnetized, but once it is
magnetized incremental increases in H produce only small incremental
increases in B.

========================

In real-world applications such as motors, this effect would be quite
significant, except that designers take steps to combat it.
-- Obviously it's not a problem for iron-oxide or rare-earth-oxide
magnets, since they are electrically nonconducting.
-- For electrically-conducting magnetic materials, the trick is to use a
laminated structure. The lamellae are mutually insulated.