Re: Symmetry of force between two magnets


Don Kelly wrote:
----------------------------
<Gbeat24@xxxxxxx> wrote in message
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Don Kelly wrote:
----------------------------
<Gbeat24@xxxxxxx> wrote in message
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At the website

http://www.magnetsales.com/index.htm

there is a calculator to give the force between two rectangular
magnets.
They caclulate both the repulsive force (magnets N-N) and attractive
(magnets N-S). However the attractive force calculates at about twice
the value of the repulsive force.


That is a very clear explanation and I see from both these answers what
the potential problems are. Thanks to all who answered.
Goerge.
Why the difference?

Thanks
George
--------------
Look at a flux plot for the two cases. In the N-N situation, the field in
the region between the magnets is weakened and the field fringes heavily.
A
small change in the distance between the magnets results in less of a
change
in the total field energy than in the case of S-N magnets where the
field
is stronger and more concentrated. The force is related to the rate of
change in field energy with distance.

Mike D has put it succinctly in laymen's terms.
--

Don Kelly dhky@xxxxxxxxxxxx
remove the X to answer


Thanks, I can see what you mean from the flux plots. However .... I
programmed a two magnet model into FEMM and although this showed a
difference between attraction and replulsion it was a much smaller
difference than the web calculator. I wonder which to believe?

The classical formula for magnetic force also shows no asymmetry.
Should this formula be modified?

George.
The classical case ignores fringing or assumes a gap cross section which is
a bit larger than the pole faces (l+g)*(w*g) where l,w, g are length, width
of pole face and g is the gap. The conventional expression is based on an
assumed constant gap flux density. In the case of N-S, this is reasonable
but in the case of N-N there is a region where the flux is perpendicular to
the gap and this is effectively ignored. The flux density is also not the
same in the two cases- hence differences are not taken into account.
Your Femm analysis accounts for this and considers the total stored energy
in calculating force.
--

Don Kelly dhky@xxxxxxxxxxxx
remove the X to answer



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