The attraction between two magnets in repulsion to a piece of metal

[alexqi]

Hi all,


I am trying to understand why the attraction between two magnets in repulsion to a piece of metal become stronger when the magnets are brought closer together than when they are further apart. I've run some femm simulations for both when the magnets are closer and further apart and attached the results. Does anyone understand why this is happening? The top(long) rectangle is the metal piece, and the bottom(shorter) rectangles are the magnets (they are in repulsion with their poles on the side or left to right).


Best regards,
Alex

 

[alexqi]

this is when the magnets are further apart.

 

[prex]

There are two effects:
- when the magnets are closer, in the region between the two, the magnetic field is forced to a shorter path because of the repulsion, also the part of this path in air is shorter, both phenomena tend to increase the field
- when the magnets are at the far end, there is no more metal extending beyond the magnets, so the magnetic flux is forced to follow a longer path in air.

prex
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[alexqi]

prex,

Thank you for your reply. I agree with you on both reasonings(if I understand them correctly). However, when I rotated the poles of the magnets 90 degrees(now the poles are on top and bottom) the opposite happens. The attraction is now stronger when the magnets are apart. Please see attached in next post...

 

[alexqi]

Links:

 

[MagBen]

the same reason: flux path is shortened when you close the gap. Your FEMM models clearly demenstrated this point: when the magnets are close, the flux of left magnet goes only from right to left through the top pole piece, while flux of right magnet goes only from left to right through the pole piece. Obviously the path length is increased.

 

[alexqi]

MagBen,

thank you. Now I am starting to see it.