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B Magnets' Repelling Force -- How to calculate it

  1. May 30, 2017 #1
    If I have 2 dissimilar magnets and push their North (or South) poles towards each other, how do I calculate how much force they will repel with. Again, I'm asking for calculations since I'd like to know before I buy magnets. I'm guessing they are either additive or they only push with a force equal to the repelling force of the smaller/weaker of the two magnets, but I'm not sure and cannot find this answer anywhere.
  2. jcsd
  3. May 30, 2017 #2


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    There's a calculator here: http://www.kjmagnetics.com/calculator.repel.asp?calcType=disc
    but it only works for two of the same magnet. For two dissimilar magnets, the result will be somewhere between the repelling force of two weaker magnets and two stronger magnets. I guess the repelling strength is approximately multiplicative, so if you have magnets A and B, you could write the repulsion between A and B using
    R(AA) = r(A)r(A) "repulsion between A and A"
    R(BB) = r(B)r(B) "repulsion between B and B"
    R(AB) = r(A)r(B) = sqrt(R(AA))sqrt(R(BB))

    This probably isn't exactly right, but definitely better than assuming they are additive.
  4. May 30, 2017 #3


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    That's an attractive intuition, but I do not believe that it can be correct. If you have a very powerful magnet adjacent to a very weak magnet such that a repulsive force would be expected, an attractive force can exist instead.

    This is similar to the way in which two objects both with a net positive static electrical charge can attract. One powerfully charged object induces a dipole in a nearby weakly charged object. The powerfully charged object then attracts the dipole because the negatively charged end is closer.
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