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Repulsion force between two magnets

  1. May 17, 2005 #1
    If I used two magnet, each being able to lift a load of 100 lb, to repel each other, what will be the total repulsive force ?
  2. jcsd
  3. May 17, 2005 #2


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    On what basis...?IIRC,the empirical formula of Coulomb gives a ~1/(r^2 ) interaction force.


    EDIT:Why did you delete your post...? :bugeye:
  4. May 17, 2005 #3
    deleted post because I ignored all other fields lines

    Physics is not my speciality but I believe that this may be an answer:

    How far away are they?

    Lets presume it is x.

    The forces will interact at every point of x. If we take an arbitary point p, it will be y from one of the magnets, and 1-y from the other.

    As we move the point y from one magnet to the other, we generate an inifinite number of points of repulsion.

    As repulsion decreases at [tex]\frac{1}{r^2}[/tex], the sum is always,

    [tex]\frac{m}{y^2} + \frac{m}{(1-y)^2}[/tex]

    (where [tex]m[/tex] is the magnetic force @ 0 distance)

    y equals 0-1 as it moves from one magnet to the other, so the solution is the sum using the values of y 1 to 0 as limits.

    I think. some physicist will tell you there is a better way but I reckon by using common sense that is pretty close! :biggrin:

    And there you go - a physicist did!


    Last edited: May 17, 2005
  5. May 17, 2005 #4


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    Can it lift 100 lbs. of paper? Might sound stupid, but the point is that the amount you lift depends on the material you are lifting. So the two (what you can lift, and force of repulsion between magnets) are unrelated.
  6. May 18, 2005 #5


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    It also depends on temperature. I saw a demonstration where a material was dipped in liquid nitrogen then placed over a magnet. The supercooled material floated over the magnet for maybe a minute until it heated up, then it dropped as if somebody cut a string holding it up.
  7. May 18, 2005 #6
    Isn't it surprising that a question that should have a simple answer, doesnt?

    I like E&M, but the way physicists understand magnetism is ovbiously inadequate for even the simplest situations involving permanent magnets.

    I wish there was a simple theory of magnetism which could be applied to the real world, instead of magnetic fields and V x B forces (which I love, but are useless in all but the simplest geometries). Yes, it is possible to answer these questions using standard E&M, but it strikes me as a square peg - round hole type of problem.
  8. May 19, 2005 #7

    Meir Achuz

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    If each magnet can pick up 100 pounds of a high mu metal, then the repulsive force if you try to push them together is 100 pounds. The high mu metal becomes magnetized so that it has the magnetization M of a magnet of the same strength as the one picking it up.
  9. May 19, 2005 #8


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    You need to be a bit careful here because what you saw might possibly be a superconductor (probably YBCO) and a magnet. This is a different concept that what is being asked in this question, I believe.

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