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Force between two magnets

  1. Nov 20, 2004 #1
    What causes the force of attraction between two permanent magnets? I'm well aware that F = qv x B, but what charges are moving?
     
  2. jcsd
  3. Nov 20, 2004 #2

    Tide

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    Charges are moving at the atomic level. In most materials the atomic currents (i.e. from spin & angular momentum) are randomly oriented so the net force is zero. However, in some materials, the atomic currents can become organized (correlated) on a macroscopic level and allow for a net magnetic force on the material.
     
  4. Nov 20, 2004 #3
    So it is the valence electrons in the metal, and not the nuclei? If it just the electrons, why does this cause a force on the entire piece of metal and not just cause a current to flow ?
     
  5. Nov 20, 2004 #4

    Tide

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    The electrons are bound to the atoms.
     
  6. Nov 20, 2004 #5

    krab

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    Magnetism in permanent magnets results from the net magnetic field created by unpaired electron spins. These are not necessarily moving; some of the magnetism can come from orbital motion, but this is not the main effect. Electron spins are dipoles and there is a force law between these spins that is proportional to the magnetic moment of each and to the reciprocal of the cube of the separation. This force law is not F=qvxB.
     
  7. Nov 21, 2004 #6

    Tide

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    Actually, the strength of the magnetic field produced by the dipole varies inversely with the cube of distance and the potential energy of a dipole in a magnetic field is [itex]U = -\vec \mu \cdot \vec B[/itex] so the force exerted by one dipole on the other will vary as the gradient of U.

    BTW - if we think about the magnetic moment of an electron classically then it amounts to a rotating charge, i.e. it constitutes an electrical current. Ultimately, these notions and forces between dipoles derive from the basic Lorentz force.
     
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