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Confusion about the nature of magnetic forces.

  1. Oct 1, 2009 #1
    Seeing as there is no net force on a magnetic dipole placed in a magnetic field ( only torque ) , and that magnetism in say a bar magnet is caused by nothing more than tiny atomic current loops ( magnetic dipoles) oriented in the same direction , i don't understand how a bar magnet would get attracted to say a refrigerator door, as this would require a net force as opposed to a net torque (Am i right in assuming that the bar magnet or refrigerator magnet magnetizes a certain area of the refrigerator door , and this area then sets up a magnetic field which influences the refrigerator magnet)
  2. jcsd
  3. Oct 1, 2009 #2


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    Staff: Mentor

    There is no net force on a magnetic dipole placed in a uniform magnetic field.
  4. Oct 1, 2009 #3
    The Stern Gerlach experiment with a beam of neutral silver atoms in a magnetic field (~1923) with a gradient won the Nobel Prize in physics, because there was a force on the atoms..

    [Edit] They won the Nobel Prize because there was a quantized force on the neutral atoms.
    Bob S
    Last edited: Oct 1, 2009
  5. Oct 1, 2009 #4
    The dipole potential energy U(r) in a magnetic field is -µB. If there is a space-dependence of B, i.e., B(r), then the -grad(U) is the force that makes work.
    Last edited: Oct 1, 2009
  6. Oct 8, 2009 #5
    No, magnetic field was only uniform along on plane. It was in fact placed in a way that magnetic field was in fact non uniform in certain direction. Otherwise gradient of field will be zero.
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