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I Narrow external magnetic field poking through a disk magnet

  1. Sep 9, 2016 #1
    Let's say I have a disk magnet that is a centimeter thick and 1 meter in diameter. Now let's apply an external field confined to a cylinder of arbitrary position, angle, and length. Let's say it has a diameter of << 1 meter, so very, very narrow. Let's presume that the external field lines are closed outside this cylinder with return paths that too far from the disk magnet to affect it.

    The questions I ask below pertain to the force on the disk magnet only:

    1) Is the force density by this external magnetic field on the disk magnet that of Lorentz force densities on "equivalent sheet currents" on the perimeter of the disk magnet?

    2) Or is the force density equivalent to that of what would be experienced by magnetic monopoles spread across each circular face of the disk?

    If the former (1) were the case, I would imagine situations where there was no external magnetic field interacting with the "equivalent sheet currents" at the perimeter of the disk magnet, and the result would be no force on the magnet despite the obvious possibility of dipole coupling energy that would be a function of the angle between the disk magnet and the applied field.

    If the latter (2) were the case, then assuming the applied field were straight, then there would be no net torque, although there would be equal and opposite forces. This would be despite the dipole coupling energy still being dependent on the angle of the magnet with respect to the applied field.

    Now let's consider the possibility that the applied field could be confined to a narrow cylinder that is bent in such a way that it "pokes" through one face at one angle and the other face at a different angle. In this case, it would be possible for the latter case (2) to yield a torque that would act on the magnet, a torque that does not appear to show up for the former case (1) under the condition that the applied magnetic field is completely confined to a spot far from the "equivalent current sheets" at the perimeter of the disk magnet.


    Kevin M.
  2. jcsd
  3. Sep 15, 2016 #2
    Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.
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