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Homework Help: Magnetic moments rules of thumbs

  1. May 11, 2008 #1
    not totally sure this should be in the advanced physics section.

    its a general test your knowledge question of everything youve learnt, so naturally i cant remember anything.....

    a magnet of magnetic moment U is droped such that it falls down metal tube of length L. the magnet's magnetic moment points down. indicate the sense of the induced current.

    obviously you can use maxwells equations if you want, but i just wanna check whether my rough and ready line of reasoning works as well. (as in, using what i could remember at the time, this is the way i solved it, becuase i havent quite got maxwell stuck in my head yet)

    first i quoted lenz' rule, any induced current is induced such that the resultant magnetic field opposes the change. (or words to that effect)

    by this reasoning i figured that if the magnetic moment is pointing down ( North to south? i.e. north pole points down???) then any current induced would have a magnetic moment that points in the opposite direction, since this coincides with the magnetic field, then field points up the tube. thus using the Right hand grip rule, thumb pointing up i worked out the current.

    this gives me the right answer, but i was wondering if its a physically valid line of reasoning. i mean i am familiar with the idea of a magnetic moment, and how the dot product of a field with a magnetic moment gives the interaction energy, but as to the forces and how magnetic moments directly interact with one another i am abit shady.
  2. jcsd
  3. May 12, 2008 #2
    I think your conclusions were correct. Maybe it would be easier to understand what is happening if you imagined the magnetic dipole as a small current loop, which causes magnetic field determined by Biot-Savart law. That field then causes induced voltage determined by Faraday's law which causes induced current that runs in the opposite direction of induced voltage.
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