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Homework Help: Dropping magnets through pipes

  1. Apr 4, 2005 #1
    My physics teacher explained to our class the traditional "dropping a magnet through a tube" experiment. As we all know, it slows down and takes longer than usual to fall through. However, he gave us an explanation that I am unsure about. I seem to remember reading about a different explanation and a different pattern of induced currents from another book, but I don't remember it that well. His explanation is as follow:

    Suppose we drop a magnet with the south pole facing downward:
    |---| <---position A
    | N |
    | S |
    |---| <---position B
    At position A, to create a field that prevents the magnet from falling, a field similar to a magnet with north pointing up, south pointing down is created. Hence, by right hand screw rule, the current is counterclockwise, looking from the top of the tube.
    At position B, for the same reason, a field with south pointing up and north pointing down is created. Hence, the current is clockwise looking from the top.

    I can't find anything wrong with the explanation, but it sounds somewhat suspicious that the current directions at two different positions would be in opposite directions. Not to mention that as the magnet moves downward, the clockwise and counterclockwise currents move downwards too.

    Can anyone confirm if this explanation is really valid please?
  2. jcsd
  3. Apr 4, 2005 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    It is. The direction of the induced current depends on the direction of the magnetic field and the direction of motion. The direction of motion does not change but the direction of the field does (ie. N pole changes to S pole) between the magnet entering the tube and the field as it is leaving.

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