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Magnet Moving inside Copper Tube and Current Direction(s?)

  1. Oct 9, 2012 #1
    Hi, this is my first post here.

    I know that, according to Lenz's law, if you drop a magnet inside a copper tube and let it fall, its movement will cause an induced current, which will slow it down in turn. However, my drawings make me believe that the direction of such current should be one below the magnet, and the other above it. For example, if the magnet is falling with its South pole down, it will induce current in a direction such that a South pole will appear right below it. However, its North pole is falling on the other side, which would also cause a South pole to appear right above it due to an induced current. But doesn't this mean that there are opposite current directions at each side of the tube? If so, how can this happen physically?

    If we think about many individual spirals, instead of a single tube, everything seems to work fine, with current flowing in different directions for different spirals; however, I can't visualize the same happening within a continuous material.

    Any thoughts on this?

  2. jcsd
  3. Oct 9, 2012 #2
    Hi Renato, welcome to PF.

    What’s so strange about having opposite currents at different places in the tube?

    Those 2 currents run round the circumference of the tube and are roughly separated by a distance approx. the length of the magnet. I say roughly because in this space they diminish, go to zero and then start building back up in the opposite direction. The magnet is travelling length ways and both currents are parallel but at right angles with direction of travel.
    The sum of the 2 combined currents is zero, hence when a magnet falls through a long coil you will only see a current generated when the magnet is entering or leaving this coil.
  4. Oct 9, 2012 #3

    Thanks, now I understand that one current decreases smoothly to zero, and then starts again in the opposite direction, as expected.

    But shouldn't these currents sum zero only when the magnet is in the middle of the coil? It seems to be the only moment when the rate of change of the magnetic flux is the same at both sides.

    Another doubt just came to my mind: would the situation be analogous if we were talking about a solenoid? Because, in this case, even though the same logic applies to finding out the direction of each current, I don't think it's possible that two opposing currents exist in a solenoid - they would "meet" rather than loop separately.

    Thank you for your help!
  5. Oct 9, 2012 #4


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    When you drop a magnet through a long solenoid you only get a current on entry and exit with nothing whilst the magnet's in the middle (the induced emfs are in opposite directions for N and S poles so they cancel - producing no current).
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