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I Feynman's disk paradox

  1. Dec 8, 2016 #1
    There is a solenoid on top of a disk with charges attached to it. and there is a clockwise current through the solenoid.


    So by disconnecting the battery, the current will lower, and hence the flux of the magnetic field though the disk will lower, and according to faraday's law, there will be an induced emf in space.

    So this is an explanation that the angular momentum of the field is transferred to the disk. It makes sense. I can imagine the e field swirling around the disk, causing it to rotate counterclockwise. (the induced current must be counterclockwise to oppose to decrease in flux)

    But where was the angular momentum before? Was it from the decrease of clockwise swirl of the E field inside wire of the solenoid? When the current was being decreased, wouldn't there be a counterclockwise swirl of the E field to reduce the current?

    This seems very analogous to the classic rotating disk falling on another stationary disk problem. Except here it isn't internal friction that mediates the transfer of angular momentum. So it seems like the counterclockwise E field's swirl inside the wire during the I reduction was transmitted though space.
    Last edited: Dec 8, 2016
  2. jcsd
  3. Dec 8, 2016 #2


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    Staff: Mentor

    Have you tried googling around for the solution to this problem? Feynman has even supplied an answer himself.
  4. Dec 9, 2016 #3


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    https://www.researchgate.net/profile/Gabriel_Lombardi/publication/243489921_Feynman%27s_disk_paradox/links/5646523408ae9f9c13e7503e.pdf [Broken]
    Last edited by a moderator: May 8, 2017
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