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Faradays rotating disk paradox

  1. May 18, 2009 #1
    I was reading about Faradays rotating disk paradox wherein when both the copper disk and magnet are rotated together an eddy current is induced along the radius of the copper disk. My question is if a maganitized steel disk (with one side being the N pole and the other side being the S pole) is rotated on its axis would a current be induced along its radius just as it is induced in the copper disk mentioned above?
  2. jcsd
  3. May 18, 2009 #2
    This an interesting and important question in the generation of potentials and currents in moving conducting objects in magnetic fields. First, if a conducting disk freely rotates about an axis parallel to a uniform magnetic field, there is a radial electric potential (but no current) induced in the conducting disk. The disk rotates without eddy current generation, and no eddy current losses. But if an external stationary electric circuit is completed between the axle and outer rim of the rotating disk, then there is a current. Homopolar generators are based on this effect. See
    This not specifically an "eddy current" in a spacially varying magnetic field, but the radial Lorentz force F = q(v x B) produced by an azimuthally moving conductor in an axial magnetic field.
    In the situation proposed in the OP, the disk is both magnetized and rotating. There is a current, as long as the rim contact is stationary.
  4. May 18, 2009 #3
    This is a confusing illusion. The field lines of the magnet, which curl around in space to the back end of the magnet, are rotating with the magnet and inducing current in the wire that's being used to contact the rotating disk. There is no current actually being induced in the rotating disk, just in the stationary wire.
  5. May 19, 2009 #4
    Thanks for the replies.


    So I presume this current could be measured with a galvanometer. Do you know if this has been experimentally verified?

    But there is also the opnion that the field is independent of the magnet and remains stationary. I guess thats why it remains a paradox. Considering that the field has no mass or matter how exactly would you define the "rotation" of a uniform field ie what is it that is actually rotating?
  6. May 19, 2009 #5
    The illusion the field remains stationary comes from the assumption the current is induced in the disk. Once you realize the current is induced in the wire the paradox disappears.
    How is the same problem solved for "translation" of the field when a magnet is moved in and out of a coil?
  7. May 19, 2009 #6
    The misconception here I think is that the voltage and current (when there is a rim contact) is based on the Faraday induction principle. It is not. This is actually based on the Lorentz force principle, which does not require a time derivative (e.g., dB/dt). It requires a velocity perpendicular to the B field. A suble clue is that the polarity of the voltage (and direction of the current) does not change when the direction of rotation of the disk is reversed. See
  8. May 19, 2009 #7
    That wikipedia article references the DePalma N-Machine:


    The N-Machine is a "free energy" device, i.e. crackpot technology:


    That wiki article is completely untrustworthy.
  9. May 19, 2009 #8
    It is unfortunate that one reference in the Wiki article to the DePalma machine tainted the entire article. Here is another old paper on the Canberra accelerator project, including the homopolar generator:
    http://physics.anu.edu.au/History/fire_in_the_belly/Fire_in_the_Belly03.pdf [Broken]
    which shows several photos of the Canberra homopolar generator with a man standing beside it (see page 30 of article (page 10 of pdf)). This unit generated over 2 million amps for pulses up to 10 seconds. Read article for details.
    Last edited by a moderator: May 4, 2017
  10. May 20, 2009 #9
    How would you then explain the case where no current is generated in the disk when the magnet is rotated?

    Obviously any physical displacement of the magnet will cause the "magnetic effect" or field to move along with it. How ever that does not imply that the field rotates when the magnet is rotated about its polar axis.
  11. May 20, 2009 #10
    I assume you mean stationary disc, rotating magnet. The field lines emerge from the pole of the magnet, go through the disc, curl around, and come back and cut the wire going in the opposite direction. The direction of the current generated in the wire opposes the direction of the current generated in the disc and they cancel each other out.
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