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I Faraday disc: different configuration question

  1. Sep 19, 2018 #1
    I started thinking about this again because it never really left my mind.
    Now we know the three different scenarios for the Faraday homopolar disc,


    1) The disc rotates while magnet and brushes with the circuit they connect stay stationary in the laboratory reference frame = result , current generated in the circuit
    2) The disc rotates together with the magnet only the brushes with the circuit they connect stay stationary = result again is that current is generated in the circuit
    3) brushes together with the disc are stationary, only the magnet under the disc rotates with respect to the lab reference frame= result is that no current is generated,
    but here is the tricky part I haven't yet understood, many textbooks and sources say that even when both disc and brushes stay stationary there is still a E field in or should I say across the disc when only the magnet is rotated, but that means that if there is an E field there must be a work done on the charges in the disc or in other words there should be charge separation happening in the disc much like in the other examples where the disc is physically rotating.Yet when brushes are added no current is flowing.

    So here is my question , what would happen if the disc stayed stationary in the lab frame but the magnet would rotate together with the brushes and the circuit that connects the brushes, would then there be current generated in the rotating circuit the same way current is generated when the disc rotates either with or without the magnet?





    I ahve come to the conclusion that in the homopolar disc case it is not about flux change with time in an enclosed loop area like it is in induction but rather is is all about relative movement of conductors in a uniform magnetic field, the weird thing about this relative movement is that I can understand why current is generated when the disc moves with velocity X and the brushes with the circuit move at some other velocity or stay stationary but I cannot understand why there would be an E field when a magnet that has a uniform field is moved around it's axis in such a way that the field strength does not change, would that imply that the B field has some other property than merely strength vs distance?
    PS. I do realize that B field lines are just a man made visualization.
     
  2. jcsd
  3. Sep 24, 2018 #2
    Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.
     
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