Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Magnetic field rotation or not?

  1. Mar 27, 2015 #1
    Suppose for a moment you had a circular copper disc placed concentrically in the air gap between the poles of an upper and a lower circular magnet, the outer faces of the upper lower magnet being connected by a pole piece so as to complete the magnetic circuit. The edge of the copper disc is connected to a first wire and the centre of the copper disc is connected to a second wire that passes through a hole in the centre of the upper magnet and the pole piece so both wires are unaffected by any airgap flux.

    Basically a Faraday homopolar motor/generator setup. Nothing new so far.

    If now the magnets and pole piece are held stationary and the disc is rotated about the axis of the assembly we will get a small emf induced across the radius of the disc because there is relative motion between the flux and the disc.

    Conversely, if the disc is held stationary and the magnets and pole piece are rotated around the axis of the assembly, people more knowledgeable than I say that there will be no emf induced across the radius of the disc because the magnetic flux does not rotate axially with the magnet assembly so there is no relative motion between the flux and the disc.

    Let's now switch from generator mode to motor mode and where the disc is held firm and the entire magnet assembly is free to rotate (within the limits of the pole piece going from the outer faces of the upper to lower magnet colliding with the wire connected to the disc edge).

    If we now pass a current through the radius of the disc, the unmoveable disc will exhibit a torque in a certain direction and the magnet assembly may or may not exhibit a torque in the opposite direction. If the magnet assembly does exhibit a torque it will rotate until the pole piece hits the wire.

    Here is where things get interesting. If the magnet assembly does in fact exhibit a torque and rotate a certain distance then it is capable of doing work. However, established thinking says that the rotating magnet assembly will NOT induce a counter emf across the disc radius because magnetic flux does not rotate axially with an axially rotating magnet as it's source. This means that (ignoring dc resistance) there can be a supply current through the disc radius but no voltage across the disc radius because there is no counter emf generated by the rotating magnet. Watts input = ? amps x zero volts so there would be zero watts input to the system despite torque and motion = some watts output!

    The alternative to this of course is that the disc would exhibit a torque in one direction but the magnet assembly would not exhibit a counter-torque. Just what IS going on here?
     
  2. jcsd
  3. Mar 27, 2015 #2
    Magnetic fields have a rotational effect on charged particles. If the particles with a certain velocity encounter the B-field perpendicularly then the particles trace out a circular orbit, otherwise helical path.
     
  4. Mar 28, 2015 #3
    Can someone explain why my alternative conclusions would be wrong? There only seems to be two alternatives and both of them are awkward.
     
  5. Mar 28, 2015 #4
    The magnetic flux rotates with the magnet. Faraday did a series of experiments that demonstrate it does. These are in his Experimental Researches in Electricity, Series XXVIII, article 3088 and beyond. When a conducting circuit is properly oriented with respect to a cylindrical magnet (which requires a clever, unconventional set up), and that magnet is then rotated about the axis of magnetization, a current is generated. Likewise, when the same circuit is fixed to the magnet and made to rotate along with it, no current is generated. In the former case the conductor is stationary while the magnet is rotated, in the latter case there is no relative motion between conductor and magnet. In other words, the results are exactly what you'd expect.

    Now, however, and this is the surreal part, Faraday, himself, is the person responsible for creating and publishing the idea we shouldn't consider the magnetic field to be rotating along with the magnet. For reasons that aren't completely clear to me, he already believed the field doesn't rotate with the magnet, and so, when he explained the above demonstrations, he did so in such a way that his notion was preserved instead of being overturned as it should have been.

    That's hard to account for, but there is a possible clue in his wording:

    "The system of power about the magnet must not be considered as necessarily revolving with the magnet, any more than the rays of light which emanate from the sun are supposed to revolve with the sun. The magnet may even, in certain cases (3097), be considered as revolving amongst its own forces, and producing a full electric effect, sensible at the galvanometer."
    -Series XXVIII, 3090

    We know that Faraday harbored the then unprovable suspicion that light was a form of electromagnetic energy. He would, quite likely then, consider a static magnetic field to be a form of light, and would logically suppose the magnetic field around a magnet must be detached from the magnet just like light is detached from the source of light. When current was generated in the above demonstrations, he was prejudiced against seeing it as the result of the rotating magnetic field cutting the stationary conductor, and, instead, conceived of the magnet having a current induced in it by "revolving amongst its own forces."

    The set up you describe is different, of course, but the explanation for lack of current is not that the field isn't rotating.
     
  6. Mar 28, 2015 #5

    Drakkith

    User Avatar

    Staff: Mentor

    Got a link? I'm having a hard time finding this.
     
  7. Mar 28, 2015 #6
  8. Mar 28, 2015 #7
    Wow!!
    Back in the late 90's I battled with titans on the old sci.physics.electromagnetics usenet group, having come to this exact conclusion through my own experiments but I was shouted down by all and sundry, some who had Phd's in subjects I didn't know existed and others who were just plain kooks. They couldn't give an explanation at all, some invoking relativity and whatnot. My hope at the time was to try and make a brushless (true) dc motor and eventually a dc motor-generator that would have the same function as a transformer but for dc. I didn't achieve this but I learnt a lot along the way.
     
  9. Mar 28, 2015 #8
    I remember finding this interesting when I found it not long after it was published. The author copped a lot of flak from the scientific community at the time IIRC. -> http://www.google.com.au/url?sa=t&r..._DskFw0q6NJ7eVQ&bvm=bv.89381419,d.dGY&cad=rja

    Unipolar Experiments
    A. G. K ELLY
    HDS Energy, Celbridge, Co. Kildare, Ireland
    agkelly@eircom.net
    ABSTRACT Novel experiments on the relative motion of conductors and
    magnets are described. In contradiction of the currently accepted
    interpretation, it is shown that the field of a magnet rotates with the magnet
    about its North-South axis. Faraday’s Law is shown to be a particular case
     
  10. Mar 28, 2015 #9

    Drakkith

    User Avatar

    Staff: Mentor

    I don't understand how rotating a symmetrical magnetic field could even result in the generation of current. Field lines just represent the direction and magnitude of the vector field, right? How can rotating the magnetic field do anything if there's no change in the field anywhere? You can say that the field lines are moving, but that just seems to be a visual artifact of having a finite number of field lines drawn. The magnitude and direction of the vector at every point in the field stays the same, don't they?
     
  11. Mar 28, 2015 #10
    You don't need a change in the field. Think of a dynamic microphone. As the voice coil moves through the gap the air gap flux density (in a good microphone at least!) remains constant but the voice coil does produce a voltage.
     
  12. Mar 28, 2015 #11

    Drakkith

    User Avatar

    Staff: Mentor

    You got me there. I was under the impression that electromagnetic induction occurred because the field was changing as you moved a magnet in and out of a coil (or vice versa). I have no idea what happens if you move a coil in a non-changing magnetic field.
     
  13. Mar 28, 2015 #12
    I want to attach pictures of the pages in question so you and Drakith can read them and see if you agree with my reading of them. However, I can't figure out how to attach images anymore since the last change in PF format.
     
  14. Mar 28, 2015 #13
    Also, you must have come across mention of creating current in a conductor simply by moving it between the poles of a horseshoe magnet.
     
  15. Mar 29, 2015 #14
    OK, my scanner is not working so I had to photograph the pages. I uploaded the pics to this site:

    https://sites.google.com/site/mrzoobyshoe/faraday1

    Go there and you'll see three clickables, Faraday1, Faraday2, and Faraday3.

    Some of it refers to stuff on previous pages. The experiments in question get started at the top of column 2 on the first page. The diagrams are schematics of his set up. A more realistic drawing of how he actually embodied this device is on a previous page.
     
  16. Mar 29, 2015 #15

    Drakkith

    User Avatar

    Staff: Mentor

    Click the 'upload' button at the bottom right of the reply box.
     
  17. Mar 29, 2015 #16
    You need special relativity to effectively deal with problems like this. Classical physics lead us to wrong conclusions (like that the magnetic field is rotating when we rotate the magnet).
     
  18. Mar 29, 2015 #17

    Drakkith

    User Avatar

    Staff: Mentor

    When I say a 'changing' field, I mean that the magnitude of the field at any one spot in the conductor changes over time, which happens when you move it through the field of a horseshoe magnet.
     
  19. Mar 29, 2015 #18
    One of the test setups I made way back used the magnet and pole piece assembly from an ordinary loudspeaker. I cut a strip of copper and formed it into a loop and soldered the ends together. I put a length of tape on the inside and the outside of the loop but left the edges bare. Then I placed the copper loop into the speaker magnet air gap far enough to let one edge of the copper loop touch the disc shaped rear steel pole piece so that it had electrical contact. The outer edge of the copper loop had a first wire soldered to it and the disc shaped rear steel pole piece had a second wire connected to it.

    Anyway, the idea was that if you held the magnet stationary and rotated the copper loop in the gap you could measure several millivolts from one edge of the loop to the other. What's more, if you held the loop still and rotated the magnet you could also see several millivolts from one edge of the loop to the other. Practically all the flux was across the airgap, through the copper, so very little would be affecting the attached measurement wires like they were in Faraday's experiments.

    After that I insulated the inner edge of the cooper loop so that it now didn't contact the the disc shaped rear steel pole piece but instead soldered a wire to the edge and passed it through a hole in the rear pole piece. Then I rotated the assembly on the axis of the centre pole and measured zero millivolts. If the flux was NOT rotating with the magnet I should have measure some slight voltage but I got none.
     
  20. Mar 29, 2015 #19
    Ah! Too late, but now I know.
     
  21. Mar 29, 2015 #20
    Another experiment I did was to get a stack of old magnets from microwave ovens and halfway up the stack put a copper disc. Then I put one wire down the centre hole to the centre of the copper disc and a second wire through a hole in the side to the edge of the disc. Essentially all the flux went from the top of the upper magnet, through the surrounding steel enclosure, and to the bottom of the lower magnet. Again no stray flux to influence the measurement wires. Rotate the entire assembly as a unit and zero millivolts detected. Again, this seems to tell me the flux rotates with the magnet. See attached diagram.
     

    Attached Files:

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Magnetic field rotation or not?
Loading...