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

Magnetic Experiment Issue

  1. Mar 16, 2010 #1
    I have a moving magnetic field that rotates around the polar (Y) axis - see below - and I want to translate the energy in the form of electricity, what should I try.

    1) I have created a magnetic field that rotates around the polar axis from the surface of a disc shaped magnet to about 1.5" above the surface (where rotation seemingly ceases)
    a) yes I know that some argue that the uniform field does not and will not rotate around the polar axis.
    b) I have verified rotation and observe significant hysteresis and the associated torque in a conducting sheet, via eddy currents in the sheet.
    c) I have observed rotation via linking two discs together, one being driven while the other is free to spin on a bearing. When one moves, the other follows (not normal for two discs to do this)
    d) I have observed movement using a variety of methods including ferrofluid - On a normal disc, the fluid remains stationary no matter how fast the disc rotates or how close the fluid container is to the disc. In my disc, the fluid is whipped around at the same velocity as the disc.
    E) By design, rotation of the field is limited to an area from the surface of one side of the disc to 1.5" above the surface. The perimeter of the disc (side) is not affected and does not seem to rotate and the field at the center of the disc seems also to remain stationary.
    F) I have tried several coils and here are the results - Wire Coil No Core, 100 turns of 22Ga magnet wire connected from center shaft to perimeter with shaped wire- No current when disc is spun, .5 ma when the coil is moved laterally. - Toroidal coil with 100 turns 22ga magnet wire connected in the same way as above - no current when disc is spun, .5a when coil is rotated.
    g) I am a bit confused by the fact that I get such a great torque from eddy currents in a sheet of copper when the disc is spun, but nothing with a coil. My original hypothesis was that by using a coil and eliminating eddy currents, I should be able to translate that energy into electrical current. Any suggestions would be appreciated.

    That being said, I would like to create electricity from this set up. This is a unipolar set up in that only one pole will ever face the conductor. The typical disc set up like that of Faraday or the one piece HomoPolar generator does not produce electricity, only eddy currents and a significant "braking" of the device.

  2. jcsd
  3. Mar 16, 2010 #2
    My thoughts are this(and I certainly could be wrong)
    You said:
    a) yes I know that some argue that the uniform field does not and will not rotate around the polar axis.

    OK. I think much study has been done on this, so I assume their is a definitive answer somewhere. Perhaps experts can jump-in and clarify.
  4. Mar 16, 2010 #3
    Sure why not? Maybe we can get another thread locked! The question of whether a magnetic field rotates with the magnet that is rotated on it's axis has been going on from the time of Faraday. Faraday decided the magnetic field does not rotate and that answer has been widely accepted for a long time. However. Many didn't accept it and the argument continued.

    Well, in relatively recent times the question has been visited again. Two salient experiments reported in the literature are 1. When two magnets are aligned on the same axis and one is spun the other one does NOT rotate. 2. An investigation of the Faraday generator that showed that one gets the SAME (correct) answer if one either assumes the field rotates or if one assumes it does not. In fact, using coils of wire to sense the induced emf there is NO way to tell which case is actually taking place!

    Many then assumed that experiment 1. "settled" the question, but that isn't true. The reason is that if one examines the direction of the induced force vectors between the two magnets, they are in directions so that a torque cannot be produced whether the field is assumed to rotate or not.

    Experiment/paper 2. then produced a spate of interest in FINDING a way to determine which case was true. Since it couldn't be done with the usual wire/circuit/meter, it was proposed that it be investigated using ELECTROSTATIC methods to measure the induced E fields. The presumption is that with modern electronics this kind of measurement would be possible. Such sensitivity was not available in Faraday's day. I know of at least 3 different proposals to do this. One uses a specially constructed electronic device sort of like a "magic eye tube" and another uses a capacitor to store the induce emf. So far as I know these experiments have never been performed or if they have, they've never been reported.

    But as the experiments suggest, that while a standard circuit to measure emf gives the same answer if the field rotates as when it does not, what happens is not the same. Consider the one-piece Faraday generator. This one has the magnet glued to and rotating with the disk. OK? Now, if the field does NOT rotate with the magnet as it rotates then there is relative motion between magnetic field and the disk as it spins even though the magnet is spinning too. For this reason an emf is assumed generated in the disk. The rest of the wiring is in a rest frame with the field so it gets no emf induced into it.

    Now assume the field spins with the magnet. Now the magnet and the disk are in the SAME [rotating] frame so NO emf is generated in the disk! However the wiring is in the rest frame and thus has velocity (B x V) with respect to the B field. If you work it all out, you find you get the same reading on the meter as above, but WHERE the emf is produced is different. In this case the emf is in the WIRES and not the DISK! But because the circuit is a complete loop you get the same reading in either case.

    So which is true? Well the proposed experiment is one way to find out. Another hint is a theoretical derivation which points out that how the system works has NOTHING to do with a magnetic field. Replace the permanent magnet with a coil of wire carrying a current. If one rotates that coil about it's axis then one is giving the current a velocity in the direction it is flowing. If you do that it turns out the two relative frames (the moving wire and the fixed observer of the fields from the wire) create an apparent E field in addition to the magnetic field from the current. Since those two frames are between the wire velocity (a permanent magnetic can be equivalent to a current-carrying coil of wire) and the observer, the observer will find an E field induced proportional to that velocity. The value of that E field can be calculated from v x B even though the E induced E field DOES NOT come from B! For that reason one can say that a rotating magnet acts AS IF the magnetic field is attached to it, but in reality the magnetic field is NOT what is producing the emf. That comes from the relative motion of frames!

    Anyone disagree? :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook