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Changing or moving magnetic field

  1. Nov 6, 2007 #1
    is an emf produced by a changing magnetic field or by a moving magnetic field?
    Last edited: Nov 6, 2007
  2. jcsd
  3. Nov 6, 2007 #2


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    The induced emf will be produced if the magnetic flux is changing through the element. Whether it field is moving wrt to the element or the field is changing, it does not matter. Just the flux density has to change.
  4. Nov 6, 2007 #3
    what if the magnetic field is moving but the flux density does not change?
  5. Nov 6, 2007 #4


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  6. Nov 6, 2007 #5

    Einstein's 1905 paper that introduced the world to relativity opens with a description of the magnet/conductor problem.[1]

    “ It is known that Maxwell's electrodynamics--as usually understood at the present time--when applied to moving bodies, leads to asymmetries which do not appear to be inherent in the phenomena. Take, for example, the reciprocal electrodynamic action of a magnet and a conductor. The observable phenomenon here depends only on the relative motion of the conductor and the magnet, whereas the customary view draws a sharp distinction between the two cases in which either the one or the other of these bodies is in motion. For if the magnet is in motion and the conductor at rest, there arises in the neighbourhood of the magnet an electric field with a certain definite energy, producing a current at the places where parts of the conductor are situated. But if the magnet is stationary and the conductor in motion, no electric field arises in the neighbourhood of the magnet. In the conductor, however, we find an electromotive force, to which in itself there is no corresponding energy, but which gives rise--assuming equality of relative motion in the two cases discussed--to electric currents of the same path and intensity as those produced by the electric forces in the former case.

    At the time of Einstein in 1905, the field equations as represented by Maxwell's equations were properly consistent. The Lorentz force equation, however, had to be modified in order to provide unique particle trajectories upon which all observers could agree.
    Last edited: Nov 6, 2007
  7. Nov 6, 2007 #6
    It is an error to think that field lines move. It can lead to calulations which are wrong. For example: Consider a rotating magnet like the one in this page


    If one were to claim that the field lines are rotating with the magnet then calculated the force per unit charge and called that an "electric field" then when you take the curl you will not get zero as is reqwuired by Maxwell's equations. I have two articles on the idea of moving field lines and how it is meaningless to speak of them. Say the word and I'll make them available.

    Best wishes

  8. Nov 6, 2007 #7
    rotating magnet


    considering how magnetic fields are produced i am not sure i would expect the field of a rotating magnet like that in the picture to be moving anyway.
  9. Nov 6, 2007 #8
    Wait a minute... The Lorentz force equation is Lorentz invariant too.
  10. Nov 7, 2007 #9
    Please will you upload them.
  11. Nov 7, 2007 #10
    Sure. It'd be my pleasure to. :smile: Here are the two of them -



    This one I just found now and is straight to the point.


    As such it appears like a good article. The author states
    Another example of how the concept of
    moving magnetic field lines can be deceptive
    is that of a homogeneously magnetized conducting
    sphere surrounded by vacuum and
    rotating around its axis. For someone thinking
    of magnetic field lines as entities that
    can ‘move,’ it is a not an uncommon fallacy
    to believe that the magnetic field lines outside
    the spherical magnet ‘rotate with the magnet’
    and that this rotating field is capable of exerting
    a force on a test charge at rest, due to the
    percieved ‘relative’ motion between the test
    charge and the magnetic field.
    What really happens is very different. As
    the spherical magnet is conducting, a charge
    on its surface will be subject to a magnetic
    force. This causes a redistribution of charges
    until the electric field from these charges
    gives a force that precisely balances the
    magnetic force. The result is an electrostatic
    potential on the surface of the sphere with
    the equator having a potential opposite to
    that of the poles.


    I have no others so if someone knows of any which are related to this topic please let me know. I'm seeking such articles right now. There is mention of this in the problem section of Chapter11 in Classical Electromagnetic Theory, by Jack Vanderlinde, John Wiley & Sons, Inc., (1993). See page 316 problem 11-8
    11-8 Find the electric field appearing around a uniformly magnetized sphere rotating at an angular frequency [itex]\omega[/itex] << r/a about the central axis parallal to the magnetization.
    Please let me know if I can help with anything else. :smile:

    Best regards

    Last edited: Nov 7, 2007
  12. Nov 7, 2007 #11
    imagine a superconducting ring with a current flowing around it and therefore a magnetic field surrounding it. if i spin the ring around an axis passing through its center then i see no reason to think that the magnetic field will rotate.

    furthermore if the magnecic field extends to infinity then wouldnt the velocity of the field increase without limit as one went further from the magnet?
    Last edited: Nov 7, 2007
  13. Nov 7, 2007 #12
    And in this case there would be no magnetic field generated either.

  14. Nov 7, 2007 #13


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    You need to be a bit carefull when talking about superconducting rings in this case. Quite a few people have probably seen demonstrations where a YBCO "tablet" is levitating above a permanent magnet (cooled by LN2).
    The "problem" with YBCO is that it is a type II supeconductor meaning it will be penetrated by flux lines when cooled in a magnetic field. This is the reason why the YBCO is "stuck" on top of the magnet (if you try the same experiment with a zero-field cooled piece of YBCO there is no stable position).

    Anyway, my point is that the the field in this configuration is NOT symmetric and you can actually rotate the magnet my rotating the piece YBCO even if the latter is perfectly round; but this is ONLY due to the fact that the flux lines are pinned in the YBCO.
    I.e. what granpa writes is correct in MOST cases, but not always.

    I used to do this demonstration when I was teaching a few years ago and while it is a neat experiment it can be quite confusing for the students since YBCO does not behave like the superconductors they read about in their text books.
  15. Nov 7, 2007 #14
    i can only assume that you didnt understand what i said. the ring has a current therefore has a magnetic field. spinning would have no effect on that field at all.
  16. Nov 7, 2007 #15
    superconductivity has nothing to do with my point. forget superconductivity. imagine that its a normal ring of metal with a battery producing a current in it. spinning the ring will not effect the magnetic field. there is no reason to think that the magnetic field would rotate.
  17. Nov 7, 2007 #16
    Actually I meant to say

    And in this case there would be no magnetic field generated either.

    That should have read

    And in this case there would be no electric field generated either.

    Consider the example I gave here

    Read from Eq. (14) on to see what the volume charge density and surface charge density are non-zero and yet the total charge is zero. This configuration of charge will produce an electric field. This can't happen with a ring of current.

  18. Nov 7, 2007 #17

    perhaps i am missing something but that website seems to have nothing to do with moving magnetic fields. its about the effects of relativity on moving charges in a conductor. further it assumes that there are moving charges in a permanent magnet, which seems very doubtful to me.
  19. Nov 8, 2007 #18
    The magnet has a magentic field. It used to be thought that the rotating magnet carried its field with it, i.e. that the magnetic field rotated with the magnet. In fact that's part of the subject matter in one of the articles I posted the URL to.
    Why does it seem doubtful to you???

    Besides, that is only a model of a magnet that I'm employing in order to describe the mechanism of the charge distribution. If the current loops bother you that much then ignore them. Focus on the equations for the polarization instead.

  20. Nov 8, 2007 #19


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    Yes, I understood that. But my point is that you need to be carefull about usings superconductors as an EXAMPLE when you talk about magnetic fields since quite a few people have seen the YBCO demonstration and that does not quite agree with the explanation you gave.
    It is probably less confusing to talk about e.g. a perfect conductor instead of a superconductor.
  21. Nov 8, 2007 #20
    Why does it seem doubtful to you???

    because permanent magnetization is the result of spin orientation of the electrons. no current is involved.
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