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Magnetic field moving faster than speed of light?

  1. Feb 7, 2014 #1
    Magnetic field moving faster than speed of light????

    We know the equations for speed of rotating magnetic field, as N= 120*f/P, where f is frequency of AC currents & P is number of magnetic poles.
    Say by some method we are able to produce a very high frequency AC voltage & apply that to a stator of 3 phase induction motor's stator, such that the peripheral speed(v) of magnetic field (v=r*w; where r is radius of rotating magnetic field & w is angular speed (rad/sec) = 2*pi* N/60), exceeds 'c', the old speed of light. But according to special Relativity as we know this would not be possible. So what phenomenon would occur at that time? Does the radius of motor goes on decreasing as we increase the frequency???
  2. jcsd
  3. Feb 7, 2014 #2


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    It's a fallacy to speak about a "moving" electromagnetic field. Electromagnetic fields do not move, rather they sit in one place and vary with time. E.g. for a plane wave, the E and B fields do not travel along at the speed of light, they just sit still and vary! For the field of a rotating magnetic dipole, it's the field pattern that is seen to rotate, not the field itself. And the fact that this pattern may move faster than c is the old scissors paradox, where the intersection of the blades seems to move faster than c.

    Radiation. A rotating magnetic dipole radiates, and the pattern of the far field is different because of this.
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