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Electromagnetic Waves

  1. Oct 11, 2008 #1
    I've already figured that oscillating charges produce electromagnetic waves...but if the charge was simply in translational motion through space, would that produce EM waves?
  2. jcsd
  3. Oct 11, 2008 #2
  4. Oct 11, 2008 #3
  5. Oct 11, 2008 #4


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    If a charge is stationary and not radiating in reference frame R, then in any other reference frame S moving at constant velocity relative to R, the charge will be moving at a constant velocity, but not radiating. This is because radiation is light, and light has the same speed in R and S. So if it radiates S, it must radiate in R, contradicting our initial assumption.

    A uniformly accelerating charge will radiate, since acceleration is absolute.
  6. Oct 11, 2008 #5
    I see...somewhat...thanks for the clarification

    An additional question...lets say that the reference frame S was accelerating...how would this change?
    Last edited: Oct 11, 2008
  7. Oct 11, 2008 #6


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    I don't know off the top of my head, Maxwell's equations look weird in accelerating frames. It's usually easier to do classical physics in inertial frames. In inertial frames, only accelerating charges radiate.

    In quantum field theory, there is the "Unruh effect" in which an accelerating observer sees radiation due to his acceleration.
  8. Oct 11, 2008 #7
    I see...thanks
  9. Oct 11, 2008 #8


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    A charge in uniform motion through a medium (i.e., not through vacuum) actually can radiate. This is not in contradiction with the theory of relativity simply because in the presence of a medium there *is* a prefered frame (the frame in which the medium is at rest). Thus, if we try and "boost" such that the particle is at rest then the entire medium will be in motion.

    An example of the above type of radiation is "Cherenkov radiation" (see PF library for more info). Also, a type of radiation called "transition radiation" can occur for a particle in uniform motion (but again this depends on the presence of a medium).
  10. Oct 12, 2008 #9
    A stationary charge has a stationary electromagnetic field surrounding it....You can tell because if you place another similar charge nearby, both stationary, they repel each other. The field is present regardless of motion, but what you observe changes with relative motion:

    When relative velocity occurs, electromagnetic waves induce emf....If you move in the right way relative to a moving charge you may detect the magnetic portion of the field...or not...depending on whether there is relative motion..

    For a stationary charge with a stationary observer: you see only an electric field. Start moving and you see only a magnetic field....motion tranforms between the two:

    from http://en.wikipedia.org/wiki/Electromagnetic_field: [Broken]
    Last edited by a moderator: May 3, 2017
  11. Oct 12, 2008 #10


    Staff: Mentor

    Umm, a particle emitting Cherenkov radiation is undergoing enormous acceleration as it slows down to below the speed of light relative to the medium.
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