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About EM waves

  1. Dec 11, 2005 #1
    I have a little doubt.

    EM waves are coupled electric and magentic oscillations. But, to produce an electric field you need a charge and for a magnetic field, you need a moving charge ,but,electric field doesnot exist with a moving charge. So, what is the source of Em waves and moreover how can these two fields become coupled?
  2. jcsd
  3. Dec 11, 2005 #2

    Doc Al

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    Not sure what your question is, but classically an electromagnetic wave is created by an accelerating charge. The electric and magnetic fields are coupled since a changing electric field produces a magnetic field, and vice versa, as described by Maxwell's equations.
  4. Dec 11, 2005 #3


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    The source of EM waves is charge - and it may be VERY far away from where you observe the radiated EM fields.
  5. Dec 11, 2005 #4


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    What gave you that idea? Charges produce electric fields regardless of whether they're moving or not.
  6. Dec 12, 2005 #5
    This may be a stupid question but how do electromagnetic move through space, what is there medium. I have thought abouit it and I think it is that the electricity makes its own medium, is that right?
  7. Dec 12, 2005 #6
    Someone may modify or correct this, but my understanding is that the field associated with a charge has no specific size or boundary except where the field from a like charge impinges on it. The field strength attenuates the further you get from the source, but it doesn't really ever end. That being the case, if you accelerate some electrons here on earth, the wave you create in their electric field will be all the way to the moon in under 2 seconds. Their field was already stretching from here to there.
  8. Dec 12, 2005 #7
    Whilst that isn't how I understand it, it could be one correct way to understand the situation.

    My understanding is that virtual photons propogate and cause the electric field and real photons are emitted as EM radiation when charges accelerate. You seem to be saying the inverse of this. An electric field is present and acceleration of charges cause ripples in the field which are observed as EM radiation.

    Maybe these are just two ways of looking at the same situation.

    One way of considering how EM radiation can travel through empty space (and this is just something that has occured to me so feel free to point out anything with which you disagree) is that radiation travels through space like, say, bullets would, hence, no medium is necessary. You may argue that this is not right but the only way it seems to me that this can be disputed is by observing the photon. When the photon is observed, it necessarily interacts with matter and takes the form of a wave (the medium being the observing equipment).
  9. Dec 12, 2005 #8
    The concept of "virtual" photons, is, I believe, exclusive to Feynman's QED. A biography I read of him said his motive in creating the concept was to be able to explore quantum electrodynamics without recourse to the electric field. That doesn't mean he didn't think there was any electric field, just that his method of analyzing things by replacing the activities of the electric field with the concept of "virtual photons" made things easier to grasp. They're not literally real, which is why he calls them "virtual" photons. They are an "It's as if..." device: When two electrons approach each other it's as if they emit photons at each other. Thinking of it that way makes what happens much easier to deal with than trying to deal with the complexities of an electric field. That, at any rate, is how the biographer explained it.
    The way photons propagate is not clear to me. The way basic EM waves, like radio waves, propagate was explained to me a couple years ago here by an administrator pretty much as I described it. The "medium" for lower end EM is the electric fields of the charges being accelerated. I've never been able to get a non-contested answer to when, why, and how EM switches from that broad kind of wave to what are called photons. The last time I asked there were a few different answers.
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