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Electric and magnetic fields of an electromagnetic wave

  1. Aug 15, 2003 #1
    I have simple question here that I've been wondering about for sometime now, and here it is. In classical physics when an charge is accelerated it creates an oscillating electric field which creates an oscillating magnetic field, a electromagnetic wave, and each field carries energy with it. So my question was does that energy come from the charge/electron? Does the electron give up some of it's kinetic energy? If so why would the electron loss energy if were to speed up?
  2. jcsd
  3. Aug 15, 2003 #2
    A preliminary question:

    Why is an electron accelerating? What is the energy shift associated with that acceleration?
  4. Aug 15, 2003 #3
    Let's say the source of acceleration was an electromotive force that accelerated the electron, so classically it would emit an electromagnetic wave. Where did the energy of the wave come from in this situation?
  5. Aug 15, 2003 #4


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    Erm... the point to ponder is that acceleration doesn't mean "speeded up". It means a change in the velocity vector. If it was accelerated linearly then what you say is true - you basically have inefficient addition of energy, and the energy is taken from the KE.

    The really interesting stuff of this is when the speed doesn't change, but the direction does, such as when in a magnetic field the electron moves in a circle. This phenomenon then explains how we see the electron "spiral inwards".
  6. Aug 15, 2003 #5
    sorry to say "speed up" what I really meant was any change in it's velocity
  7. Aug 16, 2003 #6
    The whole point I gather is that it may not be an electron that is accelerating at all. New research into Oliver Heaviside's theories , seem to prove that it is photons that are accelarated in a current and not electrons.
  8. Aug 16, 2003 #7
    The fields around a charged par-
    ticle at rest are static. No EM
    waves are created untill the
    charged particle is put into
    motion one way or another.

    The answer to your question, if
    I understand what you're asking
    correctly, is that the energy in
    the EM waves coming from a charged
    particle in motion, is coming
    from whatever force is putting the
    particle in motion. The particle
    is acting as a transducer, chang-
    ing one form of energy into another. The particle itself is
    not suffering any net loss of
    mass or energy.

  9. Aug 16, 2003 #8
    Thank you everyone! I'm starting to get the idea now(I think), but what if we talked about the quantum description of electromagnetic radiation? Photons are emitted in a atom when an electron drops from higher energy level to a lower energy level. In that case the electromagnetic radiation(photon) emitted was from the electrons energy ,because the emitted photon's energy was equal to the energy transitions that the electron made.
  10. Aug 16, 2003 #9
    My understanding of this situation
    is that the electron won't be in
    that higher orbit to begin with
    unless extra energy has been pump-
    ed into the atom forcing the elec- tron into the higher orbit. It drops immediately back to a position that will bring the atom into equilibrum, releasing the photon. So here again, the elect-
    ron has not lost anything that
    wasn't artificially added to

  11. Aug 16, 2003 #10

    Ivan Seeking

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    Staff Emeritus
    Science Advisor
    Gold Member

    This is a nice analogy; acting as a transducer.
  12. Aug 16, 2003 #11
    I see what you mean but my point was for a short time that electron had more energy and it gave it up by emitting radiation when it decayed back to the lower energy level.
  13. Aug 16, 2003 #12
    As far as my understanding of the
    subject goes, your last statement
    is correct.

  14. Aug 17, 2003 #13
    Thank you.
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