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How do electromagnetic waves propagate ?

  1. May 23, 2004 #1
    How do electromagnetic waves propagate ? The explanation that is normally put forward to this question is that EM propagates through the self sustaining or oscillation of the electric and magnetic fields , which both support each other. As the electric field recedes the magnetic field increases and vice versa , and this can go on almost indefinitely. Yet if we try to duplicate this process in a laboratory by using a capacitance inductance circuit , we find that the oscillations are very far from being self sustaining , coming to an immediate halt once the motivating force has been removed. Surely , given our more or less exact knowledge of how the process supposedly takes place , it should be possible to set up some kind of perpetual motion machine . Always taking into account , of course that Maxwell’s version of the propagation of electromagnetic waves are based in fact .
     
  2. jcsd
  3. May 23, 2004 #2
    This is bcause you have resistance in your circuit.

    Hmm, it seems to me that you have a far from perfect knowledge of this.

    I can set up this 'perpetual motion machine' for you. I will shine a torch into the sky and the light from that will propagate forever until it interacts with something.

    Matt
     
  4. May 23, 2004 #3
    Not immediate; after ringing the circuit there is a decay time involved.
     
  5. May 23, 2004 #4
    a light beam is not a machine - if moving particles constituted a machine
    then the atoms in a potato could be called a perpetual motion machine
    in which case I could point to a potato and proclaim: "here is a perpetual motion machine"

    as for the beam from a torch, it won't take long to interact,
    particles in the atmosphere alone would scatter the light from the torch very quickly, so it would never make it into space

    a powerful searchlight produces a highly focused beam of light - sometimes visible from space, and yet the most powerful searchlight cannot make a spotlight on the moon

    however an extremely high powered laser can make a laser dot on the moon
     
  6. May 23, 2004 #5

    krab

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    What do you mean by "dot"? Even a laser does not stay focused for long distances. Diffraction makes it impossible. For example, in the Apollo laser ranging experiments, the laser "spot" was 4 miles wide.
     
  7. May 24, 2004 #6

    krab

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    Not true at all. Any such circuit radiates waves. Set up a 1MHz oscillator with a capacitor and an inductor and you will have no trouble at all picking up the signal with an AM radio. The amount of power radiated away is very small indeed, but that is because your radiator, being a few cm across is tiny compared with the 1MHz wavelength of 300 metres. To find out how to radiate the power efficiently, look up stuff on antennas.
     
  8. May 24, 2004 #7
    Krab
    To find out how to radiate the power efficiently, look up stuff on antennas.
    I have , naturally , already done that. The point I am trying to make is . First , there is no , electric field and second there is no magnetic field. What kind of input do we get from all these experiments , some of them highly sophisticated , which speak of magnetic fields ? The field around a magnet is identical in every way to the field around a wire carrying a current. Further to get back to the point you had raised , even given that there is no resistance , there should be some dissipation of power . How then can light travel billions of kilometres.This is the point I was trying to make . Given superconductors , and remember those are very special conditions , it is always a possibility , but a very far fetched possibility. The existence of virtual photons and a virtual photon sea which permeates the whole Universe . Is a reality.
     
    Last edited: May 24, 2004
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