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Electromagnetic waves and polarity

  1. Feb 14, 2016 #1
    Say you have a transmitting whip antenna. If you send a quick DC burst into it, you will get the electrons in the antenna accelerating in one direction like this:
    field_a.gif
    If that EM wave is then absorbed by a receiving whip antenna, the electrons will also move in one direction. The polarity of the EM wave is only one direction.

    Conversely, if we do things normally and send an AC signal into the transmitting antenna, then the electrons will move back and forth which will transmit an EM wave with an alternating polarity.

    So am I right in thinking that a standard over the air radio signal is coherent light with 2 different polarities, while something like a laser is coherent light, but only one polarity?
     
    Last edited by a moderator: May 9, 2017
  2. jcsd
  3. Feb 14, 2016 #2

    Drakkith

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    That's a good question and I'm not sure what the answer is.

    The EM waves generated by both radio antennas and lasers have an e-field vector that oscillates back and forth, from positive to negative, and will cause charges to also oscillate back and forth over time.
     
  4. Feb 14, 2016 #3

    Vanadium 50

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    You need to better define "quick DC burst".

    If the antenna develops a net charge, that charge had to come from somewhere, so now there is a region of opposite charge. You've created a dipole, and the receiving antenna will see a bipolar signal.
     
  5. Feb 14, 2016 #4
    Say I have this setup:
    6ebNCa.png
    ...and the DC source is increasing at a rate of 1 V/sec. The electrons are being accelerated in one direction and will emit EM radiation.

    Now say I have this setup:
    YsbnYm.png
    ...and the AC source is at 1 Vpp, 1/4 hz. The electrons are being accelerated in both directions and will emit EM radiation.

    Assume there is some resistance so the current isn't infinite. If I put a receiving antenna in front of both these transmitters, would I get the exact same signal? The difference being the AC transmitter is accelerating it's electrons at 1 V/sec in the opposite direction of the DC transmitter for half the time.
     
  6. Feb 14, 2016 #5

    Vanadium 50

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    You get radiation when you have a changing dipole. Your "one direction" produces a changing dipole (although not as strong as the lower plot)
     
  7. Feb 14, 2016 #6
    But the changing dipole flips polarity in one and not the other. Is that insignificant?
     
  8. Feb 15, 2016 #7

    Vanadium 50

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    There are only so many times I am willing to repeat myself. Saying the same thing that I said before is unlikely to work any better than the last two times.
     
  9. Feb 15, 2016 #8
    "You get radiation when you have a changing dipole." Great, but I'm still confused.

    The EM wave's frequency is directly related to the frequency of the AC signal, but how is the frequency of the DC case determined when it emits its radiation?
     
  10. Feb 15, 2016 #9

    davenn

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    With that definition, it isn't a steady DC source any longer, as you change the voltage the current will also change through the load

    Once the applied voltage stops changing, then it will reach equilibrium and will be steady ... no emitted EM

    it doesn't have a frequency, as there is no oscillation as with an AC voltage, it's just a momentary pulse

    eg ... lightning discharge, spark gap transmitter
    because it doesn't have a defined freq, it is very broadband and is heard well across the radio spectrum


    Dave
     
  11. Feb 15, 2016 #10
    So really it's just emitting incoherent photons at a range of different frequencies? But doesn't the photon frequency depend on the rate of acceleration of the charge? So if all the electrons were under the same acceleration, wouldn't they be emitting the same frequency?
     
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