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General EM Wave Question

  1. Jan 17, 2008 #1
    1. If you took a positively charged antenna and shook it up and down 500 million times a second, would it emit a radio wave. If so, why? If not, why not?

    I would say, the EM wave that it emit is heat due to friction with the air. anyone have any other suggestion.

    2. Electric charges are what emit radio waves, so why can't the cellular telephone emit a radio wave by just putting a positive charge on its antenna permanently?

    not sure on this answer though.
  2. jcsd
  3. Jan 17, 2008 #2
    Electric charges emit radio waves only at very special conditions, when they are accelerated.
    If no acceleration, they may produce only EM field, not waves.
    If no speed, only E field, no waves, no magnetic field.

    Yes, it would. Actually it would emit a radio wave at frequency 500 MHz
    PLUS vulgar electric field, because the antenna is not electrically neutral.

    P.S. If you counted UPs and DOWNs together, when you said "shook it up and down 500 million times a second", then frequency of wave would be only 250 MHz... :smile:
    Last edited: Jan 17, 2008
  4. Jan 17, 2008 #3
    cool, i see.thanks for the explanation. how about the 1st question,is my reasoning correct?
    my reasoning is since energy is used in shooking the antenna, it must have transform to another energy which is heat energy, or is the heat energy significant in this case? anyone?
  5. Jan 17, 2008 #4
    what is producing this EM wave? lambda=c/f = 3x10^8/250x10^6. =1.2m (radio wavelength)
  6. Jan 17, 2008 #5
    If you would really shake your antenna by hand, I guess 99% of mechanical energy would be transformed into heat energy. This energy is not significant for EM waves.
    Then, 0.99% of energy would be transformed in the energy of acoustic waves.
    This energy is not significant for EM waves as well.
  7. Jan 17, 2008 #6
    When you shake your antena, charges in it are accelerated. This acceleration produces the EM wave.
  8. Jan 17, 2008 #7
    yeah, the antenna is shook by hand. so i can conclude that the dominant energy here is heat only. thanks.
  9. Jan 17, 2008 #8
    but this acceleration is insignificant if we compare to the speed of electron. hence the EM wave would be extremely small.

    if so, those particle accelerator when in operation would produce significant EM wave energy, right? thanks
  10. Jan 17, 2008 #9
    Yes, particle accelerators produce some EM waves, but not so significant, because they were designed to produce speed, not acceleration.
  11. Jan 17, 2008 #10


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    Staff: Mentor

    Some accelerators do produce significant EM radiation. Look up "synchrotron radiation," for example on Hyperphysics.
  12. Jan 17, 2008 #11
    In particle accelerators the "synchrotron radiation" is side effect. They try to avoid that. Anyway, cost of such source of EM waves is of the order $10 Billions. On the other hand, cost of a capacitor, consisting of a piece of wire and the Earth, is of the order $0.1. Thus an efficiency of such cheap device as a source of EM radiation is about [tex]10^{12}[/tex] times better... :smile:
    Last edited: Jan 17, 2008
  13. Jan 17, 2008 #12


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    Staff: Mentor

  14. Jan 17, 2008 #13
  15. Jan 17, 2008 #14
    interesting subject for further reading, thanks .;)
  16. Jan 18, 2008 #15


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    i would simply the question and removed the word "antenna". if you took a positive charge (or negative) and shook it up and down (or left and right) 500 million times per second, you would be creating a 500 MHz radio wave.

    this is what happens with a radio transmitting antenna. charge sloshes back and forth along the antenna element and that creates a radio wave.

    see the "thought experiment #1" at:

    so you think it would work differently if you were shaking this charged object up and dowm 500,000,000 times per second in outer space where there is no air to have friction with?

    how 'bout, the reason it works is that: unlike charges attract and like charges repel.
  17. Jan 18, 2008 #16

    what a interesting read, well i know understand how charges interact with each other to produce the EM wave. so in microscopic sense, we analyze the charges, in macroscopic sense, we talk about the antenna movement interact with the air friction, is that sound logical? but in vacuum, there is nothing to cause that friction.
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