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If magnetic forces resist a changing magnetic field, how do antennas radiate EM waves

  1. Sep 3, 2012 #1
    I understand a changing current in a wire gives way to an opposite magnetic field that resists the induced magnetic field, until there is no change in magnetic field, then you're left with a constant induced magnetic field.

    So my confusion with the antenna is when an electron rapidly oscillates across the span of the antenna, there would be a rapidly oscillating opposing magnetic response that would nullify the original signal, or at least considerably weaken it.

    So how do antenna's operate efficiently?
     
  2. jcsd
  3. Sep 3, 2012 #2
    Re: If magnetic forces resist a changing magnetic field, how do antennas radiate EM w

    Nevermind I guess I kind of answered my own question in there. A resisting magnetic force only comes up when the electrons accelerate, as in change direction. Then it moves at a constant speed throughout the span of the antenna so there is no opposing force.
     
  4. Sep 3, 2012 #3

    sophiecentaur

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    Re: If magnetic forces resist a changing magnetic field, how do antennas radiate EM w

    Which "electron" is moving along the antenna? At Radio Frequencies, the electrons actually can be thought of as oscillating by a minute fraction of the diameter of an atom. Just stick to 'current' for your argument.
    This induced field is, in fact, there (a reactive element) but there is also a small resistive element which represents Power radiated into space.
     
  5. Sep 3, 2012 #4
    Re: If magnetic forces resist a changing magnetic field, how do antennas radiate EM w

    <<If magnetic forces resist a changing magnetic field, how do antennas radiate EM waves>>

    There is resistance, but that does not mean the resistance is complete. When you drive your car, air hitting the windshield and elsewhere provides some resistance, but that does not imply your car cannot go faster than zero miles per hour.

    The resistance of the antenna is due to it emitting energy. Energy is leaving the antenna as radiation, and if there were no resistance then total energy would go up rather than being conserved.
     
    Last edited: Sep 3, 2012
  6. Sep 3, 2012 #5
    Re: If magnetic forces resist a changing magnetic field, how do antennas radiate EM w

    Thank you, i understand it much better now
     
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