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Electromagnetic wave.. How is it getting transmitted?

  1. Aug 28, 2015 #1
    I've heard that em wave consists of mutually perpendicular electric & magnetic oscillations.

    Imagine that I've a unit charge at any point in space. Associated with this charge, there exists an electric field which diminishes with distance from the charge. If we set forth an oscillation in this field, it can get transmitted. But since the field strength is getting diminished wrt distance, How can the oscillation get transmitted indefinitely?
  2. jcsd
  3. Aug 28, 2015 #2


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    The radiated wave does get weaker as it propagates away from its source. For your case, the electric and magnetic fields both off as ##\frac{1}{R}##, so the power density falls off as ##\frac{1}{R^2}##. Here ##R## is the distance from your oscillating charge to the point at which you are observing the wave.

  4. Aug 28, 2015 #3
    As a follow up question, can an EM wave be considered a form of perpetual energy? I 'know' instinctively that this cannot be true, but I don't know why.
  5. Aug 28, 2015 #4

    For how much length the electric field of a unit charge exist?
    Light reaches from sun to earth after travelling that much distance. You mean to say that the electric field of the sun extends up to the distances where its light can reach?
  6. Aug 28, 2015 #5


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    Sure. Until the EM wave interacts with something, it will always have some amount of energy. The phrase 'perpetual energy' is mostly meaningless though. Energy is conserved, so it never disappears, but just gets transferred around and changes form. This is in contrast to a 'Perpetual Motion Machine'. A machine is basically a device that moves energy around. Adding 'perpetual motion' implies that the machine can run forever, performing work without expending energy. This is impossible.

    The EM field from any object extends to infinity.
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