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Help with physical understanding of AC signal

  1. Jun 26, 2010 #1
    My understanding of how AC current works is that the current changes direction, so essentially the charges are oscillating back and forth along the direction of the cable. Then since accelerating charges radiate, this produces an EM wave which is what carries the energy. But my problem is, from my understanding of how and why accelerating charges radiate, they do not produce a transverse EM wave along the direction they are oscillating. So how is it that the charge oscillates in the same direction that the energy flows during the propogation of an AC signal?

    I apologize if my thinking is simplistic on this. I am not an expert in either electronics or Maxwell's equations.
     
  2. jcsd
  3. Jun 27, 2010 #2

    phyzguy

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    You are right that the oscillating charges in a power line generate an EM wave that propagates perpendicular to the wire. This energy leaves the power line and represents a loss of the power in the line. However, because the frequency of oscillation is very low, this power is very small compared to the power transmitted by the line. The power carried by the line is not carried by an EM wave, but by the moving charges themselves.
     
  4. Jun 27, 2010 #3

    HallsofIvy

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    Energy is NOT carried by the individual electrons in AC, at least not by an electron going from the source of the current to the use of the energy.

    Imagine holding a rough rod tightly in your hand. Now move it rapidly back and forth. You had will become warm as a result of friction. That is basically how A.M. electricity produces power- "friction" between the electrons moving back and forth in the filament of a light bulb or in a heater.
     
  5. Jun 27, 2010 #4

    phyzguy

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    Clearly you are right that the friction between the electrons and the surrounding solid is how the energy is dissipated in a light bulb filament. But I think the OP's question had to do with how the energy is propagated from the power plant to your house. It is not EM waves propagating down the wire as the OP originally asked. How would you explain it?
     
  6. Jun 27, 2010 #5
    the electric energy is carried like this , when i create a potential difference (voltage ) in a wire , an electric field is set-up at every part of the wire almost instantaneously and the first electron bump's the next and it's basically an instantaneous chain reaction so when the voltage is created at the power plant it pretty much moves an electron at your house almost instantaneously and then it oscillates 60 times per second when the voltage flips . This may not be 100 percent accurate .
     
  7. Jun 27, 2010 #6
    So what you're describing sounds like an electron displacement wave, which is how I initially thought of it. It seems to me though, that the fact that the signal propogates "almost isntantly" means it must be propogating as an EM wave. People say a 1 ft BNC adds 1 ns to your signal delay, so this obviously implies that the signal is propogating at least close to the speed of light. Is there any way a signal can travel at the speed of light without it being an EM wave?
     
  8. Jun 27, 2010 #7
    so is the electric field of each electron pushing the electron in front of it to transmit the signal .
     
  9. Jun 29, 2010 #8
    Close, very close. Cragar has the other part of the puzzle.

    The electrons are all interlinked through their E fields. They all jiggle back and forth together as if connected by springs.
    Since the springs in question are the electric field (which propagates at the speed of light) a jiggle started at one end of the wire passes through to the other (nearly) at the speed of light

    In a way, it IS an EM wave but it's not the same as the one as gets radiated sideways. It's just a simple kind of compression wave. Hardly even that because the coupling is very stiff and very fast.

    The sideways wave is a true EM transverse wave with E and B gracefully, constantly dancing round each other.
     
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