Help with physical understanding of AC signal

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Discussion Overview

The discussion centers around the physical understanding of alternating current (AC) signals, specifically how energy is transmitted through wires and the role of oscillating charges. Participants explore concepts related to electromagnetic waves, electron movement, and energy propagation in AC systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes AC current as oscillating charges that radiate electromagnetic waves, questioning how this relates to energy flow in the same direction as the charge oscillation.
  • Another participant agrees that oscillating charges generate EM waves but notes that the energy transmitted by the line is primarily due to the moving charges, not the EM waves, especially given the low frequency of oscillation.
  • A different viewpoint suggests that energy is not carried by individual electrons moving from source to load, but rather through interactions akin to friction in a physical system, such as a rod being moved back and forth.
  • One participant clarifies that the energy propagation from the power plant to homes does not involve EM waves traveling down the wire, prompting further inquiry into the mechanism of energy transmission.
  • Another participant proposes that when a potential difference is created, an electric field is established throughout the wire, leading to a rapid chain reaction of electron movement, which oscillates at the frequency of the AC signal.
  • One participant raises the idea of an electron displacement wave, questioning whether a signal can propagate at the speed of light without being an EM wave, and referencing signal delay in cables as evidence of this speed.
  • Another participant elaborates on the interaction of electrons through their electric fields, suggesting that the jiggling of electrons can transmit a signal nearly at the speed of light, distinguishing this from the EM waves that radiate outward.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms of energy transmission in AC signals, with no consensus reached on whether the propagation involves EM waves or other forms of energy transfer. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants acknowledge various assumptions about the nature of electron movement, the role of electric fields, and the distinction between energy propagation and radiation of EM waves. The discussion includes references to specific phenomena like signal delay and the behavior of electrons in conductive materials.

johng23
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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 propagation 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.
 
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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.
 
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.
 
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?
 
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 .
 
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 propagating 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 propagating 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?
 
so is the electric field of each electron pushing the electron in front of it to transmit the signal .
 
johng23 said:
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 propagating 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 propagating 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?

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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