Power Transmission via AC current

[timthereaper]

I'm a mechanical engineer and I'm really interested in electrical systems. I understand a fair amount about the basics of E/M and circuits. But no matter how much I read, I can't wrap my head around how AC systems generate power. I do know they work (obviously, right?), but I'm trying to imagine what is actually happening at the electron level. This is my mode of thinking and please let me know where I'm wrong:

An AC voltage source oscillates between a positive and negative voltage (I'm picturing a sine wave). The voltage difference across the circuit creates an electric field that moves charges along the conductor. The positive part of the wave moves charges one way, but the negative part moves the charges back to (roughly) their original positions. Energy is needed to do the work on the charges, but how does this transmit power to the load?

 

[v4theory]

The way I understand it is that AC is waves, DC is current. It is a colloquialism to call AC a current. If you consider a river flowing into the ocean the direct current of the river will be quickly dissipated by friction shortly after it reaches the ocean. But waves can travel a long way on the ocean without losing their energy.

At the electron level DC current is electrons jumping from atom to atom through the load to the negative. AC generator shoves the electrons in the wire which then give a shove to further electrons shove then jump back. The next electrons with the energy received from the shove jump to the next atoms and give them a shove and back again. So on and so forth down the wire.

 

[timthereaper]

I'm confused, because doesn't AC stand for "alternating current"? I liked your wave analogy, but I don't quite understand it. I don't see how an alternating voltage source can "shove" and "jump back" and still produce energy transmission one way. For me, that's like a zero-sum gain. I guess I'm not picturing it quite the way you are. I would see how you could produce power that way, but only if the wave is unbalanced (e.g. the area under the "positive" part of the wave curve is more than the area under the "negative part").

I was just thinking that maybe I can picture AC as a longitudinal wave instead of a transverse wave and power transmission can occur that way. Does that seem like a good way to describe it?

 

[Integral]

P = I²R

P = IE
Can either of you find in either of those equations where direction of current enters in?

Current flow is current flow, the power delivered does not depend on direction or changing of direction of current.

 

[timthereaper]

K, I think I understand. For some reason, I never thought of it that way. Thanks!

 

[Lsos]

In both AC and DC there are electrons moving in the wire, and moving electrons produce a magnetic field which can be exploited to do work.

Whether the electrons just keep moving forward (DC) or back and forth (AC) doesn't really matter, as long as they're moving.

 

[The_Duck]

A specific example of how to power something with AC: If you shove electrons through a resistive material (like, say, a light bulb filament) it heats up, regardless of which way the electrons are moving. So by jiggling the electrons in a light bulb back and forth you can make it glow even though the electrons don't experience any net displacement.