Q & A: ac current descriptions

I read the following about the movement of electrons during AC flow could you tell me if it is correct, and if so could you explain it in more detail please. "This explanation of AC is completely erroneous,The first electrons get charged and expand and contracts. This expansion and contraction fo the elctron gets passed on to the elctron beside it and and that is how AC works."
- mike (age 35)
birmingham uk

Thanks to the miracle of Google, I found the original youtube clip on which that comment was made. The part of the clip describing electrons flowing back and forth in ac current was correct. (Of course in addition to that systematic flow, the electrons have a lot of random thermal motion, but that's not especially relevant.) I have no idea what point the commenter you quoted was trying to make.

There were some fairly major errors in the youtube clip, but they weren't in the parts criticized by the quoted commenter!

Mike W.

p.s. Since Lee H has just run off to Paris for a week, I'm posting this without our usual check.

(published on 11/26/2011)

Thankyou for answering I believe the point the person in question was trying to make is that even though electrons are shown moving in a direction of flow they actually just vibrate and the charge or current is what actually flows through the electrons, I am new to electricity and electrons so could you comfirm do the electrons actually move across from higher potential or is this a representation, I have seen on a few sites now that the electrons merely vibrate and move very little. Ps I am quite concerned about what to watch on you tube now you have pointed out there were major errors in the video.
- mike (age 35)
birmingham uk

The current really does consist of a flow of electrons. It's true that under typical conditions (say a 10 A current at 50 or 60 Hz in a wire with a few square mm cross section) the electrons don't go very far before reversing directions. Under the conditions mentioned, roughly 10^-3 cm would be a good ballpark estimate of the oscillation distance. That's still large compared to an atomic size (~10^-8 cm) or the distance an electron travels in room temperature copper before scattering (~10^-6 cm). Nevertheless, it's small compared to the length of a typical power cord.

The errors in that youtube, by the way, mainly concern the statements about the alleged enormous costs of generating and transmitting dc power compared to ac power. The generation costs are very similar. Cars, for example, used to have dc generators although now most have switched to slightly more efficient ac generators. Small motors (which are generators when you turn them and extract current can easily be found in cheap ac or dc models.  The main reason for using ac power is that it allows a simple device (a transformer) to convert between high and low voltages. Since in order to transmit a lot of power through small wire you need high voltage (letting the current be small) the ability to convert to high voltage for transmission and back to lower voltage for use is very important. More elaborate devices are now being used in a few places to allow conversion to and from high voltage dc. Other than the conversion difficulties, dc makes even more efficient long-range transmission, because it does not emit electromagnetic radiation.

Mike W.

p.s.-Temporarily posted without the usual check, since Lee's in Paris.

(published on 11/27/11)