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# Physics/understanding alternating current

Question

I have struggled with understanding electricity, particularly a/c, for years, perhaps because I have difficulty understanding concepts I can't visualize.

I think I basically get direct current, visualizing electrons flowing from one side of a power source, through a circuit and load, into the other side.

Alternating current I have trouble visualizing, and therefore understanding.  As I think I understand it, the electrons flow one direction for half a cycle, then reverse and flow the other way.  But at all the critical points of this concept, I cannot picture in my head what the electrons are doing.

At the source, lets say power plant, as the turbines spin, the sawing motion of the electrons begins?  Do the electrons the entire length of the power lines move back and forth with each cycle, including through each transformer and into the home?  If I turn on one light in the whole house, are the electrons moving back and forth in one great movement traced all the way back to the power plant, and if so, what are the electrons doing in all the other wires throughout the house?  If the other end of the a/c circuit is the ground, do electrons get pushed into the earth and pulled back out again?

If I could picture in my mind what the electrons are doing in an alternating current circuit, especially at the 'ends' of the circuit, I think I might be able to get past the hurdles I have had understanding electricity.

Thanks again for your help!  Any web sites you could refer me to would also be appreciated.

Hello Mark,

You have two other questions:
"what are the electrons doing in all the other wires throughout the house?"
If all of the other of the parallel branches are turned off, there is an open switch in that branch. So those electrons can't flow.

"If the other end of the a/c circuit is the ground, do electrons get pushed into the earth and pulled back out again?"
This varies somewhat depending on the country you're in. See the Wikipedia sites that discuss Mains Electricity and Earthing Systems. Imagine your house is the only customer connected to your power plant. The power plant can be pictured as having 2 output terminals, one called "hot" and the other called "neutral". Both of these output terminals has a wire connected to them that goes to your house. The neutral output is generally tied to the earth at the plant. And at your home, your end of the neutral is again tied to earth. The reason it's tied to earth at your end mainly has to do with lightning (I guess, not my expertise). The current in the neutral wire should be approximately equal in magnitude to the current in the hot wire except in the opposite direction. (I don't know how much of the neutral current might actually flow through the earth.)

And then you expressed the main area of confusion -- at the 'ends'.
Imagine 2 pipes full of marbles between the power plant and your house. Basically it's a long pipe that goes to your house and turns around and goes back to the power plant. The power plant alternately pushes the marbles in their end of the hot pipe toward your house or pushes the marbles in the return pipe toward your house. Pushing on one or the other 120 times per second. And you want to turn on a fan. A gear is inserted into the pipe while it goes through your house. Each gap between teeth of the gear fits one marble. The gear turns Clockwise, CounterClockwise, CW, CCW, etc. And this feather fan is waved up, down, up, down, etc. It's not a perfect analogy. (My analogies never are. Sorry.) But during the 1/2 cycle when electrons are leaving the plant through the hot wire, the same number should be arriving on the neutral wire.

You wonder about the other branches in your house. So the pipe divides into several parallel branches that rejoin into the single neutral pipe as the marble pipe leaves your house. Each branch has an appliance that could be turned on. There's a valve in the off position in each branch if you don't need that appliance at the moment. So those marbles don't move.

I found several web sites. The wikipedia sites are well done but get deeper into it than you need. The other sites are fair, they have some sections that I would have said differently.
http://en.wikipedia.org/wiki/AC_power
http://en.wikipedia.org/wiki/Mains_electricity
http://en.wikipedia.org/wiki/Earthing_system
http://www.anoldman.com/power_systems/understanding_ac_power
http://www.ehow.com/how_5175805_understand-ac-dc.html
http://www.ehow.com/facts_6310325_ac-vs_-dc-currents.html

I hope this helps,
Steve
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Excellent and thorough response. Way more than I had hoped for, and greatly increased my understanding. Thanks!

Physics

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#### Steve Johnson

##### Expertise

I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

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I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

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BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University