How AC really works versus the way people describe it

I'm new to these things, but AC to me doesn't mean the black wire swings 120v rms above and 120v rms below ground (I know it does from "ground's" point of view), but instead conventional current comes out on "black" and returns on "white" and then comes out on "white" and returns on "black" 60 times a second. Is that right or am I just way off beam here?

Thanks for any help you can offer because I read things like, "The white wire is the return. It carries current, but has no voltage."

I know they must mean "no voltage relative to ground", but they never say that and to call the white the return only, makes me think that I don't understand AC at all.

I think I understand how most, if not all, people refer to the hot vs neutral wire (they see it as in reference to ground), but my "misunderstanding" is that I believe that this is a convenient way to look at AC, but not reality.

I will try to say what I understand and would very much appreciate it if someone could tell me where I am wrong:

Theoretically, there is a spinning wire in a magnetic field that emerges from some turbine as two ends of the same wire (no load connected). These two ends alternate in a push-pull of electric potential. If neither of these ends were grounded then they would indistinguishable, but a load could be put across them and current would flow back and forth with neither one considered the source or return or better yet, one is the source and one the return and then they switch roles. Now, in our AC system, one is earth grounded for safety and so it is color coded white, but that doesn't change the fact that it is a source 60 times per second just as the black wire is.

Is that right? And if it is, why do people always say the hot wire is sourcing positive and then negative current? I understand there is nothing wrong with saying this in reference to ground, but I thought most people are taught to follow conventional current, which would be the flow of positive charge, which to me means seeing it come from one side of the turbine and the coming from the other side.

Thanks for any help.

You are right on all counts.

But understand that much of "conventional" wisdom is a collective confusion of fact(s) to simplify concepts.

Yes the convention for labeling wires can be confusing.

All voltages are relative, so a voltmeter across the black and white wires will see 120Vrms (about 339Vpp). Makes no difference whether the white is grounded or not. When you ground the white wire then you will see the same voltage whether measured between black and white or black and ground (ignoring any voltage drop in the white due to current flowing). In either case the current is flowing back and forth in both wires -- out the black wire and into the white wire when the black is positive with respect to white, and into the black wire and out the white wire when the black is negative with respect to the white.

The white is commonly cause the "return" because it is at ground potential, but it doesn't define or limit the direction of the current in the return. The current flows in both directions, once each cycle.

Thanks for reassuring me on this. I couldn't see how it could be any other way, but when I have posted questions similar to this I have had people "correct" me with phrases like "no voltage on the neutral" even though I explained my understanding of "no voltage relative to ground". So I was beginning to doubt myself.

Thanks again.

I don't agree with a few things in your original post. but these concepts are the most critical:

1. 120 RMS. If I put 120 VDC or 120 VAC RMS to the same resistor, it will dissipate the same amount of power. 120 VAC does not swing 120 VRMS each way, but rather (+) sqrt(2)*120 and (-) sqrt(2)* 120 in a sin wave shape.

2. Meters unless TRMS ASSUME that the input is a sin wave and display the RMS value.

3. In the US Neutral and Ground are connected together at ONE POINT only. Ground is only supposed to conduct fault currents and provide a reference potential.

4. The Earth itself is ground, but the potential is not the same everywhere. It changes dramatically during a lightning storm.

5. You also touched on "conventional current", good. It goes positive to negative. However, there are times that one neds to know the flow of electrons: (Valve amplifiers, solid state devices and chemistry). I think Ben Franklin got the sign wrong.

6. Although I said flow of electrons, use that a visualization tool. Think of a circular string of "bumping cars" where the "bumping cars" are the electrons. One bumps into another with a force of a "voltage" and they appear to flow.

7. when doing real calculations, power Dissipated is always positive. Power delivered is always negative. But we don't say we have -12 MW power plant do we? For the equations to work, we actully do.

8. We call residential power "Single phase" and it is and it isn't. The voltage delivered by the power company is indeed single phase, but the voltage delivered to the residence is really "Bi-phase". This is not two phase power, which did exist at one time. My friend's place of business had it at one time and I've seen the motor nameplate. Bi-phase is because the center-tap of a say a 9 KV (from the power company) to 240 V (CT) Center Tapped transformer is grounded.

crutshow said:

The white is commonly cause the "return"

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I believe he meant cause to be called.

cbb said:

surprised you didn't cathch ther swing 120 RMS issue.

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NEUTRAL - At the same potential as ground
HOT - You'll get a nasty shock if a part of your body is grounded.

L1- Line #1 or Power Line #1
L2 - Line #2, or Power line #2

GROUND - A reference potential. EARTH - Literally the Earth. They do have different symbols.

Thanks for all the good information, I think I got it now.

Originally Posted by crutshow
The white is commonly cause the "return"

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KeepItSimpleStupid said:

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I believe he meant cause to be called.

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Yup, just a little typo I missed.

cowboybob said:

You are right on all counts.

But understand that much of "conventional" wisdom is a collective confusion of fact(s) to simplify concepts.

Click to expand...

Hi there cowboybob,


I have to say that's quite a good explanation of the current state of knowledge in the world, sad but true
I like to call it an "intuitive bridge" as we're constantly looking for ways to explain things we dont really know how they work.

To the main point:
The ground wire was introduced back when everything was made of metal, and before that there were only two wires the black and the white. I think the ground wire was intended to connect to the chassis so that if anything came loose it would contact the metal chassis and short out and blow the fuse thus disconnecting the circuit and preventing shock.
But as i think about this a bit more, and realizing that what cowboybob points out is actually true, could it be that adding the ground wire actually just complicates matters now that everything is made of plastic (and maybe even when it's metal) ?
The only thing a ground fault interrupter detects is the loss of current from the black wire to the white wire. If the current from/to the black wire does not equal the current to/from the white wire then a ground fault is detected (with some error margin). But that doesnt require a ground wire. And the only way a ground fault can be detected is if the current is drawn away from the black or white wire through an external ground (ie the human who might get shocked). So we have to wonder if the ground wire actually does anything anymore.