Amperage and Alternating Current

[tommygdawg]

Hey all.

This may be a really basic question...but how does current reverse direction in one AC cycle? I'm having a hard time with this conceptually. I know that coming into my house (USA) are two hot legs of 120V that are 180 degrees out of phase. When one leg is at a nominal 120V the other is at a nominal -120V. The fact that its an AC sinewave means the current is reversing direction 60 times a second. This also means that when both phases are evenly balanced there's effectively 0 amps on the neutral, right?

So my question is...what exactly is meant by the current reversing direction? How do the electrons go in an opposite direction? Maybe I'm taking the water hose analogy too far with trying yo visualize this but its just not making sense to me.

Is my question making sense? In DC it makes total sense to me. The negatively charged electrons are drawn to the positively charged terminal of the battery. But what are electrons drawn to in AC? And also, they obviously aren't becoming positrons on half the cycle... So what's going on??

Any advice would be appreciated!

[dacman]

AC, or Alternating Current, reverses current direction because of a spinning generator (although this can be simulated).  The hot wire goes both + and - in one cycle.  A Sine Wave is basically a circle spread out over time.  (Vice wrapping back around to the start, it continues to progress forward in time.)  120 Vrms is the RMS value, and the + peak of a 120 Vrms sine wave is about + 170 V and the negative peak is about - 170 V.  If a 120 Vrms line is being used, then the return path is the neutral wire (same current).  If 240 Vrms is being used, then the current flow is between the two hot wires (and not the neutral).  The output voltage to a US household comes from a 240 Vac center tapped transformer.  The center tap is grounded and called neutral (in US households).

[BobsURuncle]

The short answer is: the current reverses direction because the voltage reverses direction, just as it would in a DC circuit if the battery were reversed.

[rstofer]

For the moment, ignore the 240V situation and just think about one hot leg and the grounded neutral.  If you were to put a scope on the hot wire (don't really do this!), you would see a sine wave traversing both above and below the 0V center line.  Now add a resistor to your mental model.  On the positive half wave, the current flows in one direction through the resistor and on the other half wave, the other direction.

Now, for the 240V situation.  As you stated, the sine waves are 180 degrees apart.  Mentally color one red and the other yellow.  At some point, the red is positive with respect to the yellow and our resistor (yup, we had to wire it up again) sees current flow in one direction.  A short time later, the yellow wire is positive with respect to the yellow wire and the current is flowing in the other direction.

Pretty simple when you color the wires!

[Signal32]

Maybe a visualization would help. They don't talk directly about current but it's implied and you can easily figure it out.
There are several videos describing what you are asking.
youtube.com/watch?v=BcIDRet787k
youtube.com/watch?v=xUSSTV0Hs0s

[tommygdawg]

Thanks so much, all. I understand it now. I was a bit confused and didn't realize that electrons actually move very little along the conductor, versus direct current. So I have another question following this:

How do you complete a circuit from the power company to your house to your appliance and then back? I think my problem is I'm still visualizing this like an LED circuit with a 9V battery. You connect the ends of the bulb (with a resistor of course) to the battery leads, the circuit path is complete and electrons flow. But when a power company supplies AC to my house...does the circuit back to the power company get completed in the same way? Or does the unbalanced current from my two phases just go straight into the ground (since ground and neutral are connected at the panel)? If so...I go back to my original question: if the circuit isn't completed by going back into the power company, then how does this whole power grid thing work at all?

[Signal32]

Yes, electrons do move little in AC, also in all cases they move slowly:
"For example, for a copper wire of radius 1 mm carrying a steady current of 10 Amps, the drift velocity is only about 0.024 cm/sec"

About power companies: at 4:10 he describes exactly how power companies work, accompanied by graphic visualisation.
youtube.com/watch?v=BcIDRet787k

[dacman]

There is a complete loop from the power company to the primary of the power transformer.  On the secondary side of the power transformer, there is a complete loop to your house.  It is only grounded on the secondary side for your safety (and for interference reasons).  The center tapped secondary would supply power whether or not it was grounded.  But because it is grounded, a circuit can be completed through literal earth ground on the secondary side.