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How does alternating current get anywhere if it travels "back and forth" in the wire?. I understand that Direct Current is a flow of electrons through a wire, but I have always been confused with how Alternating Current works. I expect I am missing something simple but could anyone point me to a resource that does a good job of explaining AC, or perhaps offer an explanation themselves? Thanks!

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  • \$\begingroup\$ Alternating current doesn't actually need to "go" anywhere to do its work. What matters is that the wave itself transfers the energy while propagating over the electrons. Also note that the speed of the wave is not the speed of the electrons traveling through the wires (if they are traveling at all). \$\endgroup\$ – AndrejaKo May 1 '12 at 6:20
  • \$\begingroup\$ Thanks everyone. Some great explanations. So if I am understanding things properly I shouldn't think of electricity as a flow of electrons but rather as NRG flowing along the electrons. The electrons are just the conduit that the NRG is flowing through? \$\endgroup\$ – webworm May 1 '12 at 14:48
  • \$\begingroup\$ Work is mostly done by the electromagnetic fields induced by moving charge. The most common way of moving charge is moving electrons through a conductor. In that sense, yes, electrons are just the conduit, like oil in a hydraulic drive system. \$\endgroup\$ – posipiet May 1 '12 at 17:25
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Current or anything else energy-carrying doesn't need to "get anywhere" long term to deliver energy. Think of how pistons work in a gasoline engine. They only go back and forth and don't "go anywhere" yet still deliver power to elsewhere (the crank shaft).

Alternating current is a bit like the pistons. You can still extract useful work from it.

Another way to look at alternating current is instantaneous direct current that happens to change over time. Let's say the current if following a sine function with a peak amplitude of 1.41 A. At any point in the cycle there is some amount of instantaneous current flowing, which is anywhere from -1.41 A to +1.41 A. Sometimes the current is 0 and you can't get any work from it. Other times it is non-zero and you can. If you break the cycle up into lots and lots of instantaneous snapshots, you can find the equivalent average steady current level you could extract the same work from. That is the RMS (Root Mean Square) value, which in this case is 1 A. At any one time you might get a bit more or a bit less, but averaged over a cycle this AC current will be equivalent to 1 A DC for the purpose of extracting work. This averaging of the instantaneous snapshots is really the integral. You can write it down yourself and see the result. Keep in mind that the work a current can do is proportional to the square of that current, which is why the negative parts don't cancel the positive parts.

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And now, your moment of Zen.

puts on hat of Zen

Think of waves on a beach. They go in, they come out. They go in, they come out. Now, if you're a human with your feet in the sand, it is relatively easy to pretend that nothing is moving. If you're a hermit crab or a zebra mussel, however...

Now, let us examine the motion of the waves for a few seconds. When they reach the extremes of motion (ie all the way in or all the way out) they seem to be standing perfectly still. Hmmm, interesting...

Now you might ask yourself, how can gentle, rhythmic, periodic motion "do anything"? Well, consider the fact that every single grain of sand on that beach used to be part of a mountain.

Well, OK, at least a boulder.

Physics - making us feel bloody insignificant since 650 BCE.

removes Zen hat

To answer your question more specifically, the alternating current consists of a wave which goes in and out a lot faster than the one on the beach - in general at least 60 times faster.

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Instead of thinking of electricity travelling from point A to point B think of it as the result of reactions among electrons. A good visual representation is Newton's Cradle. enter image description here

The locations of the bearings don't change but the momentum (energy) is still transferred.

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  • \$\begingroup\$ So, how about AC, which was OP's question? \$\endgroup\$ – Federico Russo May 1 '12 at 8:42
  • \$\begingroup\$ Wait for the other bearing to come back? \$\endgroup\$ – Bryan Boettcher May 2 '12 at 19:00
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I see this question all the time and would like to offer this input. Everyone seems to understand DC, so for a moment, let’s consider a battery circuit with a positive terminal and a negative terminal. The positive terminal has a positive voltage and the negative terminal is usually considered the "ground" and a positive to negative connection completes the circuit.

What is the voltage at the positive terminal? 5VDC? 9VDC? 12VDC? It doesn't have to be fixed. The "voltage" at the positive terminal might be fixed, but it might also be variable.

In an AC voltage source, all of the voltage appears on the HOT wire, in the form of a sine wave. It is variable from 0V to +Vpeak back to 0V then negative to -Vpeak then back to 0V. The other wire required to complete the circuit is the NEUTRAL and its entire purpose is to provide a return path. It is not a “ground”, there is no “ground” in an AC source. All of the voltage in an AC source is coming from the HOT wire which is why it is called HOT. In an AC source, the voltage signal on the HOT wire alternates from 0V to +vPeak back to 0V then it goes negative to -vPeak then back to 0V.

People have a hard time grasping the idea that the HOT can go negative, because they try to compare it to the principles of DC voltages that use the return (negative terminal) as GROUND. An AC source has no "ground". The HOT wire carries a sine wave that is constantly changing from 0V to +vPeak back to 0V then negative to -vPeak then back to 0V – usually alternating around 60 times per second in the U.S., ot 60hZ

The 3rd wire that you see in an AC plug, called the ground, is not like the ground in a DC circuit. In an AC circuit, this "ground" is an additional wire that is usually connected to the device internally on the other end, and provides a safety path so that consumers don't get electrocuted in the event that something inside the device comes into contact with the HOT wire. Unlike DC, in an AC circuit, the GROUND wire is not needed at all and has nothing to do with the flow of AC current in the device.

In an AC circuit, the NEUTRAL wire is the return for the alternating voltage that flows from the HOT wire. If we connect a Center Tapped Transformer between the HOT and NEUTRAL AC wires, the center tap then becomes a "VOLTAGE REFERENCE POINT" which allows us to see the +Voltage of the sine wave where the HOT side goes into the transformer, and the -Voltage of the sine wave where the NEUTRAL wire goes into the transformer. The voltage is not switching back and forth between the NEUTRAL and HOT, the HOT wire is carrying a sine wave from 0V to +vPeak then back to 0 down to -vPeak then back to 0. Once again, the NEUTRAL wire is there to complete the circuit - It has no source voltages. All of the voltage in an AC circuit comes from the HOT wire.

This is why, in an AC circuit, the wires are labeled HOT and NEUTRAL and are required for completing the circuit. The third wire, GROUND, is there for safety purposes only. HOT is carrying a SINE WAVE, NEUTRAL is the return, and GROUND is there strictly for safety purposes.

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One more analogy.

Direct current is like a chainsaw - the sharp bits travel in one direction doing work (slicing wood) and then return to their original location. The motion of the chain is constant.

Alternating current is like a hand saw - the sharp bits travel in one direction, then briefly stop, then travel in the opposite direction.

This analogy breaks down with three phase alternating current. Three phase is good because it does not have the zero spots (current is always flowing between at least two wires) and makes it possible to design efficient and reliable motors without the need of complex electronics.

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