Does current make the full return trip? In an AC system, the neutral wire is said to provide a return path for the current back to it's source. Given that the current is said to be alternating back and forth, how does this work, does the exact same current leaving the source truly return all the way back along the nuatral wire? Also, where voltage is said to be zero along the neutral wire, what is providing the pressure to move the electons along the wire, if no potential difference exists? Surely this requires energy.
 A: The picture of charges travelling through the wire is to make this simple and easy to explain, electrons don't make round trip through the wire back to the source. When you apply a voltage electrons feel it and experience and electromagnetic force so they move but they pump into the structure of the material itself. In the absence of voltage, they don't move in straight line inside the metal, they move randomly, with voltage applied they start to shift, and this accumulated shift from all the electron is the current you measure. It's the electromagnetic energy that travels around the wire.
A: It is not the exact same electrons that were introduced into the wire that pop out again with the current reversal. All electrons experience some random motion at temperatures above absolute zero, so this in itself guarantees that some electons will be the same and some won't.
In fact, the electron drift velocity in copper is only .5 m/s per V/m electric field. Unless you maintain a non-reversing current for quite a long time, an electron you introduce into one end of the wire will only make it a few millimeters into the wire before the current reverses. For example, a 60Hz current in a 10m long wire with 1 V drop (about typical for 14 AWG house wire carrying 10A current), the average electron drift velocity is .5* (1/10) = 50 mm/s. For a 60Hz AC current, the average electron drift in the example is 50/120 = .4 mm before the current reverses.
Why does current travel in the neutral wire if it's voltage is 0? It's because the neutral wire voltage is in fact NOT zero along it's entire length. The neutral wire will actually have an increasing voltage (relative to the main electical panel neutral voltage) the further away from the panel you travel. Let's assume that the neutral wire voltage is actually zero at the electrical panel (a gross simplification, but good enough for now). Then if there is a current of 10 A flowing in the neutral for a distance of 10 m, and the wire has a resistance of .011 Ω/m (which is about right for 14 AWG house wire), then the neutral wire will actually be at a voltage of 10*10*.011 = 1.1 volts at the other end of the 10 m wire.
A: The electrons go back and forth in AC current. So it is not the case that they return to the source.
It is interesting to note that in the case of 3-phase AC supply, it is possible that the AC current is zero through the neutral wire. Each current in the live wires is $120^{\circ}$ out of phase with respect to the others:
$$I_N = I_Rsen(\omega t) + I_Ssen(\omega t + \frac{2\pi}{3}) + I_Tsen(\omega t + \frac{4\pi}{3}) $$
If $I_R = I_S = I_T$, the phases are balanced, and the above sum is zero.
