A common question that comes up on YouTube, and recently over email: How does AC change direction?
Let's simplify and talk about the AC used to distribute electricity. "Mains" power is typically generated using some form of electric generator. As massive coils of wire spin around permanent magnets, voltage builds up, and current begins to flow.
The part of the generator with the coils is called the armature. As the armature passes through the different parts of the magnetic field, the induced on the wires changes as well. This cycle is also why you see the waveform build up then down, and then in the "opposite direction." That's how fast the armature is spinning around.
After the episodes on motors are finished, I'll make an animation for what is going on. (I'm sure there's something on the internet already.)