How can current alternate

How can current alternate — why doesn’t it cancel itself out.

Actually, it does cancel out on the average. When you plug a toaster into the AC power line and turn it on, current begins to flow back and forth through that toaster. At first it flows out one wire of the outlet, through the toaster, and returns into the other wire of the outlet. About 1/120th of a second later, the current has reversed direction and is now flowing out of the second wire of the outlet, through the toaster, and into the first wire. It continues flowing back and forth so that, on the average, it heads nowhere. But the toaster receives energy with every cycle of the current so that there is a net flow of power to the toaster even if there is no net flow of current through it.

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