The plus and minus actually indicates the phase difference in a 240v alternating current circuit. One leg is in the "negative" phase whenever the other in in the "positive" phase. Mathematically, the plus and minus signs make the calculation work, i.e. the voltage difference between them is 240v ( +120 and -120). Clear as mud, right?
It is merely convenient to think about the legs as plus and minus, as if it were two DC circuits. We use a similar mathematical convenience in DC circuits when we talk about plus and minus. The plus and minus actually represent direction of current flow rather than the physical charge present, but the arithmetic works and it makes it easier to comprehend the effects. And it all gives electrical engineers and physicists something to argue about over cocktails!
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