So root mean square is just an average but we get rid of the minuses before we take the average.

So AC has positive and negative components that are equal to each other, if we actually average this at all points we get 0 volts, clearly 0 volts of electricity is not going to power everything so something must be wrong.

The trick is that electricity does work in both directions, whether current is flowing forward or backwards it still will light up a lightbulb or heat your water.

So for all useful measures of say energy use or something, we care about the total amount voltage with both positive and negative voltages contributing positively. To get rid of these negatives, we square the voltage first (since negative times negative = positive), take the average, then square root it to undo the square.

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