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Voltage or current whose direction changes periodically over time. When the direction changes, it passes through a zero point, and the positive and negative values offset each other on average to become zero. The basic form is a sine wave, but when harmonic components are added, it becomes a distorted wave. When converting AC to DC, the average value is zero, so the root mean square (RMS) value is used. RMS value is calculated by integrating the square of the voltage or current over one cycle, dividing the result by the cycle, and then taking the square root of that value.