In AC circuits, which statement correctly describes RMS value?

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RMS measures the effective value of an AC signal in terms of heating effect. It is calculated as the square root of the average of the squares of the instantaneous values over time: Vrms = sqrt( (1/T) ∫ v(t)^2 dt ). For a sinusoidal voltage, Vrms equals the peak value divided by sqrt(2). This is the quantity used to compute power in a resistor since P = Vrms^2 / R. The statement describing RMS as the square root of the average of the instantaneous values is missing the crucial squaring step, so the precise description is the square root of the average of the squares of the instantaneous values. Other options refer to different waveform properties: the average value of the absolute value is the average rectified value, the peak value is the maximum amplitude, and the plain average value is the DC average (which is zero for a balanced AC waveform).