In an AC circuit, unless otherwise specified, voltages and currents are assumed to be

In an AC circuit, unless otherwise specified, voltages and currents are assumed to be

• A. Peak values
• B. Effective (RMS) values
• C. Average values
• D. Instantaneous values

Answer: (B)

AC voltages and currents are described by their RMS (root-mean-square) values because this value reflects the actual heating or power-delivering effect of the varying signal. The RMS value is the DC-equivalent value that would dissipate the same average power in a resistor. For a sinusoidal waveform, Vrms equals V peak divided by the square root of two, and the same relation holds for current. Using RMS values lets us apply simple power relationships like P = Vrms × Irms × cosφ, and for a purely resistive load cosφ is 1, so P = Vrms × Irms. Peak values describe the maximum amplitude, not the steady heating effect; instantaneous values change with time; and average values over a cycle don’t capture the ongoing power transfer (for a symmetric AC signal the signed average is zero). Hence the standard default is RMS.

AC voltages and currents are described by their RMS (root-mean-square) values because this value reflects the actual heating or power-delivering effect of the varying signal. The RMS value is the DC-equivalent value that would dissipate the same average power in a resistor. For a sinusoidal waveform, Vrms equals V peak divided by the square root of two, and the same relation holds for current. Using RMS values lets us apply simple power relationships like P = Vrms × Irms × cosφ, and for a purely resistive load cosφ is 1, so P = Vrms × Irms. Peak values describe the maximum amplitude, not the steady heating effect; instantaneous values change with time; and average values over a cycle don’t capture the ongoing power transfer (for a symmetric AC signal the signed average is zero). Hence the standard default is RMS.