In an ac circuit, the effective voltage is

• A. Equal to the maximum instantaneous voltage.
• B. Less than the maximum instantaneous voltage.
• C. Greater than the maximum instantaneous voltage.
• D. Always equal to the RMS value of the supply.

Answer: (B)

In AC circuits, the term effective voltage usually means the RMS (root-mean-square) value—the DC-equivalent voltage that would produce the same heating in a resistor. For a sinusoidal voltage v(t) = V peak · sin(ωt), the RMS value is V peak divided by √2, which is about 0.707 of the peak. Since the peak is the maximum instantaneous value the waveform reaches, the RMS value is inherently smaller than that maximum. So the effective voltage is less than the maximum instantaneous voltage. The other statements don’t match this relationship for a typical AC sinusoid: the instantaneous voltage reaches higher values than the RMS, and while RMS is the recognized effective value, it is not equal to the instantaneous peak.