When voltage and current are out of phase, which quantity represents the power in the circuit?

• A. Capacitance
• B. Current
• C. Reactive power
• D. Frequency

Answer: (C)

When voltage and current are out of phase in an AC circuit, some energy is momentarily stored and released by reactive elements like inductors and capacitors rather than doing work. The quantity that captures this back-and-forth energy is reactive power. It reflects how much power is circulating between the source and the reactive components, and it is defined with the sine of the phase angle: Q = V I sin(phi). In contrast, real (active) power P = V I cos(phi) is the portion that actually does work, and apparent power S = V I combines both as the total power flow. When there is no phase difference, reactive power is zero and all power is real.

Capacitance is a property of a component, not a power quantity; current is a flow rate, not power; frequency is how often the waveform repeats, not a measure of power.