What is true power in an AC circuit?

• A. The product of circuit voltage and the current in phase with that voltage
• B. The product of voltage and current in any phase
• C. The reactive power in the circuit
• D. The total apparent power

Answer: (A)

True power is the portion of electrical power that actually does useful work. In an AC circuit with sinusoidal voltages and currents, it equals the product of the circuit voltage and the portion of current that is in phase with that voltage: P = V_rms I_rms cos(phi), where phi is the phase angle between voltage and current. This in‑phase current component is what transfers energy to load as heat, light, motion, etc. The current that lags or leads due to reactive elements stores energy temporarily and doesn’t contribute to average work, which is represented by reactive power Q = V_rms I_rms sin(phi). The total apparent power S = V_rms I_rms combines both real and reactive power as a vector magnitude. So the statement that true power is the product of circuit voltage and the current in phase with that voltage is the correct description because it isolates the energy actually doing work.