What is power factor in AC circuits?

• A. The ratio of reactive power to real power
• B. The ratio of apparent power to real power
• C. The ratio of real power to apparent power
• D. The phase angle between current and voltage

Answer: (C)

Power factor measures how effectively the circuit converts electrical power into useful work. It is the ratio of real power to apparent power: PF = P / S. Real power (P) is the portion that actually does work, measured in watts. Apparent power (S) is the total power flow, found by multiplying RMS voltage by RMS current (V_rms × I_rms), measured in volt-amperes. The power factor also equals the cosine of the phase angle between voltage and current: PF = cos(phi). A power factor close to 1 means most of the current is doing useful work; a lower power factor means more energy is being stored and returned by reactive elements (inductors and capacitors). It helps to remember that the ratio of reactive power to real power is Q/P, and the ratio of apparent power to real power is S/P—neither of these is the power factor itself, which is specifically P divided by S.