What is meant by the power factor in an AC circuit?

• A. The ratio of current to voltage.
• B. The ratio of impedance to resistance.
• C. The ratio of resistance to impedance.
• D. The ratio of true power to apparent power.

Answer: (D)

Power factor is about how efficiently the circuit uses the supplied power. It is the ratio of true power to apparent power. True power, measured in watts, is the real work the circuit does. Apparent power, measured in volt-amperes, is the product of voltage and current. The ratio pf = P / S, and it also equals cos φ, where φ is the phase angle between voltage and current. When a load is purely resistive, voltage and current are in phase, so pf is 1 and all the power does useful work. With inductive or capacitive elements, voltage and current are out of phase, so the power factor drops below 1, meaning some power is stored temporarily in the field rather than doing work. The description that the power factor is the ratio of true power to apparent power captures this relationship. The other options describe different electrical relationships (like a simple ratio of current to voltage or ratios involving impedance) and do not define power factor.