In an AC circuit, the ratio of power in watts to the total volt-amps is known as the:

• A. Power factor is the ratio of reactive power to apparent power
• B. Voltage divided by current
• C. Power factor is the ratio of power in watts to the total volt-amps
• D. Resistance times current

Answer: (C)

Power factor is the ratio of real power to apparent power. Real power P (watts) is the part of the power that does useful work, while apparent power S (volt-amps) is the product of rms voltage and rms current, regardless of whether it does useful work. Because P = VI cos(phi) and S = VI, the ratio PF = P/S = cos(phi). This is the quantity that tells how effectively the circuit uses the supplied power; a PF of 1 means all the power goes into useful work, while a lower PF indicates more energy is wasted in reactive components.

In the statement given, the ratio of power in watts to the total volt-amps is exactly P/S, which is the power factor. The other ideas don’t describe PF: reactive power to apparent power is sin(phi), not PF; voltage divided by current is impedance; and resistance times current is a voltage drop across a resistor.