If the power factor PF equals cos(phi) = 0.8, what does the angle phi represent in the voltage-current relationship?

• A. The phase angle between voltage and current; negative for inductive lag
• B. The ratio of reactive power to real power
• C. The phase angle between voltage and current; positive for inductive lag
• D. The supply frequency

Answer: (C)

The angle phi is the phase difference between the voltage and the current in an AC circuit. Since the power factor is PF = cos(phi), a PF of 0.8 means phi = arccos(0.8) ≈ 36.9 degrees. A positive phi indicates the current lags the voltage, which is typical for inductive loads. In other words, the voltage reaches its peak first and the current follows about 36.9 degrees later. For context, the ratio of reactive power to real power is related to phi by Q/P = tan(phi), not equal to phi itself. The supply frequency is a different quantity altogether and not represented by phi.