Final answer:
A power factor is the cosine of the phase angle between voltage and current in an AC circuit, reflecting the efficiency of power usage. It ranges from 0 to 1, with values near 1 indicating efficient power utilization.
Step-by-step explanation:
A power factor is not an angular measure but a numerical ratio with a value between 0 and 1, equal to the cosine of the phase angle between the voltage and current in an AC circuit.
The power factor represents the amount by which the power delivered in the circuit is less than the theoretical maximum of the circuit due to voltage and current being out of phase.
For a purely resistive load or at the resonant frequency of an RLC circuit, the power factor is 1, and this is the ideal condition as it indicates efficient power usage. When designing something like an efficient motor, having a power factor close to 1 is highly desirable.