What is the power factor in a purely resistive circuit?

In a purely resistive circuit, the power factor is defined as the cosine of the angle between the voltage and the current. Since resistors do not introduce any phase difference between voltage and current, this angle is 0 degrees in a purely resistive circuit. The cosine of 0 degrees is 1, which means that the power factor is 1.0.

A power factor of 1.0 indicates that all the power supplied by the source is being used for useful work (real power), with no reactive power involved. This is a key characteristic of purely resistive circuits, distinguishing them from inductive or capacitive circuits where the power factor would be less than 1 due to the presence of reactive components that cause current to lag or lead the voltage.