The power factor is a useful topic covered as part of the curriculum on alternating current circuits. The first exposure is typically in introductory courses. It is revisited in upper level network theory or circuit analysis courses in physics and engineering. It is standard practice to include the power factor in the curriculum of the circuits courses taught to all engineers. While the power factor is simply the cosine of the phase angle between the voltage and current, it has practical application for circuits containing reactive loads. Working scientists and engineers are concerned about the power factor in a broad variety of contexts ranging from electromechanical systems, e.g. motors, to impedance matching networks in audio and broadcast systems. The power company cares about a user's power factor and sets rates accordingly. We have developed a simple demonstration of an alternating current circuit using electric lamps as a proxy for resistive loads. The demonstration can be adapted to a laboratory experiment in either a second semester general physics course or a more advanced laboratory course on circuits. The pedagogical value of the activity is two-fold. First, it provides a clear visual representation of the power factor, and second it serves as a springboard for further discussion of the nature of the phase relationship of reactive circuit elements.
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