Which type of circuit does the power wheel apply to?

The power wheel specifically applies to AC circuits with a unity power factor. A unity power factor means that the voltage and current are in phase; all the power supplied by the source is being effectively used by the load. In these conditions, the measurements of power are straightforward since the real power, reactive power, and apparent power are in perfect correlation, allowing for efficient energy transfer.

When discussing power systems, unity power factor is ideal as it minimizes the losses in the electrical system, leading to better efficiency. The power wheel is designed to measure or demonstrate the performance of circuits operating under these optimal conditions.

In contrast, other types of circuits listed would not align with the functionality of the power wheel. A circuit with low resistance may affect the current flow and indirectly influence the power factor but does not guarantee unity. DC circuits do not apply here because the principles illustrated by the power wheel are specifically tailored to AC systems, where phase relationships between current and voltage are significant. Circuits with variable power factors introduce complexity that the power wheel would not effectively illustrate, as they may represent non-linear loads or other inefficiencies. Thus, the correct choice highlights the ideal scenario where the power wheel operates most effectively.