When the voltage applied to a circuit and the current through the circuit both reach their maximums and minimums at the same time, the voltage and current are:

When the voltage and current in a circuit reach their maximum and minimum values simultaneously, they are described as being "in phase." This means that the peaks and troughs of the voltage waveform align perfectly with those of the current waveform.

In an in-phase relationship, the energy transfer is efficient because both the voltage and the current are contributing to the power in a synchronized manner. This significantly affects the power factor of the circuit, leading to optimal power usage since power is maximized when both quantities align in timing.

In contrast, other terms such as "additive," "regenerative," or "out of phase" do not accurately describe this relationship. While "out of phase" refers to a scenario where voltage and current do not align—resulting in energy losses—"additive" does not pertain to phase relationships but rather to the sum of voltages or currents in a circuit. "Regenerative" is associated with systems that can return energy to the source, which is unrelated to the synchronization of voltage and current.

Therefore, being in phase indicates that both waveforms achieve their extremities in unison, optimizing power delivery within the circuit.