In a purely capacitive circuit, how is current related to the applied source voltage?

In a purely capacitive circuit, the current leads the applied source voltage. This occurs because capacitors store energy in an electric field and release it later, causing a delay in the response to a change in voltage. When an alternating current (AC) voltage is applied to a capacitor, the capacitor will begin charging and discharging cycles that correspond to the voltage waveform, yet the current reaches its maximum values before the voltage does.

This leading relationship means that if you were to plot both the voltage across the capacitor and the current flowing through it on a graph, the current waveform would peak before the voltage waveform. This phase difference is an essential concept in understanding how reactive components, like capacitors, influence the behavior of AC circuits.

The other options describe different relationships that apply to other types of circuits: a lagging current is characteristic of inductive circuits, in-phase relationships apply to purely resistive circuits, and the term "alternates" doesn’t accurately describe the relationship between current and voltage in reactive circuits. Understanding these distinctions is key to effectively analyzing AC circuits and designing electrical systems.