When a sinusoidal AC voltage is applied to an ideal capacitor, which correctly describes the phase relationship between the current and the voltage?

Answer: C. The current leads the voltage by 90°

A capacitor's current is i = C dv/dt, proportional to how fast the voltage changes. The voltage changes fastest as it crosses zero, where the current peaks, so the current leads the voltage by 90° (a quarter cycle). In an inductor, v = L di/dt makes the current lag by 90° instead, and in a resistor they are in phase. Because voltage and current are 90° apart, an ideal capacitor dissipates no power averaged over a cycle.

• A. A current lagging the voltage by 90° describes an ideal inductor (coil).
• B. Current and voltage are in phase in a resistor.
• C. Correct. Since i = C dv/dt, the current leads the voltage by 90°.