In a capacitive circuit, which quantity leads?

In a capacitive circuit, which quantity leads?

• A. Voltage leads current
• B. Current lags voltage
• C. Current and voltage are in phase
• D. Current leads voltage

Answer: (D)

In a capacitor, the current leads the voltage. This happens because the current is tied to how fast the voltage across the capacitor changes: i = C dv/dt. If the voltage varies sinusoidally as v = Vm sin(ωt), then i = C dv/dt = C ω Vm cos(ωt) = C ω Vm sin(ωt + 90°). So the current waveform reaches its peak a quarter cycle before the voltage waveform, meaning the current leads the voltage by 90 degrees. This is why, in contrast, an inductor causes the voltage to lead the current, and a resistor has current and voltage in phase. The lead/lag relationship is a fundamental sign of capacitive behavior in AC circuits.

In a capacitor, the current leads the voltage. This happens because the current is tied to how fast the voltage across the capacitor changes: i = C dv/dt. If the voltage varies sinusoidally as v = Vm sin(ωt), then i = C dv/dt = C ω Vm cos(ωt) = C ω Vm sin(ωt + 90°). So the current waveform reaches its peak a quarter cycle before the voltage waveform, meaning the current leads the voltage by 90 degrees. This is why, in contrast, an inductor causes the voltage to lead the current, and a resistor has current and voltage in phase. The lead/lag relationship is a fundamental sign of capacitive behavior in AC circuits.