In a capacitive AC circuit, the current leads the voltage.

In a capacitive AC circuit, the current leads the voltage.

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

Answer: (B)

In a capacitor, the current is tied to how fast the voltage across the capacitor is changing. That relationship is i = C dv/dt. If the voltage is a sine wave, v(t) = Vp sin(ωt), then dv/dt = ωVp cos(ωt), which is a sine wave shifted by +90 degrees. So i(t) = C dv/dt = ωC Vp sin(ωt + 90°). The current reaches its peak a quarter cycle before the voltage, meaning the current leads the voltage by 90 degrees. This is the characteristic behavior of a pure capacitive circuit. For contrast, a inductor makes current lag the voltage, and a resistor makes them in phase.

In a capacitor, the current is tied to how fast the voltage across the capacitor is changing. That relationship is i = C dv/dt. If the voltage is a sine wave, v(t) = Vp sin(ωt), then dv/dt = ωVp cos(ωt), which is a sine wave shifted by +90 degrees. So i(t) = C dv/dt = ωC Vp sin(ωt + 90°). The current reaches its peak a quarter cycle before the voltage, meaning the current leads the voltage by 90 degrees. This is the characteristic behavior of a pure capacitive circuit. For contrast, a inductor makes current lag the voltage, and a resistor makes them in phase.