At any time, the charge Q on the capacitor is related to the potential difference V across it by Q=CV. If an alternating voltage is applied across the capacitor, then the current and voltage are ou...At any time, the charge Q on the capacitor is related to the potential difference V across it by Q=CV. If an alternating voltage is applied across the capacitor, then the current and voltage are out of phase, and that V lags behind I by 90°,