Suppose you run an AC-current through an RC-circuit. Writing it all up in terms of the complex impedance you find that the current amplitude is decreased due to the capacitor. Physically I do not understand this. How does a capacitor decrease the amplitude of the current, when the current is alternating. Upon charging up the capacitor the current becomes less and less, but don't you get back this sink in the current when the capacitor discharges and works with you?