Capacitor and AC

• With an alternating source, in the first half cycle the capacitor charges until its voltage equals the peak of the source; as the source voltage falls, the capacitor (now at higher voltage than the source) discharges back into it; in the second half cycle the plates charge with the opposite polarity, and so on.
• Charge flows in and out of the plates, so an alternating current passes in the circuit (although no charge crosses the insulator).

• The current at any instant is proportional to the rate of change of charge, I = ΔQ/Δt = C ΔV/Δt.
• It is maximum when the voltage is passing through zero (steepest slope) and zero when the voltage is at its maximum.

In a capacitor the voltage lags the current by 90° (the current leads the voltage by 90°).