Discharging a capacitor

  The blue graph shows how the current (I) decreases as the capacitor discharges. The initial current (Io) is determined by the initial voltage across the capacitor (Vo) and resistance (R):

                        Graphs showing the current and
                                   voltage for a capacitor discharging

Initial current,  Io = Vo / R.

• Note that the current graphs are the same shape for both charging and discharging a capacitor. This type of graph is an example of exponential decay.

  The green graph shows how the voltage (V) decreases as the capacitor discharges.

    At first the current is large because the voltage is large, so charge is lost quickly and the voltage decreases rapidly. As charge is lost the voltage is reduced making the current smaller so the rate of discharging becomes progressively slower.

   After 5 time constants (5RC) the voltage across the capacitor is almost zero and we can reasonably say that the capacitor is fully discharged, although really discharging continues for ever (or until the circuit is changed).