Circuits that are composed of batteries and resistors only produce steady currents and voltages that do not change with time. However, for many practical applications, time-dependent currents and voltages are desired. A circuit involving a resistor and a capacitor connected in series is called an $RC$ circuit. In a closed $RC$ circuit, the capacitor goes through a transient period of charging (or discharging). As a result, the flowing current changes gradually from an initial value, when the circuit is closed, to a final steady-state value, when the capacitor is fully charged (or discharged). The length of the transient period depends on the sizes of the capacitor and resistor.