The graphs in Figure 18-20 show the charge \(q\) and current \(i\) as given by Equations 18-30. Both functions are proportional to \(e^{-t/RC}\), so both decrease by a factor of \(1/e\) whenever time \(t\) increases by one time constant \(RC\). So the value of \(RC\) determines both how rapidly the capacitor charges and how rapidly it discharges.