A capacitance $C$ with a charge $q_{0}$ at $t=0$ discharges through a resistance $R .$ Use the voltage equation $q / C+I R=0$ to show that the relaxation time of this process is $R C \mathrm{~s}$; that is,
$$
q=q_{0} \mathrm{e}^{-t / R C}
$$
(Note that $t / R C$ is non-dimensional.)