Two identical capacitors $A$ and $B$, charged to the same potential 5 V are connected in two differe

Question

Two identical capacitors $A$ and $B$, charged to the same potential 5 V are connected in two different circuits as shown below at time $t=0$. If the charge on capacitors $A$ and $B$ at time $t=C R$ is $Q_A$ and $Q_B$ respectively, then (Here $e$ is the base of natural logarithm)

Professor PC Thomas Classes · Answer

(c) : During discharging, charge on the capacitor at any time, $t$ is, 
$Q=Q_0 e^{-t / R C}$ ...(i)
 In the first circuit, after connections are made diode become reverse biased and hence the charge on capacitor remains same, i.e., $Q_A=5 C V$ For the second circuit, charge at time $t=R C$ is\ $ Q_B=Q_0 e^{-R C / R C} $\ [using equation (i)] So, $Q_B=\frac{5 C V}{e}$