Question

Two identical capacitors A and B are charged to the same potential V and are connected in two circuits at t = 0 as shown in figure.

The charges on the capacitors at a time t = CR are, respectively,
(a) VC, VC
(b) VC/e, VC
(c) VC, VC/e
(d) VC/e, VC/e.

Answer 1

In circuit (a), the diode is forward biassed. So, it offers negligible resistance to the flow of current and can thus be replaced by a short circuit. Now, the capacitor charge will leak through the resistance and decay exponentially with time.
Capacitor charge = $\frac{VC}{\mathrm{e}}$
In circuit (b), the diode is reverse biassed. So, it offers infinite resistance to the current flow and can thus be replaced by an open circuit. As the circuit is open now, no current can flow across the resistance. So, the charge in the capacitor cannot leak through the resistor.
Capacitor charge = VC