A 220 V AC supply is connected between points A and B (figure). What will be the potential difference across the capacitor?

As p-n junction conducts during the positive half cycle only, so the diode connected

$$\begin{gathered} \mathrm{Potential} \mathrm{difference} \mathrm{across} \mathrm{C}=\mathrm{peak} \mathrm{voltage} \mathrm{of} \mathrm{the} \mathrm{given} \mathrm{AC} \mathrm{voltage} \\ {\mathrm{V}}_0={\mathrm{V}}_{\mathrm{rms}}\sqrt{2}=220\sqrt{2}\mathrm{V} \end{gathered}$$here will work in positive half cycle.