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

Capacitor once gets charged upto maximum potential .After that for any other lesser value of p.d across A and B diode is reverse biased and it does not allow charge to flow in opposite direction.
From ${\mathrm{V}}_{\mathrm{rms}}=\frac{{\mathrm{V}}_{max}}{\sqrt{2}}$
$\begin{array}{l}\Rightarrow {\mathrm{V}}_{max}={\mathrm{V}}_{\mathrm{rms}}\sqrt{2}=220\sqrt{2}\mathrm{volts}\\ out put voltage \frac{V_{max}}{\sqrt{2}}=110\sqrt{2}\end{array}$