Charge Q is given to the upper plate of an isolated parallel plate capacitor of capacitance C. The potential difference between the plates

$\frac{Q/2}{C}$

In general, for charge Q­₁ and Q₂on upper and lower plate respectively the charge distributions on outer and inner part of the plates are shown in figure.

                        Here Q­₁ =Q,  Q₂ = 0

\                     Charge on inner side of plate are $\frac{Q}{2}$ and $\frac{-Q}{2}$  respectively.

                        $\text{Hence } V = \frac{Q}{2C}$