Four metallic plates, each with surface area of one side, A, are placed at a distance d from each other. The plates are connected as shown in fig. Then the capacitance of the system between P and Q is

The equivalent circuit is shown in fig. The condensers Y and Z are in parallel. Hence, their equivalent capacitance is 2 C. This combination is in series with capacitor X. Hence the equivalent capacitance between P and Q is given byCPQ=C×2CC+2C=23C=23ε0Ad