A spherical condenser has inner and outer spheres of radii $a$ and $b$ respectively. The space between the two is filled with air. The difference between the capacities of two condensers formed when the outer sphere is earthed and when the inner sphere is earthed will be

The expression for the capacity of the capacitor formed by compound dielectric placed between the plates of a parallel plate capacitor as shown in figure, will be (area of plate $=A$)

Four plates of equal area$A$ are separated by equal distances $d$ and are arranged as shown in the figure. The equivalent capacity is

Five capacitors of $10 \mu \mathrm{F}$ capacity each are connected to a $DC$ potential of $100 \mathrm{V}$ as shown in the below figure. The equivalent capacitance between the points $A$ and $B$ will be equal to

In the circuit diagram shown in the below figure, the resultant capacitance between points $P$ and $Q$ is

Four capacitors each of capacity $3 \mu \mathrm{F}$ are connected as shown in the below figure. The ratio of equivalent capacitance between $A$ and $B$ and between $A$ and $C$ will be

Three equal capacitors, each with capacitance $C$ are connected as shown in figure. Then the equivalent capacitance between point $A$ and $B$ is