One of the two identical capacitors having the same capacitance $C$, is charged to a potential $V_1$ and the other is charged to a potential $V_2$. If they are connected with their like plates together, then the decrease in the electrostatic potential energy of the combined system is

If 27 indentical charged conducting spheres each of capacitance $10 \mu \mathrm{~F}$ combine to form a big sphere, then the capacitance of the big sphere is

The capacitance of a spherical capacitor is 100 pF . If the spacing between the two spheres is 1 cm , then the radius of the inner sphere of the capacitor is

The energy stored in a capacitor is $W$. To double the charge on the plates of the capacitor, the additional work to be done is