In air, a charged soap bubble of radius $r$ is in equilibrium, outside and inside pressures being equal. The charge on the drop is $\left(\varepsilon_{0}=\right.$ permittivity of free space, $T=$ surface tension of soap solution),

A positive point charge $Q$ is kept (as shown in the figure) inside a neutral conducting shell whose centre is at $C$. An external uniform electric field $E$ is applied. Then,

The S.I. unit of capacitance of capacitor is

When a conducting slab fills the space between the two plates of a capacitor, its capacitance