The unit of electric permittivity is

The capacitance of an isolated sphere of radius $r_1$ is increased by $5$ times,when it is enclosed by an earthed concentric sphere of radius $r_2$. The ratio of their radii is

Sixty-four drops are joined together to form a bigger drop. If each small drop has a capacitance $C$,a potential $V$,and a charge $q$,then the capacitance of the bigger drop will be

$A$ capacitor is formed by two concentric spherical shells. The potential of the shell with radius $R_1$ is $V_1$ and the potential of the shell with radius $R_2$ is $V_2$. What is the potential at a point at a distance $x$ from the center? $(R_2 > x > R_1)$

The capacitance of a spherical capacitor is $1 \mu F$. If the spacing between the two spheres is $1 \text{ mm}$,the radius of the outer sphere is

Assume each oil drop has a capacitance of $C$. If $n$ drops are combined to form a bigger drop,then the capacitance of the bigger drop $C^{\prime}$ would be