The potentials of the two plates of a capacitor are $+10\,V$ and $-10\,V$. The charge on one of the plates is $40\,C$. The capacitance of the capacitor is........$F$.

$A$ solid conducting sphere of radius $R_1$ is surrounded by another concentric hollow conducting sphere of radius $R_2$. The capacitance of this assembly is proportional to

The stratosphere acts as a conducting layer for the Earth. If the stratosphere extends up to $50 \ km$ from the Earth's surface,calculate the capacitance of the spherical capacitor formed between the Earth's surface and the stratosphere in $F$. Take the radius of the Earth as $6400 \ km$.

Two similar conducting balls having charges $+q$ and $-q$ are placed at a separation $d$ from each other in air. The radius of each ball is $r$ and the separation between their centres is $d$ $(d \gg r)$. Calculate the capacitance of the two-ball system.

$A$ spherical capacitor has an outer sphere of radius $5 \text{ cm}$ and an inner sphere of radius $2 \text{ cm}$. When the inner sphere is earthed,its capacity is $C_1$,and when the outer sphere is earthed,its capacity is $C_2$. Then $\frac{C_1}{C_2}$ is

Eight drops of mercury, each of same radius and same charge, combine to form a bigger drop. The ratio of the capacitance of the bigger drop to that of each smaller drop is: (in $ : 1$)