Two conducting spheres of radii $R_1$ and $R_2$ are charged with charges $Q_1$ and $Q_2$ respectively. On bringing them in contact there is

A long, hollow conducting cylinder is kept coaxially inside another long, hollow conducting cylinder of larger radius. Both the cylinders are initially electrically neutral.

At the centre of a half ring of radius $R=10 \mathrm{~cm}$ and linear charge density $4 \mathrm{n} \mathrm{C} \mathrm{m}^{-1}$, the potential is $x \pi V$. The value of $x$ is  . . . . . 

Two identical metal balls of radius $r$ are at a distance $a (a >> r)$ from each other and are charged, one with potential $V_1$ and other with potential $V_2$. The charges $q_1$ and $q_2$ on these balls in $CGS$ esu are

As shown in the figure, charges $ + q$ and $ – q$ are placed at the vertices $B$ and $C$ of an isosceles triangle. The potential at the vertex $A$ is