Calculate the electrostatic potential energy of a system of two charges $7\ \mu C$ and $-2\ \mu C$ placed at points $(-9\ cm, 0, 0)$ and $(9\ cm, 0, 0)$ respectively (in the absence of an external field).

Along the $X$-axis,three charges $\frac{q}{2}, -q$ and $\frac{q}{2}$ are placed at $x=0, x=a$ and $x=2a$ respectively. The resultant electric potential at $x=a+r$ (if $a << r$) is: ($\varepsilon_0$ is the permittivity of free space)

Two thin coaxial rings,each of radius $a$ and having charges $+Q$ and $-Q$ respectively,are separated by a distance $s$. The potential difference between the centres of the two rings is:

$A$ radius of a spherical charged shell is $10 \ cm$ and the electric potential on its surface is $100 \ V$. Then,the potential at $2 \ cm$ from the centre of the shell will be $.......$ (in $V$)

$A$ ball of mass $1\, g$ and charge $10^{-8}\, C$ moves from a point $A$,where the potential is $600\, V$,to a point $B$,where the potential is $0\, V$. The velocity of the ball at point $B$ is $20\, cm/s$. The velocity of the ball at point $A$ will be:

The electric potentials at two points $P$ and $Q$ are $10 \ V$ and $-4 \ V$ respectively. The work done in moving $100$ electrons from $P$ to $Q$ is: