The given graph shows the variation (with distance $r$ from the centre) of:

$A$ point charge $q$ of mass $m$ is placed at the center of a ring of radius $R$ carrying a charge $Q$. When it is slightly displaced,the point charge accelerates to infinity along the $x$-axis. The ultimate speed of the point charge is .......

$A$ small sphere of radius $r_{1}$ and charge $q_{1}$ is enclosed by a spherical shell of radius $r_{2}$ and charge $q_{2}$. Show that if $q_{1}$ is positive,charge will necessarily flow from the sphere to the shell (when the two are connected by a wire) no matter what the charge $q_{2}$ on the shell is.

Write an equation for the electrostatic potential of a negative point charge.

Write the common meaning of voltage.

$A$ Van de Graaff generator has a spherical metal shell as an electrode which is at a potential $15 \times 10^6 \,V$. If the dielectric strength of the surrounding medium is $5 \times 10^7 \,V/m$, then the diameter of the shell is (in $\,cm$)