Two conducting spheres of radii $r_1$ and $r_2$ have the same electric field intensity near their surfaces. The ratio of their electric potentials is

$A$ thin spherical conducting shell of radius $R$ has a charge $q$. Another charge $Q$ is placed at the centre of the shell. The electrostatic potential at a point $P$ at a distance $\frac{R}{2}$ from the centre of the shell is

$A$ particle of mass $m$ and charge $q$ is accelerated through a potential difference of $V$ volt. Its energy will be:

If a charged spherical conductor of radius $10 \,cm$ has potential $V$ at a point distant $5 \,cm$ from its centre, then the potential at a point distant $15 \,cm$ from the centre will be

$A$ charge $Q$ is distributed over two concentric hollow spheres of radii $r$ and $R$ $(R > r)$ such that their surface charge densities are equal. The potential at the common centre is

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).