The electric field at a point due to a point charge is $20 \mathrm{N} {\mathrm{C}}^{-1}$ and the electric potential at that point is $10 \mathrm{J} {\mathrm{C}}^{-1}$. Calculate the distance of the point from the charge and the magnitude of the charge.

A hollow metallic sphere of radius $0.2 \mathrm{m}$ is given a charge of $+15 \mathrm{\mu C}$. Find
(i) electric potential at its surface.
(ii) potential at its centre.
(iii) electric potential at $0.3 \mathrm{m}$ from centre.

Consider a particle with charge $q=3.0 \mathrm{nC}$, point $A$ at distance $d_1=2.0 \mathrm{m}$ from $q$ and the point $B$ at distance $d_2=1.0 \mathrm{m}$. If $A$ and $B$ are diametrically opposite to each other as in the figure, (a) what is the electric potential difference $V_{\mathrm{A}}-V_{\mathrm{B}}$? (b) what is the electric potential difference if $A$ and $B$ are located as in the figure (b)?