$A$ hollow conducting sphere of radius $R$ carries a charge $(+Q)$ on its surface. Find the electric potential at a distance $r = R/3$ from its center.

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 circular rings of radius $R$ are placed with their axes coinciding. The charges on the two rings are $+q$ and $-q$. The potential difference between the centers of the two rings is:

If $27$ charged water droplets, each of radius $10^{-3} \,m$ and charge $10^{-12} \,C$ coalesce to form a single big spherical drop, then the potential of the big drop is (in $\,V$)

The concentric spheres of radii $R$ and $r$ have positive charges $q_{1}$ and $q_{2}$ with equal surface charge densities. What is the electric potential at their common centre?

Consider a sphere of radius $R$ with uniform charge density and total charge $Q$. The electrostatic potential distribution inside the sphere is given by $V(r) = \frac{Q}{4 \pi \varepsilon_{0} R} \left( a + b(r/R)^c \right)$. Note that the zero of potential is at infinity. The values of $(a, b, c)$ are