Which one of the following is not a measurable quantity?

$A, B$ and $C$ are three points in a uniform electric field as shown in the figure. The electric potential is

$A$ charge $Q$ is uniformly distributed over a long rod $AB$ of length $L$ as shown in the figure. The electric potential at the point $O$ lying at a distance $L$ from the end $A$ is

The electrostatic potential $V$ at a point on the circumference of a thin non-conducting disk of radius $r$ and uniform charge density $\sigma$ is given by the equation $V = 4 \sigma r$. Which of the following expressions correctly represents the electrostatic energy stored in the electric field of a similarly charged disk of radius $R$?

Two identical thin rings each of radius $R \text{ m}$ are kept on the same axis at a distance of $R \text{ m}$ apart. If the charges on them are $10 \text{ C}$ and $5 \text{ C}$ respectively,calculate the work done in moving a charge '$q$' coulomb from the centre of one ring to that of another.

$A$ conducting sphere of radius $R$ is given a charge $Q$. The electric potential and the electric field at the centre of the sphere respectively are