The electric potential outside a uniformly charged sphere at a distance $\text{'}r\text{'}$ is ($\text{'}a\text{'}$ being the radius of the sphere)-

At a certain distance from a point charge, the electric field is $500 \mathrm{V} {\mathrm{m}}^{-1}$ and the potential is $3000 \mathrm{V}$. What is the distance?

Figure represents a square carrying charges $+q,+q,-q,-q$ at its four corners as shown. Then the potential will be zero at points

The potential difference between points $A$ and $B$ in the given uniform electric field is: