Two charges $2 \; \mu C$ and $-2 \; \mu C$ are placed at points $A$ and $B$ which are $6 \; cm$ apart.
$(a)$ Identify an equipotential surface of the system.
$(b)$ What is the direction of the electric field at every point on this surface?

(N/A) The system consists of two equal and opposite charges separated by a distance of $6 \; cm$.
An equipotential surface is defined as a surface where the electric potential is the same at every point.
For a system of two equal and opposite charges (an electric dipole),the plane that perpendicularly bisects the line joining the two charges is an equipotential surface with a potential of $0 \; V$.
This plane is located at the mid-point of the line $AB$,i.e.,at a distance of $3 \; cm$ from each charge.
$(b)$ The electric field lines always originate from the positive charge and terminate at the negative charge.
At any point on the equipotential surface (the perpendicular bisector plane),the electric field vector is directed from the positive charge towards the negative charge.
Therefore,the direction of the electric field at every point on this surface is normal to the plane and directed along the line $AB$ (from $A$ to $B$).