Equipotential Surfaces
Any surface over which the potential is constant is called an equipotential surface.
In other words, the potential difference between any two points on an equipotential surface is zero. Equipotential lines are like contour lines on a map which trace lines of equal altitude. In this case the "altitude" is electric potential or voltage. Equipotential lines are always perpendicular to the electric field. In three dimensions, the lines form equipotential surfaces. Movement along an equipotential surface requires no work because such movement is always perpendicular to the electric field.
For example, consider two points A and B on an equipotential surface as shown in figure
VB - VA = 0VB = VA
It may be noted that an equipotential surface may be the surface of a material body or a surface drawn in an electric field.