# Why do electric field lines start and end at 90 degree at the surface of a conductor? [duplicate]

There is one property of electric lines of forces which states that:

Electric field lines start and end at 90 degree at the surface of the conductor.

But why is that so? Is there any proof for this statement or is it observed by any experiment?

The surface of the conductor has constant electrostatic potential $V$ and the electric field is proportional to the gradient of the potential: $\nabla V$. By definition the gradient of a scalar quantity is always perpendicular to the level curves (surfaces).