Why is electric field normal to an equipotential surface?

Becasue if it had not been so, then there would be a component of field along the surface which would create a net potential difference between every point on the equipotential surface. But the surface is equipotential.

=>This cannot happen, E is normal to the equipotential surface.

Property by virtue!

at equipotential surface potential remains same and electric field is perpendicular to an equi. surface.

U can not ask A ke baad B kyu aata hai C kyu nahi aata?

ohk!

The electric field in relation to potential is Ex = dV/dx. Along an equipotential surface there is no change in V with position x parallel to the surface, so dV/dx = 0, making Ex = 0. There is no component of field parallel to the surface, so the field is normal to the surface.