Question

why are electric field lines normal to the equitorial surface?

Answer 1

The field vector E, and hence the electric force qE, is tangent to the field line at each point in the electric field. An equipotential line is a line along which the potential energy is the same, that is the work of qE along a segment of the the equipotential line would be 0. Therefore qE, or E, is perpendicular to the equipotential line.It follows that the field lines are perpendicular to the equipotential lines.