D.2.18 (HL)—Equipotentials and electric fields

Use the perpendicular relationship

Electric field lines cross equipotential surfaces at 90°. The field points toward decreasing electric potential, so the field-line arrow is normal to the equipotential and in the direction of the negative potential gradient.

Apply it between plates

For oppositely charged parallel plates, field lines are approximately straight and perpendicular to the plates; equipotential surfaces are parallel to the plates. This is why the potential changes across the separation but remains constant along a plate.

Predict motion

A positive charge accelerates along the electric field, toward lower potential. A negative charge accelerates opposite to the field, toward higher potential. The field direction and particle-force direction must be kept separate.

Common trap

Do not draw field lines parallel to equipotentials or assume a negative charge accelerates in the field direction. The field is defined using a positive test charge.