What is an equipotential surface? State its properties. Draw equipotential surfaces for a point charge and for a uniform field, and show that E is normal to an equipotential surface.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define. A surface on which the potential has the same value at every point.
2 · Show that E is normal. Along the surface ΔV = 0, so W = qΔV = 0. A field part along the surface would do work, so E has none: E is perpendicular.
3 · State the properties.
No work is done along the surface.
E is perpendicular to it.
Two equipotentials never intersect.
They are closer together where E is strong.
4 · Draw two cases. [Point charge: dashed concentric circles with radial field lines. Uniform field: dashed parallel lines at right angles to the field arrows]
5 · Close in one line. Field lines and equipotential surfaces always cross at right angles.
Scoring tip: Draw equipotentials dashed and field lines solid with arrows, so the examiner can tell them apart.
