D.2.17 (HL)—Electric equipotentials

Define an equipotential

An electric equipotential surface joins points with the same electric potential. Moving a charge along one surface gives $\Delta V_e=0$ , so the electric field does no work.

Charge arrangement	Equipotential surfaces
Point charge	concentric spheres
Up to four point charges	distorted closed surfaces found from the scalar sum of potentials
Solid spherical conductor	constant throughout the conductor and on its surface; concentric outside
Hollow spherical conductor	constant in the empty cavity, conductor and surface; concentric outside
Opposite parallel plates	planes parallel to the plates; approximately equally spaced in the uniform central region

Read spacing carefully

If equal potential intervals are drawn, closer equipotential lines indicate a larger potential gradient and stronger electric field. Use labels and the drawing convention; arbitrary visual spacing alone does not provide a numerical field.

Common trap

Do not confuse equipotential lines with field lines. A charge can move along an equipotential without field work, even though the electric field may be non-zero perpendicular to the path.

How it is assessed

Questions interpret motion along equipotential lines or identify common equipotential shapes.

Command terms

Comment / What is

What earns marks

Identify equal-potential surfaces, set ΔVe=0 for motion along one, and infer qualitative field strength from consistent potential spacing.

Watch for

Assuming any motion on a diagram has non-zero work, or choosing a surface shape without checking the charge arrangement.

Representative question

[Maximum number: 1]

A positively charged particle is positioned in an electric field. Three equipotential lines are shown. The particle is released.

What is the initial direction of the velocity of the particle?

D.2.17 (HL)—Electric equipotentials

Model Electric Equipotentials

D.2.17 (HL) Exam Analysis

Assessment in practice

Question 1