D.1.12 (HL)—Equipotentials and field lines

Use the perpendicular relationship

Gravitational field lines cross equipotential surfaces at right angles. The field points in the direction of decreasing gravitational potential, so the field-line arrow is normal to the equipotential and toward lower $V_g$.

Apply it to a radial field

Around an isolated spherical mass, equipotential surfaces are concentric spheres and field lines are radial. In a two-dimensional sketch, draw concentric equipotential circles and radial field arrows crossing them normally toward the mass.

Read field strength

If equal potential intervals are drawn, closer equipotential lines mean a larger potential gradient and therefore a stronger field. Farther spacing indicates a weaker field. The line/surface geometry gives direction and relative strength; the potential labels give the quantitative difference.

Common trap

Do not draw field lines along equipotentials. Moving along an equipotential has zero potential difference, while the gravitational field points across it, toward lower potential.