equipotential line

Meaning: An equipotential line is a curve (a surface in three dimensions) on which every point has the same electric potential. With no potential difference between its points, the field does no net work on a charge moved along it. So the field has no component along the line: field lines cross equipotentials at right angles and point toward lower potential. Point charges give concentric circles; parallel plates give lines parallel to the plates.

Equation: W = −qΔV = 0, where W is the work done by the field (J), q is the moved charge (C), and ΔV is the potential difference between the start and end points (V).

On the MCAT: Expect a diagram of equipotentials. You may be asked to sketch the field direction (perpendicular, toward lower V), to compare field strength (for equal steps in V, lines closer together mean a stronger field), or to find the work to move a charge between two lines. That work depends only on the endpoints, not the path.

Common confusion: Equipotential does not mean zero potential or zero field. The potential is constant along the line but can have any value, and the field there is generally nonzero, just perpendicular to the line.