electric potential difference

Meaning: Electric potential difference (voltage) between points A and B is the change in electric potential energy per unit charge when a charge moves from A to B. The field points from high to low potential, so a free positive charge moves toward lower potential and a free negative charge toward higher.

Equation: ΔV = ΔU/q = Ed (uniform field). ΔV is potential difference (V = J/C), ΔU is change in potential energy (J), q is charge (C), E is field magnitude (V/m), d is separation along the field (m). For a point charge, V = kQ/r, where k = 9.0 × 10⁹ N·m²/C², Q is the source charge (C) and r is distance (m).

On the MCAT: Expect parallel plates, or a charge accelerated through a voltage so that kinetic energy gained is qΔV. Other items ask you to rank the potential at points near several charges (potential is a scalar, so contributions add algebraically). Equipotential lines are perpendicular to field lines, and no net work is done moving a charge along one.

Common confusion: Potential (J/C) is not potential energy (J). A negative charge moving to higher potential loses potential energy, because ΔU = qΔV carries the sign of q. Also, V = 0 does not mean E = 0: midway between equal and opposite charges, V = 0 but the field is not zero.