D.2.11 (HL)—Electric potential energy

Set the reference

Electric potential energy of a system is the work done to assemble its charges from infinite separation to their present positions. Define the energy as zero at infinite separation. The sign depends on the charge combination: bringing opposite charges together lowers the energy, while bringing like charges together raises it.

Interpret energy transfer

Work done by an external agent changes the electric potential energy. If a positive charge moves toward a negative source, the electric field can do positive work while the potential energy decreases. If an electron is accelerated through a potential difference, the lost electric potential energy can become kinetic energy.

Track the system

For several charges, electric potential energy belongs to the whole charge configuration and is built from pair interactions. State the reference and the system before assigning a sign; do not confuse the energy of a charge configuration with electric potential at one point.

Common trap

Electric potential energy is not always positive. Unlike charges have negative pair energy relative to infinity; like charges have positive pair energy. The sign is determined by the interaction and reference, not by whether a charge is an electron.