electric potential energy
Summary
Electric potential energy (U) is the energy a system of charges stores because of their positions. For two point charges, U = kq₁q₂/r, positive for like charges and negative for opposite charges, taking U = 0 at infinite separation. The sign tells you which way the charges move on their own.
Detail
Meaning: Electric potential energy is the work an external agent must do to bring charges from infinite separation to their positions, with U = 0 at infinity. Like charges repel, so pushing them together stores energy and U is positive. Opposite charges attract, so U is negative, which means a bound system. It belongs to the system of charges, not to one charge alone. Charges released from rest move toward lower U, and the lost U becomes kinetic energy if no other forces act.
Equation: U = kq₁q₂/r = qV, where U is potential energy (J), k is Coulomb's constant (about 9.0 × 10⁹ N·m²/C²), q₁ and q₂ are charges (C) with their signs, r is the separation (m), and V is the electric potential at the charge's location (V, equal to J/C).
On the MCAT: Expect questions asking whether U rises or falls as charges move closer or apart, or what happens to a particle's speed, using ΔKE = −ΔU. Doubling r halves U. Energies are often given in electron volts, where 1 eV is the energy gained by one elementary charge crossing 1 V.
Common confusion: Students mix up U with electric potential V. V is in volts, belongs to a location and is set by the source charge. U is in joules and equals qV for a charge placed there. Also, U varies as 1/r, while the force varies as 1/r². Keep the signs of the charges in the calculation.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.