[[Electric Force, Field, and Potential]]
# Summary
* Electric potential energy is work required from external force to assemble charges from infinity. Infinity set is the zero reference.
* Elastic potential energy: $U_E=\frac{1}{4\pi \epsilon_0}\frac{q_1q_2}{r}=k\frac{q_1q_2}{r}$
* Positive $U_E$ means repulsive interaction from like charges.
* Negative $U_E$ means attractive interaction from unlike charges.
* For multiple charges, add potential energy of each pair.
* $U_E$ is a scalar.
* Energy is conserved, as charges move potential is traded for kinetic.