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+[[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.