Practice question

**Potential energy of two charges** U = k q₁ q₂/r, k=9×10⁹ N·m²/C², q₁,q₂ in coulombs, r separation (m), positive for like charges (repulsive, work needed to bring together), negative for opposite (attractive, work released). For 20 μC and -8 μC, 0.2 m apart, U=9×10⁹×20×10⁻⁶×(-8×10⁻⁶)/0.2= -7.2 J. The potential energy of a dipole is U = -p E cos θ . At θ = 0° , cos 0 = 1 , U = -p E , the minimum. At θ = 90° , cos 90 = 0 , U = 0 . From θ = 0° to 90° , U increases from -p E to 0