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. Distance to midpoint = 0.02 m. V = 9 × 10⁹ ( (18 × 10⁻⁶/0.02) + (-9 × 10⁻⁶/0.02) ) = 9 × 10⁹ × (9 × 10⁻⁶/0.02) . V = 9 × 10⁹ × (9 × 10⁻⁶/0.02) = 4.05 × 10⁶ V . Using V = kQ/r, U = k q₁q₂/r, E = -dV/dr, C = ε₀A/d, 1/C_eq