Two equal point charges q each are placed at distance d. The electrostatic PE of the system (zero at infinity) is:

1. Work needed to bring 2nd charge from ∞ to distance d in field of 1st = k q · q / d. 2. So U = k q₁ q₂ / d = k q² / d (with both = q). 3. For N point charges, sum over all pairs: U = Σᵢⱼ k qᵢqⱼ / rᵢⱼ. _Source: syllabus Physics Ch 1 §1.13 Potential Energy of a System_