Practice question

**Electrostatic force** described by F = (1/4π ε₀)·q₁q₂/r² obeys Newton's third law. Magnitude depends on q₁q₂ and 1/r², enabling quantitative estimation at given separation, with sign indicating attraction or repulsion. Potential energy: U = k (q₁ q₂/r) . U = 9 × 10⁹ × ((5 × 10⁻⁶) × (-5 × 10⁻⁶)/0.3) = 9 × 10⁹ × (-25 × 10⁻¹²/0.3) = -0.75 J . Substituting values gives -0.75 J, which matches expected magnitude for this electrostatic configuration, confirming Coulomb's and Gauss's principles and charge quantization consistency.