Practice question

**Inverse-square law** for charges states F ∝ 1/r² while increasing with charge product. Using k = 9×10⁹ N·m²/C², force at distance r follows F = k q₁q₂/r², forming basis for pairwise force calculation. Charge on glass rod is positive, so electrons are transferred from glass to silk. Charge of one electron e = -1.6 × 10⁻¹⁹ C . Number of electrons: n = (q/|e|) = (2.5 × 10⁻⁷/1.6 × 10⁻¹⁹) = 1.5625 × 10¹² . Substituting values gives 1.56 × 10¹², which matches expected magnitude for this electrostatic configuration, confirming Coulomb's and Gauss's principles and charge quantization consistency.