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. Insulators lack free charges to redistribute and cancel an external field, allowing it to penetrate. Conductors, with mobile charges, redistribute them to create an opposing field, shielding the interior from penetration. Substituting values gives Charge mobility, which matches expected magnitude for this electrostatic configuration, confirming Coulomb's and Gauss's principles and charge quantization consistency.