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. Using Coulomb’s law: F = k (|q₁ q₂|/r²) . k = 9 × 10⁹ N·m²/C² , q₁ = 8 × 10⁻⁷ C , q₂ = -4 × 10⁻⁷ C , r = 0.6 m . |q₁ q₂| = 8 × 4 × 10⁻¹⁴ = 32 × 10⁻¹⁴ C² . r² = (0.6)² = 0.36 m² . F = 9 × 10⁹ × (32 × 10⁻¹⁴/0.36) = 9 × 10⁹ × 8.89 ×