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. Using Coulomb’s law: F = k (|q₁ q₂|/r²) . k = 9 × 10⁹ N·m²/C² , q₁ = -3 × 10⁻⁷ C , q₂ = 5 × 10⁻⁷ C , r = 0.8 m . |q₁ q₂| = 3 × 5 × 10⁻¹⁴ = 15 × 10⁻¹⁴ C² . r² = (0.8)² = 0.64 m² . F = 9 × 10⁹ × (15 × 10⁻¹⁴/0.64) = 9 × 10⁹ × 2.34375 × 10⁻¹³ = 0.00211