Practice question

**Coulomb's law** gives force between point charges as F = k·|q₁q₂|/r², k = 1/(4π ε₀) = 9×10⁹ N·m²/C², directed along line joining charges. Like charges repel, opposite attract, magnitude scales with product of charges and inverse square of separation r². Let x be distance from +12 μC , then 0.9 - x from -4 μC . (12 × 10⁻⁶/x²) = (4 × 10⁻⁶/(0.9 - x)²) , 12 (0.9 - x)² = 4 x² . 3 (0.81 - 1.8 x + x²) = x² , 2.43 - 5.4 x + 3 x² = x² . 2 x² - 5.4 x + 2.43 = 0 , x