State Coulomb's law. Two charges of +4 μC and −5 μC are 20 cm apart in air. Find the force between them and say whether it is attractive or repulsive. (k = 9 × 10⁹ N m² C⁻²)

1. 1 · State the law. The force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.
2. 2 · Write the formula. F = kq₁q₂/r²
3. 3 · Change to SI units. q₁ = 4 × 10⁻⁶ C, q₂ = 5 × 10⁻⁶ C (use the sizes), r = 0.20 m.
4. 4 · Substitute. F = (9 × 10⁹ × 4 × 10⁻⁶ × 5 × 10⁻⁶) ÷ (0.20)² = 0.18 ÷ 0.04.
5. 5 · Write the answer with unit. F = 4.5 N.
6. 6 · State the nature. The charges are unlike (+ and −), so the force is attractive.

Scoring tip: Change μC to C (× 10⁻⁶) and cm to m before you substitute. Square the distance last, and do not forget it.