CLASS 11–12 PHYSICS · ELECTRIC CHARGES AND FIELDS

Coulomb's law

F = k q₁q₂ / r² with k = 9 × 10⁹ N m² C⁻². 1 μC = 10⁻⁶ C. Like charges repel, unlike attract.

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Worked examples

Example 1 · level 1

Two point charges of 7 μC and 1 μC are 20 cm apart in air. Find the magnitude of the force between them (in N, correct to 2 decimal places). k = 9 × 10⁹ N m² C⁻².

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F = 9 × 10⁹ × 7 × 10⁻¹² ÷ 0.04
= 1.57 N (repulsive).

Answer: 1.57 N

Example 2 · level 2

Two point charges of 3 μC and −8 μC are 20 cm apart in air. Find the magnitude of the force between them (in N, correct to 2 decimal places), and note whether it is attractive or repulsive. k = 9 × 10⁹ N m² C⁻².

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F = 9 × 10⁹ × 24 × 10⁻¹² ÷ 0.04
= 5.4 N (attractive, since the charges are unlike).

Answer: 5.4 N

Example 3 · level 3

Two point charges repel with a force of 9 N. One charge is made 2 times larger and the separation is made 3 times larger. Find the new force (in N, correct to 2 decimal places).

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F′ = 9 × 2 ÷ 9 = 2 N.

Answer: 2 N