Practice

Coulomb’s Law Formula – Practice

F = kq₁q₂/r² — Electrostatic force between point charges

The magnitude of the force between two point charges is F = k|q₁q₂|/r². Like signs repel; opposite signs attract.

Guide — how to use this formula

1. 1. Use charges in coulombs and r in metres.
2. 2. F = k|q₁q₂|/r²; square r in the denominator.
3. 3. k ≈ 9×10⁹ N·m²/C² unless told otherwise.
4. 4. Halving r multiplies F by 4.

Derivation

Coulomb’s inverse-square law for point charges in vacuum/air:

1. Force magnitude:
   F = \frac{k|q_1 q_2|}{r^{2}}
2. Solve for separation:
   r = \sqrt{\frac{k|q_1 q_2|}{F}}

Convert μC → C (×10⁻⁶) and cm → m before substituting.

Examples

Example: equal 1 μC at 0.1 m

F = 9×10⁹×(10⁻⁶)²/0.01 = 0.9 N.

Example: find r

From F = kq²/r² → r = √(kq²/F).

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