Coulomb's Law

In 1785, French physicist Charles-Augustin de Coulomb used a torsion balance to measure the force between charged objects and established one of the fundamental laws of electrostatics. His discovery revealed that the force between two point charges follows an inverse-square law — remarkably similar in structure to Newton's law of gravitation.

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Coulomb's Law

The force between two point charges is:

F = kQ₁Q₂ / r²

where:

• F is the force between the charges (N)
• Q₁ and Q₂ are the magnitudes of the two charges (C)
• r is the distance between the centres of the charges (m)
• k is Coulomb's constant = 8.99 × 10⁹ N m² C⁻² (often approximated to 9.0 × 10⁹)

The constant k is related to the permittivity of free space (ε₀) by:

k = 1 / (4πε₀)

where ε₀ = 8.85 × 10⁻¹² F m⁻¹ (farads per metre).

So Coulomb's law can also be written as:

F = Q₁Q₂ / (4πε₀r²)

Key Features