Coulomb's law
The force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them: F = kQ₁Q₂/r² (where k = 8.99 × 10⁹ N·m²·C⁻² or 1/(4πε₀)). The force is repulsive if charges have the same sign and attractive if opposite signs. This law underlies electrostatic phenomena.
Formula
F = kQ₁Q₂ / r²
Real World
Lightning occurs when the electrostatic force between separated charges in a cumulonimbus cloud, governed by Coulomb's law, becomes strong enough to ionise air — a single bolt near the Empire State Building can carry 300 million volts.
Exam Focus
Always include the sign of each charge in calculations — a negative force indicates attraction, and examiners check this.
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