Coulomb force

The Coulomb force is the electrostatic attraction or repulsion that arises when two electrically charged objects occupy the same space. Unlike mechanical forces that act through contact or gravity that acts across distance due to mass, Coulomb force depends entirely on the quantity and sign of electric charge. Two charges of the same sign repel each other; opposite signs attract. The magnitude follows an inverse square law: force drops as the square of the distance increases.

The force between two point charges is given by F = k(q₁q₂)/r², where k is Coulomb's constant (approximately 8.99 × 10⁹ N·m²/C²), q₁ and q₂ are the charges in coulombs, and r is the separation distance in meters. In practical work, this law holds well for charges separated by distances much larger than their physical size, or for spherical conductors where charge distributes uniformly on the surface.

Real-world behavior and limits

In air or vacuum, Coulomb force acts undiminished. In conducting media or near dielectric materials, the force weakens because the medium's own charges partially shield or polarize in response to the field. This shielding effect matters when designing high-voltage equipment: air at atmospheric pressure breaks down and ionizes when field strength exceeds about 3 million volts per meter, so Coulomb forces between charged surfaces can trigger electrical discharge if spacing is inadequate.

Coulomb force is the foundation of capacitor action: the attraction between opposite charges on capacitor plates stores electrical energy. It also governs the behavior of charged particles in electron beams, the spacing requirements in high-voltage switchgear, and the self-repulsion that limits charge density on conductors. Engineers must account for Coulomb forces when calculating stress on insulation, predicting electrical breakdown, or designing electrode systems.

The term honors Charles-Augustin de Coulomb, an 18th-century French physicist. His torsion-balance experiments confirmed the inverse square relationship and established electrostatics as a quantifiable field. In modern electrical work, Coulomb force is the primary concept underlying field theory, circuit safety margins, and the physics of conduction and breakdown.