Coulomb's law
the fundamental law of electrostatics - the force between two point charges is proportional to the product of their charges, and inversely proportional to the square of the distance between them
Coulomb's law: inverse-square force between electrical charges
Coulomb's law quantifies the electrostatic force between two point charges. The force is directly proportional to the product of the charge magnitudes and inversely proportional to the square of the separation distance. In SI units, the force between charges Q₁ and Q₂ separated by distance r is F = k(Q₁Q₂)/r², where k is approximately 8.99 × 10⁹ N·m²/C². Like charges repel; opposite charges attract. The law holds for point charges in a vacuum or in air, though the constant k shifts in different dielectric media.
Electrical engineers rely on Coulomb's law to predict field behavior in capacitors, electrostatic sensors, and high-voltage systems. In a parallel-plate capacitor, the uniform field between plates approximates the conditions Coulomb's law describes. In air gap breakdowns, the law helps calculate when the electric field strength will trigger ionization and arc formation. For equipment spacing and creepage distance calculations, especially in high-voltage switchgear and transformer design, Coulomb's law provides the theoretical foundation for field intensity estimation.
Practical limits and variations
Coulomb's law assumes point charges: physical conductors have finite size and surface charge density varies. Real conductors follow the law at distances much larger than their dimensions. In composite insulators or layered dielectrics, permittivity varies with material, modifying the constant k; the law remains valid but must account for relative permittivity εᵣ of each region. Charge distribution on irregular conductors requires superposition of forces from infinitesimal charge elements, transforming the simple law into a field integral problem.
The inverse-square relationship breaks down only at distances comparable to atomic scale or in quantum electrodynamics. For standard industrial work, Coulomb's law is reliable from millimeters upward. Electromagnetic shielding effectiveness, grounding system design, and electrostatic discharge protection all depend on understanding this force law. The theorem that any spherically symmetric charge distribution exerts force as if all charge were at the center (the shell theorem) emerges directly from Coulomb's law and simplifies many calculations in cable design and substation layouts.