Electromagnetic Force

The electromagnetic force acts between charged particles. When the charges are at rest, it reduces to the familiar Coulomb force — attractive between unlike charges, repulsive between like charges. Charges in motion also produce magnetic effects, and a magnetic field in turn exerts a force on a moving charge; because electric and magnetic effects are generally inseparable, the combined force is called electromagnetic. Like gravity, it acts over unlimited distances without needing any medium in between — but it is enormously stronger: the electric force between two protons at a fixed distance is about $10^{36}$ times the gravitational force between them at the same distance.

Because ordinary matter is built from charged constituents (electrons and protons), and the electromagnetic force so completely dominates gravity at atomic and molecular scales, it is the electromagnetic force — not gravity — that governs the structure of atoms and molecules, the course of chemical reactions, and the mechanical and thermal properties of materials. Even the everyday macroscopic forces of tension, friction, the "normal force" and the spring force are, underneath, electromagnetic in origin.

One key difference from gravity: gravity is always attractive (there is no negative mass), while the electromagnetic force can be attractive or repulsive, because charge comes in two varieties, positive and negative. Since ordinary matter is mostly electrically neutral overall, the net electric force is usually negligible at everyday distances and gravity dominates terrestrial-scale phenomena — except where the atmosphere gets ionised, producing effects like lightning. …