Coulomb repulsion
/ KOO-lom /
Try to press the north poles of two bar magnets together: the closer you push, the harder they shove back. Two electrons do the same thing, but with electric charge instead of magnetism. Because both carry the same negative charge, they push each other away, and the push grows fiercer the closer they get. This is Coulomb repulsion, named after the 18th-century physicist who measured the law of electric force.
Quantitatively, the repulsion between two electrons weakens with distance in a steady, reaching way: halve the separation and the force quadruples; it never quite vanishes even far away. This long reach is what makes electron correlation so demanding — an electron doesn't just feel its nearest neighbor, it feels every other electron in the material at once, near and far.
It matters because Coulomb repulsion is the root cause of essentially all correlation effects, from why atoms have their sizes to why Mott insulators refuse to conduct. The honest caveat: inside a real material the bare, long-range repulsion is heavily softened by the other electrons rearranging themselves — a process called screening — so the effective push an electron feels is far gentler and shorter-ranged than the textbook formula suggests.
When you rub a balloon on your hair and the strands stand on end repelling one another, you are watching Coulomb repulsion with your own eyes: each hair has picked up the same kind of charge, so they push apart, fanning out to get as far from each other as they can.
Hair strands repelling after a balloon charges them — everyday Coulomb repulsion at work.
Coulomb's law also describes attraction between opposite charges — an electron and a proton pull together by the same rule. 'Coulomb repulsion' simply refers to the case of like charges, which is the case that matters for electrons crowding inside a material.