The electrostatic interaction

Rub a plastic ruler and bring it near small scraps of paper: they jump toward it. This is the electrostatic interaction, due to electric charges. There are two kinds, positive (+) and negative (−), and one simple rule governs everything:

   +  +   ->  repel each other      (same signs -> repulsion)
   −  −   ->  repel each other
   +  −   ->  attract each other     (opposite signs -> attraction)

Charges of the same sign repel, those of opposite signs attract. This force acts at a distance, without contact, along the line joining the two charges.

The unit of charge is the coulomb (C). The smallest free charge is that of the electron (negative) or the proton (positive), the elementary charge e ≈ 1.6 × 10⁻¹⁹ C. A body becomes charged by gaining or losing electrons: an excess of electrons makes it negative, a deficit makes it positive.

This interaction closely resembles gravitation: it too acts at a distance and decreases with distance. But one major difference sets them apart. Gravitation is always attractive; electrostatics can attract or repel. And above all, at the scale of atoms, the electric force is immensely stronger than gravity. It holds electrons around the nucleus, binds atoms into molecules, and explains nearly all of chemistry. It remains to quantify it — that is the purpose of Coulomb's law.