There is a surprising idea hidden behind Coulomb’s law: almost all the forces we encounter every day are, deep down, electric forces in disguise.
When a book presses on a table and the table supports it (normal force), when a rope pulls a load (tension), when friction slows down a crate, or when a spring resists compression (elastic force), nothing truly “mechanical” is happening in the sense of contact between solid matter. At very small distances the surfaces never actually touch: it is the electron clouds of the atoms of the two bodies that repel each other electrically. All contact forces are therefore, at root, of an electromagnetic nature: they arise from the Coulomb interaction between the charges of the facing atoms.
The only true forces at a distance
If contact forces are disguised electromagnetic forces, only two fundamental interactions remain capable of truly acting “at a distance” in classical physics: gravity and electromagnetic forces. Everything else — pushes, tensions, friction — is a consequence of them on the atomic scale.
This observation unifies chapters that seemed separate: the normal reaction on an inclined plane, the tension in the rope of Atwood’s machine, the friction that dissipates energy as heat, are all macroscopic manifestations of the same Coulomb force that, atom by atom, holds matter together.
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Topics: Electrostatics Concepts: Coulomb’s law
Related exercises: Worked exercise — Three charges at the vertices of a triangle · Problem — Force between two 1-microcoulomb charges · Problem — Force between two spheres with the same charge