Coulomb’s law is the electric twin of Newton’s law of gravitation: same inverse-square shape, but charges can repel as well as attract.
Two point charges exert equal and opposite forces given by Coulomb’s law, F = Q₁Q₂ / (4πε₀r²). Like charges repel, opposite charges attract, and the force falls off with the square of the separation.
Vary the two charges and their separation. The force arrows stay equal and opposite, reverse direction when one charge changes sign, and shrink rapidly as the charges are pulled apart.
Because the force depends on 1/r², moving the charges twice as far apart cuts the force to a quarter. The constant 1/(4πε₀) is about 8.99 × 10⁹ N m² C⁻². A uniformly charged sphere behaves, outside its surface, exactly like a point charge at its centre.
Four quick checks on Coulomb’s law and the inverse-square relationship. Each correct answer earns XP and lights this skill on your star map.
Coulomb’s law states that the force between two point charges is:
Doubling the separation of two point charges changes the force to:
Two point charges of the same sign:
Outside its surface, a uniformly charged sphere behaves like:
The law is inverse-square, so doubling the separation quarters the force, not halves it. Remember the sign: like charges repel, opposite attract, and the two forces are always equal and opposite (Newton’s third law). Treat a charged sphere as a point charge only for points outside it.
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