A unit of electric charge in the centimeter-gram-second (CGS) system of measurement, used in physics.
Abbreviation standing for 'electrostatic unit,' created in the late 19th century as physicists developed standardized systems for measuring electricity and magnetism.
The ESU is completely abandoned in modern physics—we now use the coulomb instead—but it shows how science progresses by replacing old measurement systems when better ones come along, kind of like how countries replaced coins with paper money.
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