See study by C. S. Gilmor (1971).
Force between two electric charges. The magnitude of the force math.F is proportional to the product of the two charges, math.q1 and math.q2, divided by the square of the distance math.r between them, or math.F = math.kmath.q1math.q2/math.r2, where math.k is a constant that depends on the measurement system being used. The Coulomb force can be one of repulsion, such as the force between two objects having like charges, or it can be attractive, such as the force between two objects having opposite charges.
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In principle, the coulomb could be defined in terms of the charge of an electron or elementary charge. Since the values of the Josephson (CIPM (1988) Recommendation 1, PV 56; 19) and von Klitzing (CIPM (1988), Recommendation 2, PV 56; 20) constants have been given conventional values (KJ ≡ 4.835 979 Hz/V and RK ≡ 2.581 280 7 Ω), it is possible to combine these values to form an alternative (not yet official) definition of the coulomb. A coulomb is then equal to exactly 6.241 509 629 152 65 elementary charges. Combined with the present definition of the ampere, this proposed definition would make the kilogram a derived unit.
In everyday situations, positive and negative charges are usually balanced out. According to Coulomb's Law, two point charges of +1 C, one meter apart, would experience a repulsive force of 9 N, roughly the equivalent of 900,000 metric tons of weight.
The ampere was historically a derived unit—being defined as 1 coulomb per second. Therefore the coulomb, rather than the ampere, was the SI base electrical unit.
In 1960 the SI made the ampere the base unit.