Coulomb's Law
The electric force acting on a point charge.
Coulomb's law calculates the electric force acting between to electric charges q_{1} and q_{2} and can be expressed as
F = k q_{1} q_{2} / r^{2}
= q_{1} q_{2} / (4 π ε_{0} r^{2}) (1)
where
F = force of attraction or repulsion (N)
k = Coulomb's constant = 1 / (4 π ε_{0}) = 8.9875517873681764 × 10^{9} (Nm^{2}/C^{2}) in air
q = charge (Coulombs, C)
r = distance between charges (m)
ε_{0} = permittivity of space or vacuum_{}
The quantity of charge (number of electrons) is measured in the unit Coulomb  C  where
1 coulomb (C) = 6.24 10^{18} electrons
The smallest charge that exists is the charge carried by an electron equal to 1.602 10^{19} coulomb.
Example  Force between two Charged Bodies
Two spheres in air with distance 20 mm is both charged with 2 μC (2 10^{6} C). The force acting between the two spheres can be calculated as
F = (8.98755 10^{9} Nm^{2}/C^{2}) ((2 μC) (10^{6} C/μC)) ((2 μC) (10^{6} C/μC)) / ((20 mm)(10^{3} m/mm))^{2}
= 90 N
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