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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