Coulomb's Law
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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