Coulomb's Law
Coulomb's law calculates the electric force acting between to electric charges q1 and q 2and can be expressed as
F = k q1 q2/ r2
= q1 q2/ (4 π ε0 r2) (1)
where
F = force of attraction or repulsion (N)
k = Coulomb's constant = 1 / (4 π ε0 ) = 8.9875517873681764 × 109 (Nm2/C2) in air
q = charge (Coulombs, C)
r = distance between charges (m)
ε0 = 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 1018 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 109 Nm2/C2) ( (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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