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# Coulomb's Law

## The electric force acting on a point charge.

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)

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

## Related Topics

• ### Electrical

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## Related Documents

• ### Relative Permittivity - the Dielectric Constant

Common materials and their relative permittivity.

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## Citation

• The Engineering ToolBox (2013). Coulomb's Law. [online] Available at: https://www.engineeringtoolbox.com/coulomb-law-electric-charge-force-d_1855.html [Accessed Day Month Year].

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6.3.17

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