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In 1785, Coulomb established the fundamental law of electric force between two stationary, charged particles. Experiments show that an electric force has the following properties:

  • The force is inversely proportional to the square of the separation, r, between the two particles and directed along the line joining the particles.
  • The force proportional to the product of the charges q1 and q2 on the two particles.
  • The force is attractive if the charges are of opposite sign and repulsive if the charges have the same sign.
From these observations, we can express the magnitude of the electric force between the two charges as
Where ke is a constant called the Coulomb constant. In his experiments, Coulomb was able to show that the value of the exponent of r was 2 to within an uncertainty of a few percent. Modern experiments have shown that the exponent is 2 to a precision of a few parts in 109.

The Coulomb constant has a value that depends on the choice of units. The unit of charge in SI units is the coulomb (C).

The coulomb is defined in the terms of a unit called the ampere (A), where current equals the rate of flow of charge. When the current in a wire is 1 A, the amount of charge that flows past a given point in the wire in 1s is 1 C. The Coulomb constant ke in SI units has the value
The constant ke is also written
Where the constant is known as permittivity of free space and has the value
Where the constant is known as permittivity of free space and has the value
The smallest unit of charge known in nature is the charge on an electron or proton. The charge of an electron or proton has a magnitude
|e| = 1.60219 x 10-19C

Charge and Mass of the Electron, Proton, and Neutron
Particle
Charge (C)
Mass (kg)
Electron (e)
- 1.60219 x 10-19
9.1095 x 10-31
Proton (p) 
+ 1.60219 x 10-19
1.67261 x 10-27
Neutron (n)
0
1.67492 x 10-27