Coulomb’s Law  

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:

(1) The force is inversely proportional to the square of separation, r², between the two charged particles.

(2) The force is proportional to the product of charge q₁ and the charge q₂ on the particles.

(3) The force is attractive if the charges are of opposite sign and repulsive if the charges have the same sign.

where K is the coulomb constant = 9 × 10⁹ N.m²/C².

The above equation is called Coulomb’s law, which is used to calculate the force between electric charges. In that equation F is measured in Newton (N), q is measured in unit of coulomb (C) and r in meter (m).

The constant K can be written as

where ε_ο is known as the Permittivity constant of free space.

□ ε_ο = 8.85 × 10^-12 C²/N.m²