Coulombs law
Interaction between two point charges q1 and q2:
Two charges q1 and q2 spaced r apart placed in a vacuum. The first exerts on the second a force đč12, the second exerts on the first a force đč21. Coulomb's law can be used to determine the electrostatic force, which is, written as :

where k is a constant and equals to 1/4 Ï Î”0. Here, Δ0 is the epsilon naught and it signifies permittivity of a vacuum. The value of k =9 Ă 109 Nm2/ C2
when we take the S.I unit of value of Δ0 is 8.854 à 10-12 C2 N-1 m-2.
If the two charges have the same sign then the force is repulsion force

If the two charges have different signs, then the force is attraction force

Remarque :
In a medium other than vacuum, Δ0 will be replaced by Δ = Δ0Δr where Δr represents the relative permittivity, so the force is given by the following relationship:
