## 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 𝐹12, the second exerts on the first a force 𝐹21. Coulomb's law can be used to determine the electrostatic force, which is, written as :

Couloumb law

where k is a constant and equals to 1/4 π ε0. Here, ε0 is the epsilon naught and it signifies permittivity of a vacuum. The value of k =9 × 109 Nm2/ C2

when we take the S.I unit of value of ε0 is 8.854 × 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, ε0 will be replaced by ε = ε0εr where εr represents the relative permittivity, so the force is given by the following relationship: