what is Coulomb law? Explain briefly..

The basic law of electrostatics was formulated by Coulomb. Assuming that we have very small charged particles which can be treated as points, Coulomb’s law says:

The force between two charged particles is directly proportional to the product of the charges on these particles and inversely proportional to the square of the distance between them and acts along a straight line joining the centers of these particles.

Fig.1 will help in the precise formulation of this law.

According to the annotation on Fig.1 the mathematical formulation of Coulomb’s law is

The value of the coefficient of proportionality k depends on the medium in which the electric charges interact and on the choice of units. We consequently use SI units and for charges interacting in vacuum ( or in dry air)

where ε₀ is called the permittivity of vacuum and ε₀ = 8.85 x10^-12 C²/Nm². Just to remind you - N stands for unit of force Newton. The value of the permittivity of dry air is so close to that of a vacuum that we will treat experiments in the air as performed in a vacuum.

So, for the vacuum and for the air we will use the same numerical value

k = 8.99x10⁹ Nm²/C².

Values of ε₀ and consequently of k have been determined with much greater accuracy, but for solving problems there is no need to use these very accurate values.

Particles with charges of the same sign experience forces away from each other while particles with charges of different signs experience forces towards each other. The signs of the charges are a relative method of distinguishing two types of charges.

If the interaction of charges takes place in a medium other than vacuum/air the proportionality coefficient