If two stationary and point charges Q₁ and Q₂ are kept at a distance r, then it is searched that force of attraction

or repulsion between them is F ∝ Q₁Q₂/r² i.e., F = kQ₁Q₂/r² ; (k = Proportionality constant)

Dependence of k: Constant k relays upon system of units and medium between the two charges :-

(i) Effect of units:

(a) In C.G.S. for air k = 1, F = Q₁Q₂/r²  Dyne

(b) In S.I. for air 

Note:

• ε_o = Absolute permittivity of air or free environment    . It's Dimension is[ML^-3T⁴A²]

• ε_o Compare with absolute magnetic permeability (μ_o) and velocity of light (c) according to the following relation c = 1/√(μ_oε_o)

(ii) Effect of medium:

(a) When a dielectric field is completely full in between charges rearrangement of the charges inside the dielectric medium takes place and the force between the same tow charge decreases by a factor of K known as dielectric constant or specific inductive capacity (SIC) of the medium, K is also called relative permittivity  of the medium (relative means with respect to free space).

            Hence in the presence of medium  F_m = F_air/K =1/(4πε_oK)Q1Q₂/r²

            Here ε_oK = ε_o  ε_r = ε (permittivity of medium)

(b) If a dielectric medium is partially filled in between the charges then effective air separation between the charges becomes (r-t+t√k)

Vector form of coulomb's law: Vector type of Coulomb's law is  where   is the unit vector from first charge to second charge along the line joining the two charges.

Principle of superposition: It is defined that total force acting on a given charge due to number of charges is the vector sum of the individual forces acting on the that charge due to all the charges.

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