Coulomb's Law describes the electrostatic force (non-contact) between two charged objects, which acts over some distance. Coulomb's Law expresses quantitatively the relationship between three variables affecting the electric force. Coulomb's Law states that "the electrical force between two charged objects (whenever the objects act as point charge) is directly proportional to the product of the quantity of the charge on the objects and inversely proportional to the square of the distance between the two objects":

F = k. q₁ q₂/d².…………………………… (2-4)

Where q₁, q₂ = electric charges

F = electric force

d = distance between the charged objects

k = Coulomb's constant in air

= 9.0 x10⁹ N. m² /C²

The value of k in air is slightly greater than in vacuum (K _o). The constant k is sometimes replaced by k _o:

K _o = 1/4πέ _o ………………………….. (2-5)

Where έ _o is the permittivity of vacuum:

ε _o = 8.85 x 10^-12 C²/(N. m²)

Coulomb's Law for a point charge q located a distance d from a test charge q _o is:

F = k. q q₀/d²

Since the electric field at the position of the test charge is:

E = F/ q₀ = k q/d² …………………….. (2-6)

The units of electric field (E) are N/C, and the direction of the force is always along a line joining the two charged objects. If the two objects have the same signs (both - or both +), the force on either object is directed away from each other or repulsive force. If the two objects have opposite signs (one is + and the other is +), the force on each object is directed toward each other or attractive force.