According to Coulomb.s law, the magnitude of force of attraction or repulsion between any two point charges at rest is directly proportional to the product of magnitude of charges and inversely proportional to the square of distance between them.
Let r be the distance between two point charges `q_(1) and q_(2)`
According to Coulomb.s law
`F oo q_(1)q_(2)`
`F oo 1/r^(2)`
Combining eqn.s (1) and (2), we get
`F oo (q_(1)q_(2))/r^(2)`
`F=(Kq_(1)q_(2))/r^(2)`
where K is known as electrostatic force constant Coulomb.s constant.
`:." "K=1/(4 pi varepsilon) = 1/(4 pi varepsilon_(0) varepsilon_(r)`
`:." "varepsilon_(n)varepsilon/varepsilon_(0)" "implies" "varepsilon-varepsilon_(0)`
where `pi_(0)` is called absolute permittivity in free space and `in_(r)`, is relative permittiviy.
`F_(m)=1/(4 pi varepsilon_(0)varepsilon_(r))(q_(1)q_(2))/r^(2)" "["in medium"]`
In free space `varepsilon_(r)= 1" "("In air or vacuum")`
