According to Coulomb’s law the force between two point charges is
(a) The force is directly proportional to the product of magnitude of charges i.e.
(b) The force is inversly proportional to the square of distance between two charges i.e.
Combining both
OR
Where k is a constant of proportionality known as force constant having different value in different systems
The SI unit of quantity of electric charge is the coulomb, which is equivalent to about 6.242×1018 e (e is the charge of a proton).
Hence, the charge of an electron is approximately −1.602×10−19 C. The coulomb is defined as the quantity of charge that has passed through the cross section of an electrical conductor carrying one ampere within one second. The symbol Q is often used to denote a quantity of electricity or charge.
Electric field intensity of a charge at a point is given by the force exerted by that charge on test charge or unit charge at that point
That is The electric field intensity of charge q is