COULOMB’S LAW:
Coulomb(1736-1806) measured the electrostatic force acting between two point charges and found that
The electrostatic force between two point charges is directly proportional to the product of the magnitude of the two charges and inversely proportional to the square of the distance between them.
Or
……………….. (1)
Where k is a constant depending upon the material medium present between them.
When air/ vacuum is present between the two charges
The electrostatic force between the two charges acts along the line joining the two charges. The force is repulsive if charges are similar in nature ( q1 q2 > 0 ),whereas the force is attractive if the charges are dissimilar in nature (q1 q2 < 0 ).
Coulomb’s Law – Basic Concept
Important Points to Remember
(iii) (a) SI unit of charge is coulomb and the dimensional formula for charge is [M0L0A1 T – 1].
(b) Coulomb is that amount of charge which when placed in air or vacuum at a distance of 1 m from an identical charges repels it with a force of 9 × 109 N or coulomb is that charge crossing any section of the conductor in one second when a steady current of 1 ampere flows through it.
1 coulomb = 1 ampere × 1 second.
(c) CGS units of charges are two; (I) esu of charge or stat coul. (II) emu of charge of ab coul.
1 coulomb = 3 × 109 stat coul = absolute coul.
(iv) The force exerted by each charge on the other is the same but acts in opposite direction, i.e., 
(v) The force acting on one point charge due to the other point charge is always along the line joining these two charges. It is equal in magnitude and opposite in direction on two charges, irrespective of the medium, in which they lie.
(vi) The force is conservative in nature, i.e., work done by electrostatic force in moving a point charge along a close loop of any shape is zero.
(vii) Since the force is a central force, in the absence of any other external force, angular momentum of one particle w.r.t. the other particle (in two particle system) is conserved.
(vii) When q1 and q2 are same and the medium is same,
Then 
(ix) To have the same force both in air and in medium,
(x) When two identical conductors having charges q1 and q2 are put in contact and then separated, then each has charge equal to (q1 + q2)/2. On the other hand, if charges are q1 and –q2, then each has a charge (q1 – q2)/2.
(xi) If a charge Q is to be divided into two parts such that the force between them is to be maximum, then each part is equal to Q/2.
(xii) If two charges q1 and q2 are separated by a distance d, then the distance (x) between the charge q1 and the null point is given by: 
(xii) Two identical pitch balls each of mass m are charged with a charge q. If the two balls are suspended by a silk thread of length L from the same hook as shown in fig. then the distance between the balls is given by:
(xii) Two identical pitch balls each of mass m are charged with a charge q. If the two balls are suspended by a silk thread of length L from the same hook as shown in fig. then the distance between the balls is given by: 
(xiii) In the above problem, if the whole set-up is taken in a satellite into space where there is no gravity, then the angle between the two strings is 180° and the tension in each string is equal to F.
(xiv) In the above case, if the balls are suspended in a liquid of density d and the distance between the balls remains same, then the dielectric constant of the liquid is given by: 
Comparison Between Coulomb’s Electrostatic Force and Gravitational Force
(xv) (a) Coulomb’s electrostatic force may be attractive or repulsive but the gravitational force is always attractive.
(b) Coulomb’s electrostatic force depends on the medium between the two charges but the gravitational force is independent of the medium between the two bodies.
(c) The ratio of the coulomb’s force between two protons to the gravitational force between them separated by the same distance is 1039.