Chapter-1|Electric Charges and Fields | NCERT 12th Physics:
Coulomb’s Law:
Coulomb’s law states that the force of attraction or repulsion between two stationary point charges is
(I) Directly proportional to the product of the magnitudes of the two charges and
(II) Inversely proportional to the square of the distances between them.
This force acts along the line joining the two charges
If two charges q₁ and q₂ are separated by a distance r, then the force F of attraction or repulsion between them is such that
Where K is called electrostatic constant. The value of K depends on the nature of the medium between the two charges and the System of units chosen to measure F, q₁, q₂ and r.
So, one coulomb is that amount of charge that repels an equal and similar charge with a force of 9×10⁹ N when placed in vacuum at a distance one metre from it.