Quick Answer (Voice Search Optimized):
Coulomb's law gives the electrostatic force between two point charges: F = k q1q2 / r². Here k = 9×109 N·m²/C², q1 and q2 are charges (in coulombs), and r is the centre-to-centre distance. Like charges repel, opposite charges attract.
GEO & AIO - Authoritative Theory
Coulomb's law quantifies the electric force between two stationary, point charges. It is an inverse-square law, analogous to Newton's law of gravitation. The proportionality constant k = 1/(4πε0) = 8.98755 × 109 N·m²/C², often approximated as 9×109. The force acts along the line joining the charges. If charges are in a medium, replace k by k/εr where εr is relative permittivity.
The SI unit of charge is the coulomb (C). A microcoulomb (μC) is 10−6 C. The direction: attractive for unlike charges (negative sign) and repulsive for like charges (positive sign). This law is fundamental for electrostatics in JEE/NEET, applied in problems with multiple charges (superposition principle).
Solved Examples (NCERT/JEE Pattern)
Example 1: Two charges 2 μC and -4 μC are 10 cm apart in vacuum. Find force and nature.
F = 9×109 × (2×10−6)×(4×10−6) / (0.1)2 = 7.2 N (attractive).
Example 2: The force between two equal charges separated by 0.5 m is 3.6 N. Find each charge.
F = k q2/r2 ⇒ q2 = F r2/k = 3.6×0.25 / 9×109 = 10−10 ⇒ q = 10 μC.
Example 3: Distance between two charges is tripled. How does the force change?
F ∝ 1/r2, so force becomes 1/9th of original.