The Coulomb (C) is the SI unit of electric charge. Understanding its precise definition is essential for Class 10 and 12 Physics (Electricity and Electrostatics chapters).
The Coulomb is named after French physicist Charles-Augustin de Coulomb (1736–1806), who established the mathematical law of electric force between charges (Coulomb's Law).
1 Coulomb is defined as the amount of electric charge that flows past a point in a conductor in 1 second when a steady current of 1 Ampere flows through it.
Q = I × t 1 C = 1 A × 1 s
So: 1 Coulomb = 1 Ampere-second (A·s)
The charge on one electron = 1.6 × 10⁻¹⁹ C
Therefore: 1 Coulomb = 6.25 × 10¹⁸ electrons
(That is 6.25 billion billion electrons — an enormous number!)
F = kq₁q₂/r² — The force between two point charges is directly proportional to the product of charges and inversely proportional to the square of distance between them. k = 9×10⁹ N·m²/C²
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