Coulomb Law

When two charged bodies place very close to each other, it experience a mechanical force.

According to Coulomb’s law

• The force between charges bodies is directly proportional to product of charges, inversely proportional to square of distance between them and nature of medium surrounding them. 
• Let Q₁ and Q₂ are two point charges, d is distant between them and F is force acts on two point charges.

        F α Q₁Q₂ / d²

        F = k Q₁Q₂ / d²

        Where k = Constant

                      = 1 /4πe_oe_r

The value of constant k depends upon surrounding medium of charge.

Therefore F = Q₁Q₂ / 4πe_oe_r d²

Where e_r = Relative permittivity of medium and

           e_o = Absolute permittivity of the air

               = 8.854 × 10 ^{– 12} Farad / meter

            F = Force, vector quantity

Let Q₁ = Q₂ = one coulomb, d = 1 meter, e_r = 1 and e_o = 8.854 × 10 ^{– 12}

        F = Q₁Q₂ / ( 4 × 3.14 × 8.854 × 10 ^{– 12} d² ) Newton

          = 9 × 10⁹ Q₁Q₂ / d² Newton

One coulomb charge

• If Q₁ = Q₂ = Q , d = 1 meter , F = 9 × 10⁹ Newton

        Then Q₁ = Q₂ = Q = ± 1 coulomb

• One coulomb charge is defined as a charge which placed in air from an equal and opposite charge repels with a force of 9 × 10⁹ Newton.

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