## 25/10/2018

### 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 /4πeoer

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

Therefore F = Q1Q2 / 4πeoer d2

Where er = Relative permittivity of medium and

eo = Absolute permittivity of the air

= 8.854 × 10 – 12 Farad / meter

F = Force, vector quantity

Let Q= Q2 = one coulomb, d = 1 meter, er = 1 and eo = 8.854 × 10 – 12

F = Q1Q2 / ( 4 × 3.14 × 8.854 × 10 – 12 d) Newton

= 9 × 109 Q1Q2 / dNewton

One coulomb charge

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

Then Q= 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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