## 10.4 Coulomb's Law

The force of interaction of two stationary point charges in vacuum is directly proportional to the product of these charges and inversely proportional to the square of their separation,

where F is in newton, q1 and q2 in coulomb, r in metre and k is a constant given in SI units by

= 9 ´ 109 N m2 C-2

where Î0 = 8.85 ´ 10-12 C2 N-1 m-2 and is called the permittivity of free space (vacuum or air).

For mediums other than air or vacuum, the electrostatic force between two charges becomes

Here Î = Î0 Îr, is called the absolute permittivity or permittivity of the medium, and Îr (= Î/Î0) is the relative permittivity of the medium which is a dimensionless constant. Îr is also sometimes called dielectric constant, and is represented by letter K.

### 10.5 Comparison with Gravitation Law

Coulomb's law is analogous to Newton's law of gravitation :

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