# 1.6 Coulomb's Inverse Square Law

**Source:**1.6 Coulomb's Inverse Square Law

**Tag:**inverse square law definition

In activity 1, we have seen positive charge on wool and negative charge on the balloon exert force on each other. Leaves of electroscope will collapse after the charged rod is pulled back without touching the electroscope. What conclusions can be made from these observations?

From these observations, we can conclude that, the force between two charged bodies increase in magnitude of charges. But force between charged bodies decrease with increase in distance between them.

A French Physicist Charles Augustine de Coulomb found out a quantitative law about the force between two charges. According to Coulomb's law.

From these observations, we can conclude that, the force between two charged bodies increase in magnitude of charges. But force between charged bodies decrease with increase in distance between them.

A French Physicist Charles Augustine de Coulomb found out a quantitative law about the force between two charges. According to Coulomb's law.

**Electrostatic force (F) of attraction or repulsion between two point charges 'q1' and 'q2' separated by a distance 'r' is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.****Where k is a constant of proportionality. The value of k depends on the units of F, q and r.****Unit Charge or One coulomb of Charge****How can you define unit charge?****Suppose, two unit charges are separated by a distance 1m apart in vacuum, then what will be the force of repulsion between them?****With reference to the force experienced by unit charge, it can be defined as follows.****Unit charge is the charge which experiences a repulsive force of 9*10^9 N from an equal and similar charge, kept at a distance of 1 m in air or vacuum.****In SI system one columb is that charge which flows through any section in one second causes 1A current.****Experimentally it was found that the colomb force depends on the medium betwen the charges. In a medium, Coulomb's law differs a little****Relative permittivity or dielectric constant of a medium is defined as the ratio of the permittivity of the medium to the permittivity of the vacuum.****The value of dielectric constant for any material medium is greater than 1. This implies that electrostatic force between two point charges is maximum in vacuum.**