The Physics
Hypertextbook
Opus in profectus

# Coulomb's Law

## Discussion

### introduction

The force between two point charges is…

• directly proportional to the magnitude of each charge (q1q2)
• inversely proportional to square of the separation between their centers (r)
• directed along the separation vector connecting their centers ()

This relationship is known as Coulomb's Law. Charles-Augustin Coulomb (1736–1806) France. As an equation it is usually written in one of two forms…

 F = k q1q2 r2

where…

 F = magnitude of the electrostatic force ke = electrostatic constant = 8.99 × 109 N m2/C2 q1, q2 = charges of the two objects r = separation between the charges

or…

 F = 1 q1q2 r̂ 4πε0 r2

where…

 F = electrostatic force vector ε0 = vacuum permittivity = 8.8541878128 × 10−12 C2/N m2 q1, q2 = charges of the two objects r = separation between the charges r̂ = unit vector in the radial direction

text

• When two charges have the same sign their product is positive, which means the force vector is directed with the separation vector () and the action is repulsive.
• When two charges have the opposite sign their product is negative, which means the force vector is directed against the separation vector () and the action is attractive.