Static Electricity: Coulomb's Law

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Coulomb's Law

The electrostatic force between two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them

History

This law was first published in 1785 by French physicist Charles Augustin de Coulomb

it is analogous to Isaac Newton's inverse-square law of universal gravitation

F∝ (Q₁)(Q₂)/d²

Q₁ = charge on body one

Q₂ = charge on body two

d = distance between them

This is the formula used for numerical calculations

ε = permittivity of the medium (surrounding the charges)

r = distance between the charges

When dealing with spherical conductors, the distance is the distance between the centres of the spheres

The electrostatic force can be either attractive or repulsive

Permittivity

If the permittivity is low, the force is big

The force is greatest when the charges are in a vacuum

ε₀ = permittivity of a vacuum

All media have a greater permittivity than that of a vacuum = the relative permittivity (εᵣ)

ε = εᵣ ε₀

Direction of the Force

The direction of the force is always along the line joining the two charges

Force is attractive if the charges are opposite

Force is repulsive if the charges are the same

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