Coulomb's law establishes a general definition of the electrostatic potential, ψ. Specifically, the Coulomb potential at position ⃗ r due to a point charge q i at position ⃗ r i is given by, ψ(⃗ r) ∝ q i /|⃗ r -⃗ r i |, where the constant of proportionality is (4πϵ 0 ) -1 and ϵ 0 is the vacuum permittivity. Contributions to this potential are additive, and thus in a field of multiple point charges, the Coulomb potential is a sum of single point-charge contributions,