where n is the electron density, e is the elementary charge, ln ⁡ ⁡ Λ Λ is the Coulomb logarithm, ϵ ϵ 0 is the vacuum permittivity, m e is the electron mass and v T e is the electron thermal speed. It was derived by considering the balance between the electric field and the collisional forces acting on a single electron within the plasma.