In partial differential equation form and SI units, Maxwell's microscopic equations can be written as ∇ ∇ ⋅ ⋅ E = ρ ρ ε ε 0 ∇ ∇ ⋅ ⋅ B = 0 ∇ ∇ × × E = − − ∂ ∂ B ∂ ∂ t ∇ ∇ × × B = μ μ 0 ( J + ε ε 0 ∂ ∂ E ∂ ∂ t ) With E the electric field, B the magnetic field, ρ ρ the electric charge density and J the current density. ε ε 0 is the vacuum permittivity and μ μ 0 the vacuum permeability.