In electromagnetic field theory, vector calculus can be used to express the law in terms of charge density ρ (in coulombs per cubic meter) and electric current density J (in amperes per square meter). This is called the charge density continuity equation ∂ ∂ ρ ρ ∂ ∂ t + ∇ ∇ ⋅ ⋅ J = 0.