In terms of σ σ and S , another way to write the Lorentz force (per unit volume) is f = ∇ ∇ ⋅ ⋅ σ σ − − 1 c 2 ∂ ∂ S ∂ ∂ t where c is the speed of light and ∇ · denotes the divergence of a tensor field. Rather than the amount of charge and its velocity in electric and magnetic fields, this equation relates the energy flux (flow of energy per unit time per unit distance) in the fields to the force exerted on a charge distribution. See Covariant formulation of classical electromagnetism for more details.