If we separate the total charge and total current into their free and bound parts, we get that the density of the Lorentz force is f = ( ρ ρ f − − ∇ ∇ ⋅ ⋅ P ) E + ( J f + ∇ ∇ × × M + ∂ ∂ P ∂ ∂ t ) × × B .