In continuum mechanics, the most general form of an exact conservation law is given by a continuity equation. For example, conservation of electric charge q is ∂ ∂ ρ ρ ∂ ∂ t = − − ∇ ∇ ⋅ ⋅ j where ∇⋅ is the divergence operator, ρ is the density of q (amount per unit volume), j is the flux of q (amount crossing a unit area in unit time), and t is time.