Let Ω Ω be such a region and let ∂ ∂ Ω Ω be its boundary. Then the second law of thermodynamics states that the rate of increase of η η in this region is greater than or equal to the sum of that supplied to Ω Ω (as a flux or from internal sources) and the change of the internal entropy density ρ ρ η η due to material flowing in and out of the region.