The constitutive equations describe how the quantity responds to different stimuli via transport. The Navier-Stokes equations and Fourier's law of heat conduction, for example explain the response of heat flow to temperature gradients and the relationship between fluid flux and the forces applied to the fluid, respectively. These equations also show that transport phenomena and thermodynamics are inextricably linked, which explains why transport phenomena are irreversible. Almost all of these physical phenomena entail systems pursuing their lowest energy state in accordance with the minimal energy principle. They tend to establish real thermodynamic equilibrium as they approach this condition, at which time there are no more driving forces in the system and transit ceases. Heat transfer is the system's endeavour to establish thermal equilibrium with its environment, much as mass and momentum transit move the system toward chemical and mechanical equilibrium. Heat conduction (energy transmission), fluid flow (momentum transfer), molecular diffusion (mass transfer), radiation and electric charge transfer in semiconductors are examples of transport processes. The phenomena of transportation have a wide range of applications. The motion and interaction of electrons, holes, and phonons, for example, are studied in solid state physics as transport phenomena. Another example is in biomedical engineering, where thermoregulation, perfusion, and microfluidics are all interesting transport phenomena. Transport phenomena are examined in reactor design, analysis of molecular or diffusive transport mechanisms and metallurgy in chemical engineering. The existence of external sources can influence mass, energy, and momentum transport. When the source of the odour is still present, it disappears more slowly (and may worsen). Whether or not a heat source is applied affects the pace of cooling of a solid that conducts heat. The resistance or drag imparted by the surrounding air is counteracted by the gravitational force exerted on a rain drop. Temperature variations cause heat to flow from warmer to colder areas of the system, while pressure differences cause matter to flow from high-pressure to low-pressure regions in fluid systems characterized in terms of temperature, matter density and pressure.