Human Food Consumption: a Primer on Nonequilibrium Thermodynamics for College Physics

Students often have great difficulties with applications of the energy principle, especially those from biology, although most introductory physics texts include a brief discussion of metabolism. We point out that many of these discussions are unsatisfactory, since they often fail to mention how biological systems are thermal systems in stationary nonequilibrium states . This has important implications: in particular, that energy input is in the form of work (i.e., change of potential energy) and that energy outflux is in the form of heat, which is necessary to maintain a stationary state. We focus on some key aspects of metabolism by using a mechanical analogue: energy input is the work done by the gravitational force on an object with mass m , and heat production is modelled as the energy dissipated by turbulent air flow around the object.