## 6.2 Introduction

The purpose of this module is to illustrate several ways in which the First Law of Thermodynamics can be applied to biological systems. Recall that the First Law can be written

$$$\Delta U = Q - W \tag{6.1}$$$

where
• $$\Delta U$$ is the change in internal energy of the system,
• $$Q$$ is the heat added to the system,
• and $$W$$ is the work done by the system to the surroundings.
• The reader may find it helpful to consult the Appendix on Units, Symbols, and Dimensions and/or the module on that subject. In the examples we will make different kinds of assumptions about the terms of equation (6.1) to illustrate different applications. First, we will consider an adiabatic process in which no heat is added to the system, so $$Q = 0$$. A second type of application, with $$W = 0$$, is often used to describe the heat energy balance of organisms. Much of the energy balance literature has also been devoted to steady state analysis for which $$\Delta U = 0$$. Finally, we will extend the formulation of the First Law to nonclassical thermodynamics in which mass flows are also considered.