Thermodynamics is typically defined as a theory dealing with the flow of heat and energy between material systems. This definition is certainly applicable here, however, Callen provides (in his excellent book on the subject [Cal85]) an alternative definition that highlights another role that thermodynamics plays in continuum mechanics: “Thermodynamics is the study of the restrictions on the possible properties of matter that follow from the symmetry properties of the fundamental laws of physics.” In this chapter (and the next), we address both of these aspects of thermodynamic theory in the context of continuum mechanics.

The theory of thermodynamics boils down to three fundamental laws, deduced from empirical observation, that all physical systems are assumed to obey. The zeroth law of thermodynamics is related to the concept of thermal equilibrium. The first law of thermodynamics is a statement of the conservation of energy. The second law of thermodynamics deals with the directionality of thermodynamic processes. We will discuss each of these laws in detail, but first we describe the basic concepts in which thermodynamics is phrased.

For the purposes of thermodynamic analysis, the universe is divided into two parts: the system whose behavior is of particular interest, and the system's surroundings (everything else). The behavior of the surroundings is of interest only insofar as is necessary to characterize its interactions with the system.