Interactions and Implications

Although the law of entropy increase governs the direction in which things change, we don’t observe entropy directly. Instead we observe three quantities—temperature, pressure, and chemical potential—that tell us how the entropy of a system changes as it interacts in three different ways with its surroundings. This chapter shows how these three quantities are mathematically related to a system’s entropy, energy, volume, and number of particles. These relations complete the foundation of macroscopic thermodynamics. Moreover, for the three model systems whose entropies are calculated explicitly in the previous chapter, these relations lead to detailed testable predictions of thermal behavior.