Chapters 18 and 19 were about physics at the macroscopic level—the behavior of objects we can measure and manipulate in the laboratory. Macroscopic objects are characterized by state variables, such as pressure, temperature, and specific heat. We’ve noted how some macroscopic properties, such as an object’s compressibility, are related to the spacing between atoms, but the microscopic motions of atoms and molecules haven’t really entered our discussion of thermodynamics.
What are all those atoms and molecules doing, and how do their motions and interactions influence the macroscopic physics? This is a big topic, so we will limit ourselves to seeing how the micro/macro connection answers three important questions:
How are macroscopic state variables related to and predicted by the motions of an object’s atoms and molecules?
How is heat energy transferred from one object to another in a thermal interaction?
Why are most macroscopic processes irreversible—that is, go in one direction but not the other (e.g., heat energy “flows” from hot to cold but not from cold to hot)?
A key idea underlying the micro/macro connection, which we’ll see throughout this chapter, is that atomic-level collisions and interactions continuously redistribute a system’s thermal energy among all the atoms and molecules.