The Second Law

Abstract

Why are so many large-scale processes irreversible, happening in one direction but not the other as time passes? This chapter answers that question using three simple model systems: a collection of two-state particles such as flipped coins or elementary magnetic dipoles; the Einstein model of a solid as a collection of identical quantum oscillators; and a monatomic ideal gas such as helium or argon. For each system we learn to calculate the multiplicity: the number of possible microscopic arrangements. Taking the logarithm of the multiplicity gives the entropy. And the laws of probability then imply the second law of thermodynamics: Entropy tends to increase.