Thermodynamics and entropy

The existence of entropy follows inevitably from the first and second laws of thermodynamics. However, our purpose is not to reproduce this deduction, but rather to focus on the concept of entropy, its meaning and its applications. Entropy is a central concept for many reasons, but its chief function in thermodynamics is to quantify the irreversibility of a thermodynamic process. Each term in this phrase deserves elaboration. Here we define thermodynamics and process; in subsequent sections we take up irreversibility. We will also learn how entropy or, more precisely, differences in entropy tell us which processes of an isolated system are possible and which are not.

Thermodynamics is the science of macroscopic objects composed of many parts. The very size and complexity of thermodynamic systems allow us to describe them simply in terms of a mere handful of equilibrium or thermodynamic variables, for instance, pressure, volume, temperature, mass or mole number, internal energy, and, of course, entropy. Some of these variables are related to others via equations of state in ways that differently characterize different kinds of systems, whether gas, liquid, solid, or composed of magnetized parts.