Introduction

'Thermodynamics' comes from the Greek thermos (heat) and dynamics (flow/movement) -- literally the flow of heat. It grew out of a very practical 19th-century problem: engineers already had working steam engines and wanted to understand the underlying principles well enough to make them more efficient, particularly the question of how to turn heat into mechanical work. Solving that problem produced the laws of thermodynamics, which turned out to generalise far beyond steam engines into powerful mathematical relationships covering almost any process that exchanges energy.

We meet thermodynamics constantly without naming it: burning a fuel to release heat, current flowing through a circuit to deliver electrical energy, the metabolic reactions inside our own bodies that power every biological function. Thermodynamics is the quantitative study of how energy transforms from one of these forms into another, and it lets us make useful, numeric predictions about such processes rather than just qualitative ones.

At its core, thermodynamics is concerned with macroscopic properties -- things you can measure on a bulk sample, like heat and work -- and how they relate to each other. It deals with systems that are at (or moving between) equilibrium states, and deliberately stays independent of any assumption about the individual molecules that make up the system; you do not need to know anything about atomic or molecular structure to apply it.

The whole subject rests on three laws (plus a zeroth, added later to make the concept of temperature itself rigorous). The first and second laws essentially generalise everyday experience of energy interconverting between forms. The third law is about the behaviour of entropy as temperature approaches absolute zero, and the practical unattainability of absolute zero itself. …