Introduction

Thermodynamics is the branch of physics that studies how heat, work and internal energy are exchanged between a system and its surroundings, and what limits nature places on these exchanges. Unlike mechanics, which tracks the position and velocity of every particle, thermodynamics deliberately works only with a handful of macroscopic quantities -- pressure, volume, temperature, internal energy -- that describe a system as a whole, without needing to know what any individual molecule is doing at any instant. This makes thermodynamics extremely general: the same laws apply equally to a gas in a cylinder, a chemical reaction, a living cell, or the atmosphere of a planet.

WBCHSE's Unit 8 develops this subject in the order it is most naturally built up. It begins with thermal equilibrium and the Zeroth law of thermodynamics, which together make the everyday idea of temperature mathematically precise and physically well-defined. It then studies how a system exchanges heat, work and internal energy with its surroundings, captured in the first law of thermodynamics -- essentially, the law of conservation of energy, extended to include heat as a form of energy transfer. Two particular ways of taking a gas from one state to another are studied in detail: the isothermal process, carried out at constant temperature, and the adiabatic process, carried out with absolutely no heat exchange -- both illustrated using $P$-$V$ diagrams, and both leading to explicit formulas for the work done by the gas.

The unit then turns to a question the first law alone cannot answer: which of the many energy-conserving processes actually happen in nature, and which never do? This is answered by the second law of thermodynamics, which introduces a fundamental asymmetry into physical processes -- heat flows spontaneously only from hot to cold, never the other way round, and no engine can convert heat completely into useful work. The distinction between reversible and irreversible processes follows directly from this, and the unit closes by applying these ideas to two of the most important practical devices built around them: the heat engine, which converts heat into work (with the idealised Carnot engine setting the theoretical ceiling on how efficient any such engine can be), and the refrigerator, which does the reverse, using external work to pump heat from a colder space to a warmer one.