Thermodynamics & Statistical Mechanics
The physics of heat, energy and disorder: why heat flows from hot to cold, why no engine can be perfectly efficient, and how the behaviour of trillions of molecules adds up to temperature and pressure.
Thermodynamics is the branch of physics that describes how energy, heat and work are related in macroscopic systems through a small set of empirical laws; statistical mechanics explains those laws by applying probability to the enormous numbers of microscopic particles that make up matter.
What this covers
Thermodynamics developed in the nineteenth century from practical questions about steam engines, and its laws turned out to be among the most general in physics, applying to stars, black holes, living cells and computers alike. Statistical mechanics, founded by Maxwell, Boltzmann and Gibbs, connects those macroscopic laws to the motion of atoms, and explains entropy as a count of microscopic arrangements. This domain covers the four laws, heat and temperature, processes and engine cycles, thermodynamic potentials, gases and equations of state, phase transitions, and the ensembles of statistical mechanics.
Laws of Thermodynamics
- Zeroth Law [DONE — Session 24 — 2026-10-12]
- First Law [DONE — Session 24 — 2026-10-12]
- Second Law [DONE — Session 24 — 2026-10-12]
- Third Law [DONE — Session 24 — 2026-10-12]
- Entropy [DONE — Session 24 — 2026-10-12]
- Enthalpy [DONE — Session 25 — 2026-10-12]
Heat & Temperature
- Temperature [DONE — Session 24 — 2026-10-12]
- Heat [DONE — Session 24 — 2026-10-12]
- Specific Heat Capacity [DONE — Session 25 — 2026-10-12]
- Latent Heat [DONE — Session 25 — 2026-10-12]
- Thermal Expansion [DONE — Session 26 — 2026-10-12]
- Heat Transfer [DONE — Session 26 — 2026-10-12]
- Conduction [DONE — Session 26 — 2026-10-12]
- Convection [DONE — Session 26 — 2026-10-12]
- Thermal Radiation [DONE — Session 27 — 2026-10-12]
Thermodynamic Processes & Cycles
- Isothermal Process [DONE — Session 26 — 2026-10-12]
- Adiabatic Process [DONE — Session 26 — 2026-10-12]
- Isochoric Process [DONE — Session 26 — 2026-10-12]
- Isobaric Process [DONE — Session 26 — 2026-10-12]
- Carnot Cycle [DONE — Session 25 — 2026-10-12]
- Heat Engines [DONE — Session 25 — 2026-10-12]
- Refrigerators and Heat Pumps [DONE — Session 27 — 2026-10-12]
- Thermodynamic Efficiency [DONE — Session 27 — 2026-10-12]
Thermodynamic Potentials
- Internal Energy [DONE — Session 25 — 2026-10-12]
- Helmholtz Free Energy [DONE — Session 27 — 2026-10-12]
- Gibbs Free Energy [DONE — Session 27 — 2026-10-12]
- Maxwell Relations [DONE — Session 27 — 2026-10-12]
Equations of State & Gases
- Ideal Gas Law [DONE — Session 24 — 2026-10-12]
- Van der Waals Equation [DONE — Session 27 — 2026-10-12]
- Kinetic Theory of Gases [DONE — Session 25 — 2026-10-12]
- Maxwell-Boltzmann Distribution [DONE — Session 25 — 2026-10-12]
- Equipartition Theorem [DONE — Session 27 — 2026-10-12]
Phase Transitions
- Phase Transitions [PENDING FULL ENTRY]
- Phase Diagrams [PENDING FULL ENTRY]
- Critical Point [PENDING FULL ENTRY]
- Clausius-Clapeyron Equation [PENDING FULL ENTRY]
Statistical Mechanics
- Microstates and Macrostates [PENDING FULL ENTRY]
- Boltzmann Entropy [PENDING FULL ENTRY]
- Partition Function [PENDING FULL ENTRY]
- Canonical Ensemble [PENDING FULL ENTRY]
- Grand Canonical Ensemble [PENDING FULL ENTRY]
- Microcanonical Ensemble [PENDING FULL ENTRY]
- Fermi-Dirac Statistics [PENDING FULL ENTRY]
- Bose-Einstein Statistics [PENDING FULL ENTRY]
- Maxwell-Boltzmann Statistics [PENDING FULL ENTRY]
Already covered elsewhere in the Codex
Thirty-two entries are live (marked DONE above). These live pages in other domains also cover closely related material:
- Fermi Energy — Fermi-Dirac statistics applied to electrons in metals
- Bose-Einstein Condensation — Bose-Einstein statistics and a quantum phase transition
- Conservation of Energy — the mechanical root of the first law
- Stellar Structure — hydrostatic equilibrium, ideal-gas pressure and the virial theorem in stars
- Superconductivity — a thermodynamic phase transition at low temperature
- Quantum Harmonic Oscillator — energy levels behind heat capacities
- Cosmic Microwave Background — the most precise blackbody spectrum measured
- Max Planck — blackbody radiation and the quantum hypothesis
Sources
- Callen, H.B. (1985). Thermodynamics and an Introduction to Thermostatistics (2nd ed.). Wiley.
- Reif, F. (1965). Fundamentals of Statistical and Thermal Physics. McGraw-Hill.