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Free (Joule) Expansion of a Gas

Interactive 3D free-expansion box: an insulated gas confined to one side of a partition rushes into a vacuum when the wall is removed. Watch entropy rise, pressure fall and temperature stay exactly constant — the classic proof that ideal-gas internal energy depends only on temperature.

Thermodynamics3DEasy60 FPS📱 Mobile-adapted⇄ 2D version

An insulated box holds an ideal gas confined to one side of a removable partition, with vacuum on the other side. Because the surroundings do no work on the gas and no heat crosses the walls, the first law forces the internal energy — and therefore the temperature of an ideal gas — to stay exactly constant through the whole expansion, even though the process is thoroughly irreversible and generates real entropy. This simulator renders the gas as bouncing 3D molecules confined by a real partition mesh; removing the partition lets them diffuse into the vacuum while live readouts track occupied volume, relative pressure, mean molecular speed and the entropy generated, converging on the textbook result ΔS = N k_B ln(V₂/V₁).

⚙ Under the hood

An insulated gas confined to one side of a partition rushes into a vacuum the instant the wall is removed — watch entropy rise and pressure fall while temperature stays exactly constant, the classic proof that an ideal gas's internal energy depends on temperature alone.

thermodynamicsentropyideal gasfirst lawirreversibilitykinetic theory

3D · Three.js / WebGL renderer · 60 FPS target · runs fully client-side, no install