• id: JTHERM-01-N13
• title: Free Expansion of an Ideal Gas
• tags: free-expansion, irreversible, joule

In free expansion, a gas expands into a vacuum: W = 0 (no opposing pressure), Q = 0 (adiabatic), so $\Delta U = 0$. For ideal gas: $\Delta T = 0$. But entropy increases (irreversible). This is different from adiabatic expansion against a piston (where gas cools). Free expansion is the most extreme case of irreversibility — the gas expands without doing any useful work.