free expansion

Question

For ideal gas free expansion into vacuum: work and internal energy change are:

RankGuru · Accepted Answer

Free expansion (Joule expansion) occurs when a gas expands into a vacuum through a valve.
1. **Work**: Since there is no external pressure ($P_{ext}=0$) for the gas to push against, the work done is zero ($W = 0$).
2. **Heat**: The container is typically insulated, so $Q = 0$.
3. **Internal Energy**: From First Law $\Delta U = Q - W = 0 - 0 = 0$.
For an ideal gas, since $U$ is purely a function of temperature ($T$), $\Delta U = 0$ implies the temperature of the gas remains constant ($T_i = T_f$).

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free expansion

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