alleryWhen an ideal gas expands into a vacuum:
AInternal energy decreases
BHeat absorbed is maximum
CWork done is zero because $p_\text{ext} = 0$
DWork done is maximum
Answer & Solution
Correct answer: C. Work done is zero because $p_\text{ext} = 0$
$W = -p_\text{ext}\Delta V$.
In a vacuum $p_\text{ext} = 0$, so $W = 0$ regardless of the volume change.
For an ideal gas this also gives $q = 0$ (since $\Delta U = 0$ at constant temperature for an ideal gas)
so the process is called *free expansion*.
Both $q$ and $W$ being zero make $\Delta U = 0$ consistent with the isothermal condition.
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