SME
Thermal & Statistical Physics · Problem type
Which ensemble?
Use this for “A gas in a box with a porous wall that lets molecules through to a big reservoir: which ensemble describes it?”
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Which ensemble describes each system?
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What a system shares with its surroundings picks the ensemble.
Assumptions
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Microcanonical: nothing crosses the wall, so E, N and V are fixed and every microstate at that E is equally likely.
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Canonical: energy flows to a large reservoir at temperature T, so E varies with weight e−E ÷ kBT while N and V stay fixed.
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Grand canonical: energy and particles both cross, so T and the chemical potential μ are set by the reservoir.
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For a large system all three give the same averages; the choice is the one that makes the counting easiest.
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