Canonical versus grand canonical occupation numbers for simple systems |
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Authors: | P T Landsberg P Harshman |
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Institution: | (1) Department of Electrical Engineering, University of Florida, 32611 Gainesville, Florida;(2) Present address: Faculty of Mathematical Studies, University, Southampton, England |
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Abstract: | An ideal gas ofN indistinguishable particles is described by a canonical ensemble (c.e.) and also by a grand canonical ensemble (g.c.e.) which hasN as themean total number of particles, the temperature and volume being the same in both cases. Exact mean occupation numbersn
j(N) are found if the system has only two states 1 and 2 of energiesE
2 E
1. This should apply to quantum wells and similar simple systems. For systems which have captured one particle, the theory gives the simplest answers, and one find a maximum discrepancy of 17% between the two ensembles for the fermion case. It occurs whenE
2–E
1 53 meV at room temperature. ForN=1 the mean occupation number for the c.e. is identical for fermions and for bosons, being in both cases given byn
2(1)={exp(E
2-E
1)/kT]+1}-1,n
1(1)=1-n
2(1) For largeN one reverts to the usual situation and the discrepancy between the ensembles becomes small. |
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Keywords: | Ideal quantum gas fermions bosons canonical ensemble grand canonical ensemble occupation numbers |
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