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Unformatted text preview: M. S. Shell 2009 1/9 last modified 11/30/2009 Other ensembles ChE210A The isothermal-isobaric ensemble In our past discussion, we derived the microscopic probabilities for systems that are at constant temperature, i.e., coupled to an infinitely large energy bath. We will now find the same proba- bilities at both constant temperature and pressure, when the system is coupled to an infinitely large energy and volume bath. In doing so, we will also start to see some general features of partition functions and ensembles. Conceptually, the isothermal-isobaric derivation proceeds in a similar fashion to the canonical ensemble approach. We consider the system plus bath to constitute an isolated system, with constant total energy and volume. The probability of a microstate g in the system is given by the number of times a combined microstate gG appears, where G is any microstate of the bath. Since each combined microstate gets an equal amount of time according to the law of equal a priori probabilities, the probability that the system is in g , regardless of the state of the bath, is proportional to the number of bath microstates that are compatible with microstate g s energy and volume: , , exp , / where and are the total constant volume and energy of the combined system and bath. Note here that a microstate g in the system is characterized by both a specific configuration (and hence energy) and a value of system volume. We now Taylor-expand the logarithm of the bath density of states in both and and take the limit where the bath is infinitely large: exp , , , terms of order 1/ We can absorb the first term in the exponential into the constant of proportionality, since it does not depend on the microstate g , but only on constant properties of the bath and total system. We can also simplify the bath entropy derivatives by recognizing that they relate to the bath temperature and pressure. Our final expression for the probability of a microstate in the system is: exp M. S. Shell 2009 2/9 last modified 11/30/2009 g expG Relative to the canonical ensemble, we see that we obtain an additional term in the Boltzmann factor involving the pressure (specified by the bath) and the volume associated with micro- state . We know that the probabilities of all of the microstates in the system must sum to one. This fact enables us to find a normalizing factor for : expG ,, ,, expG all at , Here, we have introduced the isothermal-isobaric partition function which serves as a normalizing factor for the microstate probabilities. normalizing factor for the microstate probabilities....
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