Partition Function

(7.51) |

Consider a quasi-static change by which and change so slowly that the system stays close to equilibrium, and, thus, remains distributed according to the canonical distribution. It follows from Equations (7.35) and (7.45) that

(7.52) |

The last term can be rewritten

(7.53) |

giving

(7.54) |

The previous equation shows that although the heat absorbed by the system, , is not an exact differential, it becomes one when multiplied by the temperature parameter, . This is essentially the second law of thermodynamics. In fact, we know that

(7.55) |

Hence,

This expression enables us to calculate the entropy of a system from its partition function.

Suppose that we are dealing with a system consisting of two systems and that only interact weakly with one another. Let each state of be denoted by an index , and have a corresponding energy . Likewise, let each state of be denoted by an index , and have a corresponding energy . A state of the combined system is then denoted by two indices and . Because and only interact weakly, their energies are additive, and the energy of state is

(7.57) |

By definition, the partition function of takes the form

(7.58) |

Hence,

(7.59) |

giving

(7.60) |

where and are the partition functions of and , respectively. It follows from Equation (7.35) that the mean energies of , , and are related by

(7.61) |

It also follows from Equation (7.56) that the respective entropies of these systems are related via

Hence, the partition function tells us that the extensive (see Section 7.8) thermodynamic functions of two weakly-interacting systems are simply additive.

It is clear that we can perform statistical thermodynamical calculations using the partition function, , instead of the more direct approach in which we use the density of states, . The former approach is advantageous because the partition function is an unrestricted sum of Boltzmann factors taken over all accessible states, irrespective of their energy, whereas the density of states is a restricted sum taken over all states whose energies lie in some narrow range. In general, it is far easier to perform an unrestricted sum than a restricted sum. Thus, it is invariably more straightforward to derive statistical thermodynamical results using rather than , although has a far more direct physical significance than .