# 4: Some Basic Applications of Statistical Thermodynamics

• 4.1: Interpreting the Partition Function
Only quantum states whose energy is less than kT can make substantial contributions to the magnitude of a partition function. Very approximately, we can say that the partition function is equal to the number of quantum states for which the energy is less than kT . Each such quantum state will contribute approximately one to the sum that comprises the partition function; the contribution of the corresponding energy level will be approximately equal to its degeneracy.
• 4.2: Conditions under which Integrals Approximate Partition Functions
A common approximation is to substitue integrals for sums. This section looks at the constraints that must be satisfied in order to make the integral a good approximation to the sum.
• 4.3: Probability Density Functions from the Energies of Classical-mechanical Models
We could postulate probability density functions apply to other energies derived from classical-mechanical models for molecular motion. We will see that this can indeed be done. The results correspond to the results that we get from the Boltzmann equation, where we assume for both derivations that many energy levels satisfy ϵ≪kT. The point is that, at a sufficiently high temperature, the behavior predicted by the quantum mechanical model and that predicted from classical mechanics converge.
• 4.4: Partition Functions and Average Energies at High Temperatures
It is important to remember that the use of integrals to approximate Boltzmann-equation sums assumes that there are a large number of energy levels, ϵi , for which ϵi≪kT . If we select a high enough temperature, the energy levels for any motion will always satisfy this condition. The energy levels for translational motion satisfy this condition even at sub-ambient temperatures. This is the reason that Maxwell’s derivation of the probability density function for translational motion is successf
• 4.5: Energy Levels for a Three-dimensional Harmonic Oscillator
One of the earliest applications of quantum mechanics was Einstein’s demonstration that the union of statistical mechanics and quantum mechanics explains the temperature variation of the heat capacities of solid materials. The physical model underlying Einstein’s development is that a monatomic solid consists of atoms vibrating about fixed points in a lattice. The particles of this solid are distinguishable from one another, because the location of each lattice point is uniquely specified.
• 4.6: Energy and Heat Capacity of the "Einstein Crystal"
• 4.7: Applications of Other Entropy Relationships
• 4.8: Problems