In physics and mathematics, the Boltzmann distribution is a certain distribution function or probability measure for the distribution of the states of a system. It underpins the concept of the canonical ensemble, providing its underlying distribution. A special case of the Boltzmann distribution, used for describing the velocities of particles of a gas, is the Maxwell–Boltzmann distribution. In more general mathematical settings, the Boltzmann distribution is also known as the Gibbs measure.
The Boltzmann distribution for the fractional number of particles N_{i} / N occupying a set of states i possessing energy E_{i} is:
where k_{B} is the Boltzmann constant, T is temperature (assumed to be a welldefined quantity), g_{i} is the degeneracy (meaning, the number of states having energy E_{i}), N is the total number of particles and Z(T) is the partition function.
Alternatively, for a single system at a welldefined temperature, it gives the probability that the system is in the specified state. The Boltzmann distribution applies only to particles at a high enough temperature and low enough density that quantum effects can be ignored, and the particles are obeying Maxwell–Boltzmann statistics. (See that article for a derivation of the Boltzmann distribution.)
The Boltzmann distribution is often expressed in terms of β = 1/kT where β is referred to as thermodynamic beta. The term or , which gives the (unnormalised) relative probability of a state, is called the Boltzmann factor and appears often in the study of physics and chemistry.
When the energy is simply the kinetic energy of the particle
then the distribution correctly gives the Maxwell–Boltzmann distribution of gas molecule speeds, previously predicted by Maxwell in 1859. The Boltzmann distribution is, however, much more general. For example, it also predicts the variation of the particle density in a gravitational field with height, if . In fact the distribution applies whenever quantum considerations can be ignored.
In some cases, a continuum approximation can be used. If there are g(E) dE states with energy E to E + dE, then the Boltzmann distribution predicts a probability distribution for the energy:
Then g(E) is called the density of states if the energy spectrum is continuous.
Classical particles with this energy distribution are said to obey Maxwell–Boltzmann statistics.
In the classical limit, i.e. at large values of E / (kT) or at small density of states — when wave functions of particles practically do not overlap — both the Bose–Einstein or Fermi–Dirac distribution become the Boltzmann distribution.
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