concept

partition function

The partition function is a statistical mechanics tool that sums over all possible states to determine thermodynamic properties like temperature.
12 related articles · 2 formulas · 1 who worked here

Description

The partition function is like a catalog of tickets in an energy lottery: it lists all possible energy states of a system and indicates the chance of each occurring.

History

Boltzmann introduced the concept of statistical weight and entropy in the 1870s, but the explicit formula Z = Σ e^{-βE_i} for the canonical ensemble was introduced by Gibbs in 1902 in his book 'Elementary Principles in Statistical Mechanics'. Einstein and Bose in 1924 generalized it to quantum gases with integer spin, and Fermi and Dirac to half-integer spin, leading to Bose–Einstein and Fermi–Dirac statistics.

How it works

All thermodynamic quantities are expressed through Z: internal energy U = -∂ ln Z/∂β, heat capacity C_v = ∂U/∂T, pressure P = k_B T (∂ ln Z/∂V)_T. In the quantum case, Z = Tr(e^{-βĤ}), where Ĥ is the Hamiltonian. For non-interacting systems, it factorizes over single-particle states, but in general requires approximate methods.

Fun fact

Knowing just this sum, you can extract from it temperature, pressure, and even how a kettle on the stove will behave—everything that can be measured macroscopically.

Concept graph →

Formulas

$$S_t = \\max(0, S_{t-1} + z_t - k)$$
Cumulative sum statistic for detecting changes in a process.
What is in the formula
S_tvariableCUSUM statistic at time t
S_{t-1}variableCUSUM statistic at previous time
z_tvariableObservation or standardized value at time t
kvariableAllowance or reference value (positive constant)
maxoperatorTakes the maximum of two numbers — maximum
s_t=\max(0,s_{t-1}+z_t-k) · 2607.00331
$$N = \frac{4\pi S_\nu \Delta\nu Q(T_{\rm rot})}{A_{ul} \Omega h c g_u e^{-E_u/kT_{\rm rot}}}$$
Column density derived from rotational molecular line emission.
What is in the formula
Nvariablecolumn density — column density per square metre
S_νvariableline flux density — spectral flux density joule per square metre per second per he
Δνvariableline width — frequency interval hertz
Qvariablerotational partition function — partition function
T_rotvariablerotational temperature — temperature kelvin
A_ulvariableEinstein A coefficient — einstein a coefficient per second
Ωvariablesolid angle of source — solid angle steradian
hvariablePlanck constant — planck constant joule second
cvariablespeed of light — speed of light metre per second
g_uvariabledegeneracy of upper level — degeneracy
E_uvariableenergy of upper level — energy joule
kvariableBoltzmann constant — boltzmann constant joule per kelvin

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