constant

boltzmann constant

Physical constant relating temperature to energy, appearing in entropy expressions.
Value
\(k_{B} = 1.380649 \times 10^{-23}\)joule per kelvin
10 related articles · 6 formulas

Description

The Boltzmann constant (symbol: k or kB) is a fundamental physical constant that connects the microscopic world of atoms and molecules to the macroscopic world of everyday temperatures and energies. It is the proportionality factor between the average kinetic energy of particles in a gas and the absolute temperature of that gas. More deeply, it appears in the statistical definition of entropy, S = k ln W, where W is the number of microscopic configurations consistent with a macroscopic state. This makes it the bridge between thermodynamics and statistical mechanics, giving a quantitative meaning to the idea that temperature is a measure of the average random motion of particles.

History

The constant was introduced by Max Planck in 1900 while deriving the blackbody radiation law, and he named it after Ludwig Boltzmann, who had pioneered statistical mechanics but never explicitly used the constant in its modern form. The value was first measured accurately by Planck using radiation data.

How it works

In kinetic theory, the average translational kinetic energy of a particle in an ideal gas is (3/2)kT, so the constant converts temperature (in kelvin) into energy (in joules). In statistical mechanics, the Boltzmann factor exp(-E/kT) gives the relative probability of a system being in a state of energy E at temperature T. The constant sets the energy scale for thermal fluctuations: at room temperature, kT is about 1/40 of an electronvolt, which is why chemical reactions and biological processes operate at that energy scale.

In practice

It is used in every calculation involving thermal noise, from the Johnson–Nyquist noise in electronic circuits to the design of infrared detectors and the calibration of thermometers. It also defines the kelvin itself, since 2019 the kelvin is defined by fixing the value of k exactly.

Fun fact

The Boltzmann constant is the only fundamental constant that has a dedicated unit (joules per kelvin) named after a person, and its value was chosen in 2019 so that the kelvin is defined by setting k = 1.380649 × 10⁻²³ J/K exactly.

Concept graph →

Formulas

$$S = k_B \ln \Omega$$
Entropy equals Boltzmann constant times log of states.
What is in the formula
Svariableentropy of the system — entropy joule per kelvin
Ωvariablenumber of microstates corresponding to the macrostate — number of microstates
k_Bconstantrelates temperature to energy per degree of freedom — boltzmann constant = 1.380649e-23 joule per kelvin
lnoperatornatural logarithm — natural logarithm
$$S \leq k \ln W$$
Entropy is bounded by the logarithm of the number of states.
What is in the formula
SvariableEntropy — entropy joule per kelvin
WvariableNumber of microstates — number of states
kconstantRelates temperature to energy in statistical mechanics — boltzmann constant = 1.380649e-23 joule per kelvin
\leqoperatorIndicates an upper bound — less than or equal
\lnoperatorComputes the logarithm to base e — natural logarithm
$$\langle h\nu \rangle = c \cdot k_B T$$
The average energy of photons in blackbody radiation is proportional to temperature.
What is in the formula
⟨hν⟩variableAverage photon energy — energy joule
TvariableTemperature of the blackbody — temperature kelvin
cconstantA numerical factor that depends on the frequency integration limits and the specific definition of 'average' (e.g., over the full spectrum). — proportionality constant = 2.701
k_BconstantFundamental constant converting temperature to energy scale — boltzmann constant = 1.380649e-23 joule per kelvin
⟨ ⟩operatorStatistical average over the photon distribution — ensemble average
$$T = \frac{\hbar c^3}{8\pi G M k_B}$$
Black hole temperature is inversely proportional to its mass.
What is in the formula
TvariableHawking temperature of the black hole — temperature kelvin
MvariableMass of the black hole — mass kilogram
\hbarconstantReduced Planck constant, fundamental quantum of action — reduced planck constant = 1.054571817e-34 joule second
cconstantSpeed of light in vacuum, universal speed limit — speed of light = 299792458 metre per second
GconstantNewtonian constant of gravitation, sets strength of gravity — gravitational constant = 6.67430e-11 cubic metre per kilogram per second squared
k_BconstantBoltzmann constant, relates temperature to energy — boltzmann constant = 1.380649e-23 joule per kelvin
$$S = \frac{k c^3 A}{4 G \hbar}$$
Black hole entropy is proportional to its event horizon area.
What is in the formula
SvariableEntropy of the black hole — entropy joule per kelvin
AvariableArea of the event horizon — area square metre
kconstantBoltzmann constant — boltzmann constant = 1.380649e-23 joule per kelvin
cconstantSpeed of light in vacuum — speed of light = 299792458 metre per second
GconstantNewtonian gravitational constant — gravitational constant = 6.67430e-11 cubic metre per kilogram per second squared
ħconstantReduced Planck constant — reduced planck constant = 1.054571817e-34 joule second
$$c_s^2 = \frac{\gamma k_B T}{m}$$
Sound speed squared equals adiabatic index times temperature over mass.
What is in the formula
c_svariableadiabatic sound speed — sound speed metre per second
Tvariabletemperature — temperature kelvin
mvariablemean particle mass — mass kilogram
γconstantratio of specific heats (Cp/Cv) of the gas — adiabatic index = varies (e.g., 5/3 for monatomic ideal ga
k_Bconstantrelates temperature to kinetic energy of particles — boltzmann constant = 1.380649e-23 joule per kelvin

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