information:extraction, information:classification
Gases

where β {\displaystyle \beta } is a numerical constant of order 1 {\displaystyle 1}, k B {\displaystyle k_{\text{B}}} is the Boltzmann constant, and λ {\displaystyle \lambda } is the mean free path, which measures the average distance a molecule travels between collisions. Since λ {\displaystyle \lambda } is inversely proportional to density, this equation predicts that thermal conductivity is independent of density for fixed temperature. The explanation is that increasing density increases the number of molecules which carry energy but decreases the average distance λ {\displaystyle \lambda } a molecule can travel before transferring its energy to a different molecule: these two effects cancel out. For most gases, this prediction agrees well with experiments at pressures up to about 10 atmospheres. At higher densities, the simplifying assumption that energy is only transported by the translational motion of particles no longer holds, and the theory must be modified to account for the transfer of energy across a finite distance at the moment of collision between particles, as well as the locally non-uniform density in a high density gas. This modification has been carried out, yielding Revised Enskog Theory, which predicts a density dependence of the thermal conductivity in dense gases.