Assuming (by considering only the energy of the random and internal motions) that there was no macroscopic motion of the gas. Consider now a gas with not only a temperature gradient dT/dx, but also a gradient of flow velocity du/dx as in the treatment of momentum transport. By the mean-free-path methods of this section, show that the flux of molecular energy in the x-direction is now given by dT/dx + dui/dx^2.
The second term on the right is interpreted at the macroscopic level as the work done by the shear stress.