大学物理（热学） Mean Free Path Diffusion, Thermal conductivity, and Viscosity 2006-10-12

Equilibrium distribution ni  the number of molecules of dn is the number of molecules vy vy + dvy in near v

Meaning of dn/N among N molecules, how many are near v probability that one molecule is near v proportional to dvx ...

The probability should be normalized Isotropic assumption The solution is The probability should be normalized Normalization is not necessary but conventional.

each normalized a a 12

Known while

For unit volume Maxwell velocity distribution

Maxwell speed distribution Most probable speed For one molecule, what speed is most possible? Among N molecules, what is the speed most molecules have in unit interval?

Maxwellian can be written as

The average speed

Transport phenomena Heat transfer : conduction convection thermal radiation diffusion viscosity thermal conductivity Conduction Fourier’s law

Diffusion Fick’s law Viscosity 6

Microscopic theory of transport coefficients

For mass transport,

Similarly

Hydrogen under STP r = 1.12×10−10m

Experimental results: