The “2λ problem” (the mean free path of a molecule crossing a boundary is twice the mean free path in the gas molecules) in deriving the microscopic mechanism of transport coefficients for gases is a common point of confusion in thermal physics teaching. This paper first systematically reviews the treatment of this problem in domestic and foreign textbooks and domestic teaching papers, including solid angle integration and one-dimensional motion simplified models. Then, a classroom teaching scheme using line segments to represent molecular free paths is proposed, and some key issues related to it are discussed. This scheme allows students to qualitatively understand and accept the conclusions related to “2λ” in a short class time without the need for integral calculations.
Ke Zhao (Wed,) studied this question.