I was reading Chpater 13 in A&M solid state physics, which is about relaxation time approximation. But there is one fundamental expression I'm trying to understand. It's the formulation of the relaxation-time approximation (Eq. 13.3). dg = dt/[tex]\tau[/tex]g0 But most of other textbooks including Ziman's and Kittel's ones say dg/dt = -1/[tex]\tau[/tex] (g-g0) (Eq 7.17 in Ziman's priciples of the theory of solids). Here g is non-equilibrium distribution function and g0 is equilibrium distribution function. Though the expression looks different apparently, using both of expressions, the exactly same physical expressions for some physical properties such as electric current can be derived as shown in the textbooks. It's easy to understand the expression in Ziman's and Kittel's ones but not easy to understand that in A&M. Could anybody explain why the expression in A&M is different from other ones? Thank you in advance.