the wiedemann franz law (1853) states that the ratio of the thermal conduc- tivity k, to the electrical conductivity σ of metals is directly proportional to the absolute temperature with a proportionality constant independent of the metal let us see how one can derive this from elementary considerations of diffusion. go back to our elementary derivation of the relation between the particle current and the particle density in one dimension which, with the definition d δ2/t gave jp =-d(θρ/ac). now consider a similar situation, but in which the particle density is uniform whereas the particles carry energy which varies in position, e(x). show that the energy current, je which is the energy which crosses from left to right per unit time per unit area is in the one-dimensional model de dr dt dt where ρ is the number of particles per unit volume and cu is the specific heat de/dt. thus k-pc, d. in class i showed that, for a system with one carrier, the electrical conductivity was σ-pe2d/kt so that 2 which is the wiedemann franz law. if, in addition, one assumes for cy the classical value of an ideal gas, 3k/2 one obtains for the ratio (3/2) (k/e)2t. this coefficient turns out to be about 1/2 the value measured experimentally, so the basic idea is correct that both processes are simply diffusive, but there is something wrong with the value of cv