In the nineteenth century, physicists had difficulties measuring the thermal conductivity of materials. Gustave Heinrich Wiedemann (1826-1899) observed in 1853 that, at least for metals, the ratio, A/a, appeared to be constant. If so, the thermal conductivity could be inferred from the easily measure electric conductivity. Eventually, a "law" was formulated in collaboration with Rudolf Franz (1827-1902) and Ludwig Valentine Lorenz (1829-1891) expressing the relationship between thermal conductivity, electric conductivity and absolute temperature. This is the Wiedemann-Franz-Lorenz law, which can be justified based on a simple classical model of electric conduction.
Consider the heat conduction in a unidimensional gas along which there is a temperature gradient (Figure 5.11). A surface, A, at the origin of coordinates is normal to the molecular motion. We assume that there is no net mass flow. Then, nivi + n2V2 = 0. (5.33)
We now use the symbol, Í, as the mean free path (not as the length of the arms, as before). The molecules that cross A coming from the left originate, on the average, from a point of coordinate -Í/2. Their kinetic energy is U(-Í/2). Those that come from the right have an energy U(Í/2). Half the molecules move to the left and half to the right. The net energy flux, that
Was this article helpful?
Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.