Electricity directly from heating a material [duplicate]

Question

I am looking for some more information about how to obtain electricity from heat directly. This e.g. involves the Seebeck effect, as I have found it is called, where a material produces a voltage across when heated in one end and having the other end slightly cooler. This should be the princip in measuring instruments etc., since just a small voltage is created.

This Wikipedia link explains what the phenomenon is about. But it is not well explained in an understandable language (for me at least). And it doesn't dive deep enough into the reason.

Are there someone who can in a down-to-earth way explain how and why a voltage can be measured between the ends of a bar of a certain material, when it is heated in one end? My question regards what happens on the atomic scale - can heat push electrons or what?

Thanks.

Answer 1

The electrons in a metal are whizzing around due to thermal energy. Lattice vibrations excite the electrons, the electrons travel some distance then scatter off the lattice again and transfer energy back to the lattice. Just like a gas, the electrons have some average mean free path, and this depends on the temperature. You would think the mean free path would increase with temperature, because the lattice transfers more energy to the electrons, but the rate of scattering off the lattice also increases with temperature. How the mean free path behaves with temperature depends on the trade-off between these two effects, and the varation with temperature can be positive or negative.

Anyhow, if you heat one end of a metal rod and cool the other then the mean free path will be different at the two ends, and the electrons at the end with the higher mean free path will tend to diffuse into the end with the lower mean free path. The result is a net charge movement, and this creates the potential difference.