Physics · Ch 2 — Current Electricity
Thomson Effect
Thomson Effect
Thomson showed that even within a SINGLE, uniform conductor (not a junction of two different metals at all), if two points along it are held at different temperatures, the density of free electrons at those two points will differ, and this difference in electron density itself creates a potential difference between the two points. This is the Thomson effect, and like the Seebeck and Peltier effects, it too is reversible.
If a current I is passed through a copper bar AB that is heated at its midpoint C, point C is found to sit at a HIGHER electrical potential than the rest of the bar; this indicates that heat is being absorbed along the segment AC and evolved along the segment CB. Since this transfer of heat happens in the same direction as the current flow, it is called the positive Thomson effect, and it is observed in metals such as silver, zinc and cadmium in addition to copper. If the copper bar is instead replaced by an IRON bar under otherwise identical conditions, the pattern reverses completely: heat is now evolved along CA and absorbed along BC, meaning the heat transfer happens in the dir …
What this figure shows. Panel (a) shows a copper bar AB heated at its middle point C, with a current I flowing through it; the diagram labels the region from A to C as where heat is absorbed and the region from C to B as where heat is evolved, and marks that point C ends up at the higher electrical potential -- the signature of the 'positive Thomson effect', also seen in metals like silver, zinc and cadmium. Panel (b) shows the same experiment repeated with an iron bar instead, where the pattern of heat absorption and evolution is exactly reversed (heat evolved along CA, absorbed along BC) -- the 'negative Thomson effect …