JEE Main & Advanced Physics Current Electricity, Charging & Discharging Of Capacitors / वर्तमान बिजली, चार्ज और कैपेसिटर का निर Thomson's Effect

In Thomson's effect we deal with only metallic rod and not with thermocouple as in Peltiers effect and Seebeck's effect. (That's why sometimes it is known as homogeneous thermo electric effect. When a current flows thorough an unequally heated metal, there is an absorption or evolution of heat in the body of the metal. This is Thomson's effect.

(i) Positive Thomson's effect : In positive Thomson's effect it is found that hot end is at high potential and cold end is at low potential. Heat is evolved when current is passed from hotter end to the colder end and heat is absorbed when current is passed from colder end to hotter end. The metals which shows positive Thomson's effect are Cu, Sn, Ag, Cd, Zn... etc.

(ii) Negative Thomson's effect : In the elements which show negative Thomson's effect, it is found that the hot end is at low potential and the cold end is at higher potential. Heat is evolved when current is passed from colder end to the hotter end and heat is absorbed when current flows from hotter end to colder end. The metals which shows negative. Thomson's effect are Fe, Co, Bi, Pt, Hg ... etc.

Thomson's co-efficient : In Thomson's effect it is found that heat released or absorbed is proportional to \[Q\Delta \theta \] i.e. \[H\propto Q\Delta \theta \]\[\Rightarrow \]\[H=\sigma Q\Delta \theta \] where \[\sigma =\] Thomson's coefficient. It's unit is Joule/coulomb\[^{o}C\] or volt/\[^{o}C\] and \[\Delta \theta =\]temperature difference.

If \[Q=1\] and \[\Delta \theta =1\] then \[\sigma =H\] so the amount of heat energy absorbed or evolved per second between two points of a conductor having a unit temperature difference, when a unit current is passed is known as Thomson's co-efficient for the material of a conductor.

It can be proved that Thomson co-efficient of the material of conductor \[\sigma =-T\frac{{{d}^{2}}E}{d{{T}^{2}}}\]\[=-T\left( \frac{dS}{dT} \right)=T\times \beta \];  where \[\beta =\] Thermo electric constant \[=\frac{dS}{dt}\]