In 1834, a French scientist Peltier discovered that if the junctions are at the same temperature and a current is passed through the circuit of the Thermocouple, heat is produced at one Junction and absorbed at the other i.e. one junction (former) is heated and the other junction (later) is cooled. This is known as Peltier effect. This effect is reversible i.e. when the direction of current is reversed, the heat evolved and absorbed are interchanged.
Peltier effect is the converse of Seebeck effect.
Consider a Copper-Iron (Cu-Fe) thermocouple having both the junctions at the same temperature. If a current is passed through this thermocouple in the direction as shown in figure 1(a), heat is absorbed in the junction 1 and evolved at the junction 2. Therefore, the junction 1 is cooled while junction 2 is heated.
If the current in the circuit is reversed (as shown in fig. 1(b)), the thermoelectric effect is also reversed i.e. junction 1 heated and the junction 2 is cooled.
Explanation of Peltier effect
When two dissimilar metals are joined, contact potential difference is established at the junctions i.e. potential of one metal becomes more that of the other. In case of a copper-iron thermocouple, the potential of iron is more than the copper.
From figure 1(a), at the junction 1, current flows from copper to Iron i.e. from lower potential to higher potential. Hence, energy is required for this purpose. As a result, energy is absorbed from the junction 1 and it is cooled.
At the junction 2, current flows from iron to Copper i.e. from higher potential to lower potential. Hence, energy is given out at junction 2. Consequently, junction 2 become hot.
Note: Peltier heat is evolved at the junction of the thermocouple which is kept cold for Seebeck effect. On the other hand, Peltier heat is absorbed in a junction which is kept hot for Seebeck effect. Therefore, Peltier effect is complementary to Seebeck effect.
Peltier Co-efficient (π)
The amount of heat energy absorbed or evolved at one of the junctions of a thermocouple when one-ampere current flows for one second (one coulomb) is called Peltier coefficient. It is denoted by π.
The SI unit of Peltier coefficient is Joule/Coulomb or J/C.
Peltier Coefficient depends upon the nature of metals forming the thermocouple and temperature of the junctions.
Suppose the cold junction of the Thermocouple is at temperature θ and the hot junction at θ+dθ. If dE is the thermo e.m.f. produced then it is found that
But dE/dθ is a thermoelectric power or Seebeck coefficient (S).