What is Thomson Effect
When a current flows through an unequal heated conductor, heat is evolved or absorbed along the length of the conductor, depending on the direction of flow of current. This effect is called the Thomson effect. This effect is reversible.
Consider a thick copper bar AB whose is both ends are maintained at the same temperature and the center is maintained at a higher temperature. When current flows through the bar from end A and B, then it is found that the heat is absorbed at end A and is absorbed at the end B.
Thus there is a transfer of heat due to the current in the direction of the current. This is called the positive Thomson effect. The other elements showing positive. Thomson effect are Cd, Zn, Ag, and Sb.
Now let us consider the case of a Bismuth bar under similar conditions. When current flows in this bar from A to B, the heat is evolved at A, and heat is absorbed at B.
Thus there is a transfer of heat due to the flow of current and takes place in the direction opposite to the direction in which current flows. This is called the negative Thomson effect. The other elements showing a negative Thomson effect are Fe, Co, Ni, Pi, and Hg.
In lead, the Thomson effect is zero.
Origin of Thomson effect
Due to the temperature difference between the two points of the same conductor, there is a difference in the electron density. The electron density is higher at low temperatures than at higher temperatures. Thus, there will be a potential difference between two points of the same connector.
Therefore, When a current (charge) flows in the conductor, work is done against or along the direction of potential difference due to the flow of current. As a result of this heat is absorbed or evolved.
The difference in the number of energies absorbed and evolved is responsible for the production of the Thomson effect. In fact, the origin of the Thomson effect is similar to the origin of the Peltier effect.
Thomson coefficient
Thomson Coefficient is defined as the amount of energy absorbed or evolved when unit current flows for one second between two points of a conductor which differ in temperature by 1°C.
If σ = Thomson Coefficient and if a unit charge is taken from a point at temperature T in a homogeneous conductor to another point at temperature T+dT, then the energy evolved or observed = σ dT.
It may be noted that σ is considered positive when the energy is required to pass the charge through the conductor.