Peltier Effect and Thomsons Effect

Peltier Effect

When an electric current is passed through a thermocouple, heat is either absorbed or released at the junctions, depending on the direction of current flow. This effect is called Peltier effect. When the direction of current is reversed in thermocouple, the heat evolved or absorbed or interchanged at the junction. This effect is an inverse process of Seebeck effect.

Peltier Coefficient

Peltier coefficient at any temperature for the junction of two metals is `the product of absolute temperature and thermoelectric power at that temperature. Peltier’s coefficient, denoted by p

\begin{align*} \pi &= TP \\ \text {or,} \: \pi &= T\frac {dE}{dT} \\ \end{align*}

Thomson’s Effect

The phenomenon of evolution or absorption of heat along with the length of a conductor on passing current through it when its two ends are kept at a different temperature is known as Thomson’s effect.

If an electric current is passed through a copper wire from its hotter end to the colder end, the heat is evolved and the wire becomes hot. If the current is reversed, heat is absorbed along the conductor. Similarly, is an electric current is passed through an iron wire from its hotter end to the colder end, the heat is absorbed and the wire gets cooled. If the current is reversed, the heat is evolved along the conductor. So, Thomson’s effect is reversible. The substances, which behave like copper, are said to have a positive Thomson’s effect. The substances which behave like iron are said the substances which behave like iron said to have negative Thomson’s effect. Thomson’s effect of lead is nil. So it is used as the standard metal in thermoelectricity.

Cause of Seebeck effect

When two different metals are brought into contact, the free electrons tend to diffuse from the metal with greater density to the other with lower electron density. Due to diffusion, a potential difference is set up at the junction of two metals, called contact potential. When both junctions are at the same temperature, the contact potential at the junctions will be equal and opposite. Hence no current flows through the thermocouple. But if one junction is kept at a higher temperature, the rate of diffusion of free electrons at the junction will increase. Therefore, the contact potential at the two junctions will become different and hence there will be an effective emf in the circuit called the thermo emf.

Cause of Peltier Effect

If two dissimilar metals are joined, contact potential is established at the junctions i.e. the potential of one must become above that of the other. For example: in Cu-Fe thermocouple, the potential of Fe is greater than the potential of Cu. At one junction, current flows from lower potential to a higher potential and the energy are required for this purpose, which is absorbed from the junction and hence it is cooled. At another junction, current flows from higher potential to lower potential. The energy is given out at this junction and makes the junction hot.

Cause of Thomson’s Effect

When two ends of a conductor are kept at different temperatures, the number of free electrons in the higher temperature region will have higher than those in the lower region. So there is diffusion of electrons from one region to another and this gives rise to an emf which is called Thomson’s emf.

Difference between Peltier’s Effect and Joule’s Effect

Distinction between Thomson’s Effect and Joule’s Effect

Application of thermoelectric Effect

Thermopile

It is a device used for detection and measurement of heat radiation.

Principle

It is based on Seebeck effect. It is constructed on the principle that if a number of thermocouples are connected in series, then the thermo emf gets multiplied.

Construction

It consists of a number of the Bi-Sb thermocouples connected in series so that the thermo emf produced in all thermocouples are added. One set of junctions is blackened and exposed to heat radiation while the other set of the junctions is protected from heat radiation by an insulating cover. A sensitive galvanometer connected to circuit detects the thermo emf produced by the radiation. The thermopile is also used for the measurement of the high temperature of a furnace.

Thermoelectric Generator

This generator is based on thermoelectric effect. By heating one junction and keeping other junction at room temperature of a thermocouple, electric current flows through the circuit. The electric power generated in this method can be used to operate the electronic device in remote areas.

Reference

Manu Kumar Khatry, Manoj Kumar Thapa,et al..Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.