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Peltier Effect :
Thomson's Effect :
Difference between Peltier's and Joule's Effect :

Thermoelectric Effect(Seebeck Effect) : Click here
Thermoelectric Effect (Short Question and Answer) : Click here

Peltier Effect

When an electric currrent 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. This is a reversible effect. That is, when direction of current is reversed, the heat evolved or absorbed is interchanged at the junction. Peltier effect is the inverse process of Seebeck effect.
The rule which tells whether heat is evolved or absorbed at any junction is Peltier effect is that, if the direction of Seebeck emf is from Cu to Fe at the hot junction, then an external emf applied in the same direction will produce cooling at this junction and heating at the other junction.

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. Perltier's coefficient, denoted by π is given by
          π = T P
          or, π = T $\frac{\mathrm{dE}}{\mathrm{dt}}$

Thomson's Effect

In 1851, William Thomson found that when an electric current is passed through a non-uniformly heated conductor, heat is evolved or absorbed. This phenomenon of evolution or absorption of heat (other than Joule's heat) along the length of a conductor on passing current through it when its two ends are kept at 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 absorved along the conductor.
Similarly, if 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 current is revrsed, 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. some of the substances that show this effect are silver, zinc, antimony, cadmium etc. The substances which behave like iron are said to have a negative Thomson's effect. Other substances showing this effect are bismuth platinum, cobalt etc. Thomson's effect of lead is nil. So it is used as the standard metal in thermoelectricity.

Cause of Seebeck Effect

Different metals have different free electron density. When two different metals are brought into contact, the free electrons tend to diffuse from the metal with greater electron 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. As a result of it 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

When 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, potential of Fe is greater than potential of Cu. At one junction current flows from lower potential to higher potential (from Cu to Fe) and the energy is 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 due to which the junction becomes hot.

Cause of Thomson's Effect

When two ends of a conductor are kept at different temperature, the number of free electrons in the higher temperature region will have higher than those in the lower temperature region. So there is diffusion of electrons from one region into another and this gives rise to a potential difference or an emf. This emf is called Thomson's emf. When an electric current is passed through the wire either work is done on the charge carries or by the charge carriers. Therfore, the thermal energy is either evolved or absorbed depending upon the work done.

Differences between Peltier's Effect and Joule's Effect

Peltier's Effect	Joule's Effect
1. It takes place only at the junction.	1. It takes place throughout the conductor
2. Heat is evolved or absorbed	2. Heat is always produced
3. It is reversible process	3. It is irreversible process
4. Heat is produced or absorbed is proportional to the current	4. Heat produced is proportional to the square of the current
5. It depends upon the direction of current	5. It is indpendent of the direction of current
6. A mount of heat evolved or absorbed depends on the nature of the metals and temperature of conductor.	6. Amount of heat produced depends on resistance of the conductor

Distinction between Thomson's Effect and Joule's Effect

Thomsons Effect	Joule's Effect
1. Heat is evolved or absorbed along the wire	1. Heat is always produced along the length of the conductor
2. It is reversible effect	2. It is an irreversible effect
3. Heat produced in proportional to the current.	3. Heat produced is proportional to the square of the current.
4. For the evolution of heat, temperature difference is required along the length.	4. Temperature difference is not required.
5. It depends on direction of current.	5. It is independent of the direction of current.
6. Amount of heat produced depends upon temperature difference between the ends of the conductor.	6. Amount of heat produced depends upon resistance but not on temperature difference.

Application of Thermoelectric Effect

There are many applications of thermoelectric effect. Some of them are discussed below.
Thermopile
It is a device used for detection and measurement of heat radiation. 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.
It consists of a number of Bi-Sb thermocouples connected in series so that the thermo emfs produced in all thermocouples are added. One set of junctions is blackened and exposed to heat radiation while the other set of the junctions are protected from heat radiation by an ensulating cover. A sensitive galvanometer connected to the circuit detects the thermo emf produced by the rediation. The thermocouple is also used for the measurement of high temperature of a furnace.

Thermoelectric Generator It is based on the 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 electronic devices in remote areas.