The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. Conversely, when a voltage is applied to it, heat is transferred from one side to the other, creating a temperature difference. At the atomic scale, an applied temperature gradient causes charge carriers in the material to diffuse from the hot side to the cold side. This effect can be used to generate electricity, measure temperature or change the temperature of objects. Because the direction of heating and cooling is determined by the polarity of the applied voltage, thermoelectric devices can be used as temperature controllers.
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The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple.
Conversely, when a voltage is applied to it, heat is transferred from one side to the other, creating a temperature difference. At the atomic scale, an applied temperature gradient causes charge carriers in the material to diffuse from the hot side to the cold side. This effect can be used to generate electricity, measure temperature or change the temperature of objects. Because the direction of heating and cooling is determined by the polarity of the applied voltage, thermoelectric devices can be used as temperature controllers.
The term "thermoelectric effect" encompasses three separately identified effects: the Seebeck effect , Peltier effect , and Thomson effect. The Seebeck and Peltier effects are different manifestations of the same physical process; textbooks may refer to this process as the Peltier—Seebeck effect the separation derives from the independent discoveries by French physicist Jean Charles Athanase Peltier and Baltic German physicist Thomas Johann Seebeck.
Joule heating , the heat that is generated whenever a current is passed through a resistive material, is not generally termed a thermoelectric effect. The Peltier—Seebeck and Thomson effects are thermodynamically reversible ,  whereas Joule heating is not. The Seebeck effect is the build up of an electric potential across a temperature gradient. A thermocouple measures the difference in potential across a hot and cold end for two dissimilar materials. This potential difference is proportional to the temperature difference between the hot and cold ends.
First discovered in by Italian scientist Alessandro Volta ,  [note 1] it is named after the Baltic German physicist Thomas Johann Seebeck , who in independently rediscovered it.
This was because the electron energy levels shifted differently in the different metals, creating a potential difference between the junctions which in turn created an electrical current through the wires, and therefore a magnetic field around the wires.
Seebeck did not recognize that an electric current was involved, so he called the phenomenon "thermomagnetic effect". The Seebeck effect is a classic example of an electromotive force EMF and leads to measurable currents or voltages in the same way as any other EMF.
The local current density is given by. In general, the Seebeck effect is described locally by the creation of an electromotive field. The Seebeck coefficients generally vary as function of temperature and depend strongly on the composition of the conductor. This simple relationship, which does not depend on conductivity, is used in the thermocouple to measure a temperature difference; an absolute temperature may be found by performing the voltage measurement at a known reference temperature.
A metal of unknown composition can be classified by its thermoelectric effect if a metallic probe of known composition is kept at a constant temperature and held in contact with the unknown sample that is locally heated to the probe temperature.
It is used commercially to identify metal alloys. Thermocouples in series form a thermopile. Thermoelectric generators are used for creating power from heat differentials. When an electric current is passed through a circuit of a thermocouple, heat is evolved at one junction and absorbed at the other junction. This is known as the Peltier Effect. The Peltier effect is the presence of heating or cooling at an electrified junction of two different conductors and is named after French physicist Jean Charles Athanase Peltier , who discovered it in The Peltier heat generated at the junction per unit time is.
The total heat generated is not determined by the Peltier effect alone, as it may also be influenced by Joule heating and thermal-gradient effects see below.
The Peltier coefficients represent how much heat is carried per unit charge. The Peltier effect can be considered as the back-action counterpart to the Seebeck effect analogous to the back-EMF in magnetic induction : if a simple thermoelectric circuit is closed, then the Seebeck effect will drive a current, which in turn by the Peltier effect will always transfer heat from the hot to the cold junction. A typical Peltier heat pump involves multiple junctions in series, through which a current is driven.
Some of the junctions lose heat due to the Peltier effect, while others gain heat. Thermoelectric heat pumps exploit this phenomenon, as do thermoelectric cooling devices found in refrigerators. In different materials, the Seebeck coefficient is not constant in temperature, and so a spatial gradient in temperature can result in a gradient in the Seebeck coefficient.
If a current is driven through this gradient, then a continuous version of the Peltier effect will occur. This equation, however, neglects Joule heating and ordinary thermal conductivity see full equations below. Often, more than one of the above effects is involved in the operation of a real thermoelectric device. The Seebeck effect, Peltier effect, and Thomson effect can be gathered together in a consistent and rigorous way, described here; this also includes the effects of Joule heating and ordinary heat conduction.
As stated above, the Seebeck effect generates an electromotive force, leading to the current equation . To describe the Peltier and Thomson effects, we must consider the flow of energy. The first term is the Fourier's heat conduction law , and the second term shows the energy carried by currents. Using these facts and the second Thomson relation see below , the heat equation can be simplified to.
If the material is not in a steady state, a complete description needs to include dynamic effects such as relating to electrical capacitance , inductance and heat capacity. In , Lord Kelvin found relationships between the three coefficients, implying that the Thomson, Peltier, and Seebeck effects are different manifestations of one effect uniquely characterized by the Seebeck coefficient. The first Thomson relation is . This relationship is easily shown given that the Thomson effect is a continuous version of the Peltier effect.
This relation expresses a subtle and fundamental connection between the Peltier and Seebeck effects. It was not satisfactorily proven until the advent of the Onsager relations , and it is worth noting that this second Thomson relation is only guaranteed for a time-reversal symmetric material; if the material is placed in a magnetic field or is itself magnetically ordered ferromagnetic , antiferromagnetic , etc.
The Thomson coefficient is unique among the three main thermoelectric coefficients because it is the only one directly measurable for individual materials. The Peltier and Seebeck coefficients can only be easily determined for pairs of materials; hence, it is difficult to find values of absolute Seebeck or Peltier coefficients for an individual material.
If the Thomson coefficient of a material is measured over a wide temperature range, it can be integrated using the Thomson relations to determine the absolute values for the Peltier and Seebeck coefficients. This needs to be done only for one material, since the other values can be determined by measuring pairwise Seebeck coefficients in thermocouples containing the reference material and then adding back the absolute Seebeck coefficient of the reference material.
For more details on absolute Seebeck coefficient determination, see Seebeck coefficient. The Seebeck effect is used in thermoelectric generators, which function like heat engines , but are less bulky, have no moving parts, and are typically more expensive and less efficient. They have a use in power plants for converting waste heat into additional electrical power a form of energy recycling and in automobiles as automotive thermoelectric generators ATGs for increasing fuel efficiency.
Space probes often use radioisotope thermoelectric generators with the same mechanism but using radioisotopes to generate the required heat difference. Recent uses include stove fans,  lighting powered by body heat  and a smartwatch powered by body heat. The Peltier effect can be used to create a refrigerator that is compact and has no circulating fluid or moving parts. Such refrigerators are useful in applications where their advantages outweigh the disadvantage of their very low efficiency.
PCR requires the cyclic heating and cooling of samples to specified temperatures. The inclusion of many thermocouples in a small space enables many samples to be amplified in parallel. Thermocouples and thermopiles are devices that use the Seebeck effect to measure the temperature difference between two objects. Thermocouples are often used to measure high temperatures, holding the temperature of one junction constant or measuring it independently cold junction compensation.
Thermopiles use many thermocouples electrically connected in series, for sensitive measurements of very small temperature difference. From Wikipedia, the free encyclopedia. This article is about the thermoelectric effect as a physical phenomenon. For applications of the thermoelectric effect, see Thermoelectric materials and Thermoelectric cooling. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources.
Unsourced material may be challenged and removed. Thermoelectric effect Seebeck effect Peltier effect Thomson effect Seebeck coefficient Ettingshausen effect Nernst effect. Thermoelectric materials Thermocouple Thermopile Thermoelectric cooling Thermoelectric generator Radioisotope thermoelectric generator Automotive thermoelectric generator.
See also: Thermoelectric materials. Main article: Thermoelectric generator. Main article: Thermoelectric cooling. Main article: Thermocouple. Disalvo, F. Any device that works at the Carnot efficiency is thermodynamically reversible, a consequence of classical thermodynamics.
In Goupil, Christophe ed. Continuum Theory and Modeling of Thermoelectric Elements. Seebeck on electro-magnetic actions]. Annales de chimie. From pp. Seebeck in Berlin]. Annalen der Physik in German. Bibcode : AnP Annales de Chimie et de Physique in French. Proceedings of the Royal Society of Edinburgh. Retrieved Part V. Thermo-electric currents". Transactions of the Royal Society of Edinburgh.
See, for example, Rowe, D. Thermoelectrics Handbook: Macro to Nano. CRC Press. MIT Technology Review. Retrieved 7 October His apparatus consisted of two glasses of water.
If the material is not in a steady state, a complete description will also need to include dynamic effects such as relating to electrical capacitance, inductance, rfecto heat capacity. Thermoelectric effect — Wikipedia Thermoelectric generators are used for creating power from heat differentials. This was efecto seebeck in the early 20th century by the efecto seebeck compass. Thermoelectric legs are thermally in parallel and electrically in series.
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