The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates 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. Textbooks may refer to it as the Peltier–Seebeck effect. This separation derives from the independent discoveries of 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 related though it is not generally termed a thermoelectric effect. The Peltier–Seebeck and Thomson effects are thermodynamically reversible, whereas Joule heating is not.
The Peltier effect is the presence of heating or cooling at an electrified junction of two different conductors and is named for French physicist Jean Charles Athanase Peltier, who discovered it in 1834. When a current is made to flow through a junction between two conductors A and B, heat may be generated (or removed) at the junction.
Thermoelectric generators (also called Seebeck generators) are devices that convert heat (temperature differences) directly into electrical energy, using a phenomenon called the Seebeck effect (a form of thermoelectric effect).
The Peltier effect is the presence of heating or cooling at an electrified junction of two different conductors (1834). His great experimental discovery was the heating or cooling of the junctions in a heterogeneous circuit of metals according to the direction in which an electric current is made to pass round the circuit. This reversible effect is proportional directly to the strength of the current, not to its square, as is the irreversible generation of heat due to resistance in all parts of the circuit. It is found that, if a current pass from an external source through a circuit of two metals, it cools one junction and heats the other. It cools the junction if it be in the same direction as the thermoelectric current which would be caused by directly heating that junction. In other words, the passage of a current from an external source produces in the junctions of the circuit a distribution of temperature which leads to the weakening of the current by the superposition of a thermo-electric current running in the opposite direction.
When electromotive current is made to flow through a electronic junction between two conductors (A and B), heat is removed at the junction. To make a typical pump, multiple junctions are created between two plates. One side heats and the other side cools. A dissipation device is attached to the hot side to maintain cooling effect on the cold side. Typically, the use of the Peltier effect as a heat pump device 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 pumps exploit this phenomenon, as do thermoelectric cooling Peltier modules found in refrigerators.
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 (via the Peltier effect) will always transfer heat from the hot to the cold junction.
The true importance of this "Peltier effect" in the explanation of thermoelectric currents was first clearly pointed out by James Prescott Joule; and Sir William Thomson further extended the subject by showing, both theoretically and experimentally, that there is something closely analogous to the Peltier effect when the heterogeneity is due, not to difference of quality of matter, but to difference of temperature in contiguous portions of the same material. Shortly after Peltier's discovery was published, Lenz used the effect to freeze small quantities of water by the cold developed in a bismuth-antimony junction when a voltaic current was passed through the metals in the order named.
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