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SEEBECK EFFECT The Seebeck effect is the conversion of Heat differences directly into Electricity . This effect was first discovered, accidentally, by the Estonia n physicist Thomas Johann Seebeck in 1821 , who found that a voltage existed between two ends of a Metal bar when a temperature gradient existed in the bar. He also discovered that a Compass needle would be deflected when a closed loop was formed of two metals with a temperature difference between the junctions. This is because the metals respond differently to the heat difference, which creates a current loop, which produces a Magnetic Field . A Voltage , the thermoelectric EMF , is created in the presence of a Temperature difference between two different metals or Semiconductor s. This usually causes a continuous current to flow in the conductors. The voltage created is on the order of several microvolts per Degree of difference. In the circuit: (which can be in several different configurations and be governed by the same equations), the voltage developed can be derived from: : ''S''A and ''S''B are the Seebeck Coefficient s (also called ''thermoelectric power'' or ''thermopower'') of the metals A and B, and ''T''1 and ''T''2 are the temperatures of the two junctions. The Seebeck coefficients are non-linear, and depend on the conductors' absolute temperature, material, and molecular structure. If the Seebeck coefficients are effectively constant for the measured temperature range, the above formula can be approximated as: : Thus, a Thermocouple works by measuring the difference in potential caused by the dissimilar wires. It can be used to measure a temperature difference directly, or to measure an absolute temperature, by setting one end to a known temperature. Several thermocouples in series are called a thermopile. This is also the principle at work behind Thermal Diode s and Thermoelectric Generator s (such as Radioisotope Thermoelectric Generator s or RTGs) which are used for creating power from heat differentials. The Seebeck effect is due to two effects: ''charge carrier diffusion'' and ''phonon drag''. Thermopower If the temperature difference between the two nodes is small, : and a voltage Δ''V'' is seen at the terminals, then the Thermopower of the entire thermocouple is defined as: : This can also be written in relation to the electric field ''E'' and the temperature gradient , by the equation |
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