The Seebeck effect is the conversion of temperature differences directly into electricity and is named for German-Estonian physicistThomas Johann Seebeck, who, in 1821 discovered that a compass needle would be deflected by a clod loop formed by two metals joined in two places, with a temperature difference between the junctions. This was becau the metals responded differently to the temperature difference, creating a current loop and a magnetic field. Seebeck did not recognize there was an electric current involved, so he called the phenomenon the thermomagnetic effect. Danish physicist Hans Christian Ørsted rectified the mistake and coined the term "thermoelectricity". The voltage created by this effect is of the order of veral microvolts per kelvin difference. One such combination, copper-constantan, has a Seebeck coefficient of 41 microvolts per kelvin at room temperature.[2]
The voltage V developed can be derived from:
where SA and SB are the thermopowers (Seebeck coefficient) of metals A and B as a function of temperature and T1 and T2 are the temperatures of the two junctions. The Seebeck coefficients are non-linear as a function of temperature, 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:
The Seebeck effect is ud in the thermocouple to measure a temperature difference; absolute temperature may be found by tting one end to a known temperature. A metal of unknown composition can be classified by its thermoelectric effect if a metallic probe of known composition, kept at a constant temperature, and is held in contact with it. Industrial quality control instruments u this as thermoelectric alloy sorting to identify metal alloys. Thermocouples in ries form a thermopile, sometimes constructed in order to increa the output voltage, since the voltage induced over each individual couple is s
mall. Thermoelectric generators are ud for creating power from heat differentials and exploit this effect.
Peltier effect
The Peltier effect is the prence of heat at an electrified junction of two different metals and is named for French physicist Jean-Charles Peltier, who discovered it in 1834. When a current is made to flow through a junction compod of materials A and B, heat is generated at the upper junction at T2, and absorbed at the lower junction at T1. The Peltier heat absorbed by the lower junction per unit time is equal to
where ΠAB is the Peltier coefficient for the thermocouple compod of materials A and B and ΠA (ΠB) is the Peltier coefficient of material A (B). Π varies with the material's temperature and its specific composition: p-type silicon typically has a positive Peltier coefficient below ~550 K, but n-type silicon is typically negative.
The Peltier coefficients reprent how much heat current is carried per unit charge through a given material. Since charge current must be continuous across a junction, the associated heat flow will develop a discontinuity if ΠA and ΠB are different. Depending on the magnitude of the current, heat must accumulate or deplete at the junction due to a non-zero divergence there caud by the carriers attempting to return to the equilibrium that existed before the current was applied by transferring energy from one connector to another. Individual couples can be connected in ries to enhance the effect. Thermoelectric heat pumps exploit this phenomenon, as do thermoelectric cooling devices found in refrigerators
Thomson effect
The Thomson effect was predicted and subquently obrved by Lord Kelvin in 1851. It describes the heating or cooling of a current-carrying conductor with a temperature gradient.
Any current-carrying conductor (except for a superconductor) with a temperature differen
ce between two points either absorbs or emits heat, depending on the material. If a current density J is pasd through a homogeneous conductor, the heat production q per unit volume is:
where ρ is the resistivity of the material, dT/dx is the temperature gradient along the wire and μ is the Thomson coefficient. The first term is the Joule heating, which does not change in sign; the cond term is the Thomson heating, which follows J changing sign.
In metals such as zinc and copper, who temperature is directly proportional to their potential, when current moves from the hotter end to the colder end, there is a generation ofheat and the positive Thomson effect occurs.[citation needed] Converly, in metals such as cobalt, nickel, and iron, who temperature is inverly proportional to their potential, when current moves from the hotter end to the colder end, there is an absorption of heat and the negative Thomson effect occurs.
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 pairwi Seebeck coefficients in thermocouples containing the reference material and then adding back the absolute thermopower of the reference material.
Lead is commonly stated to have a Thomson coefficient of zero; in fact, it is non-zero, albeit being very small.[5] In contrast, the thermoelectric coefficients of all known superconductors are zero
Figure of merit
The figure of merit Z for thermoelectric devices is defined as
where σ is the electrical conductivity, κ is the thermal conductivity, and S is the Seebeck coefficient. The dimensionless figure of merit ZT is formed by multiplying Z with the average temperature.
