Thermoelectric circuit

The various theories which have been proposed to account for the phenomena of the thermoelectric circuit may be grouped into three classes ... 

The dynamic thermocouple, also known as the "Herbert-Gottwein" thermocouple, measures the temperature at the rubbing interface of two dissimilar metals by making it the hot junction of a thermoelectric circuit. The utilization of this principle for the measurement of the chip-tool rubbing temperature in metal cutting appeared on the scene in 1925/1926 [6, 7, 8] and for the measurement of temperature in ordinary rubbing about 10 years later [9]. 

Equation 16.18 is particularly useful in deriving the net EMF of a complex thermoelectric circuit. In many reference works, three laws—the law of homogeneous materials, the law of intermediate materials, and the law of intermediate temperature—are used to show how the EMF at a measuring device is affected by various lead wires from a thermocouple junction [33], These laws can be derived from Eq. 16.17 but are generally more difficult to apply in a complex circuit. 

Thermoelectric Circuits. A typical circuit for a single thermocouple of materials A and B is shown in Fig. 16.17. The reference temperature (at which junctions b and d are maintained) is usually the ice point, 0°C. The connecting wires C are usually copper wires. Note that, according to Eq. 16.20, the connecting (copper) wires C should not affect the EMF EAB, which, for given materials A and B, is just a function of the temperature T. 

In a thermoelectric circuit such as that used for measuring Qh > the current in the ice is carried by protons and that in the metallic leads by electrons and we noted that complications might arise at the electrodes. A much simpler circuit for analysis consists of two ice elements containing different impurities connected in series and with their junctions maintained at different temperatures, as shown in fig. 9.11. 

Fig. 9.11. Thermoelectric circuit for ice thermocouple potential Vproduced by the Seebeck effect for ice specimens A and B of different impurity content, with junction temperatures and T. ...

HOT JUNCTION - That part of thermoelectric circuit which releases heat. 

The other primary thermoelectric phenomenon is the Peltier effect, which is the generation or absorption of heat at the junction of two different conductors when a current flows in the circuit. Whether the heat is evolved or absorbed is determined by the direction of the current flow. The amount of heat involved is determined by the magnitude of the current, I, and the Peltier coefficients, 7T, of the materials ... 

Thermocouples Temperature measurements using thermocouples are based on the discovery by Seebeck in 1821 that an electric current flows in a continuous circuit of two different metalhc wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammaticaUy as shown in Fig. 8-60. A and B are the two metals, and T and To are the temperatures of the junctions. Let T and To be the reference junction (cold junction) and the measuring junc tion, respectively. If the thermoelectric current i flows in the direc tion indicated in Fig. 8-60, metal A is customarily referred to as thermoelectricaUy positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), cmromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between To and T. In control systems the reference junction is usually located at... 

Seebeck s outstanding scientific achievement was the discovei"y of one of the three classical thermoelectric effects, which are the Seebeck, the Peltier, and the Thomson effects. Seebeck s discovery was the first, dating from 1822—1823, followed by that of Jean-Charles-Athanase Peltier in 1832 and that of William Thomson in 1854. Seebeck obseiwed that an electric current in a closed circuit comprised different metallic components if he heated the junctions of the components to different temperatures. He noted that the effect increases linearly with the applied temperature difference and that it crucially depends on the choice of materials. Seebeck tested most of the available metallic materials for thermoelectricity. His studies were further systematized by the French physicist... 

Besides the reversible production of heat at the junctions, there is an evolution of heat all round the circuit due to frictional resistance, this Joule s heat being proportional to the square of the current, and hence not reversed with the latter. There is also a passage of heat by conduction from the hotter to the colder parts. But if the current strength is reduced, the Joule s heat, being proportional to its square, becomes less and less in comparison with the Peltier heat, and with very small currents is negligible. We shall further assume that the reversible thermoelectric phenomena proceed independently of the heat conduction, so that the whole circuit may be treated as a reversible heat... 

Kohlrausch s theory leaves quite unexplained the fact that no thermoelectric current is set up in a homogeneous wire along which a current of heat is flowing, whilst the theory of Lord Kelvin is difficult to reconcile with the fact that thermoelectric currents cannot be set up in a circuit of liquid metals, although these show the Thomson effect. The latter seems, therefore, to be to a certain extent independent of the Peltier effect. Theories intended to escape these difficulties have been proposed by Planck (1889), and Duhem, in which the conception of the entropy of electricity is introduced. 

THERMOCOUPLE. In 1821, Seebeck discovered that an electric current flows in a continuous circuit of two metals if the two junctions are at different temperatures, as shown in Fig. 1. A and B are two metals, T and T are the temperatures of the junctions. I is the thermoelectric current. A is thermoelectncally positive to B if 7i is the colder junction. In 1834, Peltier found that current flowing across a junction of dissimilar metals causes heat to be absorbed or liberated. The direction of heat flow reverses if current flow is reversed. Rate of heat flow is proportional to current but depends upon bodi temperature and the materials at die junction. Heat transfer rate is given by PI, where P is the Peltier coefficient in watts per ampere, or die Peltier emf in volts. Many studies of the characteristics of thermocouples have led to the formulation of three fundamental laws ... 

Such an electrical circuit is a thermocouple, and Ac/)/AT =0 is the thermoelectric power of the thermocouple,... 

Thermocouples are based on the thermoelectric Seebeck effect, which generates a voltage at the junction between two metallic conductors, which depends on temperature [13]. Thus, in the measuring circuit, two junctions are created, namely, a sensitive (or hot) junction at the point where temperature has to be measured and a nonsensitive (cold) junction, kept at a constant known temperature, where the voltage established between the conductors can be easily measured [19]. Different typologies of thermocouples exist for application in a wide range of conditions they essentially differ by the materials, the most common being J (iron/constantan), K (chromel/alumel), T (copper/constantan), and E (chromel/constantan). 

There are also two well-known thermoelectric effects resulting from the joining of dissimilar materials (forming a junction) the Seebech effect, on which thermocouples are based, and the Peltier effect, used for thermopiles. The Seebech effect results when the two junctions of the dissimilar materials are held at different temperatures. The Seebech coefficient, e, is defined as the open-circuit voltage generated per unit temperature differential of the two junctions ... 

In a thermocouple, heating one junction of a bimetallic couple and cooling the other produces electromotive force in the circuit. This observation was originally was made by Seebeck in 1821. Besides the use of thermocouples, transistor electronics and semiconductors are important areas of interest for thermoelectric phenomena. Thermocouples made of semiconductors can develop relatively large electromotive potentials and are used to convert heat into electricity. 

This expression shows that the imposition of a temperature difference dT in the absence of any current produces a difference d i in electrochemical potential i.e., d ife) = —z/a, dT. This effect is known as the thermoelectric effect, and the ratio d i/e)/dT, or A(f,/e)/Ar = —ZiCii is known as the Seebeck coefficient (1823), or thermoelectric power.Experimentally, the difference of electrochemical potential may be measured by a voltmeter under open circuit conditions, and dT, measured by means of thermocouples a, is thereby experimentally determined. As defined here for p-type (n-type) material the measured Seebeck coefficient is a positive (negative) quantity. For, and both increase in the direction of increasing hole or electron concentration, which is in a direction opposite to the increase in temperature. Comparison with (6.9.2) shows that Ui = ZiSgfe. Then Eq. (6.9.6) becomes... 

Joule s law is strictly accurate so long as the conductor in which the evolution of heat is measured is homogeneous and at a uniform temperature throughout. If these conditions are not complied with, deviations from the law are obtained, and the evolution of heat is found to be no longer completely irreversible. By raising the temperature of certain parts of the circuit, electric currents can now be obtained. These reversible phenomena are called thermoelectrical phenomena in the more restricted sense of the term. In common with all other reversible processes, they must follow certain regularities, which are deter-... 

Since electrical energy can be converted at will into mechanical energ ", a thermoelectric current, is capable of doing work at the expense of the heat which is supplied from the outside to the circuit at the hot junction (E >E. In his classical thermodynamical theory W. Thomson assumed that the thermoelectrical phenomena were strictly reversible, except for the production of Joule heat, which becomes negligible as the current strength approaches zero. In this limiting case, therefore, the heat produced or absorbed at the junctions is equal... 

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