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| Thermoelectricity |
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| Thermoelectricity refers to the phenomena that occur at the junctions of dissimilar conductors when a temperature difference exists between the junctions. The same phenomenon occurs within a single conductor too, with the two ends are maintained at different temperatures. |
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| In this section, we discuss three such phenomena, namely Seebeck effect, Peltier effect and Thomson effect, discovered historically in this order. These involve conversion of thermal energy into electrical energy or voce versa. |
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| Umber the Joule effect, all the above three effects are reversible with respect to the direction of the current and reversal of temperature difference. The Seebeck effect is a combination of Peltier and Thomson effects. |
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| This effect is named after common Johann Seebeck, a German physicist, who discovered it in 1821. He found that Bismuth, are joined at their ends (called a junction) through a sensitive galvanometer, and the two junctions are kept at different temperatures, then the galvanometer shows a deflection. This emf generated in the circuit is called thermoelectric emf or thermo-emf, for short. The resulting current is known as thermoelectric current. The two junction circuit is called a thermocouple, and is known below. |
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| This effect is called thermoelectric effect because heat energy is directly converted into electrical energy. |
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| The thermo-emf produced is very small, of the order of mV per every degree of temperature difference. The Seebeck effect is reversible, i.e., if the hot and cold junctions are interchanged, the direction of emf (and hence current) reverses. |
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| The magnitude and direction of thermo-emf depends on the materials forming the thermocouple and the temperatures of the 2 junctions. |
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| Seebeck conducted a number of experiments by forming thermocouples of different metals. He arranged the metals in a series such that in a thermocouple formed from any two of them, current will flow from a metal earlier in the series to the one later in the series through the cold junction. This is called the thermoelectric series. Part of the series is: |
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| Sb, Fe, Zn, Ag, Au, Cr, Sn, Pb, Cu, Co, Ni, Bi. |
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| The greater the separation of the metals forming the thermocouple in the series, greater is the thermo emf produced. |
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| It is found that emf of a thermocouple AB is the difference between the emfs of two thermocouples AC and BC, provided the junctions are held at the same temperatures. |
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| The thermo emf of e many thermocouples has been measured as a function of the temperature T of the hot junction, when the cold junction is maintained at 0oC. Its temperature dependence is given by |
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| Where a and b are constants (called thermoelectric coefficients) which depends on the nature of the metals. |
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| This temperature dependence is an
approximate empirical relation valid over a limited temperature range. |
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| Neutral temperature and Inversion Temperature |
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| The diagram below shows an arrangement to study the effect of temperature difference between the two junctions in a Cu-Fe thermocouple. |
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| Keeping the junction B at 0oC, the temperature of junction A is increased. The graph below shows the variation of the thermo emf with the temperature of hot junction, with the cold junction at 0oC. |
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| When both the junctions are at the same temperature, there is no thermo emf junction increases. The thermo emf increases with temperature and reaches a maximum value at a certain temperature, called the neutral temperature Tn. |
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| The value of the neutral temperature is constant for a thermocouple, depends on the nature of materials and is independent of the temperature of the cold junction. |
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| As the temperature of the hot junction is increased, the thermo emf starts decreasing instead of increasing. The particular temperature at which, the thermo emf becomes zero is called the inversion temperature. On heating slightly further, the direction of the thermo emf is reversed, as the number densities of both the metals used are reversed. Therefore, the current reverses direction. This temperature depends on the temperature of the cold junction and the nature of the material. |
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| The thermo electric power (also called Seebeck coefficient) is given by |
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| i.e. S a T |
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| When T = 0, |
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| This shows that thermoelectric power is independent of the temperature of the cold junction. |
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| The neutral temperature (Tn) is when the thermo emf is the maximum. |
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| This gives |
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| a + b Tn = 0 |
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| For Cu-Fe thermocouple, Tn is ~518oF. For most pure metal thermocouples, the neutral temperature is much higher. For those made with alloys, b is small and the thermo - emf varies almost linearly with temperature over a wide range of temperature. |
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| To obtain an expression for Ti, we have to put e = 0. |
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| Expressions for Ti and Tn show that the inversion temperature Ti is twice the neutral temperature Tn. |
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