Current Seebeck Effect
See beck in 1821 found that if two wires of different metals say copper and iron are joined at their ends A and B through a low resistance galvanometer G to from a closed circuit and if one of the junctions say A is heated and the other junction B is kept cold the galvanometer shows a deflection this must be due to a current in the circuit called thermo eclectic current the current must further be due to certain called thermo the assembly of two different metals joined at their ends to have two junctions in a circuit is called a thermocouple. This phenomenon of thermo-electricity was discovered first of all by see beck. Hence it is also called see beck effect.
Thus see beck effect is the phenomenon of generation of an electric current in a thermocouple by keeping its two junctions at different temperatures.
The direction of current in Cu – Fe thermocouple is from Cu to Fe through hot junction and can be remembered by the world chl. In Sb-Bi thermocouple the direction of current is Sb to Bi through cold junction it can be recollected by ABC.
See beck found that the magnitude and direction of thermo developed in a thermo couple depends up.
(i) The nature of metals forming the thermo couple.
(ii) Difference in temperatures of the two junctions.
The see beck effect is reversible. It means if the hot and the cold junctions are interchanged. The direction of thermoelectric current is reversed. Thus see beck effect is a reversible effect.
See beck from his experimental investigations arranged a number of metals in a series know as see beck series. Some of the metals of this series in the order see beck arranged them are given below.
Bi, Ni, Co Pd, Pt, Cu, Mn, Hg, Pb, Sn, Au, Ag, Zn, Cd, Fe, Sb, Te,.
When any two of these metals from a thermocouple current flows through the hot junction from a metal occurring earlier, to a metal occurring later, in the series.
See beck also found that for a given difference of temperatures of two junctions the larger is the gap in see beck series between the metals forming the thermo couple the greater will be the thermo emf. Generated. That is why thermo emf developed is maximum in a thermo couple of antimony and bismuth for a given difference in temperatures of the two junctions. The thermo emf is of the order of 10-3 V or less.
For a temperature difference of 100C between the two junctions the thermo produced in Cu-Fe. Thermocouple is only 0.0013V and in Sb-Bi thermocouple is 0.008V.
Origin of thermo
When two different metals are brought into contact, at the junction the free electrons tend to diffuse from the metal swath greater free electron density to the other with lower free electron density due to this diffusions a potential different is developed at the junction of the two metals called contact potential when booth the junctions are at the same temperature the contact potentials at the two junctions will be the same. Hence no current flows in the thermocouple. But if one junction id heated up the rate of diffusion of free electrons at that junction will charge. As a result of it’s the contact potentials at the two junctions will become different and there will be an effective potential difference in the circuit called thermo
Thus the term produced in a thermocouple is equal to the difference in contact potentials at the two junctions A and B and is given by VAB = VA – VB
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