Thermoelectric Emf: Investigating Temperature Dependency

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Discussion Overview

The discussion revolves around the temperature dependency of thermoelectric emf, particularly focusing on why the emf does not seem to depend on the lower temperature of thermocouples and the reasons for its decrease after reaching a certain temperature. The scope includes theoretical and conceptual aspects of thermoelectric effects.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question why the thermoelectric emf does not depend on the lower temperature of the thermocouples.
  • One participant suggests that the voltage is dependent on the temperature difference between the hot and cold ends, implying the importance of the lower temperature.
  • Another participant explains that the Seebeck voltage is influenced by both the hot and cold junctions, noting that the rate of electron transport is temperature dependent.
  • There is a claim that if the cold part is at 0°C, the emf reaches a maximum at a certain temperature in the hot part, suggesting independence from the cold part's temperature.
  • Conversely, another participant argues that increasing the temperature of the cold junction will change the voltage, indicating a dependency.
  • A participant references thermoelectric efficiency and its relation to Seebeck voltage, mentioning complexities in calculations involving integrals over Matsubara frequencies.

Areas of Agreement / Disagreement

Participants express differing views on the dependency of emf on the cold junction temperature, with some asserting independence while others argue for a dependency. The discussion remains unresolved regarding the exact nature of these relationships.

Contextual Notes

Participants reference external resources and graphs to support their claims, but the discussion includes unresolved mathematical details and complexities related to the thermoelectric effect.

Ezio3.1415
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Why the thermoelectric emf doesn't depend on the lower temperature of the thermocouples... And why does the emf start to decrease after a certain increase of temperature?
 
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Why didn't anyone answer?
I really want to know and Google isn't helping me... :(
 


I don't understand your question. The voltage is dependent on the temperature difference between the hot end and colder end. This seems to necessitate that the lower temperature of the colder end be important. Could you expand on that question maybe?
 


Did u understand what I meant?
 


The Seebeck voltage depends both the cold and the hot junction. In your text it is stated that the cold junction is at 0°C it doesn't state that the Seebeck voltage is independent of the cold junction. The reason for the Seebeck voltage is the rate at with which electrons get transported by a thermal current from one side of a conductor to the other. This rate is temperature dependent, so at some point one of the elements of the thermocouple catches up to the other one, why that is so I don't know, but the Seebeck voltage cannot increase forever.
 


say cold part is 0 degree c then the emf is highest for a certain T in the hot part... If its cold part is 5,the T for highest emf remains same... Doesn't this mean the emf is independent of the cold parts temperature?
And I want to why there's a certain T and is there a certain T for highest emf in the opposite after increasing from inversion temperature... Will the graph become a sine curve?
 


Ezio3.1415 said:
say cold part is 0 degree c then the emf is highest for a certain T in the hot part... If its cold part is 5,the T for highest emf remains same... Doesn't this mean the emf is independent of the cold parts temperature?

I don't think so. I believe that if you increase the temperature of the cold junction the voltage will change.
 
This is a bit complicated. A related value is the thermoelectric efficiency plotted here for different materials.
http://www.its.caltech.edu/~jsnyder/graphics/thermoelectrics/n-zT-Big.jpg
http://www.its.caltech.edu/~jsnyder/graphics/thermoelectrics/p-zT-Big.jpg
The Seebeck voltage is related to the difference of one of the first curves with one of the second curves (actually to the difference between square roots after multiplying with some factor and dividing by T... but those are details). You see that the Seebeck voltage can only flip twice not more. The details of the thermo electrical effect are hard to calculate, and involve integrals over Matsubara frequencies if I recall correctly. For thermocouples that are used for measuring temperatures they won't flip before your metals melt http://www.pc-oscilloscopes.com/images/thermocoupleshigh.gif
 
  • #10


Drakkith That's why I am asking you how this happens...

0xDEADBEEF Thank you for your answer... This is way above my pay grade for now,I guess...
 

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