# Why the thermoelectric emf doesn't depend on the lower temperature of the thermocoupl

1. Jan 20, 2013

### Ezio3.1415

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?

2. Jan 25, 2013

### Ezio3.1415

Re: Why the thermoelectric emf doesn't depend on the lower temperature of the thermoc

I really want to know and Google isn't helping me... :(

3. Jan 25, 2013

### Drakkith

Staff Emeritus
Re: Why the thermoelectric emf doesn't depend on the lower temperature of the thermoc

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?

4. Jan 27, 2013

### Ezio3.1415

5. Jan 29, 2013

### Ezio3.1415

Re: Why the thermoelectric emf doesn't depend on the lower temperature of the thermoc

Did u understand what I meant?

6. Jan 29, 2013

Re: Why the thermoelectric emf doesn't depend on the lower temperature of the thermoc

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.

7. Jan 29, 2013

### Ezio3.1415

Re: Why the thermoelectric emf doesn't depend on the lower temperature of the thermoc

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?

8. Jan 29, 2013

### Drakkith

Staff Emeritus
Re: Why the thermoelectric emf doesn't depend on the lower temperature of the thermoc

I don't think so. I believe that if you increase the temperature of the cold junction the voltage will change.

9. Jan 29, 2013

Re: Why the thermoelectric emf doesn't depend on the lower temperature of the thermoc

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 wont flip before your metals melt http://www.pc-oscilloscopes.com/images/thermocoupleshigh.gif

10. Jan 31, 2013

### Ezio3.1415

Re: Why the thermoelectric emf doesn't depend on the lower temperature of the thermoc

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