Thermoelectric devices and entropy

[mrblanco]

Today in my thermodynamics class my professor spoke about how a process must satisfy the laws of thermodynamics in order to work. He gave an example of current going through a wire generating heat. (See attached picture) But he also talked about how adding heat to a wire and it generating a current would violate the 2nd law of thermodynamics. My question is, how does a thermoelectric device, which takes heat and turns it into a current, follow the 2nd law?

 

[Khashishi]

Because thermoelectric devices require there to be a temperature difference between two plates. Go look it up and see what you find.

 

[mrblanco]

I've looked it up and it seems to make sense, however, why would adding heat to a wire to produce a current not work? It seems like it would be the same process. If I'm adding heat then obviously there will be a temperature difference

 

[Blackberg]

If you heat one end of a wire, there is an initial current : the electrons run away to the other end as much as they can. But you very soon (seconds) get equilibrium because the first ones push back (- charged) the rest, which are left to bake in the heat (they actually don't mind that much). You can't have current in an open circuit, so you're left with a voltage.

 

[mrblanco]

Wouldn't that eventually happen in a TEG? The plates would eventually come to thermal equilibrium and there would no longer be a ΔT.

 

[Dale]

I've looked it up and it seems to make sense, however, why would adding heat to a wire to produce a current not work? It seems like it would be the same process. If I'm adding heat then obviously there will be a temperature difference

The second law doesn't prohibit heat from producing electrical work, it just puts an upper bound on the amount of work that cAn be done. Thermoelectric devices operate well below that limit.

 

[russ_watters]

I've looked it up and it seems to make sense, however, why would adding heat to a wire to produce a current not work? It seems like it would be the same process. If I'm adding heat then obviously there will be a temperature difference

A temperature difference between what and what? Obviously, the wire is the hot reservoir: where's the cold reservoir, how is heat flowing to it and how is your thermodynamic device getting in the way to harness/capture that flow?