Thermionics disobeying laws of thermodynamics

[Alex_645]

Hi there,

Now this goes against everything I know as a physicist, but I can't get my head around how thermionics behave when you have an emitter at slightly lower temperature but much lower work function, and a collector at higher temperature and higher work function, so that thermionic current moves from cold to hot. Is this possible? And if so, it's flowing in the opposite direction to thermal radiation, and seems to violate the laws of thermodynamics...

Anyone well versed on the subject that could explain why this isn't possible?

Much appreciated,
-Alex

 

[ZapperZ]

Hi there,

Now this goes against everything I know as a physicist, but I can't get my head around how thermionics behave when you have an emitter at slightly lower temperature but much lower work function, and a collector at higher temperature and higher work function, so that thermionic current moves from cold to hot. Is this possible? And if so, it's flowing in the opposite direction to thermal radiation, and seems to violate the laws of thermodynamics...

Anyone well versed on the subject that could explain why this isn't possible?

Much appreciated,
-Alex

I'm not sure I see the problem here. There are two issues to consider:

1. Thermodynamics laws applies to heat going from hot to cold. There's nothing here that prevents electrons from going from cold to hot when there is an external factor involved. The electrons leaving the metal doesn't really care if it is leaving a colder object and entering a hotter object, since its dynamics in this case is not governed by the temperature of the source and the sink.

2. Here, you are "artificially" forcing electrons to go from cold to hot. This is no different than a heat pump where you extract heat from colder reservoir and releasing it to a hotter reservoir by applying work. You are heating the source and forcing electrons to be emitted. These electrons will go everywhere (assuming no external field, since you didn't mention any). It is just that some happens to hit the hotter element. If you instead do not heat anything but simply bring the two metals together, you will have a contact potential and the electrons will flow in the correct direction dictated by thermodynamics laws.

Zz.