EMF Production in Thermocouples: Causes & Effects

[mysteriously]

1.what is the main cause for the emf produced in a thermocouple when their two junctions are at different temperatures?

2.why is it necessary that the emf produced causes the current to pass from the metal (which is at the top of thermoelectric series) to the other metal (which is at lower position) at the cold junction?? for eg:if we use a thermocouple of iron and copper and two junctions are at temperatures (lets say) 100 and 0 degree celcius...then it is said that the current flows from iron(which has higher position in thermoelectric series) to copper (which has lower position in thermoelectric series) at cold junction(0 degre celcius)..but why can't the current pas from iron to copper in the hot junction itself? isn't it the same?

3. At inversion temperature the flow of current is reversed ...why does this happen??

 

[OhYoungLions]

1. The cause of the emf can be understood by considering a box full of gas. If you heat one side of the box, the particles near that side tend to have greater kinetic energy than those on the cold side. At equilibrium, this causes a density gradient. If the particles are charged (like electrons in a metal), this produces a voltage.

2. I don't know anything about the thermoelectric series, but the main difference between those two metals is the average velocity of the electrons. If I recall, Cu has a higher velocity. If you heat a junction of Cu and Fe, I imagine the electrons move away from the heated end. There will be a net electron current at the cold junction due to the fact that the electrons in the Cu are moving faster than the ones in the Fe. This corresponds to a conventional current going from Fe to Cu.

3. Because the electrons always go from Cu to Fe.