Thermocouple theory

Forums: 

Please explain to me the theory and principle of thermocouples, in atomic scale. Like how are electrons cycles around the loop.

Depending on how much detail

Depending on how much detail you need this topic could take up a book, or even several books. But here's one very over simplified way to look at it.

Imagine that your metal wire is like a container, a long tube, holding a very simple gas. If you make one end of the container colder than the other, the gas will tend to collect, to become more dense, at the cold end compared to the hot end. If the gas is charged, like electrons are, there will be more charge at the cold end than the hot end. And that means there has to be a voltage. Something like the way a capacitor or a battery has more charge on one plate than the other, but in this case it is the temperature difference which creates the voltage.

Indeed, you can think think of a wire in a temperature gradient as something like a battery, except instead of a chemical reaction driving electrons from one end to the other it's the temperture difference forcing the charges around the circuit.

The reason you need two different kinds of wires to make a thermocouple is now clear: if the voltage from the hot side to the cold side is the same in the two wires, there would be no tendency for a current to flow. Just as if you connected two batteries plus-to-plus and minus-to-minus. If the batteries are idenical, no current flows. But if they are different, say one is a 1.7 V battery and the other is a 1.5 V battery, there will be a leftover voltage of 0.2 V that can be used to drive a current. So long as the two wires are different, there will be some voltage. And if you can manage to turn one of the batteries around, then the voltages will add up, which would be even better.

Hope that helps some.