Coupling of two different metals

Question

I have read about thermoelectric effects such as the Seebeck effect, the Peltier effect, and the Thomson effects. I understand that if there is a temperature difference between two ends of a metal, then in the hot junction end electrons will move more vigorously than in the cold end junction and then electrons will move from hot to cold junction and eventually there become a difference in potential between the hot and the cold junction and if I connect it with a wire a current will flow.

But Seebeck says that when two dissimilar metals are being coupled and put one end in hot and another in cold junction then only an EMF will be produced.

But I don't understand previously when I used only one type of metal then also an EMF was generated, then why do I need to couple two dissimilar metals? What is the importance to couple two dissimilar metal to have an EMF in circuit?

Answer 1

You are right that in the case of the Seebeck effect, you do not need two dissimilar materials for the effect to manifest itself. If you have a straight metallic wire and you heat an end and say, remove heat from the other end, there will be a potential difference between the two ends of the wire. However if you want to measure that voltage, you need to plug in a voltmeter. If the cables of the voltmeter are of the same material than the wire, then you would measure 0 voltage, because essentially you'd measure the difference of the Seebeck coefficients of "the two materials" or more if you use more material, or less if you use a single material. This is why usually a picture is drawn with two different materials. What a voltmeter measure in that case is simply the voltage corresponding to $\Delta V = (S_A-S_B)\Delta T$.

If you choose superconducting wire at a low enough temperature for the connection to the voltmeter, you'd really measure the absolute Seebeck coefficient of the original material. That is so because the Seebeck coefficient of any superconducting material vanishes.

Thus, if you use a single material in the form of a loop (a closed circuit), keep a temperature difference between two points A and B, then the voltage between A and B exactly cancels out the voltage from B to A, because the material that makes up the branches A to B and B to A have the same Seebeck coefficient. If you use a material with a different Seebeck coefficient, you'd measure the voltage given by the expression above, which in general does not vanish.

Answer 2

then electrons will move from hot to cold junction and eventually there become a difference in potential between the hot and the cold junction and if I connect it with a wire a current will flow.

The key point here is that the current will flow only if the wire connecting two ends of the piece of metal being heated, is made of a dissimilar metal.

If both the original piece of metal and the connecting wire are made of the same metal, you can view it as just one piece of metal heated at one end, which will cause some diffusion of electrons to the other end, as you've described, but no loop current.

If, on the other hand, the wire is made out of a dissimilar metal, you have the setup which is supposed to and will generate EMF due to the Seebeck effect.

So there is no contradiction.