# How does a thermocouple work?

## Homework Statement

descibe the operational principle of a thermocouple

## The Attempt at a Solution

A thermocouple works using the seebeck affect. Two dissimilar metals joined together at one end forming a "hot" junction this is where the temperature is measured. At the other end the wires aren't joind this is the reference end (cold end). As the hot junction is heated, the heat is conducted at different speads in the two dissimlar metals. The electrons at the hot junction "diffuse" and move to the cooler end (reference end) of the wires making one of the wires more positively charged and the other more negatively charged as more electrons will be in the reference end of the better conductor wire creating a potential difference

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haruspex
Homework Helper
Gold Member
2020 Award
You are confusing two things.
In the Seebeck effect, only one metal is needed and no circuit. Electrons diffuse from the hot end to the cold, creating a potential difference.
In a thermocouple, strips of two different metals are connected at both ends, creating a circuit. With one end heated, a current flows.
See https://www.explainthatstuff.com/howthermocoupleswork.html

Daniel2244
scottdave
Homework Helper
So are you looking to see if you are correct? It's been awhile since I have worked with these, so I looked it up.
https://en.m.wikipedia.org/wiki/Thermocouple
So it looks like you sort of got it. The Seebeck Effect is the thermoelectric effect where temperature difference produces a voltage.

scottdave
Homework Helper
Nice article

Daniel2244
You are confusing two things.
In the Seebeck effect, only one metal is needed and no circuit. Electrons diffuse from the hot end to the cold, creating a potential difference.
In a thermocouple, strips of two different metals are connected at both ends, creating a circuit. With one end heated, a current flows.
See https://www.explainthatstuff.com/howthermocoupleswork.html
So, A thermocouple works by two dissimlar metals being connected at each end. One end is called the hot junction and the other end is a cold junction. When heat is applied to the hot junction, electrons move between the hot junction of the different metals at a faster rate causing the two different juctions to have a different charge producing a voltage.

rude man
Homework Helper
Gold Member
Yes, thermocouples operate on the Seebeck effect.

A typical setup is three junctions: T, T0 and TR where
T is temp. to be measured;
T0 is a reference temperature, often an ice bath;
and TR is the temperature of the measuring device, typically a potentiometer at room temperature so no current flows across the TR junctions. See attached "Seebeck effect.jpg" taken from Heat and Thermodynamics by M W Zemansky of CCNY, hopefully visible.

Each wire type A, B and C (note the same wire type C connecting T0 to TR) has its own characteristic "entropy transport parameter" S* and depends on the material and temperature of the wire. The emf's developed are across the wires so there are Δemf's developed between junctions a to c, c to e, e to d and d to b. Then the respective transport parameters are integrated over temperature over these four wires; the result after cancellation of the S*c parameters is
Δemfa,b ≈ (T - T0)(S*A - S*B) as desired.

Clearly we want to use wires A and B with ΔS* as different from one another as possible. I seem to remember copper and constantan.

There are other thermoelectric effects, e.g. Peltier and Joule. They're different phenomena.

#### Attachments

• seebeck effect.jpg
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