Physics · Chapter 15
Study notes aligned to the official NEB syllabus.
When two dissimilar metals are joined to form a closed circuit and their two junctions are kept at different temperatures, an electric current flows in the circuit. Effects of this kind, in which a temperature difference produces an electromotive force (and, conversely, a current produces heating or cooling at a junction), are called thermoelectric effects. The three important thermoelectric effects are the Seebeck effect, the Peltier effect, and the Thomson effect. This chapter deals mainly with the Seebeck effect and its use in the thermocouple, the Peltier effect, and the thermopile.
A pair of two different metals joined at two junctions to form such a circuit is called a thermocouple.
The Seebeck effect is the production of an electromotive force (and hence a current) in a thermocouple when its two junctions are kept at different temperatures.
Cause: the free-electron densities and the way electron energy varies with temperature differ from one metal to another. When the junctions are at different temperatures, this difference drives a net diffusion of electrons across the junctions, setting up the thermo-emf.
The thermo-emf depends on two factors:
The metals can be arranged in a definite order called the thermoelectric series:
$$\text{Sb} - \text{Fe} - \text{Cd} - \text{Zn} - \text{Ag} - \text{Au} - \text{Pb} - \text{Mo} - \text{Cu} - \text{Pt} - \text{Co} - \text{Ni} - \text{Bi}$$
Two rules follow:
Keep the cold junction at a fixed temperature (say $0^\circ\text{C}$) and gradually raise the temperature $\theta$ of the hot junction. The thermo-emf is found to vary with $\theta$ according to the parabolic law:
$$E = a\theta + \tfrac{1}{2}b\theta^{2}$$
where $a$ and $b$ are constants for a given thermocouple ($a$ in $\text{V},^\circ\text{C}^{-1}$, $b$ in $\text{V},^\circ\text{C}^{-2}$; usually $a>0$ and $b<0$).
As the hot-junction temperature rises:
Key relation. The neutral temperature lies midway between the cold-junction temperature $\theta_c$ and the temperature of inversion, because the $E$-$\theta$ curve is a symmetric parabola about $\theta_n$:
$$ \begin{aligned} \theta_n - \theta_c &= \theta_i - \theta_n \quad\Longrightarrow\quad \theta_n \ &= \frac{\theta_i + \theta_c}{2} \end{aligned} $$
When two dissimilar metals are joined to form a closed circuit and their two junctions are kept at different temperatures, an electric current flows in the circuit. Effects of this kind, in which a temperature difference produces an electromotive force (and, conversely, a current produces heating or cooling at a junction), are called thermoelectric effects. The three important thermoelectric effects are the Seebeck effect, the Peltier effect, and the Thomson effect. This chapter deals mainly with the Seebeck effect and its use in the thermocouple, the Peltier effect, and the thermopile.
A pair of two different metals joined at two junctions to form such a circuit is called a thermocouple.
The Seebeck effect is the production of an electromotive force (and hence a current) in a thermocouple when its two junctions are kept at different temperatures.
Cause: the free-electron densities and the way electron energy varies with temperature differ from one metal to another. When the junctions are at different temperatures, this difference drives a net diffusion of electrons across the junctions, setting up the thermo-emf.
The thermo-emf depends on two factors:
The metals can be arranged in a definite order called the thermoelectric series:
Two rules follow:
Keep the cold junction at a fixed temperature (say ) and gradually raise the temperature of the hot junction. The thermo-emf is found to vary with according to the parabolic law:
where and are constants for a given thermocouple ( in , in ; usually and ).
As the hot-junction temperature rises:
Key relation. The neutral temperature lies midway between the cold-junction temperature and the temperature of inversion, because the - curve is a symmetric parabola about :