Nernst effect

The Nernst effect describes two effects in the interaction between electrical voltage or electrical current and heat flow or temperature difference in an external magnetic field. Both were named after Walther Nernst , who discovered them partly in collaboration with Albert von Ettingshausen . Whenever the Nernst effect is mentioned in international scientific literature , the thermomagnetic (transversal) Nernst effect is always meant.

Nernst effect (thermomagnetic)

In the event of a heat flow in the magnetic field aligned perpendicular to it, an electrical voltage occurs in the transverse direction. This is a thermomagnetic effect called the Nernst effect. In more recent literature it is also referred to as the first Ettingshausen-Nernst effect . The reverse of this effect is the Ettingshausen effect .

The following applies:

{\ displaystyle E_ {y} = N \ cdot {\ frac {{\ text {d}} T} {{\ text {d}} x}} \ cdot B_ {z}}

,

d. H. the component of the electric field in the transverse direction (y) is proportional to the temperature gradient in the longitudinal direction (x) and to the magnetic flux density perpendicular to it (z) . Where is the Nernst coefficient ; alternatively, it is also referred to with the Greek letter . ${\ displaystyle E}$ ${\ displaystyle {\ text {d}} T / {\ text {d}} x}$ ${\ displaystyle B}$ ${\ displaystyle N}$ ${\ displaystyle \ nu}$

Comparison with the Hall effect

The Nernst effect is the thermal analogue of the Hall effect . With the Hall effect, an externally applied voltage causes an electric current to flow. The moving charge carriers (generally electrons) are deflected transversely by the Lorentz force (perpendicular to the direction of the current and perpendicular to the external magnetic field), as a result of which a voltage builds up in this direction.
With the Nernst effect, the temperature difference causes the mobile charge carriers to flow from the warmer end to the cooler end. Here, too, the Lorentz force causes tension to build up in the transverse direction. The sign of the voltage is the other way around: If the magnetic field runs from bottom to top and electric current flows from left to right, the Hall effect creates a negative potential in front and a positive potential in the back. If, with the same magnetic field, a heat flow flows from left to right, the Nernst effect creates a positive potential in front and a negative potential in the back. The difference is that electrons, by convention, carry a negative charge, so their direction of movement is opposite to that of the electric current.

The Nernst effect was discovered in 1886, seven years after the Hall effect.

Spin-Nernst effect

Just as there is a quantum mechanical analogue to the Hall effect with the spin Hall effect, an analogue to the (thermomagnetic) Nernst effect was also proven in 2017: the spin Nernst effect. Here, too, the deflection is "the other way around" than with the Spin Hall effect.

Nernst effect (galvanomagnetic)

In the case of an electric current in a magnetic field oriented perpendicular to it, a temperature difference occurs in the transverse direction. This is a galvanomagnetic effect known as the Ettingshausen effect . In addition, there is also a temperature difference in the longitudinal direction. This phenomenon is also known as the "Nernst effect".

Individual evidence

↑ see e.g. B. [1]
^ Bergmann, Schaefer: Elektrizitätslehre , De Gruyter 1966, p. 487
↑ Christophe Goupil: Continuum Theory and Modeling of Thermoelectric Elements . Wiley-VCH 2016, ISBN 978-3-527-41337-9 , page 21.
↑ Spectrum Lexicon of Physics, Nernst Effect

[1] see e.g. B. [1]

[2] Bergmann, Schaefer: Elektrizitätslehre , De Gruyter 1966, p. 487

[3] Christophe Goupil: Continuum Theory and Modeling of Thermoelectric Elements . Wiley-VCH 2016, ISBN 978-3-527-41337-9 , page 21.

[4] Spectrum Lexicon of Physics, Nernst Effect