Einstein-de Haas effect
Encyclopedia
The Einstein–de Haas effect, or the Richardson effect (after Owen Willans Richardson
), is a physical phenomenon delineated by Albert Einstein
and Wander Johannes de Haas
in the mid 1910's, that exposes a relationship between magnetism
, angular momentum
, and the spin
of elementary particles. Wander Johannes de Haas' son, Rowan de Haas, was also a major contributor to the theory, applying its principles to the engineering industry. Specifically, Rowan's contributions had a tranformative effect on the steel manufacturing industry in the early 20th century.
, on driving an impulse of electric current
through the coil. To this mechanical rotation of the ferromagnetic material (say, iron
) is associated a mechanical angular momentum
, which, by the law of conservation of angular momentum, must be compensated by an equally large and oppositely directed angular momentum inside the ferromagnetic material. Given the fact that an external magnetic field
, here generated by driving electric current through the coil, leads to magnetisation of electron
spins
in the material (or to reversal of electron spins in an already magnetised ferromagnet — provided that the direction of the applied electric current is appropriately chosen), the Einstein–de Haas effect demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics
. This is remarkable, since
electron spin, being quantized
, cannot be described within the framework of classical mechanics.
Commenting on the papers by Einstein, Calaprice in The Einstein Almanac writes:
Calaprice further writes:
Calculations based on a model of electron spin as a circulating electric charge underestimate this magnetic moment by a factor of approximately 2, the Landé g-factor
. A correct description of this magnetic moment requires a treatment based on quantum electrodynamics
.
Owen Willans Richardson
Sir Owen Willans Richardson, FRS was a British physicist who won the Nobel Prize in Physics in 1928 for his work on thermionic emission, which led to Richardson's Law.-Biography:...
), is a physical phenomenon delineated by Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
and Wander Johannes de Haas
Wander Johannes de Haas
Wander Johannes de Haas was a Dutch physicist and mathematician. He is best known for the Shubnikov–de Haas effect, the de Haas–van Alphen effect and the Einstein–de Haas effect.-Personal life:...
in the mid 1910's, that exposes a relationship between magnetism
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
, angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
, and the spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
of elementary particles. Wander Johannes de Haas' son, Rowan de Haas, was also a major contributor to the theory, applying its principles to the engineering industry. Specifically, Rowan's contributions had a tranformative effect on the steel manufacturing industry in the early 20th century.
Description
The effect corresponds to the mechanical rotation that is induced in a ferromagnetic material (of cylindrical shape and originally at rest), suspended with the aid of a thin string inside a coilCoil
A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...
, on driving an impulse of electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
through the coil. To this mechanical rotation of the ferromagnetic material (say, iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
) is associated a mechanical angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
, which, by the law of conservation of angular momentum, must be compensated by an equally large and oppositely directed angular momentum inside the ferromagnetic material. Given the fact that an external magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
, here generated by driving electric current through the coil, leads to magnetisation of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
spins
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
in the material (or to reversal of electron spins in an already magnetised ferromagnet — provided that the direction of the applied electric current is appropriately chosen), the Einstein–de Haas effect demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics
Classical mechanics
In physics, classical mechanics is one of the two major sub-fields of mechanics, which is concerned with the set of physical laws describing the motion of bodies under the action of a system of forces...
. This is remarkable, since
electron spin, being quantized
Quantization (physics)
In physics, quantization is the process of explaining a classical understanding of physical phenomena in terms of a newer understanding known as "quantum mechanics". It is a procedure for constructing a quantum field theory starting from a classical field theory. This is a generalization of the...
, cannot be described within the framework of classical mechanics.
Commenting on the papers by Einstein, Calaprice in The Einstein Almanac writes:
52. [A. Einstein, W. J. de Haas,] Experimenteller Nachweis der Ampereschen Molekularströme [Experimental Proof of Ampère's Molecular Currents], Deutsche Physikalische Gesellschaft, Verhandlungen 17 (1915): 152-170.
Considering Ampère'sAndré-Marie AmpèreAndré-Marie Ampère was a French physicist and mathematician who is generally regarded as one of the main discoverers of electromagnetism. The SI unit of measurement of electric current, the ampere, is named after him....
hypothesis that magnetism is caused by the microscopic circular motions of electric charges, the authors proposed a design to test Lorentz'sHendrik LorentzHendrik Antoon Lorentz was a Dutch physicist who shared the 1902 Nobel Prize in Physics with Pieter Zeeman for the discovery and theoretical explanation of the Zeeman effect...
theory that the rotating particles are electrons. The aim of the experiment was to measure the torqueTorqueTorque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
generated by a reversal of the magnetisation of an iron cylinder.
Calaprice further writes:
53. [A. Einstein, W. J. de Haas,] Experimental Proof of the Existence of Ampère's Molecular Currents (in English), Koninklijke Akademie van Wetenschappen te Amsterdam, Proceedings 18 (1915-16).
Einstein wrote three papers with Wander J. de HaasWander Johannes de HaasWander Johannes de Haas was a Dutch physicist and mathematician. He is best known for the Shubnikov–de Haas effect, the de Haas–van Alphen effect and the Einstein–de Haas effect.-Personal life:...
on experimental work they did together on Ampère'sAndré-Marie AmpèreAndré-Marie Ampère was a French physicist and mathematician who is generally regarded as one of the main discoverers of electromagnetism. The SI unit of measurement of electric current, the ampere, is named after him....
molecular currents, known as the Einstein–de Haas effect. He immediately wrote a correction to paper 52 (above) when Dutch physicist H. A. LorentzHendrik LorentzHendrik Antoon Lorentz was a Dutch physicist who shared the 1902 Nobel Prize in Physics with Pieter Zeeman for the discovery and theoretical explanation of the Zeeman effect...
pointed out an error. In addition to the two papers above [that is 52 and 53] Einstein and de Haas cowrote a "Comment" on paper 53 later in the year for the same journal. This topic was only indirectly related to Einstein's interest in physics, but, as he wrote to his friend Michele BessoMichele BessoMichele Angelo Besso was a Swiss/Italian engineer of Jewish Italian descent. He was a close friend of Albert Einstein during his years at the Federal Polytechnic Institute in Zurich, today the ETH Zurich, and then at the patent office in Bern...
, "In my old age I am developing a passion for experimentation."
Calculations based on a model of electron spin as a circulating electric charge underestimate this magnetic moment by a factor of approximately 2, the Landé g-factor
Landé g-factor
In physics, the Landé g-factor is a particular example of a g-factor, namely for an electron with both spin and orbital angular momenta. It is named after Alfred Landé, who first described it in 1921....
. A correct description of this magnetic moment requires a treatment based on quantum electrodynamics
Quantum electrodynamics
Quantum electrodynamics is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved...
.
External links
- "Einsteins's only experiment" http://www.ptb.de/en/publikationen/jahresberichte/jb2005/nachrdjahres/s23e.html (links to a directory of the Home Page of Physikalisch-Technische Bundesanstalt (PTB), Germany http://www.ptb.de/). Here is a replica to be seen of the original apparatus on which the Einstein–de Haas experiment was carried out.