The magnetic moment is the strength of the response of a piece of metal to a magnetic field. It is a measure of the degree to which an object is affected by
The magnetic field of a bar magnet aligns iron filings, demonstrating the lines of force of the magnetic field. [Image source: Newton Henry Black, Harvey N. Davis (1913) Practical Physics, The MacMillan Co., USA, p. 242, Public Domain, https://en.wikipedia.org/wiki/Magnetism]magnetism. Glass, for example, has zero magnetic moment. Were it placed in a magnetic field created by a bar magnet, it would not be affected. In contrast, a piece of iron has a magnetic moment. In the accompanying photo, the iron filings are affected by the bar magnet, demonstrating that the filings have a magnetic moment.
Quantum particle with a magnetic moment. Green arrow—particle; red lines—its magnetic field. [Image source (slightly modified): Bdushaw (Own work) from https://en.wikipedia.org/wiki/Magnetic_moment]The definition of “magnetic moment,” is, more precisely: the degree to which an external magnetic field is able to influence the object. Objects which can be magnetized, that is, create their own magnetic fields, have magnetic moments.
Magnetic Moment of a Quantum Particle
Electrons in a piece of iron are like tiny bar magnets. If they are aligned so that their north poles all point in the same direction, a magnetic field is created, and we say that the iron has been magnetized. [Image source: http://www.delsearegional. us/academic/classes /highschool/science/ physics/firstyear/Units/ Unit07/Notes/Basic Magnetism.htm]Some quantum particles have a magnetic moment, meaning they respond to magnets. Electrons and protons, for example, both have magnetic moments. It is the collective magnetic moments of all the electrons in a metallic material that accounts for its magnetism.
The magnetic moment of quantum particles is due to electrical charge and spin. Electrical charge creates an electrical field around the quantum particle. Most quantum particles have an additional property called “quantum spin.” When a particle which has electrical charge, spins, it creates a magnetic field. So, the particle has both an electrical field and a magnetic field. The degree to which the magnetic field of the particle responds to an external magnetic field, for example a bar magnet placed near the particle, is the magnetic moment of the particle. Click for more detail on the magnetic moment of electrons.
A note about spin: As a note, physicists don’t visualize quantum particle spin as a rotational motion. In fact, physicists provide no visualization for spin at all. While initially, physicists assumed quantum particles were spinning like a top, their calculations soon showed that this would be a physical impossibility. They simply define “spin” as that property, which, in the presence of the quantum particle’s electrical charge, gives it a magnetic field.
Etymology of “Moment.”
“Moment” may seem an odd usage in the phrase “magnetic moment.” It has entered the jargon of physics due to older meanings of “moment” having to do with motion. These are no longer in general use. “Moment” derives from the Latin momentum, which meant “motion.”
