# Magnetic Moment ?

I'm trying to get a better understanding of a magnetic moment . The magnetic moment of a particle is its willingness to align with the B field. Can we think of it as its magnetic moment of inertia or is this incorrect.

Staff Emeritus
No, moment of inertia is something else.

okay could you maybe explain magnetic moment in layman's terms .

Staff Emeritus
What don't you understand about, say, the Wikipedia article?

Are you trying to find differences in the magnetic moment of a particle's spin or just trying to understand the generic meaning of the magnetic moment of a magenet / charge with angular momentum

im just trying to understand the basic concept of magnetic moment , and i have read the wiki page on it .

I think this pretty much sums it up:

"The magnetic moment of a magnet is a measure of its tendency to align with a [external] magnetic field."

"The magnetic field produced by a magnet is proportional to its magnetic moment"

In other words, we define the magnetic moment from the magnetic field of the object.

Hmm I never thought of it like a moment of inertia...

As you said the magnetic moment is a measure of an objects tendency to align with a B-field.

A moment of inertia is a measure of an objects (lack of) tendency to changes in rotation.

So I mean they are *sort* of analogous...

I'm trying to get a better understanding of a magnetic moment . The magnetic moment of a particle is its willingness to align with the B field. Can we think of it as its magnetic moment of inertia or is this incorrect.

The magnetic moment is the magnetic dipole moment which is the magnetic version of the electric dipole moment. I suspect you're asking about its etymology, which I'm unsure about, but as a guess:

The moment of a vector G is in general defined as G x r, examples being moment of momentum p x r, moment of a force F x r. The electric dipole moment of two equal charges seperated by a distance l is defined as ql. So maybe "moment" was chosen in the electric case because the expression looks similar to the moment of a couple = fr.

Heald in his book Classical Electromagnetic Radiation on page23 states that by defining the magnetic dipole moment as proportional to the current and the area, you end up with an expression for the torque on a magnetic dipole in a magnetic field similar to that on an electric dipole in an electric field:

T_e = p x E

T_m = m x B

Hence, moment in magnetic moment appears to have nothing to do with the way moment was used in the past by physicists and is inseparable from magnetic-moment which appears to be based entirely on electric dipole moment.

alxm
You're on the right track, Jason:
I don't have a linguistic reference, but I'm fairly certain the etymology is from German (as so many physics words),
where (dreh-)moment is the word for 'torque'. (neuter, the masculine is the word for 'instant')

So moment = torque.

You're on the right track, Jason:
I don't have a linguistic reference, but I'm fairly certain the etymology is from German (as so many physics words),
where (dreh-)moment is the word for 'torque'. (neuter, the masculine is the word for 'instant')

So moment = torque.

http://www.etymonline.com gives the etymology of moment as coming from Latin:

moment
mid-14c., "very brief portion of time, instant," in moment of time, from O.Fr. moment, from L. momentum "movement, moving power," also "instant, importance," contraction of *movimentum, from movere "to move" (see move). Some (but not OED) explain the sense evolution of the L. word by notion of a particle so small it would just "move" the pointer of a scale, which led to the transferred sense of "minute time division." Sense of "importance, 'weight' " is attested in English from 1520s. Phrase never a dull moment first recorded 1889 in Jerome K. Jerome's "Three Men in a Boat." Phrase moment of truth first recorded 1932 in Hemingway's "Death in the Afternoon," from Sp. el momento de la verdad, the final sword-thrust in a bull-fight.