From L. momentum "movement, moving power," from movere "to move,"
→ move.

Jonbâk, from jonb present stem of jonbidan "to move"
(Mid.Pers. jumbidan, jumb- "to move,"
Lori, Laki jem "motion," related to gâm "step, pace;"
O.Pers. gam- "to come; to go," Av. gam- "to come; to go,"
jamaiti "goes," gāman-
"step, pac;" Mod.Pers. âmadan "to come;" Skt. gamati "goes;"
Gk. bainein "to go, walk, step," L. venire "to come;"
Tocharian A käm- "to come;" O.H.G. queman "to come;" E. come;
PIE root *gwem- "to go, come") + -âk noun suffix.

multipole moment

گشتاور ِ بسقطبه

gaštâvar-e basqotbé

Fr.: moment multipolaire

The quantity that gives the electric potential field due to a distribution of
charges, such as a → dipole,
→ quadrupole, → octupole, etc.
A multipole moment usually involves powers of the distance to the origin, as well as
some angular dependence.

1) Mechanics: The → angular momentum
associated with the motion of a particle about an origin, equal to the cross product
of the position vector (r) with the linear momentum (p = mv):
L = r x p. Although r and p are constantly changing
direction, L is a constant in the absence of any external force on the system.
Also known as orbital momentum.
2) Quantum mechanics: The → angular momentum
operator associated with the motion of a particle about an origin, equal to
the cross product of the position vector with the linear momentum, as opposed to the
→ spin angular momentum.
In quantum mechanics the orbital angular momentum is quantized. Its magnitude
is confined to discrete values given by the expression:
ħ &radic l(l + 1), where l is the orbital angular momentum quantum
number, or azimuthal quantum number, and is limited to positive integral values
(l = 0, 1, 2, ...). Moreover, the orientation of the direction of rotation is
quantized, as determined by the → magnetic quantum number.
Since the electron carries an electric charge, the circulation of electron constitutes
a current loop which generates a magnetic moment associated to the
orbital angular momentum.

A quantity characterizing an electric charge distribution,
determined by the product of the charge density, the second power of the
distance from the origin, and a spherical harmonic over the charge distribution.

Fr.: moment angulaire rotationnel, moment cinétique ~

The → angular momentum of a body rotating about an axis.
The rotational angular momentum of a solid homogeneous sphere of mass
M and radius R rotating about an axis passing through its center
with a period of T is given by:
L = 4πMR^{2}/5T.

An intrinsic quantum mechanical characteristic of a particle that has no classical
counterpart but may loosely be likened to the classical
→ angular momentum of a particle
arising from rotation about its own axis.
The magnitude of spin angular momentum is given by the expression
S = ħ √ s(s + 1), where s is the
→ spin quantum number. As an example, the spin of an electron
is s = 1/2; this means that its spin angular momentum is
(ħ /2) √ 3 or 0.91 x 10^{-34} J.s. In addition, the projection of
an angular momentum onto some defined axis is also quantized, with a z-component
S_{z} = m_{s}ħ. The only values of m_{s}
(magnetic quantum number) are ± 1/2. See also
→ Stern-Gerlach experiment.

The magnetic moment associated with the → spin angular momentum
of a charged particle. The direction of the magnetic moment is opposite to the direction
of the angular momentum. The magnitude of the magnetic moment is given by:
μ = -g(q / 2m)J, where q is the charge, m is the mass,
and J the angular momentum. The parameter g is a characteristic of the
state of the atom. It would be 1 for a pure orbital moment, or 2 for a spin moment, or some
other number in between for a complicated system like an atom. The quantity
in the parenthesis for the electron is the → Bohr magneton.
The electron spin magnetic moment is important in the → spin-orbit
interaction which splits atomic energy levels and gives rise to
→ fine structure in the spectra of atoms.
It is also a factor in the interaction of atom with external fields,
→ Zeeman effect.