Everything about Magnetic Moment totally explained
In
physics,
astronomy,
chemistry, and
electrical engineering, the term
magnetic moment of a system (such as a loop of
electric current, a bar
magnet, an
electron, a
molecule, or a
planet) usually refers to its
magnetic dipole moment, and is a measure of the strength of the system's net
magnetic source. Specifically,
magnetic dipole moment quantifies the contribution of the system's internal magnetism to the external
dipolar magnetic field produced by the system (for example the component of the external magnetic field that drops off with distance as the inverse cube). Any
dipolar magnetic field pattern is symmetric with respect to rotations around a particular axis, therefore it's customary to describe the magnetic dipole moment that creates such a field as a
vector with a direction along that axis. For
quadrupolar,
octupolar, and higher-order
multipole magnetic moments, see
Multipole expansion.
Two kinds of magnetic sources
Fundamentally, contributions to any system's magnetic moment may come from sources of two kinds: (1) motion of
electric charges, such as
electric currents and (2) the
intrinsic magnetism of
elementary particles, such as the
electron.
Contributions due to the sources of the first kind can be calculated from knowing the distribution of all the electric currents (or, alternatively, of all the electric charges and their velocities) inside the system, by using the formulas below. On the other hand, the
magnitude of each elementary particle's intrinsic magnetic moment is a fixed number, often measured experimentally to a great precision. For example, any electron's magnetic moment is measured to be −9.284764×10
-24 J/T. The
direction of the magnetic moment of any elementary particle is entirely determined by the direction of its
spin (the
minus in front of the value above indicates that any electron's magnetic moment is antiparallel to its spin).
Finally, the net magnetic moment of any system is a
vector sum of contributions from one or both types of sources.
For example, a hydrogen atom's magnetic moment is a vector sum of the following contributions: the intrinsic moment of the electron, the orbital motion of the electron around the proton, and the intrinsic moment of the proton.
Formulas and values for calculating magnetic moments
In the simplest case of a
planar loop of
electric current, its magnetic moment is defined as:
» is a negative constant multiplied by the
spin, so the magnetic moment is antiparallel to the spin angular momentum. This can be understood with the following classical picture: if we imagine that the spin angular momentum is created by the electron mass spinning around some axis, the electric current that this rotation creates spins in the
opposite direction, because of the negative charge of the electron; such current loops produce a magnetic moment which is antiparallel to the spin angular momentum.
Magnetic moments of nuclei
» Also see nuclear magnetic moment.
The nuclear system is a complex physical system consisting of nucleons, for example, protons and neutrons. The quantum mechanical properties of the nucleons include the spin among others. Since the electromagnetic moments of the nucleus depends on the spin of the individual nucleons, one can look at these properties with measurements of nuclear moments, and more specifically the nuclear magnetic dipole moment.
The nuclear magnetic moment is very sensitive to the individual contributions from nucleons and a measurement or prediction of its value can reveal important information about the content of the nuclear wavefunction. There are several theoretical models that predict the value of the magnetic dipole moment and a number of experimental techniques aiming to carry out measurements in nuclei along the nuclear chart.
Magnetic moments of molecules
Any molecule has a well-defined magnitude of magnetic moment, which may depend on the molecule's
energy state. Typically, the overall magnetic moment of a molecule is a combination of the following contributions, in the order of their typical strength:
Examples of molecular magnetism
Oxygen molecule, O2, exhibits strong paramagnetism, due to unpaired spins of its outermost two electrons.
Carbon dioxide molecule, CO2, mostly exhibits diamagnetism, a much weaker magnetic moment of the electron orbitals that's proportional to the external magnetic field. In the rare instance when a magnetic isotope, such as 13C or 17O, is present, it'll contribute its nuclear magnetism to the molecule's magnetic moment.
Hydrogen molecule, H2, in a weak (or zero) magnetic field exhibits nuclear magnetism, and can be in a para- or an ortho- nuclear spin configuration.Further Information
Get more info on 'Magnetic Moment'.
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