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In physics, moment is a combination of a physical quantity and a distance. Moments are usually defined with respect to a fixed reference point; they deal with physical quantities as measured at some distance from that reference point. For example, a moment of force is the product of a force and its distance from an axis, which causes rotation about that axis. In principle, any physical quantity can be combined with a distance to produce a moment; commonly used quantities include forces, masses, and electric charge distributions.
In its most simple and basic form, a moment is the product of the distance to some point, raised to some power, multiplied by some physical quantity such as the force, charge, etc. at that point:
where Q is the physical quantity such as a force applied at a point, or a point charge, or a point mass, etc. If the quantity is not concentrated solely at a single point, the moment is the integral of that quantity's density over space:
where \rho is the distribution of the density of charge, mass, or whatever quantity is being considered.
More complex forms take into account the angular relationships between the distance and the physical quantity, but the above equations capture the essential feature of a moment, namely the existence of an underlying r^n\,\rho(r) or equivalent term. This implies that there are multiple moments (one for each value of n) and that the moment generally depends on the reference point from which the distance r is measured, although for certain moments (technically, the lowest non-zero moment) this dependence vanishes and the moment becomes independent of the reference point.
Each value of n corresponds to a different moment: the 1st moment corresponds to n=1; the 2nd moment to n=2, etc. The 0th moment (n=0) is sometimes called the monopole moment; the 1st moment (n=1) is sometimes called the dipole moment, and the 2nd moment (n=2) is sometimes called the quadrupole moment, especially in the context of electric charge distributions.
Examples:
Assuming a density function that is finite and localized to a particular region, outside that region a 1/r potential may be expressed as a series of spherical harmonics:
The coefficients q_{lm} are known as multipole moments, and take the form:
where \mathbf{r}' expressed in spherical coordinates (r',\phi',\theta') is a variable of integration. A more complete treatment may be found in pages describing multipole expansion or spherical multipole moments. (Note: the convention in the above equations was taken from Jackson.^{[1]} The conventions used in the referenced pages may be slightly different.)
When \rho represents an electric charge density, the q_{lm} are, in a sense, projections of the moments of electric charge: q_{00} is the monopole moment; the q_{1m} are projections of the dipole moment, the q_{2m} are projections of the quadrupole moment, etc.
The multipole expansion applies to 1/r scalar potentials, examples of which include the electric potential and the gravitational potential. For these potentials, the expression can be used to approximate the strength of a field produced by a localized distribution of charges (or mass) by calculating the first few moments. For sufficiently large r, a reasonable approximation can be obtained from just the monopole and dipole moments. Higher fidelity can be achieved by calculating higher order moments. Extensions of the technique can be used to calculate interaction energies and intermolecular forces.
The technique can also be used to determine the properties of an unknown distribution \rho. Measurements pertaining to multipole moments may be taken and used to infer properties of the underlying distribution. This technique applies to small objects such as molecules,^{[2]}^{[3]} but has also been applied to the universe itself,^{[4]} being for example the technique employed by the WMAP and Planck experiments to analyze the Cosmic microwave background radiation.
The concept of moment in physics is derived from the mathematical concept of moments.^{[5]} . The principle of moments is derived from Archimedes' discovery of the operating principle of the lever. In the lever one applies a force, in his day most often human muscle, to an arm, a beam of some sort. Archimedes noted that the amount of force applied to the object, the moment of force, is defined as M = rF, where F is the applied force, and r is the distance from the applied force to object.^{[6]} However, historical evolution of the term 'moment' and its use in different branches of science, such as mathematics, physics and engineering, is unclear.
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