We use electromagnetic notations and nomenclature here (with SI units) because that's the prime application. However, some of the discussion is really of a more general mathematical nature. Dipole moments are what you get when you pack a globally neutral finite variation into an infinitesimal amount of space. An electric dipole is a charge multiplied into a length, a magnetic dipole is a current into a surface area.
In 1912, Peter Debye (1884-1966)
pioneered the study of the electric dipole moments (EDM)
of asymmetrical molecules
(i.e., molecules without a center of symmetry).
He was awarded the Nobel prize for chemistry in 1936.
The unit of dipolar moment most commonly used by chemists is the debye (D) which is defined as a decimal submultiple of the franklin-centimeter, the standard cgs unit (esu). The franklin is a unit of electric charge also known as statcoulomb (statC) and is worth exactly 0.1 C / 299792458. One debye is equal to one attofranklin-centimeter (this particular use of a metric prefix with a non-SI unit is especially dubious, as the "atto" prefix was only introduced in 1975).
1 D = 10-18 statC.cm = (10-21 J/T) / c = 3.33564095198... 10-30 C.m
As the elementary charge (e) is 1.602176487(40) 10-19 C, an electric dipole moment (EDM) of 1 D corresponds to two opposite elementary charges separated by a distance of about 0.2082 Å (or 0.02082 nm).
Hydrogen Peroxide
and Polarity by Vince Calder
The Dipole Moment of
Nitric Oxide by C. P. Smyth & K. B. McAlpine (1933).
Microwave Detection of
Interstellar NO by H. S. Liszt & B. E. Turner (1978).
Although many atoms have a permanent magnetic moment, no permanent electric dipole moment (EDM) has ever been detected for any atom.
In 2000, the search for a nonzero atomic EDM has led a team at the University of Washington to one of the most precise measurements ever made (cf. Romalis et al., Phys. Rev. Lett. 86 (2001) pp.2505-2508). The EDM of a mercury atom, if it has any, would correspond to a displacement of its electronic cloud (80 electrons) less than 2 10-30 m. This is about 18 orders of magnitude less than what's observed for the simple polar molecules listed in the above table.
This result was obtained by looking for a possible shift due to strong electric fields of the precession frequency of 199Hg atoms in a weak magnetic field. No such frequency shift was observed at a precision of 0.4 nHz.
Even more fundamentally, the same type of investigation was carried out about the electron itself at the Department of Physics of Imperial College London. The results, published in Nature in May 2011, indicate that the charge of the electron has a perfect spherical symmetry to a precision of 15 orders of magnitude (one part in a million billion) which was widely advertised as "hair's width compared to the size of the solar system".
The new experimental accuracy (corresponding to atto-electronvolt energy shifts, on a pulsed beam of ytterbium fluoride) represents only a relatively small improvement (a factor of 1.5) over the remarkable result obtained in 2002 by Eugene Commins et al. at UC Berkeley.
However, the British team expects to improve their current accuracy by a factor of 10 or 100 "over the next few years". A lack of symmetry is expected to occur at that level of accuracy if our fundamental theories are correct (concerning, in particular, the breaking of symmetry between matter and antimatter in the early universe).
New limit on the permanent EDM of 199Hg
talk by W. Clark Griffith (2009).
Improved measurement of the shape of the electron
Nature 473, pp. 493-496 (26 May 2011)
by Jony J. Hudson, D. M. Kara, I. J. Smallman, B. E. Sauer, M. R. Tarbutt & Edward A. Hinds
Spherical Electron by Ed Copeland
(in "Sixty Symbols" by Brady Haran)
The net force an electric field E exerts on an electric dipole p is:
F = grad (p.E) - ( div E ) p
In the similar expression for the force exerted on a magnetic dipole m, the second term vanishes because B is divergence-free:
F = grad (m.B) - ( div B ) m = grad (m.B)
Originally, Coulomb defined what we now call the magnetic induction B and the magnetic moment m of a compass needle in terms of each other, using essentially the following expression of the torque applied by the magnetic field to the needle. He measured that mechanical torque directly with the delicate torsion balance which he invented. (Coulomb would later use that instrument to establish the basic law of electrostatics which now bears his name.)
Torque on a Magnetic Dipole mThe following expressions could be obtained from the general expressions of electrodynamic potentials and/or fields in the limit of dipolar distributions.
However, I fondly remember establishing both sets of dipolar formulas (as an undergraduate student, in June of 1975 or 1976) by proving that, if there are no sources at a nonzero distance from the origin, linear superpositions of these two are the only "spherical and dipolar" solutions of Maxwell's equations. Loosely speaking, this is to say that there's no other way to build solutions of Maxwell's equations where the value of each field component at position r is a sum of products of k(r)r by a vector Z(t-r/c) or by one of its derivatives...
The two types of dynamic solutions that emerge from such an analysis are readily identified from the respective static parts of the electric and magnetic dipolar fields. (These well-known static fields are obtained as the limiting cases of simple distributions [two point charges, or a current loop] whose moments are kept constant as their sizes tend to zero.)
My long-forgotten motivation was to use such solutions as rigorous building blocks for dealing with interference using Huygens' principle.
The potentials listed below both satisfy the Lorenz gauge.
The electric dipolar moment p of a charge distribution is:
p = òòò r r dV - r òòò r dV
The second term is zero if there's no net electric charge, in which case the value of the first term does not depend on the origin chosen for positions. The dipolar moment of a neutral distribution of point charges is p = å qi ri.
The counterpart of the above for magnetic dipoles is discussed below.
The magnetic dipolar moment m of a current distribution is:
m = ½ òòò r ´ j dV - ½ r ´ òòò j dV
The second term is zero for confined currents, in which case the value of the first term does not depend on the origin chosen for positions. The dipolar moment for a current I flowing in a loop of vectorial area S is m = I S.
Quantitative magnetic moments were introduced in 1777 by Charles de Coulomb (1736-1806) for compass needles. Coulomb studied them with the torsion balance which he devised and would later put to good use to establish the law of electrostatics named after him.
Elsewhere on this site, we discuss the electromagnetic properties of matter, using the symbols P and M to denote the changing densities of electric and magnetic dipoles per unit of volume. The above lowercase symbols p and m can be construed as denoting those densities integrated at a point.
In the following sections, we discuss the electromagnetic fields which are found in the midst of static dipoles, distributed with densities M and P.
Jefimenko's equations (retarded potential) | Advanced potential
Let's consider how an electromagnetic planar wave (progressing along the direction of the x-axis) can be generated from a source consisting of a uniform distribution of synchronized electromagnetic dipoles in the yOz plane. At time t, each infinitesimal dipole in that plane is equal to the elementary area dy dz multiplied into an areal moment density (a vector) which depends on t only.
Using the the equations of the above section, we compute the fields produces by a synchronized sheet of dipoles at pulsatance w.
We first examine the contribution to the fields at location (x,0,0) of an infinitesimal crown of radius R on the source sheet at x = 0. This involves only a constant value of the moments on the sheet (namely the value at time t-d/c where d2 = x2+R2 ).
[画像: Come back later, we're still working on this one... ]
Elements of diffraction theory:
2015年02月07日
by Svetoslav S. Ivanov (U. of Sofia).
Huygens-Fresnel-Kirchhoff
wave-front diffraction formulation. by Hal G. Kraus (March 1989).
Wikipedia :
Huygens-Fresnel principle
|
Kirchhoff-Fresnel diffraction formula
|
Kirchhoff integral theorem
A particularly simple case is that of a uniform sheet of magnetic dipoles, namely an open surface (not necessarily a planar one) where each element of surface dS carries a normal magnetic moment I dS proportional to it (the vector dS points northward and its magnitude is equal to the infinitesimal surface area it represents). The constant I (the density of magnetization per unit of surface area) is homogeneous to a current and, indeed, such a magnetic sheet generates everywhere exactly the same static magnetic induction as would a current I circulating around the oriented loop which borders the surface!
This can be established by triangulating the surface (the tinier the triangles, the better the approximation). The coarse triangulation at left is enough to visualize the situation: Each triangle carries a dipole moment equal to I times its (vectorial) area, which is exactly the same as the dipole moment of a triangular circuit with current I flowing through its 3 edges. Because all inner edges in this decomposition belong to two adjoining triangles, the total current flowing through each of them is zero ! Thus, no inner edge contributes anything to the magnetic field, which is thus the same as the field produced by a current I flowing through the loop bordering the triangulated surface. QED
Stacking vertically (with uniform spacing) such horizontal magnetic sheets, we see that a uniform distribution of magnetic dipole moments with density M inside an infinite vertical cylinder is magnetically equivalent to a long solenoid. Thus, there's no magnetic field outside the cylinder whereas, inside the cylinder, we have:
B = mo M
More generally, the magnetic field produced by any static distribution M of magnetic dipoles is the same as the field produced by a current density:
j = rot M
To establish this, notice that the above can be construed as the elementary cases (in integrated form) whose superpositions yield the general case.
Incidentally, this implies that two distributions of static magnetic dipoles which have the same rotational (curl) generate the same magnetic induction. Such distributions differ by the gradient of some scalar field, which is a very special type of "magnetization" that doesn't produce any magnetic induction !
Just like any static distribution of magnets can be simulated by a distribution of currents, it can be shown that any static distribution P of electric dipoles produces the same electric field as the following distribution of charges:
r = - div P
In particular, an infinite slab of uniformly distributed electric dipoles creates the same electric field as two parallel plates with opposing charges, namely:
E = - P / eo
The minus sign need not be surprising: In an horizontal slab of vertical dipoles, each dipole contributes only equatorially with a vector whose direction is opposite to that of the dipole itself ! What's less intuitive is that all the polar and equatorial contributions cancel each other perfectly outside the plane of the slab.
The term electret is used (by analogy with magnet) to denote something endowed with a remanent electric dipole moment ( ferroelectric substances are analogous to ferromagnetic ones). More commonly, a net density of electric dipole moments is induced by an external electric field in a dielectric material.
The word "electret" was coined in 1885 by Oliver Heaviside (1850-1925), to whom we owe so many other electromagnetic terms, including: conductance (Sept. 1885), permeability (Sept. 1885), inductance (Feb. 1886), impedance (July. 1886), admittance (Dec. 1887) and reluctance (May 1888). Heaviside also used the term "permittance" in June 1887, for what is now known as susceptance.
In full generality, a dynamic distribution of electric and magnetic dipoles would create the same electromagnetic fields as the following distribution of charges and currents. In the classical description of the electromagnetic properties of matter (by H.A. Lorentz) bound charges and currents are expressed this way.
j =
rot M +
¶P/¶t
r =
- div P
Conversely, any distribution of charges and currents can be shown to have the same electromagnetic field as some distribution of magnetic and dipole moments (which is not uniquely determined by the above equations).
Bluntly speaking, an observer in the midst of a static distribution of upward dipoles will receive upward field contributions from the dipoles of both polar regions (above and below herself) but downward field contributions from the equatorial region (in the vicinity of her own horizontal plane).
The polar and equatorial contributions of all the dipoles at a fixed distance cancel perfectly, since the integral of 3 (u.P) u - P vanishes. Only the axial component (parallel to P) could require a computation:
However, even that simple computation is made useless by a simple scaling argument: Since static dipolar fields vary inversely as the cube of distances, two spheres of different sizes carrying the same density of dipoles will create the same field at the center. Thus, the difference between two such spheres is a thick spherical shell which contributes absolutely nothing to the field at the center. QED
Although the singularity at the origin makes it impossible to obtain the field by integrating down to a zero distance shell-by-shell (you'd obtain different results for different shapes of the shells) we can use our previous physical results for uniform magnetic rods or electric slabs (containing the origin) and subtract from those quantities the convergent integrals corresponding to contributions that stay clear from the origin. With this kind of subtraction, we may obtain the field at the center of a uniform sphere of electric or magnetic dipoles...
At the center of a uniformly magnetized sphere, the magnetic induction is:
B = ( 2/3 ) mo M
At the center of a uniformly polarized sphere, the electric field is:
E = - ( 1/3 ) P / eo
A uniformly magnetized sphere by Richard Fitzpatrick | Weisstein
There is a nice paradox in the results of the previous sections for the fields created by uniform distributions of dipoles: The mathematical expression of the electric field (E) created by an electric dipole is exactly the same as the expression for the magnetic induction (B) of a magnetic dipole. Yet, we've just established that, in the main, uniformly distributed magnetic dipoles create a magnetic induction parallel to them, whereas uniformly distributed electric dipoles create an electric field antiparallel to them.
What's going on here?
Well, the mathematical expressions we obtained for ideal dipoles (zero-size dipoles specified only by their dipolar moments) are the limits of the fields created by actual finite dipoles (either tiny loops of current or tight pairs of opposite charges). No paradox occurs with uniform distributions of finite dipoles which are formed in a physically sensible way:
You can easily stack magnetic dipoles (think of little magnets) to form a long bar magnet equivalent to a solenoid. The magnetic induction inside a long solenoid is simply given by Ampère's law and there's no induction outside of them which would hinder a side-by-side assembly to create essentially a uniform distribution of magnetic dipoles throughout a larger volume of space. To actually feel with your muscles the problems that would occur if you tried to assemble magnetic dipoles the other way (stacking slabs, instead of bunching rods) just try to assemble two short bar magnets sideways (e.g., two flat disks with their north sides up).
The opposite is true for electric dipoles which can easily form thin polarized membranes when aggregated side by side (such things are important in biology). Those flat membranes are stacked effortlessly into thick slabs because there's virtually no electric field outside of them (except near the borders). Don't even think that elementary electric dipoles would align into a rod like little magnets do.
Actually, uniform distribution of "zero-size" dipoles (with the fields given above in the magnetic and electric cases) yield indeterminate fields because of divergences at short distances. To settle the issue, you must go back to the physics: Although the magnetic field of a magnetic dipole has the same expression as the electric field of an electric dipole, the respective fields still retain their particular nature. Thus, we can only use Ampère's law to integrate a magnetic field and Gauss's law to integrate an electric field. Those two laws are totally different and so are the orientations of the fields they yield in uniform distributions of their respective types of dipoles.
I find it absolutely wonderful that the distinct characteristics of the electric and magnetic fields translate so directly into this puzzling reversal of sign...
