Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These areas of physics are the basis for all electric, optical and radio technologies like power generation, electric motors, wireless communication, cameras, televisions, computers etc. Maxwell's equations describe how electric and magnetic fields are generated by charges, currents and changes of each other. One important consequence of the equations is that fluctuating electric and magnetic fields can propagate at the speed of light, and this electromagnetic radiation manifests itself in manifold ways from radio waves to light and X- or γ-rays. The equations are named after the physicist and mathematician James Clerk Maxwell, who published an early form of the equations between 1861 and 1862, and first proposed that light is an electromagnetic phenomenon.
The equations have two major variants. The "microscopic" set of Maxwell's equations uses total charge and total current, including the complicated charges and currents in materials at the atomic scale. The microscopic equations have universal applicability but may be infeasible to calculate with. The "macroscopic" set of Maxwell's equations defines two new auxiliary fields that describe the large-scale behaviour of matter without having to consider atomic scale details, but their use requires experimentally determining parameters that describe the electromagnetic response of materials phenomenologically.
The term "Maxwell's equations" is often used for equivalent alternative formulations. Versions of Maxwell's equations based on the electric and magnetic potentials are preferred for explicitly solving the equations as a boundary value problem, analytical mechanics, or for use in quantum mechanics. The space-time formulations (i.e. on space-time rather than space and time separately), are commonly used in high energy and gravitational physics because they are manifestly compatible with special and general relativity.[note 1] In fact, historically, Einstein developed special and general relativity to accommodate the absolute speed of light that drops out of the Maxwell equations with the principle that only relative movement has physical consequences.
Since the mid-20th century, it has been understood that Maxwell's equations are not exact but are a classical field theory approximation to the more accurate and fundamental theory of quantum electrodynamics. In many situations, though, deviations from Maxwell's equations are immeasurably small. Exceptions include nonclassical light, photon-photon scattering, quantum optics, and many other phenomena related to photons or virtual photons.
- 1 Formulation in terms of electric and magnetic fields (microscopic or in vacuum version)
- 2 Relationship between differential and integral formulations
- 3 Conceptual descriptions
- 4 Charge Conservation
- 5 Vacuum equations, electromagnetic waves and speed of light
- 6 Macroscopic formulation
- 7 Alternative formulations
- 8 Solutions
- 9 Limitations of the Maxwell equations as a theory of electromagnetism
- 10 Variations
- 11 See also
- 12 Notes
- 13 References
- 14 Historical publications
- 15 External links
Formulation in terms of electric and magnetic fields (microscopic or in vacuum version)
In the electric and magnetic field formulation there are four equations. Two of them describe how the fields vary in space due to sources, if any; electric fields emanating from electric charges in Gauss's law, and magnetic fields as closed field lines not due to magnetic monopoles in Gauss's law for magnetism. The other two describe how the fields "circulate" around their respective sources; the magnetic field "circulates" around electric currents and time varying electric fields in Ampère's law with Maxwell's addition, while the electric field "circulates" around time varying magnetic fields in Faraday's law. A separate law of nature, the Lorentz force law, describes how the electric and magnetic field act on charged particles and currents. A version of this law was included in the original equations by Maxwell but, by convention, is no longer.
The precise formulation of Maxwell's equations depends on the precise definition of the quantities involved. Conventions differ with the unit systems, because various definitions and dimensions are changed by absorbing dimensionful factors like the speed of light c. This makes constants come out differently. The most common form is based on conventions used when quantities measured using SI units, but other commonly used conventions are used with other units including Gaussian units based on the cgs system, Lorentz–Heaviside units (used mainly in particle physics), and Planck units (used in theoretical physics).
The vector calculus formulation below has become standard. It is mathematically much more convenient than Maxwell's original 20 equations and is due to Oliver Heaviside  The differential and integral equations formulations are mathematically equivalent and are both useful. The integral formulation relates fields within a region of space to fields on the boundary and can often be used to simplify and directly calculate fields from symmetric distributions of charges and currents. On the other hand, the differential equations are purely local and are a more natural starting point for calculating the fields in more complicated (less symmetric) situations, for example using finite element analysis.
Formulation in SI units convention
Name Integral equations Differential equations Meaning Gauss's law The electric flux leaving a volume is proportional to the charge inside. Gauss's law for magnetism There are no magnetic monopoles; the total magnetic flux through a closed surface is zero. Maxwell–Faraday equation (Faraday's law of induction) The voltage induced in a closed circuit is proportional to the rate of change of the magnetic flux it encloses. Ampère's circuital law (with Maxwell's addition) The magnetic field induced around a closed loop is proportional to the electric current plus displacement current (rate of change of electric field) it encloses.
Formulation in Gaussian units convention
Gaussian units are a popular system of units, that are part of the centimetre–gram–second system of units (cgs). When using cgs units it is conventional to use a slightly different definition of electric field Ecgs = c−1 ESI. This implies that the modified electric and magnetic field have the same units (in the SI convention this is not the case making dimensional analysis of the equations different: e.g. for an electromagnetic wave in vacuum , ). The CGS system uses a unit of charge defined in such a way that the permittivity of the vacuum ε0 = 1/, hence μ0 = 4π/. These units are sometimes preferred over SI units in the context of special relativity,:vii since when using them, the components of the electromagnetic tensor, the Lorentz covariant object describing the electromagnetic field, have the same unit without constant factors. Using these different conventions, the Maxwell equations become:
|Name||Integral equations||Differential equations||Meaning|
|Gauss's law||The electric flux leaving a volume is proportional to the charge inside.|
|Gauss's law for magnetism||There are no magnetic monopoles; the total magnetic flux through a closed surface is zero.|
|Maxwell–Faraday equation (Faraday's law of induction)||The voltage induced in a closed circuit is proportional to the rate of change of the magnetic flux it encloses.|
|Ampère's law (with Maxwell's extension)||The magnetic field induced around a closed loop is proportional to the electric current plus displacement current (rate of change of electric field) it encloses.|
Key to the notation
- the electric charge density (charge per unit volume), ρ, and
- the electric current density (current per unit area), J.
The universal constants appearing in the equations are
In the differential equations,
- the nabla symbol, ∇, denotes the three-dimensional gradient operator,
- the ∇⋅ symbol denotes the divergence operator,
- the ∇× symbol denotes the curl operator.
In the integral equations,
- Ω is any fixed volume with closed boundary surface ∂Ω, and
- Σ is any fixed surface with closed boundary curve ∂Σ,
Here a fixed volume or surface means that it does not change over time. The equations are correct, complete and a little easier to interpret with time-independent surfaces. However, since the surface is time-independent, we can bring the differentiation under the integral sign in Faraday's law:
The Maxwell's equations can be formulated with possibly time dependent surfaces and volumes by substituting the lefthand side with the righthand side in the integral equation version of the Maxwell equations.
- is a surface integral over the surface ∂Ω, (the loop indicates that the boundary surface is closed)
- is a volume integral over the volume Ω,
- is a surface integral over the surface Σ,
- is a line integral around the curve ∂Σ (the loop indicates that the boundary curve is closed).
- The volume integral over Ω of the total charge density ρ, is the total electric charge Q contained in Ω:
- where dV is the volume element.
- The net electric current I is the surface integral of the electric current density J passing through a fixed surface, Σ:
- where dS denotes the vector element of surface area, S, normal to surface, Σ. (Vector area is also denoted by A rather than S, but this conflicts with the magnetic potential, a separate vector field).
Relationship between differential and integral formulations
Flux and divergence
where the last equality uses the Gauss divergence theorem. Using the integral version of Gauss's equation we can rewrite this to
Since Ω can be chosen arbitrary, e.g. as an arbitrary small ball with arbitrary centre, this implies that the integrand must be zero, which is the differential equations formulation of Gauss equation up to a trivial rearrangement. Gauss's law for magnetism in differential equations form follows likewise from the integral form by rewriting the magnetic flux
Circulation and curl
where we used the Kelvin-Stokes theorem. Using the modified Ampere law in integral form and the writing the time derivative of the flux as the surface integral of the partial time derivative of E we conclude that
Since Σ can be chosen arbitrary, e.g. as an arbitrary small, arbitrary oriented, and arbitrary centred disk, we conclude that the integrand must be zero. This is Ampere's modified law in differential equations form up to a trivial rearrangement. Likewise, the Faraday law in differential equations form follows from rewriting the integral form using the Kelvin-Stokes theorem.
The line integrals and curls are analogous to quantities in classical fluid dynamics: the circulation of a fluid is the line integral of the fluid's flow velocity field around a closed loop, and the vorticity of the fluid is the curl of the velocity field.
Gauss's law describes the relationship between a static electric field and the electric charges that cause it: The static electric field points away from positive charges and towards negative charges. In the field line description, electric field lines begin only at positive electric charges and end only at negative electric charges. 'Counting' the number of field lines passing through a closed surface, therefore, yields the total charge (including bound charge due to polarization of material) enclosed by that surface divided by dielectricity of free space (the vacuum permittivity). More technically, it relates the electric flux through any hypothetical closed "Gaussian surface" to the enclosed electric charge.
Gauss's law for magnetism
Gauss's law for magnetism states that there are no "magnetic charges" (also called magnetic monopoles), analogous to electric charges. Instead, the magnetic field due to materials is generated by a configuration called a dipole. Magnetic dipoles are best represented as loops of current but resemble positive and negative 'magnetic charges', inseparably bound together, having no net 'magnetic charge'. In terms of field lines, this equation states that magnetic field lines neither begin nor end but make loops or extend to infinity and back. In other words, any magnetic field line that enters a given volume must somewhere exit that volume. Equivalent technical statements are that the sum total magnetic flux through any Gaussian surface is zero, or that the magnetic field is a solenoidal vector field.
The Maxwell-Faraday's equation version of Faraday's law describes how a time varying magnetic field creates ("induces") an electric field. This dynamically induced electric field has closed field lines just as the magnetic field, if not superposed by a static (charge induced) electric field. This aspect of electromagnetic induction is the operating principle behind many electric generators: for example, a rotating bar magnet creates a changing magnetic field, which in turn generates an electric field in a nearby wire.
Ampère's law with Maxwell's addition
Ampère's law with Maxwell's addition states that magnetic fields can be generated in two ways: by electric current (this was the original "Ampère's law") and by changing electric fields (this was "Maxwell's addition").
Maxwell's addition to Ampère's law is particularly important: it makes the set of equations mathematically consistent for non static fields, without changing the laws of Ampere and Gauss for static fields. However, as a consequence, it predicts that a changing magnetic field induces an electric field and vice versa. Therefore, these equations allow self-sustaining "electromagnetic waves" to travel through empty space (see electromagnetic wave equation).
The speed calculated for electromagnetic waves, which could be predicted from experiments on charges and currents,[note 2] exactly matches the speed of light; indeed, light is one form of electromagnetic radiation (as are X-rays, radio waves, and others). Maxwell understood the connection between electromagnetic waves and light in 1861, thereby unifying the theories of electromagnetism and optics.
The lefthand side of the modified Ampere's law has zero divergence by the div-curl-identity. Therefore the right handside, Gauss's law and interchanging derivatives give
By the Gauss divergence theorem that means that the rate of change of the charge in a fixed volume equals the current flowing in or out of the boundary
In particular, in an isolated system the total charge is conserved.
Vacuum equations, electromagnetic waves and speed of light
In a region with no charges (ρ = 0) and no currents (J = 0), such as in a vacuum, Maxwell's equations reduce to:
which is usually[note 3] less than c.
In addition, E and B are mutually perpendicular to each other and the direction of wave propagation, and are in phase with each other. A sinusoidal plane wave is one special solution of these equations. Maxwell's equations explain how these waves can physically propagate through space. The changing magnetic field creates a changing electric field through Faraday's law. In turn, that electric field creates a changing magnetic field through Maxwell's addition to Ampère's law. This perpetual cycle allows these waves, now known as electromagnetic radiation, to move through space at velocity, c.
The microscopic variant of Maxwell's equation is the version given above. It expresses the electric E field and the magnetic B field in terms of the total charge and total current present, including the charges and currents at the atomic level. The "microscopic" form is sometimes called the "general" form of Maxwell's equations. The macroscopic variant of Maxwell's equation is equally general, however, with the difference being one of bookkeeping.
The "microscopic" variant is sometimes called "Maxwell's equations in a vacuum". This refers to the fact that the material medium is not built into the structure of the equation; it does not mean that space is empty of charge or current. They are also known as the "Maxwell-Lorentz equations". Lorentz tried to use these equations to predict the macroscopic properties of bulk matter from the physical behavior of its microscopic constituents.:5
"Maxwell's macroscopic equations", also known as Maxwell's equations in matter, are more similar to those that Maxwell introduced himself.
Name Integral equations (SI convention) Differential equations (SI convention) Differential equations (Gaussian convention) Gauss's law Gauss's law for magnetism Maxwell–Faraday equation (Faraday's law of induction) Ampère's circuital law (with Maxwell's addition)
Unlike the "microscopic" equations, the "macroscopic" equations separate out the bound charge Qb and bound current Ib to obtain equations that depend only on the free charges Qf and currents If. This factorization can be made by splitting the total electric charge and current as follows:
Correspondingly, the total current density J splits into free Jf and bound Jb components, and similarly the total charge density ρ splits into free ρf and bound ρb parts.
The cost of this factorization is that additional fields, the displacement field D and the magnetizing field H, are defined and need to be determined. Phenomenological constituent equations relate the additional fields to the electric field E and the magnetic B-field, often through a simple linear relation.
For a detailed description of the differences between the microscopic (total charge and current including material contributes or in air/vacuum)[note 4] and macroscopic (free charge and current; practical to use on materials) variants of Maxwell's equations, see below.
Bound charge and current
When an electric field is applied to a dielectric material its molecules respond by forming microscopic electric dipoles – their atomic nuclei move a tiny distance in the direction of the field, while their electrons move a tiny distance in the opposite direction. This produces a macroscopic bound charge in the material even though all of the charges involved are bound to individual molecules. For example, if every molecule responds the same, similar to that shown in the figure, these tiny movements of charge combine to produce a layer of positive bound charge on one side of the material and a layer of negative charge on the other side. The bound charge is most conveniently described in terms of the polarization P of the material, its dipole moment per unit volume. If P is uniform, a macroscopic separation of charge is produced only at the surfaces where P enters and leaves the material. For non-uniform P, a charge is also produced in the bulk.
Somewhat similarly, in all materials the constituent atoms exhibit magnetic moments that are intrinsically linked to the angular momentum of the components of the atoms, most notably their electrons. The connection to angular momentum suggests the picture of an assembly of microscopic current loops. Outside the material, an assembly of such microscopic current loops is not different from a macroscopic current circulating around the material's surface, despite the fact that no individual charge is traveling a large distance. These bound currents can be described using the magnetization M.
The very complicated and granular bound charges and bound currents, therefore, can be represented on the macroscopic scale in terms of P and M which average these charges and currents on a sufficiently large scale so as not to see the granularity of individual atoms, but also sufficiently small that they vary with location in the material. As such, Maxwell's macroscopic equations ignore many details on a fine scale that can be unimportant to understanding matters on a gross scale by calculating fields that are averaged over some suitable volume.
Auxiliary fields, polarization and magnetization
The definitions (not constitutive relations) of the auxiliary fields are:
where P is the polarization field and M is the magnetization field which are defined in terms of microscopic bound charges and bound currents respectively. The macroscopic bound charge density ρb and bound current density Jb in terms of polarization P and magnetization M are then defined as
If we define the total, bound, and free charge and current density by
and use the defining relations above to eliminate D, and H, the "macroscopic" Maxwell's equations reproduce the "microscopic" equations.
In order to apply 'Maxwell's macroscopic equations', it is necessary to specify the relations between displacement field D and the electric field E, as well as the magnetizing field H and the magnetic field B. Equivalently, we have to specify the dependence of the polarisation P (hence the bound charge) and the magnetisation M (hence the bound current) on the applied electric and magnetic field. The equations specifying this response are called constitutive relations. For real-world materials, the constitutive relations are rarely simple, except approximately, and usually determined by experiment. See the main article on constitutive relations for a fuller description.:44–45
For materials without polarisation and magnetisation, the constitutive relations are (by definition):2
An alternative viewpoint on the microscopic equations is that they are the macroscopic equations together with the statement that vacuum behaves like a perfect linear "material" without additional polarisation and magnetisation. More generally, for linear materials the constitutive relations are:44–45
where ε is the permittivity and μ the permeability of the material. For the displacement field D the linear approximation is usually excellent because for all but the most extreme electric fields or temperatures obtainable in the laboratory (high power pulsed lasers) the interatomic electric fields of materials of the order of 1011 V/m are much higher than the external field. For the magnetizing field , however, the linear approximation can break down in common materials like iron leading to phenomena like hysteresis. Even the linear case can have various complications, however.
- For homogeneous materials, ε and μ are constant throughout the material, while for inhomogeneous materials they depend on location within the material (and perhaps time).:463
- For isotropic materials, ε and μ are scalars, while for anisotropic materials (e.g. due to crystal structure) they are tensors.:421:463
- Materials are generally dispersive, so ε and μ depend on the frequency of any incident EM waves.:625:397
Even more generally, in the case of non-linear materials (see for example nonlinear optics), D and P are not necessarily proportional to E, similarly H or M is not necessarily proportional to B. In general D and H depend on both E and B, on location and time, and possibly other physical quantities.
In applications one also has to describe how the free currents and charge density behave in terms of E and B possibly coupled to other physical quantities like pressure, and the mass, number density, and velocity of charge-carrying particles. E.g., the original equations given by Maxwell (see History of Maxwell's equations) included Ohms law in the form
Following is a summary of some of the numerous other ways to write the microscopic Maxwell's equations, showing they can be formulated using different points of view and mathematical formalisms that describe the same physics. Often, they are also called the Maxwell equations. The direct space–time formulations make manifest that the Maxwell equations are relativistically invariant (in fact studying the hidden symmetry of the vector calculus formulation was a major source of inspiration for relativity theory). In addition, the formulation using potentials was originally introduced as a convenient way to solve the equations but with all the observable physics contained in the fields. The potentials play a central role in quantum mechanics, however, and act quantum mechanically with observable consequences even when the fields vanish (Aharonov–Bohm effect). See the main articles for the details of each formulation. SI units are used throughout.
Formalism Formulation Homogeneous equations Inhomogeneous equations Vector calculus Fields
3D Euclidean space + time
Potentials (any gauge)
3D Euclidean space + time
Potentials (Lorenz gauge)
3D Euclidean space + time
Tensor calculus Fields Potentials (any gauge) Potentials (Lorenz gauge) Fields
Potentials (any gauge)
Any space–time (with topological restrictions)
Potentials (Lorenz gauge)
Any space–time (with topological restrictions)
Differential forms Fields
Potentials (any gauge)
Any space–time (with topological restrictions)
Potentials (Lorenz gauge)
Any space–time (with topological restrictions)
- In the vector formulation on Euclidean space + time, φ is the electrical potential, A is the vector potential and ◻ = 1/ ∂2/ − ∇2 is the d'Alembert operator.
- In the tensor calculus formulation, the electromagnetic tensor Fαβ is an antisymmetric covariant rank 2 tensor; the four-potential, Aα, is a covariant vector; the current, Jα, is a vector; the square brackets, [ ], denote antisymmetrization of indices; ∂α is the derivative with respect to the coordinate, xα. In Minkowski space coordinates are chosen with respect to an inertial frame; (xα) = (ct,x,y,z), so that the metric tensor used to raise and lower indices is ηαβ = diag(1,−1,−1,−1). The d'Alembert operator on Minkowski space is ◻ = ∂α∂α as in the vector formulation. In general spacetimes, the coordinate system xα is arbitrary, the covariant derivative ∇α, the Ricci tensor, Rαβ and raising and lowering of indices are defined by the Lorentzian metric, gαβ and the d'Alembert operator is defined as ◻ = ∇α∇α. The topological restriction is that the second real cohomology group of the space vanishes (see the differential form formulation for an explanation). Note that this is violated for Minkowski space with a line removed, which can model a (flat) space-time with a point-like monopole on the complement of the line.
- In the differential form formulation on arbitrary space times, F = Fαβdxα ∧ dxβ is the electromagnetic tensor considered as a 2-form, A = Aαdxα is the potential 1-form, J is the current 3-form, d is the exterior derivative, and is the Hodge star on forms defined by the Lorentzian metric of space–time. Note that in the special case of 2-forms such as F, the Hodge star only depends on the metric up to a local scale . This means that, as formulated, the differential form field equations are conformally invariant, but the Lorenz gauge condition breaks conformal invariance. The operator is the d'Alembert–Laplace–Beltrami operator on 1-forms on an arbitrary Lorentzian space–time. The topological condition is again that the second real cohomology group is trivial. By the isomorphism with the second de Rham cohomology this condition means that every closed 2 form is exact.
Maxwell's equations are partial differential equations that relate the electric and magnetic fields to each other and to the electric charges and currents. Often, the charges and currents are themselves dependent on the electric and magnetic fields via the Lorentz force equation and the constitutive relations. These all form a set of coupled partial differential equations, which are often very difficult to solve. In fact, the solutions of these equations encompass all the diverse phenomena in the entire field of classical electromagnetism. A thorough discussion is far beyond the scope of the article, but some general notes follow.
Like any differential equation, boundary conditions and initial conditions are necessary for a unique solution. For example, even with no charges and no currents anywhere in spacetime, many solutions to Maxwell's equations are possible, not just the obvious solution E = B = 0. Another solution is E = constant, B = constant, while yet other solutions have electromagnetic waves filling spacetime. In some cases, Maxwell's equations are solved through infinite space, and boundary conditions are given as asymptotic limits at infinity. In other cases, Maxwell's equations are solved in just a finite region of space, with appropriate boundary conditions on that region: For example, the boundary could be an artificial absorbing boundary representing the rest of the universe, or periodic boundary conditions, or (as with a waveguide or cavity resonator) the boundary conditions may describe the walls that isolate a small region from the outside world.
Jefimenko's equations (or the closely related Liénard–Wiechert potentials) are the explicit solution to Maxwell's equations for the electric and magnetic fields created by any given distribution of charges and currents. It assumes specific initial conditions to obtain the so-called "retarded solution", where the only fields present are the ones created by the charges. Jefimenko's equations are not so helpful in situations when the charges and currents are themselves affected by the fields they create.
Numerical methods for differential equations can be used to approximately solve Maxwell's equations when an exact solution is impossible. These methods usually require a computer, and include the finite element method and finite-difference time-domain method. For more details, see Computational electromagnetics.
Maxwell's equations seem overdetermined, in that they involve six unknowns (the three components of E and B) but eight equations (one for each of the two Gauss's laws, three vector components each for Faraday's and Ampere's laws). (The currents and charges are not unknowns, being freely specifiable subject to charge conservation.) This is related to a certain limited kind of redundancy in Maxwell's equations: It can be proven that any system satisfying Faraday's law and Ampere's law automatically also satisfies the two Gauss's laws, as long as the system's initial condition does. This explanation was first introduced by Julius Adams Stratton in 1941. Although it is possible to simply ignore the two Gauss's laws in a numerical algorithm (apart from the initial conditions), the imperfect precision of the calculations can lead to ever-increasing violations of those laws. By introducing dummy variables characterizing these violations, the four equations become not overdetermined after all. The resulting formulation can lead to more accurate algorithms that take all four laws into account.
Limitations of the Maxwell equations as a theory of electromagnetism
While Maxwell's equations (along with the rest of classical electromagnetism) are extraordinarily successful at explaining and predicting a variety of phenomena, they are not exact, but approximations. In some special situations, they can be noticeably inaccurate. Examples include extremely strong fields (see Euler–Heisenberg Lagrangian) and extremely short distances (see vacuum polarization). Moreover, various phenomena occur in the world even though Maxwell's equations predict them to be impossible, such as "nonclassical light" and quantum entanglement of electromagnetic fields (see quantum optics). Finally, any phenomenon involving individual photons, such as the photoelectric effect, Planck's law, the Duane–Hunt law, single-photon light detectors, etc., would be difficult or impossible to explain if Maxwell's equations were exactly true, as Maxwell's equations do not involve photons. For the most accurate predictions in all situations, Maxwell's equations have been superseded by quantum electrodynamics.
Popular variations on the Maxwell equations as a classical theory of electromagnetic fields are relatively scarce because the standard equations have stood the test of time remarkably well.
Maxwell's equations posit that there is electric charge, but no magnetic charge (also called magnetic monopoles), in the universe. Indeed, magnetic charge has never been observed (despite extensive searches)[note 5] and may not exist. If they did exist, both Gauss's law for magnetism and Faraday's law would need to be modified, and the resulting four equations would be fully symmetric under the interchange of electric and magnetic fields.:273–275
- Maxwell's equations in any form are compatible with relativity. These space-time formulations, though, make that compatibility more readily apparent by revealing that the electric and magnetic fields blend into a single tensor, and that their distinction depends on the movement of the observer and the corresponding observer dependent notion of time.
- The quantity we would now call 1⁄√, with units of velocity, was directly measured before Maxwell's equations, in an 1855 experiment by Wilhelm Eduard Weber and Rudolf Kohlrausch. They charged a leyden jar (a kind of capacitor), and measured the electrostatic force associated with the potential; then, they discharged it while measuring the magnetic force from the current in the discharge wire. Their result was ×108 m/s, remarkably close to the speed of light. See 3.107The story of electrical and magnetic measurements: from 500 B.C. to the 1940s, by Joseph F. Keithley, p115
- There are cases (anomalous dispersion) where the phase velocity can exceed c, but the "signal velocity" will still be < c
- In some books—e.g., in U. Krey and A. Owen's Basic Theoretical Physics (Springer 2007)—the term effective charge is used instead of total charge, while free charge is simply called charge.
- See magnetic monopole for a discussion of monopole searches. Recently, scientists have discovered that some types of condensed matter, including spin ice and topological insulators, which display emergent behavior resembling magnetic monopoles. (See  and .) Although these were described in the popular press as the long-awaited discovery of magnetic monopoles, they are only superficially related. A "true" magnetic monopole is something where ∇ ⋅ B ≠ 0, whereas in these condensed-matter systems, ∇ ⋅ B = 0 while only ∇ ⋅ H ≠ 0.
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- J Rosen. "Redundancy and superfluity for electromagnetic fields and potentials". American Journal of Physics. 48 (12): 1071. Bibcode:1980AmJPh..48.1071R. doi:10.1119/1.12289.
- J.A. Stratton (1941). Electromagnetic Theory. McGraw-Hill Book Company. pp. 1–6.
- B Jiang & J Wu & L.A. Povinelli (1996). "The Origin of Spurious Solutions in Computational Electromagnetics". Journal of Computational Physics. 125 (1): 104. Bibcode:1996JCoPh.125..104J. doi:10.1006/jcph.1996.0082.
- Further reading can be found in list of textbooks in electromagnetism
- On Faraday's Lines of Force – 1855/56 Maxwell's first paper (Part 1 & 2) – Compiled by Blaze Labs Research (PDF)
- On Physical Lines of Force – 1861 Maxwell's 1861 paper describing magnetic lines of Force – Predecessor to 1873 Treatise
- James Clerk Maxwell, "A Dynamical Theory of the Electromagnetic Field", Philosophical Transactions of the Royal Society of London 155, 459–512 (1865). (This article accompanied a December 8, 1864 presentation by Maxwell to the Royal Society.)
- A Dynamical Theory Of The Electromagnetic Field – 1865 Maxwell's 1865 paper describing his 20 Equations, link from Google Books.
- J. Clerk Maxwell (1873) A Treatise on Electricity and Magnetism
- Maxwell, J.C., A Treatise on Electricity And Magnetism – Volume 1 – 1873 – Posner Memorial Collection – Carnegie Mellon University
- Maxwell, J.C., A Treatise on Electricity And Magnetism – Volume 2 – 1873 – Posner Memorial Collection – Carnegie Mellon University
The developments before relativity:
- Joseph Larmor (1897) "On a dynamical theory of the electric and luminiferous medium", Phil. Trans. Roy. Soc. 190, 205–300 (third and last in a series of papers with the same name).
- Hendrik Lorentz (1899) "Simplified theory of electrical and optical phenomena in moving systems", Proc. Acad. Science Amsterdam, I, 427–43.
- Hendrik Lorentz (1904) "Electromagnetic phenomena in a system moving with any velocity less than that of light", Proc. Acad. Science Amsterdam, IV, 669–78.
- Henri Poincaré (1900) "La théorie de Lorentz et le Principe de Réaction", Archives Néerlandaises, V, 253–78.
- Henri Poincaré (1902) La Science et l'Hypothèse
- Henri Poincaré (1905) "Sur la dynamique de l'électron", Comptes rendus de l'Académie des Sciences, 140, 1504–8.
- Catt, Walton and Davidson. "The History of Displacement Current". Wireless World, March 1979.
- Hazewinkel, Michiel, ed. (2001), "Maxwell equations", Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
- maxwells-equations.com — An intuitive tutorial of Maxwell's equations.
- Mathematical aspects of Maxwell's equation are discussed on the Dispersive PDE Wiki.
- Electromagnetism, B. Crowell, Fullerton College
- Lecture series: Relativity and electromagnetism, R. Fitzpatrick, University of Texas at Austin
- Electromagnetic waves from Maxwell's equations on Project PHYSNET.
- MIT Video Lecture Series (36 x 50 minute lectures) (in .mp4 format) – Electricity and Magnetism Taught by Professor Walter Lewin.