In classical electromagnetism, Maxwell's equations are a set of four partial differential equations that describe the properties of the electric and magnetic fields and relate them to their sources, charge density and current density. These equations are used to show that light is an electromagnetic wave. Individually, the equations are known as Gauss' law, Gauss' law for magnetism, Faraday's law of induction, and Ampère's law with Maxwell's correction.
These four equations, together with the Lorentz force law (derived by Maxwell), are the complete set of laws of classical electromagnetism.
This section will conceptually describe each of the four Maxwell's equations, and also how they link together to explain the origin of electromagnetic radiation such as light. The exact equations can be found in the subsequent section.
Maxwell's correction to Ampère's law was particularly important: With its inclusion, the laws state that a changing electric field could produce a magnetic field, and vice-versa. It follows that, even with no electric charges or currents present, it's possible to have stable, self-perpetuating waves of oscillating electric and magnetic fields, with each field driving the other. (These waves are called electromagnetic radiation.) The four Maxwell's equations describe these waves quantitatively, and moreover predict that the waves should have a particular, universal speed, which can be simply calculated in terms of two easily-measurable physical constants (called the electric constant and magnetic constant).
The speed calculated for electromagnetic radiation exactly matches the speed of light; indeed, light is one form of electromagnetic radiation (as are X-rays, radio waves, and others). In this way, Maxwell unified the hitherto separate fields of electromagnetism and optics.
The equations in this section are given in SI units. Unlike the equations of mechanics (for example), Maxwell's equations are not unchanged in other unit systems. Though the general form remains the same, various definitions get changed and different constants appear at different places. Other than SI (used in engineering), the units commonly used are Gaussian units (based on the cgs system and considered to have some theoretical advantages over SI), Lorentz-Heaviside units (used mainly in particle physics) and Planck units (used in theoretical physics). See below for CGS-Gaussian units.
Two equivalent, general formulations of Maxwell's equations follow. The first separates bound charge and bound current (which arise in the context of dielectric and/or magnetized materials) from free charge and free current (the more conventional type of charge and current). This separation is useful for calculations involving dielectric or magnetized materials. The second formulation treats all charge equally, combining free and bound charge into total charge (and likewise with current). This is the more fundamental or microscopic point of view, and is particularly useful when no dielectric or magnetic material is present. More detail, and a proof that these two formulations are mathematically equivalent, are given in section 3.
Symbols in bold represent vector quantities, whereas symbols in italics represent scalar quantities. The definitions of terms used in the two tables of equations are given in another table immediately following.
|Name||Differential form||Integral form|
|Gauss' law for magnetism:|
|Maxwell-Faraday equation(Faraday's law of induction):|
| Ampère's circuital law|
(with Maxwell's correction):
|Name||Differential form||Integral form|
|Gauss' law for magnetism:|
|Maxwell-Faraday equation(Faraday's law of induction):|
| Ampère's circuital law|
(with Maxwell's correction):
The following table provides the meaning of each symbol and the SI unit of measure:
|Symbol||Meaning (first term is the most common)||SI Unit of Measure|
|the divergence operator||per meter (factor contributed by applying either operator)|
|the curl operator|
|partial derivative with respect to time||per second (factor contributed by applying the operator)|
|electric field|| volt per meter or, equivalently, |
newton per coulomb
| magnetic field |
also called the magnetic induction
also called the magnetic field density
also called the magnetic flux density
| tesla, or equivalently, |
weber per square meter
volt•second per square meter
| electric displacement field |
also called the electric flux density
| coulombs per square meter or, equivalently, |
newton per volt-meter
| magnetizing field |
also called auxiliary magnetic field
also called magnetic field intensity
also called magnetic field
|ampere per meter|
|permittivity of free space, officially the electric constant, a universal constant||farads per meter|
|permeability of free space, officially the magnetic constant, a universal constant||henries per meter, or newtons per ampere squared|
|free charge density (not including bound charge)||coulomb per cubic meter|
|total charge density (including both free and bound charge)||coulomb per cubic meter|
|the flux of the electric field through any closed gaussian surface S||joule-meter per coulomb|
|net unbalanced free electric charge enclosed by theGaussian surface S (not including bound charge)||coulombs|
|net unbalanced electric charge enclosed by the Gaussiansurface S (including both free and bound charge)||coulombs|
|the flux of the magnetic field through any closed surface S||tesla meter-squared or weber|
|line integral of the electric field along the boundary ∂S(therefore necessarily a closed curve) of the surface S||joule per coulomb|
|magnetic flux through any surface S (not necessarily closed)|| weber or equivalently, |
|free current density (not including bound current)||ampere per square meter|
|total current density (including both free and bound current)||ampere per square meter|
|line integral of the magnetic field over the closed boundary ∂S of the surface S||tesla-meter|
|net free electrical current passing throughthe surface S (not including bound current)||amperes|
|net electrical current passing through thesurface S (including both free and bound current)||amperes|
|electric flux through any surface S, not necessarily closed||joule-meter per coulomb|
|flux of electric displacement field through any surface S, not necessarily closed||coulombs|
| differential vector element of surface area A, with infinitesimally |
small magnitude and direction normal to surface S
|differential vector element of path length tangential to the path/curve||meters|
Maxwell's equations are generally applied to macroscopic averages of the fields, which vary wildly on a microscopic scale in the vicinity of individual atoms (where they undergo quantum mechanical effects as well). It is only in this averaged sense that one can define quantities such as the permittivity and permeability of a material. At the microscopic level, Maxwell's equations, ignoring quantum effects, describe fields, charges and currents in free space — but at this level of detail one must include all charges, even those at an atomic level, generally an intractable problem.
Although James Clerk Maxwell was not the originator of these equations, he nevertheless derived three of them again independently in conjunction with his molecular vortex model of Faraday's "lines of force", along with the full version of Faraday's law of induction. In doing so he made an important addition to Ampère's circuital law.
Controversy has always surrounded the term Maxwell's equations concerning the extent to which Maxwell himself was involved in these equations. The term Maxwell's equations nowadays applies to a set of four equations that were grouped together as a distinct set in 1884 by Oliver Heaviside, in conjunction with Willard Gibbs.
The importance of Maxwell's role in these equations lies in the correction he made to Ampère's circuital law in his 1861 paper On Physical Lines of Force.pdf. He added the displacement current term to Ampère's circuital law and this enabled him to derive the electromagnetic wave equation in his later 1865 paper A Dynamical Theory of the Electromagnetic Field and demonstrate the fact that light is an electromagnetic wave. This fact was then later confirmed experimentally by Heinrich Hertz in 1887.
The reason that these equations are called Maxwell's equations is disputed. Some say that these equations were originally called the Hertz-Heaviside equations but that Einstein for whatever reason later referred to them as the Maxwell-Hertz equations. see pages 110-112 of Nahin's book
These equations are based on the works of James Clerk Maxwell, and Heaviside made no secret of the fact that he was working from Maxwell's papers. Heaviside aimed to produce a symmetrical set of equations that were crucial as regards deriving the telegrapher's equations. The net result was a set of four equations, three of which had appeared in substance throughout Maxwell's previous papers, in particular Maxwell's 1861 paper On Physical Lines of Force.pdf and 1865 paper A Dynamical Theory of the Electromagnetic Field. The fourth was a partial time derivative version of Faraday's law of induction that doesn't include motionally induced EMF.
Of Heaviside's equations, the most important in deriving the telegrapher's equations was the version of Ampère's circuital law that had been amended by Maxwell in this 1861 paper to include what is termed the displacement current.
Three of Heaviside's four equations appeared throughout Maxwell's 1861 paper On Physical Lines of Force:
(i) At equation (56) of Maxwell's 1861 paper we see .
(ii) At equation (112) we see Ampère's circuital law with Maxwell's correction. It is this correction called displacement current which is the most significant aspect of Maxwell's work in electromagnetism as it enabled him to later derive the electromagnetic wave equation in his 1865 paper A Dynamical Theory of the Electromagnetic Field, and hence show that light is an electromagnetic wave. It is therefore this aspect of Maxwell's work which gives Heaviside's equations their full significance. (Interestingly, Kirchhoff derived the telegrapher's equations in 1857 without using displacement current. But he did use Poisson's equation and the equation of continuity which are the mathematical ingredients of the displacement current. Nevertheless, Kirchhoff believed his equations to be applicable only inside an electric wire and so he is not credited with having discovered that light is an electromagnetic wave).
(iii) At equation (113) we see Gauss' law.
(iv) Heaviside's fourth equation introduced a restricted partial time derivative version of Faraday's law of induction. (A full version of Faraday's law of induction had appeared at equation (54) of Maxwell's 1861 paper). It is important however to note that Heaviside's partial time derivative notation, as opposed to the total time derivative notation used by Maxwell at equations (54), resulted in the loss of the v × B term that appeared in Maxwell's equation (77). Nowadays, the v × B term appears in the force law F = q (E + v × B ) which sits adjacent to Maxwell's equations and bears the name Lorentz force. The Lorentz Force corresponds in effect to Maxwell's equation (77), but it appeared in this paper when Lorentz was still a young boy.
Confusion over the term "Maxwell's equations" is further increased because it is also sometimes used for a set of eight equations that appeared in Part III of Maxwell's 1865 paper A Dynamical Theory of the Electromagnetic Field, entitled "General Equations of the Electromagnetic Field" (page 480 of the article and page 2 of the pdf link), a confusion compounded by the writing of six of those eight equations as three separate equations (one for each of the Cartesian axes), resulting in twenty equations in twenty unknowns. (As noted above, this terminology is not common: Modern references to the term "Maxwell's equations" usually refer to the Heaviside restatements.)
These original eight equations are nearly identical to the Heaviside versions in substance, but they have some superficial differences. In fact, only one of the Heaviside versions is completely unchanged from these original equations, and that is Gauss' law (Maxwell's equation G below). Another of Heaviside's four equations is an amalgamation of Maxwell's law of total currents (equation A below) with Ampère's circuital law (equation C below). This amalgamation, which Maxwell himself originally made at equation (112) in his 1861 paper "On Physical Lines of Force" (see above), is the one that modifies Ampère's circuital law to include Maxwell's displacement current.
The eight original Maxwell's equations can be written in modern vector notation as follows:(A) The law of total currents
It is interesting to note the term that appears in equation D. Equation D is therefore effectively the Lorentz force, similarly to equation (77) of his 1861 paper (see above).
When Maxwell derives the electromagnetic wave equation in his 1865 paper, he uses equation D to cater for electromagnetic induction rather than Faraday's law of induction which is used in modern textbooks. (Faraday's law itself does not appear among his equations.) However, Maxwell drops the term from equation D when he is deriving the electromagnetic wave equation, as he considers the situation only from the rest frame.
If an electric field is applied to a dielectric material, each of the molecules responds by forming a microscopic dipole -- its atomic nucleus will move a tiny distance in the direction of the field, while its electrons will move a tiny distance in the opposite direction. This is called polarization of the material. The distribution of charge that results from these tiny movements turn out to be identical to having a layer of positive charge on one side of the material, and a layer of negative charge on the other side -- a macroscopic separation of charge, even though all of the charges involved are "bound" to a single molecule. This is called bound charge. Likewise, in a magnetized material, there is effectively a "bound current" circulating around the material, despite the fact that no individual charge is travelling a distance larger than a single molecule.
In this section, a simple proof is outlined which shows that the two alternate general formulations of Maxwell's equations given in Section 1 are mathematically equivalent.
The relation between polarization, magnetization, bound charge, and bound current is as follows:
In order to apply Maxwell's equations (the formulation in terms of free charge and current, and D and H), it is necessary to specify the relations between D and E, and H and B. These are called constitutive relations, and correspond physically to specifying the response of bound charge and current to the field, or equivalently, how much polarization and magnetization a material acquires in the presence of electromagnetic fields.
In the absence of magnetic or dielectric materials, the relations are simple:
For real-world materials, the constitutive relations are not simple proportionalities, except approximately. The relations can usually still be written:
If further there are dependencies on:
then the constitutive relations take a more complicated form:
Substituting in the constitutive relations above, Maxwell's equations in a linear material (differential form only) are:
These are formally identical to the general formulation in terms of E and B (given above), except that the permittivity of free space was replaced with the permittivity of the material (see also displacement field, electric susceptibility and polarization density), the permeability of free space was replaced with the permeability of the material (see also magnetization, magnetic susceptibility and magnetic field), and only free charges and currents are included (instead of all charges and currents).
Starting with the equations appropriate in the case without dielectric or magnetic materials, and assuming that there is no current or electric charge present in the vacuum, we obtain the Maxwell equations in free space:
These equations have a solution in terms of traveling sinusoidal plane waves, with the electric and magnetic field directions orthogonal to one another and the direction of travel, and with the two fields in phase, traveling at the speed
In fact, Maxwell's equations explains specifically how these waves can physically propagate through space. The changing magnetic field creates a changing electric field through Faraday's law. That electric field, in turn, creates a changing magnetic field through Maxwell's correction to Ampère's law. This perpetual cycle allows these waves, known as electromagnetic radiation, to move through space, always at velocity c0.
Maxwell's equations of electromagnetism relate the electric and magnetic fields to the motions of electric charges. The standard form of the equations provide for an electric charge, but posit no magnetic charge (in accordance with the fact that magnetic charge has never been seen and may not exist). Except for this, the equations are symmetric under interchange of electric and magnetic field. In fact, symmetric equations can be written when all charges are zero, and this is how the wave equation is derived (see immediately above).
Fully symmetric equations can also be written if one allows for the possibility of "magnetic charges" analogous to electric charges. With the inclusion of a variable for these magnetic charges, say , there will also be "magnetic current" variable in the equations, . The extended Maxwell's equations, simplified by nondimensionalization, are as follows:
|Name||Without magnetic monopoles||With magnetic monopoles (hypothetical)|
|Gauss' law for magnetism:|
| Maxwell-Faraday equation|
(Faraday's law of induction):
| Ampère's law|
(with Maxwell's extension):
|Note: the Bivector notation embodies the sign swap, and these four equations can be written as only one equation.|
If magnetic charges do not exist, or if they exist but where they are not present in a region, then the new variables are zero, and the symmetric equations reduce to the conventional equations of electromagnetism such as . Classically, the question is "Why does the magnetic charge always seem to be zero?"
where q is the charge on the particle and v is the particle velocity. (It also should be remembered that the Lorentz force is not the only force exerted upon charged bodies, which also may be subject to gravitational, nuclear, etc. forces.) Therefore, in both classical and quantum physics, the precise dynamics of a system form a set of coupled differential equations, which are almost always too complicated to be solved exactly, even at the level of statistical mechanics. This remark applies to not only the dynamics of free charges and currents (which enter Maxwell's equations directly), but also the dynamics of bound charges and currents, which enter Maxwell's equations through the constitutive equations, as described next.
Commonly, real materials are approximated as "continuum" media with bulk properties such as the refractive index, permittivity, permeability, conductivity, and/or various susceptibilities. These lead to the macroscopic Maxwell's equations, which are written (as given above) in terms of free charge/current densities and D, H, E, and B (rather than E and B alone ) along with the constitutive equations relating these fields. For example, although a real material consists of atoms whose electronic charge densities can be individually polarized by an applied field, for most purposes behavior at the atomic scale is not relevant and the material is approximated by an overall polarization density related to the applied field by an electric susceptibility.
Continuum approximations of atomic-scale inhomogeneities cannot be determined from Maxwell's equations alone. but require some type of quantum mechanical analysis such as quantum field theory as applied to condensed matter physics. See, for example, density functional theory, Green–Kubo relations and Green's function (many-body theory). Various approximate transport equations have evolved, for example, the Boltzmann equation or the Fokker–Planck equation or the Navier-Stokes equations. Some examples where these equations are applied are magnetohydrodynamics, fluid dynamics, electrohydrodynamics, superconductivity, plasma modeling. An entire physical apparatus for dealing with these matters has developed. A different set of homogenization methods (evolving from a tradition in treating materials such as conglomerates and laminates) are based upon approximation of an inhomogeneous material by a homogeneous effective medium (valid for excitations with wavelengths much larger than the scale of the inhomogeneity).
Theoretical results have their place, but often require fitting to experiment. Continuum-approximation properties of many real materials rely upon measurement, for example, ellipsometry measurements.
In practice, some materials properties have a negligible impact in particular circumstances, permitting neglect of small effects. For example: optical nonlinearities can be neglected for low field strengths; material dispersion is unimportant where frequency is limited to a narrow bandwidth; material absorption can be neglected for wavelengths where a material is transparent; and metals with finite conductivity often are approximated at microwave or longer wavelengths as perfect metals with infinite conductivity (forming hard barriers with zero skin depth of field penetration).
And, of course, some situations demand that Maxwell's equations and the Lorentz force be combined with other forces that are not electromagnetic. An obvious example is gravity. A more subtle example, which applies where electrical forces are weakened due to charge balance in a solid or a molecule, is the Casimir force from quantum electrodynamics.
The connection of Maxwell's equations to the rest of the physical world is via the fundamental sources of charges and currents and the forces on them, and also by the properties of physical materials.
Where c is the speed of light in a vacuum. For the electromagnetic field in a vacuum, the equations become:
where is the charge on the particle and is the particle velocity. This is slightly different from the SI-unit expression above. For example, here the magnetic field has the same units as the electric field .
Maxwell's equations have a close relation to special relativity: Not only were Maxwell's equations a crucial part of the historical development of special relativity, but also, special relativity has motivated a compact mathematical formulation Maxwell's equations, in terms of covariant tensors.
Maxwell's electromagnetic wave equation only applied in what he believed to be the rest frame of the luminiferous medium because he didn't use the vXB term of his equation (D) when he derived it. Maxwell's idea of the luminiferous medium was that it comprised of aethereal vortices aligned solenoidally along their rotation axes.
The American scientist A.A. Michelson set out to determine the velocity of the earth through the luminiferous medium aether using a light wave interferometer that he had invented. When the Michelson-Morley experiment was conducted by Edward Morley and Albert Abraham Michelson in 1887, it produced a null result for the change of the velocity of light due to the Earth's motion through the hypothesized aether. This null result was in line with the theory that was proposed in 1845 by George Stokes which suggested that the aether was entrained with the Earth's orbital motion.
Hendrik Lorentz objected to Stokes' aether drag model and in along with George FitzGerald and Joseph Larmor, he suggested another approach. Both Larmor (1897) and Lorentz (1899, 1904) derived the Lorentz transformation (so named by Henri Poincaré) as one under which Maxwell's equations were invariant. Poincaré (1900) analyzed the coordination of moving clocks by exchanging light signals. He also established mathematically the group property of the Lorentz transformation (Poincaré 1905).
This culminated in Albert Einstein's revolutionary theory of special relativity, which postulated the absence of any absolute rest frame, dismissed the aether as unnecessary (a bold idea, which did not come to Lorentz nor to Poincaré), and established the invariance of Maxwell's equations in all inertial frames of reference, in contrast to the famous Newtonian equations for classical mechanics. But the transformations between two different inertial frames had to correspond to Lorentz' equations and not - as former believed - to those of Galileo (called Galilean transformations). Indeed, Maxwell's equations played a key role in Einstein's famous paper on special relativity; for example, in the opening paragraph of the paper, he motivated his theory by nothing that a description of a conductor moving with respect to a magnet must generate a consistent set of fields irrespective of whether the force is calculated in the rest frame of the magnet or that of the conductor.
General relativity has also had a close relationship with Maxwell's equations. For example, Kaluza and Klein showed in the 1920s that Maxwell's equations can be derived by extending general relativity into five dimensions. This strategy of using higher dimensions to unify different forces continues to be an active area of research in particle physics.
In special relativity, in order to more clearly express the fact that Maxwell's equations in vacuum take the same form in any inertial coordinate system, Maxwell's equations are written in terms of four-vectors and tensors in the "manifestly covariant" form. The purely spatial components of the following are in SI units.
and the result of raising its indices
With these ingredients, Maxwell's equations can be written:
which is equivalent to
where is the contravariant version of the Levi-Civita symbol, and
is the 4-gradient. Repeated indices are summed over according to Einstein summation convention. We have displayed the results in several common notations. Upper and lower components of a vector, and respectively, are interchanged with the fundamental tensor g, e.g., g=η=diag(-1,+1,+1,+1).
The first tensor equation is an expression of the two inhomogeneous Maxwell's equations, Gauss' law and Ampere's law with Maxwell's correction. The second equation is an expression of the two homogeneous equations, Faraday's law of induction and Gauss's law for magnetism.
Alternative covariant presentations of Maxwell's equations also exist, for example in terms of the four-potential; see Covariant formulation of classical electromagnetism for details.
Maxwell's equations can be written in an alternative form, involving the electric potential (also called scalar potential) and magnetic potential (also called vector potential), as follows. (The following equations are valid in the absence of dielectric and magnetic materials; or if such materials are present, they are valid as long as bound charge and bound current are included in the total charge and current densities.)
First, Gauss' law for magnetism states:
Second, plugging this into Faraday's law, we get:
Combining these with the remaining two Maxwell's equations yields the four relations:
These equations, taken together, are as powerful and complete as Maxwell's equations. Moreover, the problem has been reduced somewhat, as the electric and magnetic fields each have three components which need to be solved for (six components altogether), while the electric and magnetic potentials have only four components altogether. On the other hand, these equations appear more complicated than Maxwell's equations using just the electric and magnetic fields.
In fact, these equations can be simplified a good deal by taking advantage of gauge freedom—i.e., the fact that there are many different choices of A and consistent with a given E and B. For more information, see the article gauge freedom.
In free space, where ε = ε0 and μ = μ0 are constant everywhere, Maxwell's equations simplify considerably once the language of differential geometry and differential forms is used. In what follows, cgs units, not SI units are used, however. The electric and magnetic fields are now jointly described by a 2-form F in a 4-dimensional spacetime manifold. Maxwell's equations then reduce to the Bianchi identity
In a linear, macroscopic theory, the influence of matter on the electromagnetic field is described through more general linear transformation in the space of 2-forms. We call
When the fields are expressed as linear combinations (of exterior products) of basis forms ,
In this formulation, electromagnetism generalises immediately to any 4-dimensional oriented manifold or with small adaptations any manifold, requiring not even a metric. Thus the expression of Maxwell's equations in terms of differential forms leads to a further notational and conceptual simplification. Whereas Maxwell's Equations could be written as two tensor equations instead of eight scalar equations, from which the propagation of electromagnetic disturbances and the continuity equation could be derived with a little effort, using differential forms leads to an even simpler derivation of these results.
Often, the time derivative in the third law motivates calling this equation "dynamical", which is somewhat misleading; in the sense of the preceding analysis, this is rather an artifact of breaking relativistic covariance by choosing a preferred time direction. To have physical degrees of freedom propagated by these field equations, one must include a kinetic term F *F for A; and take into account the non-physical degrees of freedom which can be removed by gauge transformation A→A' = A-dα: see also gauge fixing and Fadeev-Popov ghosts.
An elegant and intuitive way to formulate Maxwell's equations is to use complex line bundles or principal bundles with fibre U(1). The connection on the line bundle has a curvature which is a two-form that automatically satisfies and can be interpreted as a field-strength. If the line bundle is trivial with flat reference connection d we can write and F = dA with A the 1-form composed of the electric potential and the magnetic vector potential.
In quantum mechanics, the connection itself is used to define the dynamics of the system. This formulation allows a natural description of the Aharonov-Bohm effect. In this experiment, a static magnetic field runs through a long magnetic wire (e.g. an Fe wire magnetized longitudinally). Outside of this wire the magnetic induction is zero, in contrast to the vector potential, which essentially depends on the magnetic flux through the cross-section of the wire and does not vanish outside. Since there is no electric field either, the Maxwell tensor F = 0 throughout the space-time region outside the tube, during the experiment. This means by definition that the connection is flat there.
However, as mentioned, the connection depends on the magnetic field through the tube since the holonomy along a non-contractible curve encircling the tube is the magnetic flux through the tube in the proper units. This can be detected quantum-mechanically with a double-slit electron diffraction experiment on an electron wave traveling around the tube. The holonomy corresponds to an extra phase shift, which leads to a shift in the diffraction pattern. (See Michael Murray, Line Bundles, 2002 (PDF web link) for a simple mathematical review of this formulation. See also R. Bott, On some recent interactions between mathematics and physics, Canadian Mathematical Bulletin, 28 (1985) no. 2 pp 129-164.)''
is a Christoffel symbol that characterizes the curvature of spacetime and is the covariant derivative.
Here g is as usual the determinant of the metric tensor . A small computation that uses the symmetry of the Christoffel symbols (i.e. the torsion-freeness of the Levi Civita connection) and the covariant constantness of the Hodge star operator then shows that in this coordinate neighborhood we have:
The developments before relativity