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There are various **mathematical descriptions of the electromagnetic field** that are used in the study of electromagnetism, one of the four fundamental interactions of nature. In this article, several approaches are discussed, although the equations are in terms of electric and magnetic fields, potentials, and charges with currents, generally speaking.

**Electromagnetism** is a branch of physics involving the study of the **electromagnetic force**, a type of physical interaction that occurs between electrically charged particles. The electromagnetic force is carried by electromagnetic fields composed of electric fields and magnetic fields, is responsible for electromagnetic radiation such as light, and is one of the four fundamental interactions in nature. The other three fundamental interactions are the strong interaction, the weak interaction, and gravitation. At high energy the weak force and electromagnetic force are unified as a single electroweak force.

In physics, the **fundamental interactions**, also known as **fundamental forces**, are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist: the gravitational and electromagnetic interactions, which produce significant long-range forces whose effects can be seen directly in everyday life, and the strong and weak interactions, which produce forces at minuscule, subatomic distances and govern nuclear interactions. Some scientists hypothesize that a fifth force might exist, but the hypotheses remain speculative.

- Vector field approach
- Maxwell's equations in the vector field approach
- Potential field approach
- Maxwell's equations in potential formulation
- Gauge freedom
- Extension to quantum electrodynamics
- Geometric algebra formulations
- Differential forms approach
- Field 2-form
- Current 3-form, dual current 1-form
- Alternate metric signature
- Curved spacetime
- Traditional formulation
- Formulation in terms of differential forms
- Classical electrodynamics as the curvature of a line bundle
- Discussion
- Potential formulation
- Manifestly covariant (tensor) approach
- Differential forms approach 2
- Geometric calculus approach
- See also
- Notes
- References

The most common description of the electromagnetic field uses two three-dimensional vector fields called the electric field and the magnetic field. These vector fields each have a value defined at every point of space and time and are thus often regarded as functions of the space and time coordinates. As such, they are often written as **E**(*x*, *y*, *z*, *t*) (electric field) and **B**(*x*, *y*, *z*, *t*) (magnetic field).

In vector calculus and physics, a **vector field** is an assignment of a vector to each point in a subset of space. A vector field in the plane, can be visualised as: a collection of arrows with a given magnitude and direction, each attached to a point in the plane. Vector fields are often used to model, for example, the speed and direction of a moving fluid throughout space, or the strength and direction of some force, such as the magnetic or gravitational force, as it changes from one point to another point.

An **electric field** surrounds an electric charge, and exerts force on other charges in the field, attracting or repelling them. Electric field is sometimes abbreviated as **E**-field. The electric field is defined mathematically as a vector field that associates to each point in space the force per unit of charge exerted on an infinitesimal positive test charge at rest at that point. The SI unit for electric field strength is volt per meter (V/m). Newtons per coulomb (N/C) is also used as a unit of electric field strengh. Electric fields are created by electric charges, or by time-varying magnetic fields. Electric fields are important in many areas of physics, and are exploited practically in electrical technology. On an atomic scale, the electric field is responsible for the attractive force between the atomic nucleus and electrons that holds atoms together, and the forces between atoms that cause chemical bonding. Electric fields and magnetic fields are both manifestations of the electromagnetic force, one of the four fundamental forces of nature.

A **magnetic field** is a vector field that describes the magnetic influence of electric charges in relative motion and magnetized materials. Magnetic fields are observed in a wide range of size scales, from subatomic particles to galaxies. In everyday life, the effects of magnetic fields are often seen in permanent magnets, which pull on magnetic materials and attract or repel other magnets. Magnetic fields surround and are created by magnetized material and by moving electric charges such as those used in electromagnets. Magnetic fields exert forces on nearby moving electrical charges and torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. Both the strength and direction of a magnetic field vary with location. As such, it is an example of a vector field.

If only the electric field (**E**) is non-zero, and is constant in time, the field is said to be an electrostatic field. Similarly, if only the magnetic field (**B**) is non-zero and is constant in time, the field is said to be a magnetostatic field. However, if either the electric or magnetic field has a time-dependence, then both fields must be considered together as a coupled electromagnetic field using Maxwell's equations.

**Maxwell's equations** are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits. The equations provide a mathematical model for electric, optical, and radio technologies, such as power generation, electric motors, wireless communication, lenses, radar etc. Maxwell's equations describe how electric and magnetic fields are generated by charges, currents, and changes of the fields. One important consequence of the equations is that they demonstrate how fluctuating electric and magnetic fields propagate at the speed of light. Known as electromagnetic radiation, these waves may occur at various wavelengths to produce a spectrum from radio waves to γ-rays. The equations are named after the physicist and mathematician James Clerk Maxwell, who between 1861 and 1862 published an early form of the equations that included the Lorentz force law. He also first used the equations to propose that light is an electromagnetic phenomenon.

The behaviour of electric and magnetic fields, whether in cases of electrostatics, magnetostatics, or electrodynamics (electromagnetic fields), is governed by Maxwell's equations:

**Maxwell's equations**(*vector fields*)**Gauss's law****Gauss's law for magnetism****Faraday's law****Ampère–Maxwell law**

where *ρ* is the charge density, which can (and often does) depend on time and position, *ε*_{0} is the electric constant, *μ*_{0} is the magnetic constant, and **J** is the current per unit area, also a function of time and position. The equations take this form with the International System of Quantities.

The **International System of Quantities** (ISQ) is a system based on seven base quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. Other quantities such as area, pressure, and electrical resistance are derived from these base quantities by clear, non-contradictory equations. The ISQ defines the quantities that are measured with the SI units and also includes many other quantities in modern science and technology. The ISQ is defined in the international standard ISO/IEC 80000, and was finalised in 2009 with the publication of ISO 80000-1.

When dealing with only nondispersive isotropic linear materials, Maxwell's equations are often modified to ignore bound charges by replacing the permeability and permittivity of free space with the permeability and permittivity of the linear material in question. For some materials that have more complex responses to electromagnetic fields, these properties can be represented by tensors, with time-dependence related to the material's ability to respond to rapid field changes (dispersion (optics), Green–Kubo relations), and possibly also field dependencies representing nonlinear and/or nonlocal material responses to large amplitude fields (nonlinear optics).

In optics, **dispersion** is the phenomenon in which the phase velocity of a wave depends on its frequency.

The **Green–Kubo relations** give the exact mathematical expression for transport coefficients in terms of integrals of time correlation functions:

**Nonlinear optics** (**NLO**) is the branch of optics that describes the behaviour of light in *nonlinear media*, that is, media in which the polarization density **P** responds non-linearly to the electric field **E** of the light. The non-linearity is typically observed only at very high light intensities (values of atomic electric fields, typically 10^{8} V/m) such as those provided by lasers. Above the Schwinger limit, the vacuum itself is expected to become nonlinear. In nonlinear optics, the superposition principle no longer holds.

Many times in the use and calculation of electric and magnetic fields, the approach used first computes an associated potential: the electric potential, *, for the electric field, and the magnetic potential, ***A**, for the magnetic field. The electric potential is a scalar field, while the magnetic potential is a vector field. This is why sometimes the electric potential is called the scalar potential and the magnetic potential is called the vector potential. These potentials can be used to find their associated fields as follows:

These relations can be substituted into Maxwell's equations to express the latter in terms of the potentials. Faraday's law and Gauss's law for magnetism reduce to identities (e.g., in the case of Gauss's Law for magnetism, 0 = 0). The other two of Maxwell's equations turn out less simply.

**Maxwell's equations**(*potential formulation*)

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 together had six components to solve for.^{ [1] } In the potential formulation, there are only four components: the electric potential and the three components of the vector potential. However, the equations are messier than Maxwell's equations using the electric and magnetic fields.

These equations can be simplified by taking advantage of the fact that only the electric and magnetic fields are physically meaningful quantities that can be measured; the potentials are not. There is a freedom to constrain the form of the potentials provided that this does not affect the resultant electric and magnetic fields, called gauge freedom. Specifically for these equations, for any choice of a twice-differentiable scalar function of position and time *λ*, if (*φ*, **A**) is a solution for a given system, then so is another potential (*φ*′, **A**′) given by:

This freedom can be used to simplify the potential formulation. Either of two such scalar functions is typically chosen: the Coulomb gauge and the Lorenz gauge.

The Coulomb gauge is chosen in such a way that , which corresponds to the case of magnetostatics. In terms of *λ*, this means that it must satisfy the equation

- .

This choice of function results in the following formulation of Maxwell's equations:

Several features about Maxwell's equations in the Coulomb gauge are as follows. Firstly, solving for the electric potential is very easy, as the equation is a version of Poisson's equation. Secondly, solving for the magnetic vector potential is particularly difficult. This is the big disadvantage of this gauge. The third thing to note, and something which is not immediately obvious, is that the electric potential changes instantly everywhere in response to a change in conditions in one locality.

For instance, if a charge is moved in New York at 1 pm local time, then a hypothetical observer in Australia who could measure the electric potential directly would measure a change in the potential at 1 pm New York time. This seemingly goes violates causality in special relativity, i.e. the impossibility of information, signals, or anything travelling faster than the speed of light. The resolution to this apparent problem lies in the fact that, as previously stated, no observers can measure the potentials; they measure the electric and magnetic fields. So, the combination of **∇***φ* and ∂**A**/∂*t* used in determining the electric field restores the speed limit imposed by special relativity for the electric field, making all observable quantities consistent with relativity.

A gauge that is often used is the Lorenz gauge condition. In this, the scalar function *λ* is chosen such that

meaning that *λ* must satisfy the equation

The Lorenz gauge results in the following form of Maxwell's equations:

The operator is called the d'Alembertian (some authors denote this by only the square ). These equations are inhomogeneous versions of the wave equation, with the terms on the right side of the equation serving as the source functions for the wave. As with any wave equation, these equations lead to two types of solution: advanced potentials (which are related to the configuration of the sources at future points in time), and retarded potentials (which are related to the past configurations of the sources); the former are usually disregarded where the field is to analyzed from a causality perspective.

As pointed out above, the Lorenz gauge is no more valid than any other gauge since the potentials cannot be measured. Despite this, there are certain quantum mechanical phenomena in which potentials appear to affect particles in regions where the observable field vanishes throughout the region, for example as in the Aharonov–Bohm effect. However, these phenomena do not provide a means to directly measure the potentials nor to detect a difference between different but mutually gauge equivalent potentials. The Lorenz gauge has the further advantage of the equations being Lorentz invariant.

Canonical quantization of the electromagnetic fields proceeds by elevating the scalar and vector potentials; *φ*(**x**), **A**(**x**), from fields to field operators. Substituting 1/*c*^{2} = *ε*_{0}*μ*_{0} into the previous Lorenz gauge equations gives:

Here, **J** and *ρ* are the current and charge density of the * matter field *. If the matter field is taken so as to describe the interaction of electromagnetic fields with the Dirac electron given by the four-component Dirac spinor field *ψ*, the current and charge densities have form:^{ [2] }

where * α* are the first three Dirac matrices. Using this, we can re-write Maxwell's equations as:

**Maxwell's equations**(*QED*)

which is the form used in quantum electrodynamics.

Analogous to the tensor formulation, two objects, one for the field and one for the current, are introduced. In geometric algebra (GA) these are multivectors. The field multivector, known as the Riemann–Silberstein vector, is

and the current multivector is

where, in the algebra of physical space (APS) with the vector basis . The unit pseudoscalar is (assuming an orthonormal basis). Orthonormal basis vectors share the algebra of the Pauli matrices, but are usually not equated with them. After defining the derivative

Maxwell's equations are reduced to the single equation^{ [3] }

**Maxwell's equations***(APS formulation)*

In three dimensions, the derivative has a special structure allowing the introduction of a cross product:

from which it is easily seen that Gauss's law is the scalar part, the Ampère–Maxwell law is the vector part, Faraday's law is the pseudovector part, and Gauss's law for magnetism is the pseudoscalar part of the equation. After expanding and rearranging, this can be written as

We can identify APS as a subalgebra of the spacetime algebra (STA) , defining and . The s have the same algebraic properties of the gamma matrices but their matrix representation is not needed. The derivative is now

The Riemann–Silberstein becomes a bivector

and the charge and current density become a vector

Owing to the identity

Maxwell's equations reduce to the single equation

**Maxwell's equations***(STA formulation)*

In free space, where *ε* = *ε*_{0} and *μ* = *μ*_{0} are constant everywhere, Maxwell's equations simplify considerably once the language of differential geometry and differential forms is used. In what follows, cgs-Gaussian units, not SI units are used. (To convert to SI, see here.) The electric and magnetic fields are now jointly described by a 2-form **F** in a 4-dimensional spacetime manifold. The Faraday tensor (electromagnetic tensor) can be written as a 2-form in Minkowski space with metric signature (− + + +) as

which, as the curvature form, is the exterior derivative of the electromagnetic four-potential,

The source free equations can be written by the action of the exterior derivative on this 2-form. But for the equations with source terms (Gauss's law and the Ampère-Maxwell equation), the Hodge dual of this 2-form is needed. The Hodge star operator takes a *p*-form to a (*n*−*p*)-form, where *n* is the number of dimensions. Here, it takes the 2-form (*F*) and gives another 2-form (in four dimensions, *n*−*p* = 4 − 2 = 2). For the basis cotangent vectors, the Hodge dual is given as (see Hodge star operator § Four dimensions)

and so on. Using these relations, the dual of the Faraday 2-form is the Maxwell tensor,

Here, the 3-form **J** is called the *electric current form* or * current 3-form *:

with the corresponding dual 1-form:

Maxwell's equations then reduce to the Bianchi identity and the source equation, respectively:

**Maxwell's equations**(*current 3-form*)

where d denotes the exterior derivative – a natural coordinate- and metric-independent differential operator acting on forms, and the (dual) Hodge star operator is a linear transformation from the space of 2-forms to the space of (4 − 2)-forms defined by the metric in Minkowski space (in four dimensions even by any metric conformal to this metric). The fields are in natural units where 1/4π*ε*_{0} = 1.

Since d^{2} = 0, the 3-form **J** satisfies the conservation of current (continuity equation):

The current 3-form can be integrated over a 3-dimensional space-time region. The physical interpretation of this integral is the charge in that region if it is spacelike, or the amount of charge that flows through a surface in a certain amount of time if that region is a spacelike surface cross a timelike interval. As the exterior derivative is defined on any manifold, the differential form version of the Bianchi identity makes sense for any 4-dimensional manifold, whereas the source equation is defined if the manifold is oriented and has a Lorentz metric. In particular the differential form version of the Maxwell equations are a convenient and intuitive formulation of the Maxwell equations in general relativity.

*Note:* In much of the literature, the notations and are switched, so that is a 1-form called the current and is a 3-form called the dual current.^{ [4] }

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

the constitutive transformation. The role of this transformation is comparable to the Hodge duality transformation. The Maxwell equations in the presence of matter then become:

where the current 3-form **J** still satisfies the continuity equation d**J** = 0.

When the fields are expressed as linear combinations (of exterior products) of basis forms *θ*^{p},

the constitutive relation takes the form

where the field coefficient functions are antisymmetric in the indices and the constitutive coefficients are antisymmetric in the corresponding pairs. In particular, the Hodge duality transformation leading to the vacuum equations discussed above are obtained by taking

which up to scaling is the only invariant tensor of this type that can be defined with the metric.

In this formulation, electromagnetism generalises immediately to any 4-dimensional oriented manifold or with small adaptations any manifold.

In the particle physicist's sign convention for the metric signature (+ − − −), the potential 1-form is

- .

The Faraday curvature 2-form becomes

and the Maxwell tensor becomes

- .

The current 3-form **J** is

and the corresponding dual 1-form is

- .

The current norm is now positive and equals

- ,

with the canonical volume form .

Matter and energy generate curvature of spacetime. This is the subject of general relativity. Curvature of spacetime affects electrodynamics. An electromagnetic field having energy and momentum also generates curvature in spacetime. Maxwell's equations in curved spacetime can be obtained by replacing the derivatives in the equations in flat spacetime with covariant derivatives. (Whether this is the appropriate generalization requires separate investigation.) The sourced and source-free equations become (cgs-Gaussian units):

and

Here,

is a Christoffel symbol that characterizes the curvature of spacetime and ∇_{α} is the covariant derivative.

The formulation of the Maxwell equations in terms of differential forms can be used without change in general relativity. The equivalence of the more traditional general relativistic formulation using the covariant derivative with the differential form formulation can be seen as follows. Choose local coordinates *x*^{α} which gives a basis of 1-forms d*x*^{α} in every point of the open set where the coordinates are defined. Using this basis and cgs-Gaussian units we define

- The antisymmetric field tensor
*F*_{αβ}, corresponding to the field 2-form**F**

- The current-vector infinitesimal 3-form
**J**

The epsilon tensor contracted with the differential 3-form produces 6 times the number of terms required.

Here *g* is as usual the determinant of the matrix representing the metric tensor, *g*_{αβ}. 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 Bianchi identity

- the source equation

- the continuity equation

An elegant and intuitive way to formulate Maxwell's equations is to use complex line bundles or a principal U(1)-bundle, on the fibers of which U(1) acts regularly. The principal U(1)-connection ∇ on the line bundle has a curvature **F** = ∇^{2} which is a two-form that automatically satisfies d**F** = 0 and can be interpreted as a field-strength. If the line bundle is trivial with flat reference connection *d* we can write ∇ = d + **A** and **F** = d**A** 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 iron 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.^{ [5] }^{ [6] }

Following are the reasons for using each of such formulations.

In advanced classical mechanics it is often useful, and in quantum mechanics frequently essential, to express Maxwell's equations in a *potential formulation* involving the electric potential (also called scalar potential) *φ*, and the magnetic potential (also called vector potential) **A**. For example, the analysis of radio antennas makes full use of Maxwell's vector and scalar potentials to separate the variables, a common technique used in formulating the solutions of differential equations. The potentials can be introduced by using the Poincaré lemma on the homogeneous equations to solve them in a universal way (this assumes that we consider a topologically simple, e.g. contractible space). The potentials are defined as in the table above. Alternatively, these equations define **E** and **B** in terms of the electric and magnetic potentials which then satisfy the homogeneous equations for **E** and **B** as identities. Substitution gives the non-homogeneous Maxwell equations in potential form.

Many different choices of **A** and *φ* are consistent with given observable electric and magnetic fields **E** and **B**, so the potentials seem to contain more, (classically) unobservable information. The non uniqueness of the potentials is well understood, however. For every scalar function of position and time *λ*(*x*, *t*), the potentials can be changed by a gauge transformation as

without changing the electric and magnetic field. Two pairs of gauge transformed potentials (*φ*, **A**) and (*φ*′, **A**′) are called *gauge equivalent*, and the freedom to select any pair of potentials in its gauge equivalence class is called gauge freedom. Again by the Poincaré lemma (and under its assumptions), gauge freedom is the only source of indeterminacy, so the field formulation is equivalent to the potential formulation if we consider the potential equations as equations for gauge equivalence classes.

The potential equations can be simplified using a procedure called gauge fixing. Since the potentials are only defined up to gauge equivalence, we are free to impose additional equations on the potentials, as long as for every pair of potentials there is a gauge equivalent pair that satisfies the additional equations (i.e. if the gauge fixing equations define a slice to the gauge action). The gauge-fixed potentials still have a gauge freedom under all gauge transformations that leave the gauge fixing equations invariant. Inspection of the potential equations suggests two natural choices. In the Coulomb gauge, we impose **∇**⋅**A** = 0 which is mostly used in the case of magneto statics when we can neglect the *c*^{−2}∂^{2}**A**/∂*t*^{2} term. In the Lorenz gauge (named after the Dane Ludvig Lorenz), we impose

The Lorenz gauge condition has the advantage of being Lorentz invariant and leading to Lorentz-invariant equations for the potentials.

Maxwell's equations are exactly consistent with special relativity—i.e., if they are valid in one inertial reference frame, then they are automatically valid in every other inertial reference frame. In fact, Maxwell's equations were crucial in the historical development of special relativity. However, in the usual formulation of Maxwell's equations, their consistency with special relativity is not obvious; it can only be proven by a laborious calculation.

For example, consider a conductor moving in the field of a magnet.^{ [7] } In the frame of the magnet, that conductor experiences a *magnetic* force. But in the frame of a conductor moving relative to the magnet, the conductor experiences a force due to an *electric* field. The motion is exactly consistent in these two different reference frames, but it mathematically arises in quite different ways.

For this reason and others, it is often useful to rewrite Maxwell's equations in a way that is "manifestly covariant"—i.e. *obviously* consistent with special relativity, even with just a glance at the equations—using covariant and contravariant four-vectors and tensors. This can be done using the EM tensor **F**, or the 4-potential **A**, with the 4-current **J** – see covariant formulation of classical electromagnetism.

Gauss's law for magnetism and the Faraday–Maxwell law can be grouped together since the equations are homogeneous, and be seen as geometric *identities* expressing the *field***F** (a 2-form), which can be derived from the *4-potential***A**. Gauss's law for electricity and the Ampere–Maxwell law could be seen as the *dynamical equations of motion * of the fields, obtained via the Lagrangian principle of least action, from the "interaction term" **AJ** (introduced through gauge covariant derivatives), coupling the field to matter. For the field formulation of Maxwell's equations in terms of a principle of extremal action, see electromagnetic tensor.

Often, the time derivative in the Faraday–Maxwell equation 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 that can be removed by gauge transformation **A** ↦ **A** − d*α*. See also gauge fixing and Faddeev–Popov ghosts.

This formulation uses the algebra that spacetime generates through the introduction of a distributive, associative (but not commutative) product called the geometric product. Elements and operations of the algebra can generally be associated with geometric meaning. The members of the algebra may be decomposed by grade (as in the formalism of differential forms) and the (geometric) product of a vector with a *k*-vector decomposes into a (*k* − 1)-vector and a (*k* + 1)-vector. The (*k* − 1)-vector component can be identified with the inner product and the (*k* + 1)-vector component with the outer product. It is of algebraic convenience that the geometric product is invertible, while the inner and outer products are not. The derivatives that appear in Maxwell's equations are vectors and electromagnetic fields are represented by the Faraday bivector **F**. This formulation is as general as that of differential forms for manifolds with a metric tensor, as then these are naturally identified with *r*-forms and there are corresponding operations. Maxwell's equations reduce to one equation in this formalism. This equation can be separated into parts as is done above for comparative reasons.

- ↑ Introduction to Electrodynamics by Griffiths
- ↑ Quantum Electrodynamics, Mathworld
- ↑ Oersted Medal Lecture David Hestenes "Reforming the Mathematical Language of Physics" (Am. J. Phys. 71 (2), February 2003, pp. 104–121) Online:http://geocalc.clas.asu.edu/html/Oersted-ReformingTheLanguage.html p26
- ↑ Misner, Charles W.; Thorne, Kip; Wheeler, John Archibald (1973).
*Gravitation*. W. H. Freeman. p. 81. ISBN 978-0-7167-0344-0. - ↑ M. Murray (5 September 2008). "Line Bundles. Honours 1996" (PDF). University of Adelaide . Retrieved 2010-11-19.
- ↑ R. Bott (1985). "On some recent interactions between mathematics and physics".
*Canadian Mathematical Bulletin*.**28**(2): 129–164. doi:10.4153/CMB-1985-016-3. - ↑ Albert Einstein (1905)
*On the electrodynamics of moving bodies*

In physics the **Lorentz force** is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge *q* moving with a velocity *v* in an electric field **E** and a magnetic field **B** experiences a force of

In physics, the **Navier–Stokes equations**, named after Claude-Louis Navier and George Gabriel Stokes, describe the motion of viscous fluid substances.

The **stress–energy tensor**, sometimes **stress–energy–momentum tensor** or **energy–momentum tensor**, is a tensor quantity in physics that describes the density and flux of energy and momentum in spacetime, generalizing the stress tensor of Newtonian physics. It is an attribute of matter, radiation, and non-gravitational force fields. The stress–energy tensor is the source of the gravitational field in the Einstein field equations of general relativity, just as mass density is the source of such a field in Newtonian gravity.

In thermodynamics, the **Onsager reciprocal relations** express the equality of certain ratios between flows and forces in thermodynamic systems out of equilibrium, but where a notion of local equilibrium exists.

An **electromagnetic four-potential** is a relativistic vector function from which the electromagnetic field can be derived. It combines both an electric scalar potential and a magnetic vector potential into a single four-vector.

In mathematics, the **Hamilton–Jacobi equation** (**HJE**) is a necessary condition describing extremal geometry in generalizations of problems from the calculus of variations, and is a special case of the Hamilton–Jacobi–Bellman equation. It is named for William Rowan Hamilton and Carl Gustav Jacob Jacobi.

The term **magnetic potential** can be used for either of two quantities in classical electromagnetism: the *magnetic vector potential*, or simply *vector potential*, **A**; and the *magnetic scalar potential**ψ*. Both quantities can be used in certain circumstances to calculate the magnetic field **B**.

The **rigid rotor** is a mechanical model of rotating systems. An arbitrary rigid rotor is a 3-dimensional rigid object, such as a top. To orient such an object in space requires three angles, known as Euler angles. A special rigid rotor is the *linear rotor* requiring only two angles to describe, for example of a diatomic molecule. More general molecules are 3-dimensional, such as water, ammonia, or methane.

In differential geometry, the **four-gradient** is the four-vector analogue of the gradient from vector calculus.

In electromagnetism, the **electromagnetic tensor** or **electromagnetic field tensor** is a mathematical object that describes the electromagnetic field in spacetime. The field tensor was first used after the four-dimensional tensor formulation of special relativity was introduced by Hermann Minkowski. The tensor allows related physical laws to be written very concisely.

In quantum chemistry, the potential energy surfaces are obtained within the adiabatic or Born–Oppenheimer approximation. This corresponds to a representation of the molecular wave function where the variables corresponding to the molecular geometry and the electronic degrees of freedom are separated. The non separable terms are due to the nuclear kinetic energy terms in the molecular Hamiltonian and are said to couple the potential energy surfaces. In the neighbourhood of an avoided crossing or conical intersection, these terms cannot be neglected. One therefore usually performs one unitary transformation from the adiabatic representation to the so-called **diabatic representation** in which the nuclear kinetic energy operator is diagonal. In this representation, the coupling is due to the electronic energy and is a scalar quantity which is significantly easier to estimate numerically.

A **theoretical motivation for general relativity**, including the motivation for the geodesic equation and the Einstein field equation, can be obtained from special relativity by examining the dynamics of particles in circular orbits about the earth. A key advantage in examining circular orbits is that it is possible to know the solution of the Einstein Field Equation *a priori*. This provides a means to inform and verify the formalism.

The **covariant formulation of classical electromagnetism** refers to ways of writing the laws of classical electromagnetism in a form that is manifestly invariant under Lorentz transformations, in the formalism of special relativity using rectilinear inertial coordinate systems. These expressions both make it simple to prove that the laws of classical electromagnetism take the same form in any inertial coordinate system, and also provide a way to translate the fields and forces from one frame to another. However, this is not as general as Maxwell's equations in curved spacetime or non-rectilinear coordinate systems.

In physics, **Maxwell's equations in curved spacetime** govern the dynamics of the electromagnetic field in curved spacetime or where one uses an arbitrary coordinate system. These equations can be viewed as a generalization of the vacuum Maxwell's equations which are normally formulated in the local coordinates of flat spacetime. But because general relativity dictates that the presence of electromagnetic fields induce curvature in spacetime, Maxwell's equations in flat spacetime should be viewed as a convenient approximation.

In electromagnetism and applications, an **inhomogeneous electromagnetic wave equation**, or **nonhomogeneous electromagnetic wave equation**, is one of a set of wave equations describing the propagation of electromagnetic waves generated by nonzero source charges and currents. The source terms in the wave equations makes the partial differential equations *inhomogeneous*, if the source terms are zero the equations reduce to the homogeneous electromagnetic wave equations. The equations follow from Maxwell's equations.

The **gradient theorem**, also known as the **fundamental theorem of calculus for line integrals**, says that a line integral through a gradient field can be evaluated by evaluating the original scalar field at the endpoints of the curve.

The theory of special relativity plays an important role in the modern theory of classical electromagnetism. First of all, it gives formulas for how electromagnetic objects, in particular the electric and magnetic fields, are altered under a Lorentz transformation from one inertial frame of reference to another. Secondly, it sheds light on the relationship between electricity and magnetism, showing that frame of reference determines if an observation follows electrostatic or magnetic laws. Third, it motivates a compact and convenient notation for the laws of electromagnetism, namely the "manifestly covariant" tensor form.

The **Kelvin–Stokes theorem**, named after Lord Kelvin and George Stokes, also known as the **Stokes' theorem**, the **fundamental theorem for curls** or simply the **curl theorem**, is a theorem in vector calculus on . Given a vector field, the theorem relates the integral of the curl of the vector field over some surface, to the line integral of the vector field around the boundary of the surface.

In continuum mechanics, a **compatible** deformation **tensor field** in a body is that *unique* tensor field that is obtained when the body is subjected to a continuous, single-valued, displacement field. **Compatibility** is the study of the conditions under which such a displacement field can be guaranteed. Compatibility conditions are particular cases of integrability conditions and were first derived for linear elasticity by Barré de Saint-Venant in 1864 and proved rigorously by Beltrami in 1886.

**Lagrangian field theory** is a formalism in classical field theory. It is the field-theoretic analogue of Lagrangian mechanics. Lagrangian mechanics is used for discrete particles each with a finite number of degrees of freedom. Lagrangian field theory applies to continua and fields, which have an infinite number of degrees of freedom.

- Warnick, Karl; Russer, Peter (2014). "Differential Forms and Electromagnetic Field Theory" (PDF).
*Progress in Electromagnetics Research*.**148**: 83–112. doi:10.2528/PIER14063009. - Russer, Peter (2006).
*Electromagnetics, Microwave Circuit and Antenna Design for Communications Engineering*(2nd ed.). Artech House. ISBN 978-1-58053-907-4. (with worked problems in Warnick, Russer 2006 ISBN 1-59693-096-9) - Hehl, Friedrich; Obukhov, Yuri (2003).
*Foundations of Classical Electrodynamics*. Birkhäuser. ISBN 978-0-8176-4222-8. - Doran, Chris; Lasenby, Anthony (2007).
*Geometric Algebra for Physicists*. Cambridge Univ. Press. ISBN 978-0-521-71595-9.

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