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{{electromagnetism|cTopic=Electrodynamics}}

In [[physics]] (specifically in [[electromagnetism]]) the '''Lorentz force''' (or '''electromagnetic force''') is the combination of electric and magnetic [[force]] on a [[point charge]] due to [[electromagnetic field]]s. A particle of charge ''q'' moving with a velocity '''v''' in an [[electric field]] '''E''' and a [[magnetic field]] '''B''' experiences a force of
: <math>\mathbf{F} = q\mathbf{E} + q\mathbf{v} \times \mathbf{B}</math>
(in [[International System of Units|SI units]]<ref name=units>In SI units, {{math|'''B'''}} is measured in [[tesla (unit)|teslas]] (symbol: T). In [[Gaussian units|Gaussian-cgs units]], {{math|'''B'''}} is measured in [[gauss (unit)|gauss]] (symbol: G). See e.g. {{cite web |url=http://www.ngdc.noaa.gov/geomag/faqgeom.shtml |title=Geomagnetism Frequently Asked Questions |publisher=National Geophysical Data Center |accessdate=21 October 2013}})</ref><ref name=units2>The {{math|'''H'''}}-field is measured in [[ampere]]s per metre (A/m) in SI units, and in [[oersted]]s (Oe) in cgs units. {{cite web|title=International system of units (SI) |url=http://physics.nist.gov/cuu/Units/units.html |work=NIST reference on constants, units, and uncertainty |publisher=National Institute of Standards and Technology |accessdate=9 May 2012}}</ref>). Variations on this basic formula describe the magnetic force on a current-carrying wire (sometimes called [[#Force on a current-carrying wire|Laplace force]]), the [[electromotive force]] in a wire loop moving through a magnetic field (an aspect of [[Faraday's law of induction]]), and the force on a charged particle which might be traveling near the [[speed of light]] ([[special relativity|relativistic]] form of the Lorentz force).

Historians suggest that the law is implicit in a paper by [[James Clerk Maxwell]], published in 1865.<ref name=Huray/> [[Hendrik Lorentz]] arrived in a complete derivation in 1895,<ref name=Dahl/> identifying the contribution of the electric force a few years after [[Oliver Heaviside]] correctly identified the contribution of the magnetic force.<ref name=Nahin/>

==Equation==

===Charged particle===

[[File:Lorentz force particle.svg|200px|thumb|Lorentz force '''F''' on a [[charged particle]] (of charge ''q'') in motion (instantaneous velocity '''v'''). The [[electric field|'''E''' field]] and [[magnetic field|'''B''' field]] vary in space and time.]]

The force '''F''' acting on a particle of [[electric charge]] ''q'' with instantaneous velocity '''v''', due to an external electric field '''E''' and magnetic field '''B''', is given by (in [[SI units]]<ref name=units/>):<ref name=Jackson2/>

{{Equation box 1
|indent =:
|equation = <math>\mathbf{F} = q(\mathbf{E} + \mathbf{v} \times \mathbf{B})</math>
|cellpadding
|border
|border colour = #50C878
|background colour = #ECFCF4
}}

where <big>×</big> is the vector cross product (all boldface quantities are vectors). In terms of cartesian components, we have:

: <math>F_x = q (E_x + v_y B_z - v_z B_y),</math>
: <math>F_y = q (E_y + v_z B_x - v_x B_z),</math>
: <math>F_z = q (E_z + v_x B_y - v_y B_x).</math>

In general, the electric and magnetic fields are functions of the position and time. Therefore, explicitly, the Lorentz force can be written as:

:<math>\mathbf{F}\left(\mathbf{r},\mathbf{\dot{r}},t,q\right) = q\left[\mathbf{E}(\mathbf{r},t) + \mathbf{\dot{r}} \times \mathbf{B}(\mathbf{r},t)\right]</math>

in which '''r''' is the position vector of the charged particle, ''t'' is time, and the overdot is a time derivative.

A positively charged particle will be accelerated in the ''same'' linear orientation as the '''E''' field, but will curve perpendicularly to both the instantaneous velocity vector '''v''' and the '''B''' field according to the [[right-hand rule]] (in detail, if the fingers of the right hand are extended to point in the direction of '''v''' and are then curled to point in the direction of '''B''', then the extended thumb will point in the direction of '''F''').

The term ''q'''''E''' is called the '''electric force''', while the term ''q''('''v''' <big>×</big> '''B''') is called the '''magnetic force'''.<ref name=Griffiths1>See Griffiths, page 204.</ref> According to some definitions, the term "Lorentz force" refers specifically to the formula for the magnetic force,<ref name=Griffiths2>For example, see the [http://ilorentz.org/history/lorentz/lorentz.html website of the Lorentz Institute] or Griffiths.</ref> with the ''total'' electromagnetic force (including the electric force) given some other (nonstandard) name. This article will ''not'' follow this nomenclature: In what follows, the term "Lorentz force" will refer to the expression for the total force.

The magnetic force component of the Lorentz force manifests itself as the force that acts on a current-carrying wire in a magnetic field. In that context, it is also called the [[#Force on a current-carrying wire|Laplace force]].

The Lorentz force is a force exerted by the electromagnetic field on the charged particle, that is, it is the rate at which linear momentum is transferred from the electromagnetic field to the particle. Associated with it is the power which is the rate at which energy is transferred from the electromagnetic field to the particle. That power is
:<math>\mathbf{v} \cdot \mathbf{F} = q \, \mathbf{v} \cdot \mathbf{E}</math>.
Notice that the magnetic field does not contribute to the power because the magnetic force is always perpendicular to the velocity of the particle.

===Continuous charge distribution===

[[File:Lorentz force continuum.svg|200px|thumb|Lorentz force (per unit 3-volume) '''f''' on a continuous [[charge distribution]] ([[charge density]] ρ) in motion. The 3-[[current density]] '''J''' corresponds to the motion of the charge element ''dq'' in [[volume element]] ''dV'' and varies throughout the continuum.]]

For a continuous [[charge distribution]] in motion, the Lorentz force equation becomes:

:<math>\mathrm{d}\mathbf{F} = \mathrm{d}q\left(\mathbf{E} + \mathbf{v} \times \mathbf{B}\right)\,\!</math>

where ''d'''''F''' is the force on a small piece of the charge distribution with charge ''dq''. If both sides of this equation are divided by the volume of this small piece of the charge distribution ''dV'', the result is:

:<math>\mathbf{f} = \rho\left(\mathbf{E} + \mathbf{v} \times \mathbf{B}\right)\,\!</math>

where '''f''' is the ''force density'' (force per unit volume) and ''ρ'' is the [[charge density]] (charge per unit volume). Next, the [[current density]] corresponding to the motion of the charge continuum is

:<math>\mathbf{J} = \rho \mathbf{v} \,\!</math>

so the continuous analogue to the equation is<ref name="Electrodynamics 1999">{{cite book|last=Griffiths|first=David J.|title=Introduction to electrodynamics|year=1999|publisher=Prentice Hall|location=Upper Saddle River, New Jersey [u.a.]|isbn=978-0-13-805326-0|edition=3rd|others=reprint. with corr.|url=https://archive.org/details/introductiontoel00grif_0}}</ref>

{{Equation box 1
|indent =:
|equation = <math>\mathbf{f} = \rho \mathbf{E} + \mathbf{J} \times \mathbf{B}\,\!</math>
|cellpadding= 6
|border
|border colour = #0073CF
|background colour=#F5FFFA}}

The total force is the [[volume integral]] over the charge distribution:

:<math> \mathbf{F} = \iiint \! ( \rho \mathbf{E} + \mathbf{J} \times \mathbf{B} )\,\mathrm{d}V. \,\!</math>

By eliminating ρ and '''J''', using [[Maxwell's equations]], and manipulating using the theorems of [[vector calculus]], this form of the equation can be used to derive the [[Maxwell stress tensor]] '''σ''', in turn this can be combined with the [[Poynting vector]] '''S''' to obtain the [[electromagnetic stress–energy tensor]] '''T''' used in [[general relativity]].<ref name="Electrodynamics 1999"/>

In terms of '''σ''' and '''S''', another way to write the Lorentz force (per unit volume) is<ref name="Electrodynamics 1999"/>

:<math> \mathbf{f} = \nabla\cdot\boldsymbol{\sigma} - \dfrac{1}{c^2} \dfrac{\partial \mathbf{S}}{\partial t}  \,\!</math>

where ''c'' is the [[speed of light]] and [[Nabla symbol|∇]]· denotes the divergence of a [[tensor field]]. Rather than the amount of charge and its velocity in electric and magnetic fields, this equation relates the [[energy flux]] (flow of ''energy'' per unit time per unit distance) in the fields to the force exerted on a charge distribution. See [[Covariant formulation of classical electromagnetism#Charge continuum|Covariant formulation of classical electromagnetism]] for more details.

The density of power associated with the Lorentz force in a material medium is
:<math>\mathbf{J} \cdot \mathbf{E}</math>.

If we separate the total charge and total current into their free and bound parts, we get that the density of the Lorentz force is
:<math>\mathbf{f} = (\rho_f - \nabla \cdot \mathbf P) \mathbf{E} + (\mathbf{J}_f + \nabla\times\mathbf{M} + \frac{\partial\mathbf{P}}{\partial t}) \times \mathbf{B}</math>.

where: &rho;<sub>f</sub> is the density of free charge; ''P'' is the [[polarization density]]; ''J''<sub>f</sub> is the density of free current; and ''M'' is the [[magnetization]] density. In this way, the Lorentz force can explain the torque applied to a permanent magnet by the magnetic field. The density of the associated power is
:<math>\left(\mathbf{J}_f + \nabla\times\mathbf{M} + \frac{\partial\mathbf{P}}{\partial t}\right) \cdot \mathbf{E}</math>.

===Equation in cgs units===
The above-mentioned formulae use [[SI units]] which are the most common among experimentalists, technicians, and engineers. In [[Gaussian units|cgs-Gaussian units]], which are somewhat more common among theoretical physicists as well as condensed matter experimentalists, one has instead
: <math>\mathbf{F} = q_\mathrm{cgs} \left(\mathbf{E}_\mathrm{cgs} + \frac{\mathbf{v}}{c} \times \mathbf{B}_\mathrm{cgs}\right).</math>
where ''c'' is the [[speed of light]]. 
Although this equation looks slightly different, it is completely equivalent, since
one has the following relations:<ref name=units/>

:<math>q_\mathrm{cgs}=\frac{q_\mathrm{SI}}{\sqrt{4\pi \epsilon_0}},\quad \mathbf E_\mathrm{cgs} =\sqrt{4\pi\epsilon_0}\,\mathbf E_\mathrm{SI},\quad \mathbf B_\mathrm{cgs} ={\sqrt{4\pi /\mu_0}}\,{\mathbf B_\mathrm{SI}},\quad c=\frac{1}{\sqrt{ \epsilon_0\mu_0}}.</math>

where ε<sub>0</sub> is the [[vacuum permittivity]] and μ<sub>0</sub> the [[vacuum permeability]]. In practice, the subscripts "cgs" and "SI" are always omitted, and the unit system has to be assessed from context.

==History==
{{multiple image
   | align = right
   | position
   | direction = horizontal
   | footer = [[Charged particle]]s experiencing the Lorentz force.
   | image1    = Lorentz force.svg
   | caption1  = Trajectory of a particle with a positive or negative charge ''q'' under the influence of a magnetic field ''B'', which is directed perpendicularly out of the screen.
   | width1     = 250
   | image2    = Cyclotron motion.jpg
   | caption2  = Beam of electrons moving in a circle, due to the presence of a magnetic field. Purple light is emitted along the electron path, due to the electrons colliding with gas molecules in the bulb. A [[Teltron tube]] is used in this example.
   | width2     = 250
  }}
Early attempts to quantitatively describe the electromagnetic force were made in the mid-18th century. It was proposed that the force on magnetic poles, by [[Johann Tobias Mayer]] and others in 1760,<ref>
{{Cite book
|first = Michel
|last = Delon
|title = Encyclopedia of the Enlightenment
|place = Chicago, IL
|publisher = Fitzroy Dearborn Publishers
|year = 2001
|pages = 538
|isbn = 157958246X}}</ref> and electrically charged objects, by [[Henry Cavendish]] in 1762,<ref>
{{Cite book
|first = Elliot H.
|last = Goodwin
|title = The New Cambridge Modern History Volume 8: The American and French Revolutions, 1763–93
|place = Cambridge
|publisher = Cambridge University Press
|year = 1965
|pages = 130
|isbn = 9780521045469}}</ref> obeyed an [[inverse-square law]]. However, in both cases the experimental proof was neither complete nor conclusive. It was not until 1784 when [[Charles-Augustin de Coulomb]], using a [[torsion balance]], was able to definitively show through experiment that this was true.<ref>
{{Cite book
|first = Herbert W.
|last = Meyer
|title = A History of Electricity and Magnetism
|place = Norwalk, Connecticut
|publisher = Burndy Library
|year = 1972
|pages = 30–31
|isbn = 0-262-13070-X
|url = https://archive.org/details/AHistoryof_00_Meye
}}</ref> Soon after the discovery in 1820 by [[H. C. Ørsted]] that a magnetic needle is acted on by a voltaic current, [[André-Marie Ampère]] that same year was able to devise through experimentation the formula for the angular dependence of the force between two current elements.<ref>
{{Cite book
|first = Gerrit L.
|last = Verschuur
|title = Hidden Attraction : The History And Mystery Of Magnetism
|place = New York
|publisher = Oxford University Press
|isbn = 0-19-506488-7
|year = 1993
|pages = 78–79
|url = https://archive.org/details/hiddenattraction00vers
}}</ref><ref>
{{Cite book
|first = Olivier
|last = Darrigol
|title = Electrodynamics from [[André Ampère|Ampère]] to [[Albert Einstein|Einstein]]
|place = Oxford, [England]
|publisher = Oxford University Press
|isbn = 0-19-850593-0
|year = 2000
|pages = 9, 25
|postscript = <!--None-->}}</ref> In all these descriptions, the force was always given in terms of the properties of the objects involved and the distances between them rather than in terms of electric and magnetic fields.<ref>
{{Cite book
|first = Gerrit L.
|last = Verschuur
|title = Hidden Attraction : The History And Mystery Of Magnetism
|place = New York
|publisher = Oxford University Press
|isbn = 0-19-506488-7
|year = 1993
|page = 76
|url = https://archive.org/details/hiddenattraction00vers
}}</ref>

The modern concept of electric and magnetic fields first arose in the theories of [[Michael Faraday]], particularly his idea of [[lines of force]], later to be given full mathematical description by [[William Thomson, 1st Baron Kelvin|Lord Kelvin]] and [[James Clerk Maxwell]].<ref>
{{Cite book
|first = Olivier
|last = Darrigol
|title = Electrodynamics from [[André Ampère|Ampère]] to [[Albert Einstein|Einstein]]
|place = Oxford, [England]
|publisher = Oxford University Press
|isbn = 0-19-850593-0
|year = 2000
|pages = 126–131, 139–144
|postscript = <!--None-->}}</ref> From a modern perspective it is possible to identify in Maxwell's 1865 formulation of his field equations a form of the Lorentz force equation in relation to electric currents,<ref name=Huray>
{{Cite book
|first = Paul G.
|last = Huray
|title = Maxwell's Equations
|publisher = Wiley-IEEE
|isbn = 0-470-54276-4
|year = 2010
|page = 22
|url = https://books.google.com/books?id=0QsDgdd0MhMC&pg=PA22#v=onepage&q&f=false}}</ref> however, in the time of Maxwell it was not evident how his equations related to the forces on moving charged objects. [[J. J. Thomson]] was the first to attempt to derive from Maxwell's field equations the electromagnetic forces on a moving charged object in terms of the object's properties and external fields. Interested in determining the electromagnetic behavior of the charged particles in [[cathode ray]]s, Thomson published a paper in 1881 wherein he gave the force on the particles due to an external magnetic field as<ref name=Nahin>Paul J. Nahin, [https://books.google.com/books?id=e9wEntQmA0IC&source=gbs_navlinks_s ''Oliver Heaviside''], JHU Press, 2002.</ref>
:<math>\mathbf{F} = \frac{q}{2}\mathbf{v} \times \mathbf{B}.</math>
Thomson derived the correct basic form of the formula, but, because of some miscalculations and an incomplete description of the [[displacement current]], included an incorrect scale-factor of a half in front of the formula.  [[Oliver Heaviside]] invented the modern vector notation and applied it to Maxwell's field equations; he also (in 1885 and 1889) had fixed the mistakes of Thomson's derivation and arrived at the correct form of the magnetic force on a moving charged object.<ref name=Nahin/><ref>
{{Cite book
|first = Olivier
|last = Darrigol
|title = Electrodynamics from [[André Ampère|Ampère]] to [[Albert Einstein|Einstein]]
|place = Oxford, [England]
|publisher = Oxford University Press
|isbn = 0-19-850593-0
|year = 2000
|pages = 200, 429–430
|postscript = <!--None-->}}</ref><ref>{{cite journal | author= Heaviside, Oliver| title=On the Electromagnetic Effects due to the Motion of Electrification through a Dielectric | journal=Philosophical Magazine |date=April 1889 |page=324 |url=http://en.wikisource.org/wiki/Motion_of_Electrification_through_a_Dielectric}}</ref> Finally, in 1895,<ref name=Dahl>Per F. Dahl, ''Flash of the Cathode Rays: A History of J J Thomson's Electron'', CRC Press, 1997, p. 10.</ref><ref>Lorentz, Hendrik Antoon, ''Versuch einer Theorie der electrischen und optischen Erscheinungen in bewegten Körpern'', 1895.</ref> [[Hendrik Lorentz]] derived the modern form of the formula for the electromagnetic force which includes the contributions to the total force from both the electric and the magnetic fields. Lorentz began by abandoning the Maxwellian descriptions of the ether and conduction. Instead, Lorentz made a distinction between matter and the [[luminiferous aether]] and sought to apply the Maxwell equations at a microscopic scale. Using Heaviside's version of the Maxwell equations for a stationary ether and applying [[Lagrangian mechanics]] (see below), Lorentz arrived at the correct and complete form of the force law that now bears his name.<ref>
{{Cite book
|first = Olivier
|last = Darrigol
|title = Electrodynamics from [[André Ampère|Ampère]] to [[Albert Einstein|Einstein]]
|place = Oxford, [England]
|publisher = Oxford University Press
|isbn = 0-19-850593-0
|year = 2000
|page = 327
|postscript = <!--None-->}}</ref><ref>
{{cite book
|last = Whittaker
|first = E. T.
|title = A History of the Theories of Aether and Electricity: From the Age of Descartes to the Close of the Nineteenth Century
|publisher = Longmans, Green and Co.
|year = 1910
|pages = 420–423
|url = https://books.google.com/books?id=CGJDAAAAIAAJ&printsec=frontcover#v=onepage&q&f=false
|isbn = 1-143-01208-9}}</ref>

==Trajectories of particles due to the Lorentz force==
{{Main|Guiding center}}
[[File:charged-particle-drifts.svg|300px|thumbnail|right|'''Charged particle drifts''' in a homogeneous magnetic field. (A) No disturbing force (B) With an electric field, E (C) With an independent force, F (e.g. gravity) (D) In an inhomogeneous magnetic field, grad H]]
In many cases of practical interest, the motion in a [[magnetic field]] of an [[electric charge|electrically charged]] particle (such as an [[electron]] or [[ion]] in a [[Plasma (physics)|plasma]]) can be treated as the [[Quantum superposition|superposition]] of a relatively fast circular motion around a point called the '''guiding center''' and a relatively slow '''drift''' of this point. The drift speeds may differ for various species depending on their charge states, masses, or temperatures, possibly resulting in electric currents or chemical separation.

==Significance of the Lorentz force==

While the modern Maxwell's equations describe how electrically charged particles and currents or moving charged particles give rise to electric and magnetic fields, the Lorentz force law completes that picture by describing the force acting on a moving point charge ''q'' in the presence of electromagnetic fields.<ref name=Jackson2>See Jackson, page 2. The book lists the four modern Maxwell's equations, and then states, "Also essential for consideration of charged particle motion is the Lorentz force equation, '''F''' = ''q'' ('''E'''+ '''v <big>×</big> B'''), which gives the force acting on a point charge ''q'' in the presence of electromagnetic fields."</ref><ref name=Griffiths50>See Griffiths, page 326, which states that Maxwell's equations, "together with the [Lorentz] force law...summarize the entire theoretical content of classical electrodynamics".</ref> The Lorentz force law describes the effect of '''E''' and '''B''' upon a point charge, but such electromagnetic forces are not the entire picture. Charged particles are possibly coupled to other forces, notably gravity and nuclear forces. Thus, Maxwell's equations do not stand separate from other physical laws, but are coupled to them via the charge and current densities. The response of a point charge to the Lorentz law is one aspect; the generation of '''E''' and '''B''' by currents and charges is another.

In real materials the Lorentz force is inadequate to describe the collective behavior of charged particles, both in principle and as a matter of computation. The charged particles in a material medium not only respond to the '''E''' and '''B''' fields but also generate these fields. Complex transport equations must be solved to determine the time and spatial response of charges, for example, the [[Boltzmann equation]] or the [[Fokker–Planck equation]] or the [[Navier–Stokes equations]]. For example, see [[magnetohydrodynamics]], [[fluid dynamics]], [[electrohydrodynamics]], [[superconductivity]], [[stellar evolution]]. An entire physical apparatus for dealing with these matters has developed. See for example, [[Green–Kubo relations]] and [[Green's function (many-body theory)]].

==Lorentz force law as the definition of E and B==

In many textbook treatments of classical electromagnetism, the Lorentz force Law is used as the ''definition'' of the electric and magnetic fields '''E''' and '''B'''.<ref name=Jackson20>See, for example, Jackson, pp. 777–8.</ref><ref>{{cite book|title=Gravitation|author1=J.A. Wheeler |author2=C. Misner |author3=K.S. Thorne |publisher=W.H. Freeman & Co|year=1973|pages=72–73|isbn=0-7167-0344-0}}. These authors use the Lorentz force in tensor form as definer of the [[electromagnetic tensor]] ''F'', in turn the fields '''E''' and '''B'''.</ref><ref>{{cite book|title=Electromagnetism|edition=2nd|author1=I.S. Grant |author2=W.R. Phillips |author3=Manchester Physics |publisher=John Wiley & Sons|year=1990|page=122|isbn=978-0-471-92712-9}}</ref> To be specific, the Lorentz force is understood to be the following empirical statement:

:''The electromagnetic force '''F''' on a [[test charge]] at a given point and time is a certain function of its charge ''q'' and velocity '''v''', which can be parameterized by exactly two vectors '''E''' and '''B''', in the functional form'':

::<math>\mathbf{F}=q(\mathbf{E}+\mathbf{v}\times\mathbf{B})</math>

This is valid, even for particles approaching the speed of light (that is, [[Norm (mathematics)#Euclidean norm|magnitude]] of '''v''' = |'''v'''| ≈ ''c'').<ref>{{cite book|title=Electromagnetism (2nd Edition)|author1=I.S. Grant |author2=W.R. Phillips |author3=Manchester Physics |publisher=John Wiley & Sons|year=1990|page=123|isbn=978-0-471-92712-9}}</ref> So the two [[vector field]]s '''E''' and '''B''' are thereby defined throughout space and time, and these are called the "electric field" and "magnetic field". The fields are defined everywhere in space and time with respect to what force a test charge would receive regardless of whether a charge is present to experience the force.

As a definition of '''E''' and '''B''', the Lorentz force is only a definition in principle because a real particle (as opposed to the hypothetical "test charge" of infinitesimally-small mass and charge) would generate its own finite '''E''' and '''B''' fields, which would alter the electromagnetic force that it experiences. In addition, if the charge experiences acceleration, as if forced into a curved trajectory by some external agency, it emits radiation that causes braking of its motion. See for example [[Bremsstrahlung]] and [[synchrotron light]]. These effects occur through both a direct effect (called the [[Abraham–Lorentz force|radiation reaction force]]) and indirectly (by affecting the motion of nearby charges and currents). Moreover, net force must include [[gravity]], [[Electroweak interaction|electroweak]], and any other forces aside from electromagnetic force.

==Force on a current-carrying wire==
[[File:Regla mano derecha Laplace.svg|right|thumb|250px|Right-hand rule for a current-carrying wire in a magnetic field ''B'']]
When a wire carrying an electric current is placed in a magnetic field, each of the moving charges, which comprise the current, experiences the Lorentz force, and together they can create a macroscopic force on the wire (sometimes called the '''Laplace force'''). By combining the Lorentz force law above with the definition of electric current, the following equation results, in the case of a straight, stationary wire:<ref>{{Cite web|url=http://www.physicsexperiment.co.uk/content/bil.html|title=Physics Experiments|website=www.physicsexperiment.co.uk|language=en|access-date=2018-08-14}}</ref>

:<math>\mathbf{F} = I \boldsymbol{\ell} \times \mathbf{B}</math>

where '''ℓ''' is a vector whose magnitude is the length of wire, and whose direction is along the wire, aligned with the direction of [[conventional current]] charge flow ''I''.

If the wire is not straight but curved, the force on it can be computed by applying this formula to each [[infinitesimal]] segment of wire ''d'''''ℓ''', then adding up all these forces by [[integration (calculus)|integration]]. Formally, the net force on a stationary, rigid wire carrying a steady current ''I'' is

:<math>\mathbf{F} = I\int \mathrm{d}\boldsymbol{\ell}\times \mathbf{B}</math>

This is the net force. In addition, there will usually be [[torque]], plus other effects if the wire is not perfectly rigid.

One application of this is [[Ampère's force law]], which describes how two current-carrying wires can attract or repel each other, since each experiences a Lorentz force from the other's magnetic field. For more information, see the article: [[Ampère's force law]].

==EMF==

The magnetic force ({{nowrap|''q'''''v''' × '''B'''}}) component of the Lorentz force is responsible for ''motional'' [[electromotive force]] (or ''motional EMF''), the phenomenon underlying many electrical generators. When a conductor is moved through a magnetic field, the magnetic field exerts opposite forces on electrons and nuclei in the wire, and this creates the EMF. The term "motional EMF" is applied to this phenomenon, since the EMF is due to the ''motion'' of the wire.

In other electrical generators, the magnets move, while the conductors do not. In this case, the EMF is due to the electric force (''q'''''E''') term in the Lorentz Force equation. The electric field in question is created by the changing magnetic field, resulting in an ''induced'' EMF, as described by the [[Electromagnetic induction#Maxwell–Faraday equation|Maxwell–Faraday equation]] (one of the four modern [[Maxwell's equations]]).<ref name=Griffiths302>See Griffiths, pages 301–3.</ref>

Both of these EMFs, despite their apparently distinct origins, are described by the same equation, namely, the EMF is the rate of change of [[magnetic flux]] through the wire. (This is Faraday's law of induction, see [[Lorentz force#Lorentz force and Faraday.27s law of induction|below]].) Einstein's [[special theory of relativity]] was partially motivated by the desire to better understand this link between the two effects.<ref name=Griffiths302/> In fact, the electric and magnetic fields are different facets of the same electromagnetic field, and in moving from one inertial frame to another, the [[solenoidal vector field]] portion of the ''E''-field can change in whole or in part to a ''B''-field or ''vice versa''.<ref name=Chow>
{{cite book
|author=Tai L. Chow
|title=Electromagnetic theory
|year= 2006
|page =395
|publisher=Jones and Bartlett
|location=Sudbury MA
|isbn=0-7637-3827-1
|url=https://books.google.com/books?id=dpnpMhw1zo8C&pg=PA153&dq=isbn=0763738271}}
</ref>

==Lorentz force and Faraday's law of induction==
[[File:LeidenLorentz1.jpg|{{largethumb}}|Lorentz force -image on a wall in Leiden]]
{{main|Faraday's law of induction}}

Given a loop of wire in a [[magnetic field]], Faraday's law of induction states the induced [[electromotive force]] (EMF) in the wire is:

:<math>\mathcal{E} = -\frac{\mathrm{d}\Phi_B}{\mathrm{d}t}</math>

where

:<math> \Phi_B = \iint_{\Sigma(t)} \mathrm{d} \mathbf{A} \cdot \mathbf{B}(\mathbf{r}, t)</math>

is the [[magnetic flux]] through the loop, '''B''' is the magnetic field, Σ(''t'') is a surface bounded by the closed contour ∂Σ(''t''), at all at time ''t'', d'''A''' is an infinitesimal [[vector area]] element of Σ(''t'') (magnitude is the area of an infinitesimal patch of surface, direction is [[orthogonal]] to that surface patch).

The ''sign'' of the EMF is determined by [[Lenz's law]]. Note that this is valid for not only a ''stationary'' wire{{snd}}but also for a ''moving'' wire.

From [[Faraday's law of induction]] (that is valid for a moving wire, for instance in a motor) and the [[Maxwell Equations]], the Lorentz Force can be deduced. The reverse is also true, the Lorentz force and the [[Maxwell Equations]] can be used to derive the [[Faraday's law of induction|Faraday Law]].

Let Σ(''t'') be the moving wire, moving together without rotation and with constant velocity '''v''' and Σ(''t'') be the internal surface of the wire. The EMF around the closed path ∂Σ(''t'') is given by:<ref name=Landau>
{{cite book
|author=Landau, L. D., Lifshitz, E. M., & Pitaevskiĭ, L. P.
|title=Electrodynamics of continuous media; Volume 8  ''Course of Theoretical Physics''
|year= 1984
|page =§63 (§49 pp. 205–207 in 1960 edition)
|edition=Second
|publisher=Butterworth-Heinemann
|location=Oxford
|isbn=0-7506-2634-8
|url=http://worldcat.org/search?q=0750626348&qt=owc_search}}
</ref>

:<math>\mathcal{E} =\oint_{\partial \Sigma (t)} \mathrm{d} \boldsymbol{\ell} \cdot \mathbf{F} / q</math>

where

:<math>\mathbf{E} = \mathbf{F} / q</math>

is the electric field and d'''ℓ''' is an [[infinitesimal]] vector element of the contour ∂Σ(''t'').

NB: Both d'''ℓ''' and d'''A''' have a sign ambiguity; to get the correct sign, the [[right-hand rule]] is used, as explained in the article [[Kelvin–Stokes theorem]].

The above result can be compared with the version of Faraday's law of induction that appears in the modern Maxwell's equations, called here the ''Maxwell–Faraday equation'':

:<math>\nabla \times \mathbf{E} = -\frac{\partial \mathbf{B}}{\partial t} \ .</math>

The Maxwell–Faraday equation also can be written in an ''integral form'' using the [[Kelvin–Stokes theorem]].<ref name=Harrington>
{{cite book
|author=Roger F Harrington
|title=Introduction to electromagnetic engineering
|year= 2003
|page =56
|publisher=Dover Publications
|location=Mineola, New York
|isbn=0-486-43241-6
|url=https://books.google.com/books?id=ZlC2EV8zvX8C&pg=PA57&dq=%22faraday%27s+law+of+induction%22}}
</ref>

So we have, the Maxwell Faraday equation:

:<math> \oint_{\partial \Sigma(t)}\mathrm{d} \boldsymbol{\ell} \cdot \mathbf{E}(\mathbf{r},\ t) = - \  \iint_{\Sigma(t)}  \mathrm{d} \mathbf {A} \cdot {{\mathrm{d} \,\mathbf {B}(\mathbf{r},\ t)} \over \mathrm{d}t } </math>

and the Faraday Law,

:<math> \oint_{\partial \Sigma(t)}\mathrm{d} \boldsymbol{\ell} \cdot \mathbf{F}/q(\mathbf{r},\ t) = - \frac{\mathrm{d}}{\mathrm{d}t}  \iint_{\Sigma(t)}  \mathrm{d} \mathbf {A} \cdot \mathbf{B}(\mathbf{r},\ t). </math>

The two are equivalent if the wire is not moving. Using the [[Leibniz integral rule]] and that ''div'' '''B''' = 0, results in,

:<math> \oint_{\partial \Sigma(t)} \mathrm{d} \boldsymbol{\ell} \cdot \mathbf{F}/q(\mathbf{r}, t) = 
- \iint_{\Sigma(t)}  \mathrm{d} \mathbf{A} \cdot \frac{\partial}{\partial t} \mathbf{B}(\mathbf{r}, t) +
\oint_{\partial \Sigma(t)} \!\!\!\!\mathbf{v} \times \mathbf{B} \,\mathrm{d} \boldsymbol{\ell}
</math>

and using the Maxwell Faraday equation,

:<math> \oint_{\partial \Sigma(t)} \mathrm{d} \boldsymbol{\ell} \cdot \mathbf{F}/q(\mathbf{r},\ t) = 
\oint_{\partial \Sigma(t)} \mathrm{d} \boldsymbol{\ell} \cdot \mathbf{E}(\mathbf{r},\ t)  +
\oint_{\partial \Sigma(t)}\!\!\!\! \mathbf{v} \times \mathbf{B}(\mathbf{r},\ t)\, \mathrm{d} \boldsymbol{\ell}
</math>

since this is valid for any wire position it implies that,

:<math> \mathbf{F}= q\,\mathbf{E}(\mathbf{r},\ t)  + q\,\mathbf{v} \times \mathbf{B}(\mathbf{r},\ t).</math>

Faraday's law of induction holds whether the loop of wire is rigid and stationary, or in motion or in process of deformation, and it holds whether the magnetic field is constant in time or changing. However, there are cases where Faraday's law is either inadequate or difficult to use, and application of the underlying Lorentz force law is necessary. See [[Faraday paradox#Inapplicability of Faraday's law|inapplicability of Faraday's law]].

If the magnetic field is fixed in time and the conducting loop moves through the field, the magnetic flux Φ<sub>''B''</sub> linking the loop can change in several ways. For example, if the '''B'''-field varies with position, and the loop moves to a location with different '''B'''-field, Φ<sub>''B''</sub> will change. Alternatively, if the loop changes orientation with respect to the '''B'''-field, the {{nowrap|'''B''' &sdot; d'''A'''}} differential element will change because of the different angle between '''B''' and d'''A''', also changing Φ<sub>''B''</sub>. As a third example, if a portion of the circuit is swept through a uniform, time-independent '''B'''-field, and another portion of the circuit is held stationary, the flux linking the entire closed circuit can change due to the shift in relative position of the circuit's component parts with time (surface ∂Σ(''t'') time-dependent). In all three cases, Faraday's law of induction then predicts the EMF generated by the change in Φ<sub>''B''</sub>.

Note that the Maxwell Faraday's equation implies that the Electric Field '''E''' is non conservative when the Magnetic Field '''B''' varies in time, and is not expressible as the gradient of a [[scalar field]], and not subject to the [[gradient theorem]] since its rotational is not zero.<ref name="Landau"/><ref>{{cite book
|author=M N O Sadiku
|title=Elements of electromagnetics
|year= 2007
|page =391
|edition=Fourth
|publisher=Oxford University Press
|location=NY/Oxford
|isbn=0-19-530048-3
|url=https://books.google.com/books?id=w2ITHQAACAAJ&dq=ISBN0-19-530048-3}}</ref>

==Lorentz force in terms of potentials==

{{see also|Mathematical descriptions of the electromagnetic field|Maxwell's equations|Helmholtz decomposition}}

The '''E''' and '''B''' fields can be replaced by the [[magnetic vector potential]] '''A''' and ([[Scalar (mathematics)|scalar]]) [[electrostatic potential]] ''ϕ''  by

:<math> \mathbf{E} = - \nabla \phi - \frac { \partial \mathbf{A} } { \partial t }</math>
:<math>\mathbf{B} = \nabla \times \mathbf{A}</math>

where ∇ is the gradient, ∇⋅ is the divergence, ∇× is the [[Curl (mathematics)|curl]].

The force becomes

:<math>\mathbf{F} = q\left[-\nabla \phi- \frac{\partial \mathbf{A}}{\partial t}+\mathbf{v}\times(\nabla\times\mathbf{A})\right]</math>

and using an [[Triple product#Vector triple product|identity for the triple product]] simplifies to

{{Equation box 1
|indent =:
|equation = <math>\mathbf{F} = q\left[-\nabla \phi- \frac{\partial \mathbf{A}}{\partial t}+ \nabla (\mathbf{v}\cdot\mathbf{A})-(\mathbf{v}\cdot\nabla)\mathbf{A} \right]</math>
|cellpadding= 6
|border
|border colour = #0073CF
|background colour=#F5FFFA}}

('''v''' has no dependence on position, so there's no need to use [[Vector calculus identities#Special notations|Feynman's subscript notation]]). Using the chain rule, the [[total derivative]] of '''A''' is:

:<math>\frac{\mathrm{d}\mathbf{A}}{\mathrm{d}t} = \frac{\partial\mathbf{A}}{\partial t}+(\mathbf{v}\cdot\nabla)\mathbf{A} </math>

so the above expression can be rewritten as:

:<math>\mathbf{F} = q\left[-\nabla (\phi-\mathbf{v}\cdot\mathbf{A})- \frac{\mathrm{d}\mathbf{A}}{\mathrm{d}t}\right]</math>.

With '''v''' = '''ẋ''', we can put the equation into the convenient Euler–Lagrange form

{{Equation box 1
|indent =:
|equation = <math>\mathbf{F} = q\left[-\nabla_{\mathbf{x}}(\phi-\dot{\mathbf{x}}\cdot\mathbf{A})+ \frac{\mathrm{d}}{\mathrm{d}t}\nabla_{\dot{\mathbf{x}}}(\phi-\dot{\mathbf{x}}\cdot\mathbf{A})\right]</math>
|cellpadding= 6
|border
|border colour = #0073CF
|background colour=#F5FFFA}}

where

<math>\nabla_{\mathbf{x}} = \hat{x} \dfrac{\partial}{\partial x} + \hat{y} \dfrac{\partial}{\partial y} + \hat{z} \dfrac{\partial}{\partial z}</math>

and

<math>\nabla_{\dot{\mathbf{x}}} = \hat{x} \dfrac{\partial}{\partial \dot{x}} + \hat{y} \dfrac{\partial}{\partial \dot{y}} + \hat{z} \dfrac{\partial}{\partial \dot{z}}</math>.

==Lorentz force and analytical mechanics==<!--this section links from [[Momentum#Lagrangian and Hamiltonian formulation]]-->
{{see also|Momentum#Lagrangian and Hamiltonian formulation|l1=Momentum}}
The [[Lagrangian mechanics|Lagrangian]] for a charged particle of mass ''m'' and charge ''q'' in an electromagnetic field equivalently describes the dynamics of the particle in terms of its ''energy'', rather than the force exerted on it. The classical expression is given by:<ref>Classical Mechanics (2nd Edition), T.W.B. Kibble, European Physics Series, McGraw Hill (UK), 1973, {{ISBN|0-07-084018-0}}.</ref>

:<math>L=\frac{m}{2}\mathbf{\dot{r}}\cdot\mathbf{\dot{r}}+q\mathbf{A}\cdot\mathbf{\dot{r}}-q\phi</math>

where '''A''' and ''ϕ'' are the potential fields as above. Using [[Lagrangian mechanics|Lagrange's equations]], the equation for the Lorentz force can be obtained.

:{| class="toccolours collapsible collapsed" width="80%" style="text-align:left"
!Derivation of Lorentz force from classical Lagrangian (SI units)
|-
|For an '''A''' field, a particle moving with velocity '''v''' = '''ṙ''' has [[potential momentum]] <math>q\mathbf{A}(\mathbf{r},t)</math>, so its potential energy is <math>q\mathbf{A}(\mathbf{r},t)\cdot\mathbf{\dot{r}}</math>. For a ''ϕ'' field, the particle's potential energy is <math>q\phi(\mathbf{r},t)</math>.

The total [[potential energy]] is then:

:<math>V=q\phi-q\mathbf{A}\cdot\mathbf{\dot{r}}</math>

and the [[kinetic energy]] is:

:<math>T=\frac{m}{2}\mathbf{\dot{r}}\cdot\mathbf{\dot{r}}</math>

hence the Lagrangian:

:<math>L=T-V=\frac{m}{2}\mathbf{\dot{r}}\cdot\mathbf{\dot{r}}+q\mathbf{A}\cdot\mathbf{\dot{r}}-q\phi</math>

:<math>L=\frac{m}{2}(\dot{x}^2+\dot{y}^2+\dot{z}^2) + q(\dot{x}A_x+\dot{y}A_y+\dot{z}A_z) - q\phi</math>

Lagrange's equations are

:<math>\frac{\mathrm{d}}{\mathrm{d}t}\frac{\partial L}{\partial \dot{x}}=\frac{\partial L}{\partial x}</math>

(same for ''y'' and ''z''). So calculating the partial derivatives:

:<math>\begin{align}\frac{\mathrm{d}}{\mathrm{d}t}\frac{\partial L}{\partial \dot{x}} & =m\ddot{x}+q\frac{\mathrm{d} A_x}{\mathrm{d}t} \\
& = m\ddot{x}+ \frac{q}{\mathrm{d}t}\left(\frac{\partial A_x}{\partial t}dt+\frac{\partial A_x}{\partial x}dx+\frac{\partial A_x}{\partial y}dy+\frac{\partial A_x}{\partial z}dz\right) \\
& = m\ddot{x}+ q\left(\frac{\partial A_x}{\partial t}+\frac{\partial A_x}{\partial x}\dot{x}+\frac{\partial A_x}{\partial y}\dot{y}+\frac{\partial A_x}{\partial z}\dot{z}\right)\\
\end{align}</math>

:<math>\frac{\partial L}{\partial x}= -q\frac{\partial \phi}{\partial x}+ q\left(\frac{\partial A_x}{\partial x}\dot{x}+\frac{\partial A_y}{\partial x}\dot{y}+\frac{\partial A_z}{\partial x}\dot{z}\right)</math>

equating and simplifying:

:<math>m\ddot{x}+ q\left(\frac{\partial A_x}{\partial t}+\frac{\partial A_x}{\partial x}\dot{x}+\frac{\partial A_x}{\partial y}\dot{y}+\frac{\partial A_x}{\partial z}\dot{z}\right)= -q\frac{\partial \phi}{\partial x}+ q\left(\frac{\partial A_x}{\partial x}\dot{x}+\frac{\partial A_y}{\partial x}\dot{y}+\frac{\partial A_z}{\partial x}\dot{z}\right)</math>

:<math>\begin{align} F_x & = -q\left(\frac{\partial \phi}{\partial x}+\frac{\partial A_x}{\partial t}\right) + q\left[\dot{y}\left(\frac{\partial A_y}{\partial x} - \frac{\partial A_x}{\partial y}\right)+\dot{z}\left(\frac{\partial A_z}{\partial x}-\frac{\partial A_x}{\partial z}\right)\right] \\
& = qE_x + q[\dot{y}(\nabla\times\mathbf{A})_z-\dot{z}(\nabla\times\mathbf{A})_y] \\
& = qE_x + q[\mathbf{\dot{r}}\times(\nabla\times\mathbf{A})]_x \\
& = qE_x + q(\mathbf{\dot{r}}\times\mathbf{B})_x
\end{align}</math>

and similarly for the ''y'' and ''z'' directions. Hence the force equation is:

:<math>\mathbf{F}= q(\mathbf{E} + \mathbf{\dot{r}}\times\mathbf{B})</math>
|}

The potential energy depends on the velocity of the particle, so the force is velocity dependent, so it is not conservative.

The relativistic Lagrangian is

:<math>L = -mc^2\sqrt{1-\left(\frac{\dot{\mathbf{r}}}{c}\right)^2} + q \mathbf{A}(\mathbf{r})\cdot\dot{\mathbf{r}} - q \phi(\mathbf{r}) \,\!</math>

The action is the relativistic [[arclength]] of the path of the particle in [[space time]], minus the potential energy contribution, plus an extra contribution which [[Quantum Mechanics|quantum mechanically]] is an extra [[phase (waves)|phase]] a charged particle gets when it is moving along a vector potential.

:{| class="toccolours collapsible collapsed" width="80%" style="text-align:left"
!Derivation of Lorentz force from relativistic Lagrangian (SI units)
|-
|
The equations of motion derived by [[calculus of variations|extremizing]] the action (see [[matrix calculus]] for the notation):

:<math> \frac{\mathrm{d}\mathbf{P}}{\mathrm{d}t} =\frac{\partial L}{\partial \mathbf{r}} = q {\partial \mathbf{A} \over \partial \mathbf{r}}\cdot \dot{\mathbf{r}} - q {\partial \phi \over \partial \mathbf{r} }\,\!</math>

:<math>\mathbf{P} -q\mathbf{A} = \frac{m\dot{\mathbf{r}}}{\sqrt{1-\left(\frac{\dot{\mathbf{r}}}{c}\right)^2}}\,</math>

are the same as [[Hamiltonian mechanics|Hamilton's equations of motion]]:

:<math> \frac{\mathrm{d}\mathbf{r}}{\mathrm{d}t} = \frac{\partial}{\partial \mathbf{p}}\left ( \sqrt{(\mathbf{P}-q\mathbf{A})^2 +(mc^2)^2} + q\phi \right ) \,\!</math>
:<math> \frac{\mathrm{d}\mathbf{p}}{\mathrm{d}t} = -{\partial \over \partial \mathbf{r}}\left ( \sqrt{(\mathbf{P}-q\mathbf{A})^2 + (mc^2)^2} + q\phi \right ) \,\!</math>

both are equivalent to the noncanonical form:

:<math> \frac{\mathrm{d}}{\mathrm{d}t}\left ( {m\dot{\mathbf{r}} \over \sqrt{1-\left(\frac{\dot{\mathbf{r}}}{c}\right)^2}} \right ) = q\left ( \mathbf{E} + \dot\mathbf{r} \times \mathbf{B} \right ) . \,\!</math>

This formula is the Lorentz force, representing the rate at which the EM field adds relativistic momentum to the particle.
|}

==Relativistic form of the Lorentz force==

===Covariant form of the Lorentz force===

====Field tensor====

{{main|Covariant formulation of classical electromagnetism|Mathematical descriptions of the electromagnetic field}}

Using the [[metric signature]] {{nowrap|(1, −1, −1, −1)}}, the Lorentz force for a charge ''q'' can be written in<ref>Jackson, J.D. Chapter 11</ref> [[Lorentz covariance|covariant form]]:

{{Equation box 1
|indent =:
|equation = <math> \frac{\mathrm{d} p^\alpha}{\mathrm{d} \tau} = q  F^{\alpha \beta} U_\beta </math>
|cellpadding
|border
|border colour = #50C878
|background colour = #ECFCF4}}

where ''p<sup>α</sup>'' is the [[four-momentum]], defined as

:<math>p^\alpha = \left(p_0, p_1, p_2, p_3 \right) = \left(\gamma m c, p_x, p_y, p_z \right) \, ,</math>

''τ'' the [[proper time]] of the particle, ''F<sup>αβ</sup>'' the contravariant [[electromagnetic tensor]]

:<math>F^{\alpha \beta} = \begin{pmatrix}
0 & -E_x/c & -E_y/c & -E_z/c \\
E_x/c & 0 & -B_z & B_y \\
E_y/c & B_z & 0 & -B_x \\
E_z/c & -B_y & B_x & 0
\end{pmatrix}
</math>

and ''U'' is the covariant [[four-velocity|4-velocity]] of the particle, defined as:

:<math>U_\beta = \left(U_0, U_1, U_2, U_3 \right) = \gamma \left(c, -v_x, -v_y, -v_z \right) \, ,</math>

in which

:<math>\gamma(v)=\frac{1}{\sqrt{1- \frac{v^2}{c^2}}}=\frac{1}{\sqrt{1- \frac{v_x^2 + v_y^2+ v_z^2}{c^2}}}</math>

is the [[Lorentz factor]].

The fields are transformed to a frame moving with constant relative velocity by:

:<math> F'^{\mu \nu} = {\Lambda^{\mu}}_{\alpha} {\Lambda^{\nu}}_{\beta} F^{\alpha \beta} \, ,</math>

where Λ''<sup>μ</sup><sub>α</sub>'' is the [[Lorentz transformation]] tensor.

====Translation to vector notation====

The {{nowrap|1=''α'' = 1}} component (''x''-component) of the force is
:<math> \frac{\mathrm{d} p^1}{\mathrm{d} \tau} = q U_\beta F^{1 \beta} = q\left(U_0 F^{10} + U_1 F^{11} + U_2 F^{12} + U_3 F^{13} \right) .</math>

Substituting the components of the covariant electromagnetic tensor ''F'' yields

:<math> \frac{\mathrm{d} p^1}{\mathrm{d} \tau} = q \left[U_0 \left(\frac{E_x}{c} \right) + U_2 (-B_z) + U_3 (B_y) \right] .</math>

Using the components of covariant [[four-velocity]] yields

:<math> \begin{align}
 \frac{\mathrm{d} p^1}{\mathrm{d} \tau} & = q \gamma \left[c \left(\frac{E_x}{c} \right) + (-v_y) (-B_z) + (-v_z) (B_y) \right] \\
 &= q \gamma \left(E_x + v_y B_z - v_z B_y \right) \\
 & = q \gamma \left[ E_x + \left( \mathbf{v} \times \mathbf{B} \right)_x \right] \, .
\end{align} </math>

The calculation for {{nowrap|1=''α'' = 2}}, 3 (force components in the ''y'' and ''z'' directions) yields similar results, so collecting the 3 equations into one:

:<math> \frac{\mathrm{d} \mathbf{p} }{\mathrm{d} \tau} = q \gamma\left( \mathbf{E} + \mathbf{v} \times \mathbf{B} \right) \, , </math>

and since differentials in coordinate time ''dt'' and proper time ''dτ'' are related by the Lorentz factor,

:<math>dt=\gamma(v)d\tau\,,</math>

so we arrive at

:<math> \frac{\mathrm{d} \mathbf{p} }{\mathrm{d} t} = q \left( \mathbf{E} + \mathbf{v} \times \mathbf{B} \right)\, .</math>

This is precisely the Lorentz force law, however, it is important to note that '''p''' is the relativistic expression,

:<math>\mathbf{p}=\gamma(v) m_0 \mathbf{v} \,.</math>

===Lorentz force in spacetime algebra (STA)===

The electric and magnetic fields are [[Classical electromagnetism and special relativity|dependent on the velocity of an observer]], so the relativistic form of the Lorentz force law can best be exhibited starting from a coordinate-independent expression for the electromagnetic and magnetic fields  <math>\mathcal{F}</math>, and an arbitrary time-direction, <math>\gamma_0</math>. This can be settled through [[Spacetime algebra|Space-Time Algebra]] (or the geometric algebra of space-time), a type of Clifford's Algebra defined on a [[pseudo-Euclidean space]],<ref>{{cite web|last=Hestenes|first=David|authorlink=David Hestenes|title=SpaceTime Calculus|url=http://geocalc.clas.asu.edu/html/STC.html}}</ref> as

: <math>\mathbf{E} = (\mathcal{F}\cdot\gamma_0)\gamma_0</math>

and

: <math>i\mathbf{B} = (\mathcal{F}\wedge\gamma_0)\gamma_0</math>

<math>\mathcal F</math> is a space-time bivector (an oriented plane segment, just like a vector is an oriented line segment), which has six degrees of freedom corresponding to boosts (rotations in space-time planes) and rotations (rotations in space-space planes).  The dot product with the vector <math>\gamma_0</math> pulls a vector (in the space algebra) from the translational part, while the wedge-product creates a trivector (in the space algebra) who is dual to a vector which is the usual magnetic field vector.
The relativistic velocity is given by the (time-like) changes in a time-position vector <math>v=\dot x</math>, where

: <math>v^2 = 1,</math>

(which shows our choice for the metric) and the velocity is

: <math>\mathbf{v} = cv \wedge \gamma_0 / (v \cdot \gamma_0).</math>

The proper (invariant is an inadequate term because no transformation has been defined) form of the Lorentz force law is simply

{{Equation box 1
|indent =:
|equation = <math> F = q\mathcal{F}\cdot v</math>
|cellpadding
|border
|border colour = #50C878
|background colour = #ECFCF4}}

Note that the order is important because between a bivector and a vector the dot product is anti-symmetric. Upon a space time split like one can obtain the velocity, and fields as above yielding the usual expression.

===Lorentz force in general relativity===
In the [[general theory of relativity]] the equation of motion for a particle with mass <math>m</math> and charge <math>e</math>, moving in a space with metric tensor <math>g_{ab}</math> and electromagnetic field <math>F_{ab}</math>, is given as
:<math>
m\frac{du_c}{ds}-m\frac{1}{2}g_{ab,c}u^au^b=eF_{cb}u^b \;,
</math>
where <math>u^a= dx^a/ds</math> (<math>dx^a</math> is taken along the trajectory), <math>g_{ab,c}= \partial g_{ab}/\partial x^c</math>, and <math>ds^2=g_{ab}dx^adx^b</math>.

The equation can also be written as
:<math>
m\frac{du_c}{ds}-m\Gamma_{abc}u^au^b=eF_{cb}u^b \;,
</math>
where <math>\Gamma_{abc}</math> is the [[Levi-Civita connection#Christoffel symbols|Christoffel symbol]] (of the torsion-free metric connection in general relativity), or as
:<math>
m\frac{Du_c}{ds}=eF_{cb}u^b \;,
</math>
where <math>D</math> is the [[covariant differential]] in general relativity (metric, torsion-free).

==Applications==
The Lorentz force occurs in many devices, including:
*[[Cyclotron]]s and other circular path [[particle accelerator]]s
*[[Mass spectrometer]]s
*Velocity Filters
*[[Magnetron]]s
*[[Lorentz force velocimetry]]

In its manifestation as the Laplace force on an electric current in a conductor, this force occurs in many devices including:
{{colbegin}}
*[[Electric motor]]s
*[[Railgun]]s
*[[Linear motor]]s
*[[Loudspeaker]]s
*[[Magnetoplasmadynamic thruster]]s
*[[Electrical generator]]s
*[[Homopolar generator]]s
*[[Linear alternator]]s
{{colend}}

==See also==
{{colbegin}}
* [[Hall effect]]
* [[Electromagnetism]]
* [[Gravitomagnetism]]
* [[Ampère's force law]]
* [[Hendrik Lorentz]]
* [[Maxwell's equations]]
* [[Formulation of Maxwell's equations in special relativity]]
* [[Moving magnet and conductor problem]]
* [[Abraham–Lorentz force]]
* [[Larmor formula]]
* [[Cyclotron radiation]]
* [[Magnetic potential]]
* [[Magnetoresistance]]
* [[Scalar potential]]
* [[Helmholtz decomposition]]
* [[Guiding center]]
* [[Field line]]
* [[Coulomb's law]]
* [[Electromagnetic buoyancy]]
{{colend}}

==Footnotes==
{{Reflist|30em}}

==References==

The numbered references refer in part to the list immediately below.

*{{Cite book |first1 = Richard Phillips |last1 = Feynman |author-link = Richard Feynman |first2 = Robert B. |last2 = Leighton  |first3 = Matthew L. |last3 = Sands |title = The Feynman lectures on physics (3 vol.) |publisher = Pearson / Addison-Wesley | year= 2006 |isbn = 0-8053-9047-2 |postscript = <!--None-->}}: volume 2.
*{{Cite book |first = David J. |last = Griffiths |title = Introduction to electrodynamics |edition = 3rd |place = Upper Saddle River, [NJ.] |publisher = Prentice-Hall |year = 1999 |isbn = 0-13-805326-X |postscript = <!--None--> |url = https://archive.org/details/introductiontoel00grif_0 }}
*{{Cite book |first = John David |last = Jackson  |title = Classical electrodynamics |edition = 3rd
 |location = New York, [NY.] |publisher = Wiley | year = 1999 |isbn = 0-471-30932-X |postscript = <!--None-->}}

*{{Cite book |first1 = Raymond A. |last1 = Serway |first2 = John W., Jr. |last2 = Jewett |title = Physics for scientists and engineers, with modern physics |place = Belmont, [CA.]  |publisher = Thomson Brooks/Cole |year = 2004 |isbn = 0-534-40846-X |postscript = <!--None-->}}
*{{Cite book |first = Mark A. |last = Srednicki |title= Quantum field theory |url=https://books.google.com/books?id=5OepxIG42B4C&pg=PA315&dq=isbn=9780521864497 |place = Cambridge, [England] ; New York [NY.] |publisher = Cambridge University Press | year=2007  |isbn = 978-0-521-86449-7 |postscript = <!--None-->}}

==External links==
{{Commons|Lorentz force}}
*[https://web.archive.org/web/20150713153934/https://www.youtube.com/watch?v=mxMMqNrm598 Lorentz force (demonstration)]
*[https://web.archive.org/web/20090226225302/http://www.nadn.navy.mil/Users/physics/tank/Public/FaradaysLaw.pdf Faraday's law: Tankersley and Mosca]
*[http://hyperphysics.phy-astr.gsu.edu/HBASE/hframe.html Notes from Physics and Astronomy HyperPhysics at Georgia State University]; see also [http://hyperphysics.phy-astr.gsu.edu/HBASE/hframe.html home page]
* [http://chair.pa.msu.edu/applets/Lorentz/a.htm Interactive Java applet on the magnetic deflection of a particle beam in a homogeneous magnetic field] by Wolfgang Bauer
* [http://ilorentz.org/history/wallformulas/images/pages/page_5.html The Lorentz force formula on a wall directly opposite Lorentz's home in downtown Leiden]

{{Authority control}}

[[Category:Concepts in physics]]
[[Category:Electromagnetism]]
[[Category:Maxwell's equations]]
[[Category:Hendrik Lorentz]]