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| [[File:DipoleRadiation.gif|300px|thumb|[[Dipole radiation]] of a dipole vertically in the page showing electric field strength (colour) and Poynting vector (arrows) in the plane of the page.]]
| | Greetings! I am Marvella and I really feel comfy when people use the complete name. One of the issues she loves most is to do aerobics and now she is trying to make money with it. Bookkeeping is what I do. California is our birth location.<br><br>Here is my webpage [http://www.fb2.cy3.com.br/index.php?do=/profile-1201/info/ www.fb2.cy3.com.br] |
| {{Electromagnetism|cTopic=Electrodynamics}}
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| In [[physics]], the '''Poynting vector''' represents the directional [[energy flux]] density (the rate of energy transfer per unit area, in units of [[watt]]s per square [[metre]] (W·m<sup>−2</sup>)) of an [[electromagnetic field]]. It is named after its inventor [[John Henry Poynting]]. [[Oliver Heaviside]] and [[Nikolay Umov]] independently co-invented the Poynting vector.
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| ==Definition==
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| In Poynting's original paper and in many textbooks, it is usually denoted by '''S''' or '''N''', and defined as:<ref>Electromagnetism (2nd Edition), I.S. Grant, W.R. Phillips, Manchester Physics, John Wiley & Sons, 2008, ISBN 978-0471927129</ref><ref>Introduction to Electrodynamics (3rd Edition), D.J. Griffiths, Pearson Education, Dorling Kindersley, 2007, ISBN 81-7758-293-3</ref>
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| :<math>\mathbf{S} = \mathbf{E}\times\mathbf{H},</math>
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| which is often called the Abraham form;
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| where '''E''' is the [[electric field]] and '''H''' the [[magnetic field]].<ref name="Poynting">{{cite journal
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| | author=Poynting, J. H.
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| | authorlink=John_Henry_Poynting
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| | year=1884
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| | journal=Philosophical Transactions of the Royal Society of London
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| | volume=175
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| | pages=343–361
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| | title=[[s:On the Transfer of Energy in the Electromagnetic Field|On the Transfer of Energy in the Electromagnetic Field]]
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| | doi=10.1098/rstl.1884.0016
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| | ref=harv
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| }}
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| </ref><ref name=Jackson>
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| {{cite book
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| |author=John David Jackson
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| |title=Classical electrodynamics
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| |edition=Third
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| |publisher= Wiley
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| |location=New York
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| |year=1998
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| |isbn=0-471-30932-X
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| |url=http://worldcat.org/isbn/047130932X}}
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| </ref> (All bold letters represent vectors.)
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| Occasionally an alternative definition in terms of electric field '''E''' and the [[magnetic flux density]] '''B''' is used. It is even possible to combine the displacement field '''D''' with the magnetic flux density '''B''' to get the Minkowski form of the Poynting vector, or use '''D''' and '''H''' to construct another.<ref name=kinslerfavaromccall>
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| {{cite journal
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| | author=Kinsler, P.
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| | coauthors=Favaro, A.; McCall M.W.
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| | year=2009
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| | title=Four Poynting theorems
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| | journal=Eur. J. Phys.
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| | volume=30
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| | issue=5
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| | pages=983
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| | doi=10.1088/0143-0807/30/5/007
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| | arxiv=0908.1721
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| | ref=harv
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| |bibcode = 2009EJPh...30..983K }}
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| </ref>
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| The choice has been controversial: Pfeifer et al.<ref name=PfeiferNHR2007>
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| {{cite journal
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| | author=Pfeifer, R.N.C.
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| | coauthors=Nieminen, T.A.; Heckenberg N. R.; Rubinsztein-Dunlop H.
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| | year=2007
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| | title=Momentum of an electromagnetic wave in dielectric media
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| | journal=Rev. Mod. Phys.
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| | volume=79
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| | issue=4
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| | pages=1197
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| | doi=10.1103/RevModPhys.79.1197
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| | ref=harv
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| |bibcode = 2007RvMP...79.1197P }}
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| </ref>
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| summarize the century-long dispute between proponents of the Abraham and Minkowski forms.
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| The Poynting vector represents the particular case of an energy flux vector for electromagnetic energy. However, any type of energy has its direction of movement in space, as well as its density, so energy flux vectors can be defined for other types of energy as well, e.g., for [[Poynting's_theorem#Generalization|mechanical energy]]. The Umov-Poynting vector<ref name="Umov-Poynting">{{cite journal
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| | author=Umov, N. A.
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| | authorlink=Nikolay_Umov
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| | year=1874
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| | journal=Zeitschrift für Mathematik und Physik
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| | volume=XIX
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| | pages=97
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| | title=Ein Theorem über die Wechselwirkungen in Endlichen Entfernungen
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| | ref=harv
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| }}
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| </ref> discovered by [[Nikolay Umov]] in 1874 describes energy flux in liquid and elastic media in a completely generalized view.
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| ===Interpretation===
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| The Poynting vector appears in [[Poynting's theorem]] (see this article for the derivation of the theorem and vector), an energy-conservation law,<ref name=Jackson/>
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| :<math>\frac{\partial u}{\partial t} = - \mathbf{\nabla}\cdot\mathbf{S} -\mathbf{J}_{f} \cdot \mathbf{E},</math>
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| where '''J'''<sub>''f''</sub> is the [[current density]] of [[Maxwell's equations#Formulation in terms of free charge and current|free charges]] and ''u'' is the electromagnetic energy density,
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| :<math>u = \frac{1}{2}\left(\mathbf{E}\cdot\mathbf{D} + \mathbf{B}\cdot\mathbf{H}\right)</math>
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| where '''B''' is the [[magnetic flux density]] and '''D''' the [[electric displacement field]].
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| The first term in the right-hand side represents the net electromagnetic energy flow into a small volume, while the second term represents the subtracted portion of the work done by free electrical currents that are not necessarily converted into electromagnetic energy ([[dissipation]], heat). In this definition, bound electrical currents are not included in this term, and instead contribute to '''S''' and ''u''.
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| Note that ''u'' can only be given if linear, [[dispersion (optics)|nondispersive]] and uniform materials are involved, i.e., if the [[Maxwell's_equations#Constitutive_relations|constitutive relations]] can be written as
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| :<math>\mathbf{D} = \varepsilon\mathbf{E}\,\quad \mathbf{H} = \mathbf{B}/\mu</math>
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| where ε and μ are constants (which depend on the material through which the energy flows), called the [[permittivity]] and [[permeability (electromagnetism)|permeability]], respectively, of the material.<ref name=Jackson/>
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| This practically limits Poynting's theorem in this form to fields in vacuum. A generalization to dispersive materials is possible under certain circumstances at the cost of additional terms and the loss of their clear physical interpretation.<ref name=Jackson/>
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| The Poynting vector is usually interpreted as an energy flux, but this is only strictly correct for [[electromagnetic radiation]]. The more general case is described by Poynting's theorem above, where it occurs as a [[divergence]], which means that it can only describe the ''change'' of energy density in space, rather than the flow.
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| ===Invariance to adding a curl of a field===
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| Since the Poynting vector only occurs in Poynting's theorem as a [[divergence]] ∇ • '''S''', the Poynting vector '''S''' is arbitrary to the extent that one can add a [[Curl (mathematics)|curl]] of a field '''F''' to '''S''',<ref name="Jackson"/>
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| :<math>\mathbf{S}' = \mathbf{S} + \nabla\times\mathbf F\,\Rightarrow\,\nabla\cdot\mathbf{S}' = \nabla\cdot\mathbf{S}\,,</math>
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| since the divergence of the curl term is zero: ∇ • (∇ × '''F''') = 0 for an arbitrary field '''F''' (see [[vector calculus identities#Divergence of the curl|Vector calculus identities]]).
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|
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| This property is used in quasi-electrostatic regimes to describe for instance energy propagating through waves in piezoelectric materials. In such cases magnetic fields are negligible and a local flux of energy can be defined based on electrical quantities only.
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| In the general case we can express the divergence of the Poynting vector as:
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| :<math>\nabla\cdot\left(\mathbf{E}\times\mathbf{H}\right) = \mathbf{H}\cdot\nabla\times\mathbf{E}- \mathbf{E}\cdot\nabla\times\mathbf{H} </math>
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|
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| The fourth of the [[Maxwell's equations#"Microscopic" versus "macroscopic"|Maxwell's equations]] writes:
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| :<math>\nabla \times \mathbf{H} = \mathbf{J}_\mathrm{f} + \frac{\partial \mathbf{D}} {\partial t}</math> where <math> \mathbf{J}_\mathrm{f} </math> is the current density due to free charges. In dielectric materials it reduces to: <math>\nabla \times \mathbf{H} = \frac{\partial \mathbf{D}} {\partial t}</math>.
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| Combining the two previous results, leads to the following quasi-electrostatic divergence:
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| :<math>\nabla\cdot\mathbf{S} = -\mathbf{E}\cdot\frac{\partial \mathbf{D}} {\partial t}</math>
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|
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| A new “magnetic free” Poynting vector leading to the same divergence can be defined as:
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| :<math>\mathbf{S}' = -\mathsf{V} \frac{\partial \mathbf{D}} {\partial t}</math> where V is the electrostatic potential.
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| A demonstration in the case of the parallel-plate capacitor that both '''S''' and ''' S’''', although being orthogonal, lead to the same overall energy balance is provided by Bondar & Bastien.<ref name=bondarbastienaflb>
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| {{cite journal
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| | author=Bondar, H.
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| | coauthors=Bastien, F.
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| | year=2008
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| | title=Quelques remarques sur la transmission de l’énergie électromagnétique en champ proche
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| | journal=Annales de la Fondation Louis de Broglie.
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| | volume=33
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| | issue=3-4
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| | pages=283–305
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| |url=http://aflb.ensmp.fr/AFLB-333/aflb333m577.htm}}
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| </ref>
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| It is often thought that using a different vector than the classical Poynting vector will lead to inconsistencies in a [[theory of relativity|relativistic]] description of electromagnetic fields where energy and momentum should be defined locally in terms of the [[stress-energy tensor]] .{{Citation needed|date=July 2011}}
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|
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| However such a transformation is consistent with [[Quantum electrodynamics#Introduction|Quantum electrodynamics]] where photon particles have no defined trajectories but only a probability of being emitted or absorbed.
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| ==Formulation in terms of microscopic fields==
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| In some cases, it may be more appropriate to define the Poynting vector '''S''' as
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| :<math>\mathbf{S} = \frac{1}{\mu_0}\mathbf{E} \times \mathbf{B},</math>
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| where μ<sub>0</sub> is the [[magnetic constant]].
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| It can be derived directly from [[Maxwell's equations#Formulation in terms of free charge and current|Maxwell's equations in terms of ''total'' charge and current]] and the [[Lorentz force]] law only.
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| The corresponding form of [[Poynting's theorem]] is
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| :<math>\frac{\partial u}{\partial t} + \nabla\cdot\mathbf{S} = - \mathbf{J}\cdot\mathbf{E},</math>
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| where '''J''' is the ''total'' [[current density]] and the energy density ''u'' is
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| :<math>u = \frac{1}{2}\left(\varepsilon_0 \mathbf{E}^2 + \frac{1}{\mu_0}\mathbf{B}^2\right)</math>
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| where ε<sub>0</sub> is the [[electric constant]].
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| The two alternative definitions of the Poynting ''vector'' are equivalent in vacuum or in non-magnetic materials, where '''B''' = μ<sub>0</sub> '''H'''. In all other cases, they differ in that
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| :<math>\mathbf{S}=1/\mu_0 \mathbf{E}\times\mathbf{B}\,,</math>
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| and the corresponding ''u'' are purely radiative, since the dissipation term, (−'''J''' • '''E''') covers the total current, while the definition in terms of '''H''' has contributions from bound currents which then lack in the dissipation term.<ref name=Richter>
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| {{cite journal
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| | author=Richter, F.
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| | coauthors=Florian, M.; Henneberger, K.
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| | year=2008
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| | title=Poynting's theorem and energy conservation in the propagation of light in bounded media
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| | journal=Europhys. Lett.
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| | volume=81
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| | issue=6
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| | pages=67005
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| | doi=10.1209/0295-5075/81/67005
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| | arxiv=0710.0515
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| | ref=harv
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| |bibcode = 2008EL.....8167005R }}</ref>
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| Since only the microscopic fields '''E''' and '''B''' are needed in the derivation of
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| :<math>\mathbf{S}=1/\mu_0 \mathbf{E}\times\mathbf{B}\,,</math>
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| assumptions about any material possibly present can be completely avoided, and Poynting's vector as well as the theorem in this definition are universally valid, in vacuum as in all kinds of material. This is especially true for the electromagnetic energy density, in contrast to the case above.<ref name=Richter/>
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| ==Time-averaged Poynting vector==
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| For time-periodic [[sinusoidal]] electromagnetic fields, the [[average]] power flow per unit time is often more useful, and can be found by treating the electric and magnetic fields as [[complex number|complex]] vectors as follows (star * denotes the [[complex conjugate]]):
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| :<math>\begin{align}\mathbf{S} & = \mathbf{E} \times \mathbf{H}\\
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| & = \mathrm{Re}\left(\mathbf{\widetilde E}\right) \times \mathrm{Re}\left(\mathbf{\widetilde H} \right)\\
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| & = \mathrm{Re}\left(\mathbf{E_c} e^{j\omega t}\right) \times \mathrm{Re}\left(\mathbf{H_c} e^{j\omega t}\right)\\
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| & = \frac{1}{2}\left(\mathbf{E_c} e^{j\omega t} + \mathbf{E_c}^* e^{-j\omega t}\right) \times \frac{1}{2}\left(\mathbf{H_c} e^{j\omega t} + \mathbf{H_c}^* e^{-j\omega t}\right)\\
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| & = \frac{1}{4}\left(\mathbf{E_c} \times \mathbf{H_c}^* + \mathbf{E_c}^* \times \mathbf{H_c} + \mathbf{E_c} \times \mathbf{H_c} e^{2j\omega t} + \mathbf{E_c}^* \times \mathbf{H_c}^* e^{-2j\omega t}\right)\\
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| & = \frac{1}{4}\left(\mathbf{E_c} \times \mathbf{H_c}^* + \left(\mathbf{E_c} \times \mathbf{H_c}^*\right)^* + \mathbf{E_c} \times \mathbf{H_c} e^{2j\omega t} + \left(\mathbf{E_c} \times \mathbf{H_c} e^{2j\omega t}\right)^*\right)\\
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| & = \frac{1}{2}\mathrm{Re}\left(\mathbf{E_c} \times \mathbf{H_c}^*\right) + \frac{1}{2}\mathrm{Re}\left(\mathbf{E_c} \times \mathbf{H_c} e^{2j\omega t}\right).
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| \end{align}</math>
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| The average over time is given as
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| :<math>\langle\mathbf{S}\rangle = \frac{1}{T}\int_0^T \mathbf{S}(t)dt = \frac{1}{T}\int_0^T \left[\frac{1}{2}\mathrm{Re}\left(\mathbf{E_c} \times \mathbf{H_c}^*\right) + \frac{1}{2}\mathrm{Re}\left(\mathbf{E_c} \times \mathbf{H_c} e^{2j\omega t}\right) \right]dt.</math>
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| The second term is a sinusoidal curve
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| :<math>\mathrm{Re}\left(e^{2j\omega t}\right) = \cos 2\omega t</math> | |
| and its average is zero, giving
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| :<math>\langle\mathbf{S}\rangle = \frac{1}{2}\mathrm{Re}\left(\mathbf{E_c} \times \mathbf{H_c}^*\right)
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| =\frac{1}{2}\mathrm{Re}\left(\left[\mathbf{E_c}e^{j\omega t}\right] \times \left[\mathbf{H_c}^* e^{-j\omega t}\right]\right)
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| =\frac{1}{2}\mathrm{Re}\left(\mathbf{\widetilde E} \times \mathbf{\widetilde H}^*\right).</math>
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| ==Examples and applications==
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| ===In a coaxial cable===
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| [[File:Poynting vector coaxial cable.svg|300px|"300px"|right|thumb|Poynting vector in a coaxial cable, shown in red]]
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| For example, the Poynting vector within the [[dielectric]] [[Insulator (electrical)|insulator]] of a [[coaxial cable]] is nearly parallel to the wire axis (assuming no fields outside the cable and a wavelength longer than the diameter of the cable, including DC). Electrical energy delivered to the load is flowing entirely through the dielectric between the [[Electrical conductor|conductor]]s. Very little energy flows in the conductors themselves, since the electric field strength is nearly zero. The energy flowing in the conductors flows radially into the conductors and accounts for energy lost to resistive heating of the conductor. No energy flows outside the cable, either, since there the magnetic fields of inner and outer conductors cancel to zero.
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| === Resistive dissipation ===
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| If a conductor has significant resistance, then, near the surface of that conductor, the Poynting vector would be tilted toward and impinge upon the conductor. Once the Poynting vector enters the conductor, it is bent to a direction that is almost perpendicular to the surface.<ref>{{Harvtxt|Harrington|1981|p=61}}</ref> This is a consequence of [[Snell's law]] and the very slow speed of light inside a conductor. See Hayt page 402<ref>{{Harvtxt|Hayt|1993|p=402}}</ref> for the definition and computation of the speed of light in a conductor. Inside the conductor, the Poynting vector represents energy flow from the [[electromagnetic field]] into the wire, producing resistive [[Joule heating]] in the wire. For a derivation that starts with Snell's law see Reitz page 454.<ref>{{Harvtxt|Reitz|1993|p=454}}</ref>
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| ===In plane waves===
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| In a propagating ''sinusoidal'' [[Linear polarisation|linearly polarized]] electromagnetic [[plane wave]] of a fixed frequency, the Poynting vector always points in the direction of propagation while oscillating in magnitude. The time-averaged magnitude of the Poynting vector is
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| :<math>\langle S \rangle = \frac{1}{2 \mu_0 c} E_0^2 = \frac{\varepsilon_0 c}{2} E_0^2,</math>
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| where ''E''<sub>0</sub> is the peak value of the electric field and ''c'' is the [[speed of light]] in free space. This time-averaged value is also called the [[irradiance]] or intensity ''I''.
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| ====Derivation====
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| In an electromagnetic plane wave, '''E''' and '''B''' are always perpendicular to each other and the direction of propagation. Moreover, their amplitudes are related according to
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| :<math>B_0 = \frac{1}{c}E_0,</math>
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| and their time and position dependences are
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| :<math>E\left(t,{\mathbf r}\right) = E_0\,\cos\left(\omega\,t- {\mathbf k} \cdot {\mathbf r} \right),</math>
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| :<math>B\left(t,{\mathbf r}\right) = B_0\,\cos\left(\omega\,t- {\mathbf k} \cdot {\mathbf r} \right),</math>
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| where ''ω'' is the frequency of the wave and '''k''' is [[wave vector]].
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| The time-dependent and position magnitude of the Poynting vector is then
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| :<math>S(t) = \frac{1}{\mu_0} E_0\,B_0\,\cos^2\left(\omega t-{\mathbf k} \cdot {\mathbf r}\right) =
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| \frac{1}{\mu_0 c} E_0^2 \cos^2\left(\omega t-{\mathbf k} \cdot {\mathbf r} \right) =
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| \varepsilon_0 c E_0^2 \cos^2\left(\omega t-{\mathbf k} \cdot {\mathbf r} \right).</math>
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| In the last step, we used the equality ε<sub>0</sub>μ<sub>0</sub> = ''c''<sup>−2</sup>. Since the time- or space-average of cos<sup>2</sup>(ω''t'' − '''k''' • '''r''') is 1/2, it follows that
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| :<math>\left\langle S \right\rangle = \frac{\varepsilon_0 c}{2} E_0^2.</math>
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| It will be appreciated that quantitatively the Poynting vector is evaluated only from a prior knowledge of the distribution of electric and magnetic fields, which are calculated by applying boundary conditions to a particular set of physical circumstances, for example a [[dipole antenna]]. Therefore the '''E''' and '''H''' field distributions form the primary object of any analysis, while the Poynting vector remains an interesting by-product.
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| ===Radiation pressure===
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| The density of the linear momentum of the electromagnetic field is ''S''/''c''<sup>2</sup> (the speed of light in free space). The [[radiation pressure]] exerted by an electromagnetic wave on the surface of a target is given by:
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| :<math>P_{rad}=\frac{\langle S\rangle}{c},</math>
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| where <math>\langle S\rangle</math> the time-averaged intensity above.
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| ===In static fields===
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| [[Image:Poynting-Paradoxon.svg|200px|"200px"|left|thumb|Poynting vector in a static field, where E is electric field, H is magnetic field, and S is Poynting vector]]
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| {{-}}
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| The consideration of the Poynting vector in static fields shows the relativistic nature of the Maxwell equations and allows a better understanding of the magnetic component of the [[Lorentz force]], ''q''('''v''' × '''B'''). To illustrate, the accompanying picture is considered, which describes the Poynting vector in a cylindrical capacitor, which is located in an ''H'' field (pointing into the page) generated by a permanent magnet. Although there are only static electric and magnetic fields, the calculation of the Poynting vector produces a clockwise circular flow of electromagnetic energy, with no beginning or end.
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| While the circulating energy flow may seem nonsensical or paradoxical, it proves to be absolutely necessary to maintain [[conservation of momentum]]. Momentum density is proportional to energy flow density, so the circulating flow of energy contains an ''angular'' momentum. This is the cause of the magnetic component of the Lorentz force which occurs when the capacitor is discharged. During discharge, the angular momentum contained in the energy flow is depleted as it is transferred to the charges of the discharge current crossing the magnetic field.<ref>Feynman Lectures on Physics, Sections 17-4 and Volume 2, Chapter 17, section 4 and the end of Chapter 27, Section 6.</ref>
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| ==Notes==
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| {{Reflist}}
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| == References ==
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| *{{Cite journal|last=Harrington |first= Roger F. |year= 1961 |title= Time-Harmonic Electromagnetic Fields |edition= |publisher= McGraw-Hill |isbn= |doi= |ref=harv |postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}
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| *{{Cite book |last=Hayt |first= William |year= 1981 |title= Engineering Electromagnetics |edition= 4th |publisher= McGraw-Hill |isbn= 0-07-027395-2 |ref=harv |postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}} }}
| |
| *{{Cite book |last= Reitz |first= John R. |last2= Milford |first2= Frederick J. |last3= Christy |first3= Robert W. |year= 1993 |title= Foundations of Electromagnetic Theory|edition= 4th |publisher= Addison-Wesley |isbn= 0-201-52624-7 |ref= harv |postscript= <!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}
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| == Further reading ==
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| *[http://scienceworld.wolfram.com/physics/PoyntingVector.html "Poynting Vector" from ScienceWorld (A Wolfram Web Resource)] by Eric W. Weisstein
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| *{{cite book
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| |author=Richard Becker & Sauter, F
| |
| |title=Electromagnetic fields and interactions
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| |publisher= Dover
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| |location=New York
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| |year=1964
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| |isbn=0-486-64290-9
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| |url=http://worldcat.org/isbn/0486642909}}
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| *{{cite book
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| |author=Joseph Edminister
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| |title=Schaum's outline of theory and problems of electromagnetics
| |
| |publisher= McGraw-Hill Professional
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| |location=New York
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| |page=225
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| |year=1995
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| |isbn=0-07-021234-1
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| |url=http://books.google.com/?id=xV97IDOqBZIC&pg=PA225&dq=%22Poynting+vector%22}}
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| [[Category:Electromagnetic radiation]]
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| [[Category:Optics]]
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| [[Category:Vectors]]
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