Editing Poynting vector (section)

==Static fields==
[[File:Poynting-Paradoxon.svg|upright=1.15|thumb|Poynting vector in a static field, where '''E''' is the electric field, '''H''' the magnetic field, and '''S''' the Poynting vector.]]

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]], {{nowrap|''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.

While the circulating energy flow may seem unphysical, its existence is necessary to maintain [[conservation of angular momentum]]. The momentum of an electromagnetic wave in free space is equal to its power divided by ''c'', the speed of light. Therefore, the circular flow of electromagnetic energy implies an ''angular'' momentum.<ref name="Feynman">{{cite book
| last = Feynman
| first = Richard Phillips
| author-link = Richard Feynman
| title = The Feynman Lectures on Physics
| volume = II: Mainly Electromagnetism and Matter
| edition = The New Millennium
| publisher = Basic Books
| place = New York
| year = 2011
| isbn = 978-0-465-02494-0
| url = https://feynmanlectures.caltech.edu/II_27.html
}}</ref> If one were to connect a wire between the two plates of the charged capacitor, then there would be a Lorentz force on that wire while the capacitor is discharging due to the discharge current and the crossed magnetic field; that force would be circumferential to the central axis and thus add angular momentum to the system. That angular momentum would match the "hidden" angular momentum, revealed by the Poynting vector, circulating before the capacitor was discharged.