Editing Electromagnetic radiation (section)

=== Near and far fields ===
{{Main|Near and far field|Liénard–Wiechert potential}}
[[File:FarNearFields-USP-4998112-1.svg|thumb|upright=1.35|In electromagnetic radiation (such as microwaves from an antenna, shown here) the term ''radiation'' applies only to the parts of the [[electromagnetic field]] that radiate into infinite space and decrease in intensity by an [[inverse-square law]] of power, such that the total energy that crosses through an imaginary sphere surrounding the source is the same regardless of the size of the sphere. Electromagnetic radiation thus reaches the ''[[near and far field|far]]'' part of the electromagnetic field around a transmitter. A part of the ''near'' field (close to the transmitter) includes the changing ''[[electromagnetic field]]'', but that is not electromagnetic ''radiation''.]]
Maxwell's equations established that some charges and currents (''sources'') produce local [[electromagnetic field]]s near them that do not radiate. Currents directly produce magnetic fields, but such fields of a [[magnetic dipole|magnetic-dipole]]–type that dies out with distance from the current. In a similar manner, moving charges pushed apart in a conductor by a changing electrical potential (such as in an antenna) produce an [[electric dipole|electric-dipole]]–type electrical field, but this also declines with distance. These fields make up the ''[[near and far field|near]]'' field. Neither of these behaviours is responsible for EM radiation. Instead, they only efficiently transfer energy to a receiver very close to the source, such as inside a [[transformer]]. The near field has strong effects on its source, with any energy withdrawn by a receiver causing increased ''load'' (decreased [[electrical reactance]]) on the source. The near field does not propagate freely into space, carrying energy away without a distance limit, but rather oscillates, returning its energy to the transmitter if it is not absorbed by a receiver.<ref>{{Cite web |date=2023-09-15 |title=Electromagnetic radiation {{!}} Spectrum, Examples, & Types {{!}} Britannica |url=https://www.britannica.com/science/electromagnetic-radiation |access-date=2023-10-16 |website=www.britannica.com |language=en |archive-date=2 May 2015 |archive-url=https://web.archive.org/web/20150502222537/http://www.britannica.com/EBchecked/topic/183228/electromagnetic-radiation/59182/Microwaves |url-status=live }}</ref>

By contrast, the ''[[Near and far field|far]]'' field is composed of ''radiation'' that is free of the transmitter, in the sense that the transmitter requires the same power to send changes in the field out regardless of whether anything absorbs the signal, e.g. a radio station does not need to increase its power when more receivers use the signal. This far part of the electromagnetic field ''is'' electromagnetic radiation. The far fields propagate (radiate) without allowing the transmitter to affect them. This causes them to be independent in the sense that their existence and their energy, after they have left the transmitter, is completely independent of both transmitter and receiver. Due to [[conservation of energy]], the amount of power passing through any closed surface drawn around the source is the same. The [[power density]] of EM radiation from an [[isotropic]] source decreases with the inverse square of the distance from the source; this is called the [[inverse-square law]]. Field intensity due to dipole parts of the near field varies according to an inverse-cube law,<ref>{{Cite journal |last=Capps |first=Charles |date=August 16, 2001 |title=Near field or far field? |url=https://people.eecs.ku.edu/~callen58/501/Capps2001EDNpp95.pdf |journal=designfeature |pages=96}}</ref> and thus fades with distance.

In the [[Liénard–Wiechert potential]] formulation of the electric and magnetic fields due to motion of a single particle (according to Maxwell's equations), the terms associated with acceleration of the particle are those that are responsible for the part of the field that is regarded as electromagnetic radiation. By contrast, the term associated with the changing static electric field of the particle and the magnetic term that results from the particle's uniform velocity are both associated with the near field, and do not comprise electromagnetic radiation.<ref>{{Cite web |date=2021-12-09 |title=10.1: Liénard-Wiechert Potentials |url=https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Essential_Graduate_Physics_-_Classical_Electrodynamics_(Likharev)/10%3A_Radiation_by_Relativistic_Charges/10.01%3A_Lienard-Wiechert_Potentials |access-date=2024-07-26 |website=Physics LibreTexts |language=en |archive-date=26 July 2024 |archive-url=https://web.archive.org/web/20240726225813/https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Essential_Graduate_Physics_-_Classical_Electrodynamics_(Likharev)/10%3A_Radiation_by_Relativistic_Charges/10.01%3A_Lienard-Wiechert_Potentials |url-status=live }}</ref>