Editing Energy (section)

== Energy transfer ==
{{redirect|Energy transfer|the pipeline company|Energy Transfer Partners}}

=== Closed systems ===
Energy transfer can be considered for the special case of systems which are [[closed system|closed]] to transfers of matter. The portion of the energy which is transferred by [[conservative force]]s over a distance is measured as the [[Work (thermodynamics)|work]] the source system does on the receiving system. The portion of the energy which does not do work during the transfer is called [[heat]].<ref group=note>Although heat is "wasted" energy for a specific energy transfer (see: [[waste heat]]), it can often be harnessed to do useful work in subsequent interactions. However, the maximum energy that can be "recycled" from such recovery processes is limited by the [[second law of thermodynamics]].</ref> Energy can be transferred between systems in a variety of ways. Examples include the transmission of [[electromagnetic energy]] via photons, physical collisions which transfer [[kinetic energy]],<ref group=note>The mechanism for most macroscopic physical collisions is actually [[Electromagnetism|electromagnetic]], but it is very common to simplify the interaction by ignoring the mechanism of collision and just calculate the beginning and end result.</ref> [[tidal interactions]],<ref>{{cite book | title=The Physics of Energy | first1=Robert L. | last1=Jaffe | first2=Washington | last2=Taylor | date=2018 | isbn=9781107016651 | page=611 | publisher=Cambridge University Press | url=https://books.google.com/books?id=drZDDwAAQBAJ&pg=PA611 | access-date=2022-05-22 | archive-date=2022-07-30 | archive-url=https://web.archive.org/web/20220730093040/https://www.google.com/books/edition/The_Physics_of_Energy/drZDDwAAQBAJ?gbpv=1&pg=PA611 | url-status=live }}</ref> and the conductive transfer of [[thermal energy]].

Energy is strictly conserved and is also locally conserved wherever it can be defined. In thermodynamics, for closed systems, the process of energy transfer is described by the [[first law of thermodynamics|first law]]:<ref group=note>There are several [[First law of thermodynamics#Description|sign conventions for this equation]]. Here, the signs in this equation follow the IUPAC convention.</ref>

{{NumBlk|:|<math>\Delta{}E = W + Q </math>|{{EquationRef|1}}}}

where <math>E</math> is the amount of energy transferred, <math>W</math>&nbsp; represents the work done on or by the system, and <math>Q</math> represents the heat flow into or out of the system. As a simplification, the heat term, <math>Q</math>, can sometimes be ignored, especially for fast processes involving gases, which are poor conductors of heat, or when the [[thermal efficiency]] of the transfer is high. For such [[adiabatic process]]es,

{{NumBlk|:|<math>\Delta{}E = W</math>|{{EquationRef|2}}}}

This simplified equation is the one used to define the [[joule]], for example.

=== Open systems ===
Beyond the constraints of closed systems, [[Thermodynamic system#Open system|open systems]] can gain or lose energy in association with matter transfer (this process is illustrated by injection of an air-fuel mixture into a car engine, a system which gains in energy thereby, without addition of either work or heat). Denoting this energy by <math>E_\text{matter}</math>, one may write

{{NumBlk|:|<math>\Delta E = W + Q + E_\text{matter} .</math>|{{EquationRef|3}}}}