Editing Heat transfer (section)

== Overview ==
{{See also|Heat transfer physics}}
[[File:Erbe.gif|thumb|300px|Earth's longwave thermal [[Earth's energy budget#Outgoing energy|radiation]] intensity, from clouds, atmosphere and surface.]]
Heat transfer is the energy exchanged between materials (solid/liquid/gas) as a result of a temperature difference. The [[thermodynamic free energy]] is the amount of work that a thermodynamic system can perform. [[Enthalpy]] is a [[thermodynamic potential]], designated by the letter "H", that is the sum of the [[internal energy]] of the system (U) plus the product of [[pressure]] (P) and [[volume]] (V). [[Joule]] is a unit to quantify [[energy]], work, or the amount of heat.<ref>{{Cite web |last=Abedin |first=Engineer Zain ul |date=2023-08-09 |title=What are the Basic Concepts of Engineering Thermodynamics? |url=https://mechanicalmentor.com/basic-concepts-of-thermodynamics |access-date=2023-11-20 |language=en-US}}</ref>

Heat transfer is a [[process function]] (or path function), as opposed to [[functions of state]]; therefore, the amount of heat transferred in a [[thermodynamic process]] that changes the [[thermodynamic state|state]] of a [[thermodynamic system|system]] depends on how that process occurs, not only the net difference between the initial and final states of the process.

Thermodynamic and [[Mechanical engineering|mechanical]] heat transfer is calculated with the [[heat transfer coefficient]], the [[Proportionality (mathematics)|proportionality]] between the [[heat flux]] and the thermodynamic driving force for the flow of heat. Heat flux is a quantitative, vectorial representation of heat flow through a surface.<ref name=NJIT>{{cite web |publisher=New Jersey Institute of Technology, Chemical Engineering Department |title=B.S. Chemical Engineering |url=http://catalog.njit.edu/undergraduate/programs/chemicalengineering.php |access-date=9 April 2011 |url-status=dead |archive-url=https://web.archive.org/web/20101210121252/http://catalog.njit.edu/undergraduate/programs/chemicalengineering.php |archive-date=10 December 2010}}</ref>

In engineering contexts, the term ''heat'' is taken as synonymous with thermal energy. This usage has its origin in the [[Nicolas Léonard Sadi Carnot#The second law of thermodynamics|historical interpretation]] of heat as a fluid (''caloric'') that can be transferred by various causes,<ref name=lienhard>
{{cite book
 |last1=Lienhard  |first1=John H. IV
 |last2=Lienhard  |first2=John H. V
 |year=2019
 |title=A Heat Transfer Textbook
 |url=http://ahtt.mit.edu
 |edition=5th
 |publisher=Dover Pub.
 |location=Mineola, NY
 |page=3
}}</ref> and that is also common in the language of laymen and everyday life.

The [[Transport phenomena|transport]] equations for thermal energy ([[Thermal conduction#Fourier's law|Fourier's law]]), mechanical momentum ([[Newtonian fluid|Newton's law for fluids]]), and mass transfer ([[Fick's laws of diffusion]]) are similar,<ref>
{{cite book
 |last1=Welty |first1=James R.
 |last2=Wicks |first2=Charles E.
 |last3=Wilson |first3=Robert Elliott
 |year=1976
 |title=Fundamentals of momentum, heat, and mass transfer
 |url=https://books.google.com/books?id=hZxRAAAAMAAJ
 |edition=2nd
 |publisher=Wiley
 |location=New York
 |isbn=978-0-471-93354-0
 |oclc=2213384
}}</ref><ref name=Faghri>
{{cite book
 |last1=Faghri |first1=Amir
 |last2=Zhang |first2=Yuwen
 |last3=Howell |first3=John
 |year=2010
 |title=Advanced Heat and Mass Transfer
 |publisher=Global Digital Press
 |location=Columbia, MO
 |isbn=978-0-9842760-0-4
}}</ref> and analogies among these three transport processes have been developed to facilitate the prediction of conversion from any one to the others.<ref name=Faghri/>

[[Thermal engineering]] concerns the generation, use, conversion, storage, and exchange of heat transfer. As such, heat transfer is involved in almost every sector of the economy.<ref>{{cite journal |last1=Taylor |first1=R. A. |year=2012 |title=Socioeconomic impacts of heat transfer research |journal=International Communications in Heat and Mass Transfer |volume=39 |issue=10 |pages=1467–1473 |doi=10.1016/j.icheatmasstransfer.2012.09.007|bibcode=2012ICHMT..39.1467T }}</ref> Heat transfer is classified into various mechanisms, such as [[thermal conduction]], [[Convection (heat transfer)|thermal convection]], [[thermal radiation]], and transfer of energy by [[phase changes]].