Editing Heat transfer (section)

=== Newton's law of cooling ===
[[File:Enoch_Seeman_the_younger_-_Isaac_Newton_-_NPG_558_-_National_Portrait_Gallery_(cropped).jpg|alt=Portrait of Isaac Newton|thumb|upright|Isaac Newton]]
[[File:Krzywa_ostygania.svg|alt=Graph showing Newton's law of cooling|thumb|Newton's law of cooling. ''T''{{sub|0}} = original temperature, ''T''{{sub|''R''}} = ambient temperature, ''t'' = time]]
In 1701, [[Isaac Newton]] anonymously published an article in ''[[Philosophical Transactions of the Royal Society|Philosophical Transactions]]'' noting (in modern terms) that the rate of temperature change of a body is proportional to the difference in temperatures ({{Lang|la|graduum caloris}}, "degrees of heat") between the body and its surroundings.<ref>{{Cite journal |year=1701 |title=VII. Scala graduum caloris |url=https://royalsocietypublishing.org/doi/10.1098/rstl.1700.0082 |journal=Philosophical Transactions of the Royal Society of London |language=en |volume=22 |issue=270 |pages=824–829 |doi=10.1098/rstl.1700.0082|url-access=subscription }}</ref> The phrase "temperature change" was later replaced with "heat loss", and the relationship was named Newton's law of cooling. In general, the law is valid only if the temperature difference is small and the heat transfer mechanism remains the same.

==== Thermal conduction ====
In heat conduction, the law is valid only if the [[thermal conductivity]] of the warmer body is independent of temperature. The thermal conductivity of most materials is only weakly dependent on temperature, so in general the law holds true.

==== Thermal convection ====
In convective heat transfer, the law is valid for forced air or pumped fluid cooling, where the properties of the fluid do not vary strongly with temperature, but it is only approximately true for buoyancy-driven convection, where the velocity of the flow increases with temperature difference.

==== Thermal radiation ====
In the case of heat transfer by thermal radiation, Newton's law of cooling holds only for very small temperature differences.