Editing Second law of thermodynamics (section)

== Introduction ==
[[File:Heat flow hot to cold.png|thumb|upright|Heat flowing from hot water to cold water]]

The [[first law of thermodynamics]] provides the definition of the [[internal energy]] of a [[thermodynamic system]], and expresses its change for a [[closed system]] in terms of [[Work (thermodynamics)|work]] and [[heat]].<ref>[[Max Planck|Planck, M.]] (1897/1903), pp. 40–41.</ref> It can be linked to the law of [[conservation of energy]].<ref>Munster A. (1970), pp. 8–9, 50–51.</ref> Conceptually, the first law describes the fundamental principle that systems do not consume or 'use up' energy, that energy is neither created nor destroyed, but is simply converted from one form to another.

The second law is concerned with the direction of natural processes.<ref>{{harvnb|Mandl|1988}}</ref> It asserts that a natural process runs only in one sense, and is not reversible. That is, the state of a natural system itself can be reversed, but not without increasing the entropy of the system's surroundings, that is, both the state of the system plus the state of its surroundings cannot be together, fully reversed, without implying the destruction of entropy.

For example, when a path for conduction or [[radiation]] is made available, heat always flows spontaneously from a hotter to a colder body. Such [[Phenomenon|phenomena]] are accounted for in terms of [[entropy|entropy change]].<ref>[[Max Planck|Planck, M.]] (1897/1903), pp. 79–107.</ref><ref>Bailyn, M. (1994), Section 71, pp. 113–154.</ref> A heat pump can reverse this heat flow, but the reversal process and the original process, both cause entropy production, thereby increasing the entropy of the system's surroundings. If an isolated system containing distinct subsystems is held initially in internal thermodynamic equilibrium by internal partitioning by impermeable walls between the subsystems, and then some operation makes the walls more permeable, then the system spontaneously evolves to reach a final new internal [[thermodynamic equilibrium]], and its total entropy, <math>S</math>, increases.

In a [[Reversible process (thermodynamics)|reversible]] or [[Quasistatic process|quasi-static]], idealized process of transfer of energy as heat to a [[closed system|closed]] thermodynamic system of interest, (which allows the entry or exit of energy – but not transfer of matter), from an auxiliary thermodynamic system, an infinitesimal increment (<math>\mathrm d S</math>) in the entropy of the system of interest is defined to result from an infinitesimal [[transfer of heat]] (<math>\delta Q</math>) to the system of interest, divided by the common [[thermodynamic temperature]] <math>(T)</math> of the system of interest and the auxiliary thermodynamic system:<ref>Bailyn, M. (1994), p. 120.</ref>
: <math>\mathrm dS = \frac{\delta Q}{T} \,\, \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \text {(closed system; idealized, reversible process)}.</math>

Different notations are used for an [[infinitesimal]] amount of heat <math>(\delta)</math> and infinitesimal change of entropy <math>(\mathrm d)</math> because entropy is a [[function of state]], while heat, like work, is not.

For an actually possible infinitesimal process without exchange of mass with the surroundings, the second law requires that the increment in system entropy fulfills the [[Clausius Theorem|inequality]]<ref name="MortimerBook">{{cite book | last=Mortimer | first=R.G. | title=Physical Chemistry | publisher=Elsevier Science | year=2008 | isbn=978-0-12-370617-1 | url=https://books.google.com/books?id=5CXWAQAACAAJ | page=120}}</ref><ref name="FermiBook">{{cite book | last=Fermi | first=E. | title=Thermodynamics | publisher=Dover Publications | series=Dover Books on Physics | year=2012 | isbn=978-0-486-13485-7 | url=https://books.google.com/books?id=xCjDAgAAQBAJ | page=48}}</ref>
: <math>\mathrm dS > \frac{\delta Q}{T_\text{surr}} \,\, \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \text {(closed system; actually possible, irreversible process).}</math>

This is because a general process for this case (no mass exchange between the system and its surroundings) may include work being done on the system by its surroundings, which can have frictional or viscous effects inside the system, because a chemical reaction may be in progress, or because heat transfer actually occurs only irreversibly, driven by a finite difference between the system temperature ({{math|''T''}}) and the temperature of the surroundings ({{math|''T''<sub>surr</sub>}}).<ref name=":0">Adkins, C.J. (1968/1983), p. 75.</ref><ref name="Munster 45" />

The equality still applies for pure heat flow (only heat flow, no change in chemical composition and mass),
: <math>\mathrm dS = \frac{\delta Q}{T} \,\, \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \text {(actually possible quasistatic irreversible process without composition change).}</math>
which is the basis of the accurate determination of the absolute entropy of pure substances from measured heat capacity curves and entropy changes at phase transitions, i.e. by calorimetry.<ref name="Oxtoby8th">Oxtoby, D. W; Gillis, H.P., [[Laurie Butler|Butler, L. J.]] (2015).''Principles of Modern Chemistry'', Brooks Cole. p. 617. {{ISBN|978-1305079113}}</ref><ref name="MortimerBook" />

The [[zeroth law of thermodynamics]] in its usual short statement allows recognition that two bodies in a relation of thermal equilibrium have the same temperature, especially that a test body has the same temperature as a reference thermometric body.<ref name=dugdale>{{cite book|author=J. S. Dugdale|title=Entropy and its Physical Meaning|url=https://archive.org/details/entropyitsphysic00dugd|url-access=limited|publisher=Taylor & Francis|year=1996|isbn=978-0-7484-0569-5|page=[https://archive.org/details/entropyitsphysic00dugd/page/n23 13]|quote=This law is the basis of temperature.}}</ref> For a body in thermal equilibrium with another, there are indefinitely many empirical temperature scales, in general respectively depending on the properties of a particular reference thermometric body. The second law allows{{clarify|date=August 2018}} a distinguished temperature scale, which defines an absolute, [[thermodynamic temperature]], independent of the properties of any particular reference thermometric body.<ref>[[Mark Zemansky|Zemansky, M.W.]] (1968), pp. 207–209.</ref><ref>Quinn, T.J. (1983), p. 8.</ref>