Editing Entropy (section)

=== Reversible process ===
The entropy change ''<math display="inline">\mathrm{d} S</math>'' of a system can be well-defined as a small portion of [[heat]] ''<math display="inline">\delta Q_{\mathsf{rev}}</math>'' transferred from the surroundings to the system during a reversible process divided by the [[temperature]] ''<math display="inline">T</math>'' of the system during this [[heat transfer]]:<math display="block">\mathrm{d} S = \frac{\delta Q_\mathsf{rev}}{T}</math>The reversible process is [[Quasistatic process|quasistatic]] (i.e., it occurs without any dissipation, deviating only infinitesimally from the thermodynamic equilibrium), and it may conserve total entropy. For example, in the [[Carnot cycle]], while the heat flow from a hot reservoir to a cold reservoir represents the increase in the entropy in a cold reservoir, the work output, if reversibly and perfectly stored, represents the decrease in the entropy which could be used to operate the heat engine in reverse, returning to the initial state; thus the total entropy change may still be zero at all times if the entire process is reversible.

In contrast, an irreversible process increases the total entropy of the system and surroundings.<ref>{{cite web|last1=Lower|first1=Stephen|title=What is entropy?|url=http://www.chem1.com/acad/webtext/thermeq/TE2.html|website=chem1.com|access-date=21 May 2016}}</ref> Any process that happens quickly enough to deviate from the thermal equilibrium cannot be reversible; the total entropy increases, and the potential for maximum work to be done during the process is lost.<ref>{{cite web|title=6.5 Irreversibility, Entropy Changes, and ''Lost Work''|url=http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node48.html|website=web.mit.edu|access-date=21 May 2016}}</ref>