Editing Entropy (section)

=== Isothermal expansion or compression of an ideal gas ===
For the expansion (or compression) of an [[ideal gas]] from an initial volume <math display="inline">V_0</math> and pressure <math display="inline">P_0</math> to a final volume <math display="inline">V</math> and pressure <math display="inline">P</math> at any constant temperature, the change in entropy is given by:<math display="block">\Delta S = n R \ln{\frac{V}{V_0}} = - n R \ln{\frac{P}{P_0}}</math>Here <math display="inline">n</math> is the amount of gas (in [[Mole (unit)|moles]]) and <math display="inline">R</math> is the [[ideal gas constant]]. These equations also apply for expansion into a finite vacuum or a [[throttling process (thermodynamics)|throttling process]], where the temperature, internal energy and enthalpy for an ideal gas remain constant.