对于恒定组成系统中的某些简单过程,熵的变化可由简单的公式给出。<ref>{{cite web |url=http://www.grc.nasa.gov/WWW/k-12/Numbers/Math/Mathematical_Thinking/ideal_gases_under_constant.htm |title=GRC.nasa.gov |publisher=GRC.nasa.gov |date=2000-03-27 |accessdate=2012-08-17 |archive-url=https://www.webcitation.org/6179uUEbb?url=http://www.grc.nasa.gov/WWW/K-12/Numbers/Math/Mathematical_Thinking/ideal_gases_under_constant.htm |archive-date=21 August 2011 |url-status=dead }}</ref> | 对于恒定组成系统中的某些简单过程,熵的变化可由简单的公式给出。<ref>{{cite web |url=http://www.grc.nasa.gov/WWW/k-12/Numbers/Math/Mathematical_Thinking/ideal_gases_under_constant.htm |title=GRC.nasa.gov |publisher=GRC.nasa.gov |date=2000-03-27 |accessdate=2012-08-17 |archive-url=https://www.webcitation.org/6179uUEbb?url=http://www.grc.nasa.gov/WWW/K-12/Numbers/Math/Mathematical_Thinking/ideal_gases_under_constant.htm |archive-date=21 August 2011 |url-status=dead }}</ref> |