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系统及其周围环境的总熵在理想情况下可以保持不变，在这种情况下，系统处于热力学平衡状态，或者正在经历一个假想的可逆过程。所有过程中，包括'''自发过程 spontaneous processes'''，es,<ref>Atkins and de Paula, p.78</ref> 系统及其周围环境的总熵增加，这一过程在热力学意义上是不可逆的。熵的增加解释了自然过程的不可逆性，以及未来和过去之间的不对称性<ref>{{cite book|last=Zohuri|first=Bahman|title=Dimensional Analysis Beyond the Pi Theorem|url=https://books.google.com/books?id=pRVuDQAAQBAJ|year=2016|publisher=Springer|isbn=978-3-319-45726-0|page=[https://books.google.com.ph/books?id=pRVuDQAAQBAJ&pg=PA111&dq=%22increase+in+entropy+accounts+for+the+irreversibility+of+natural+processes+and+the+asymmetry+between+future+and+past.%22&hl=en&sa=X&ved=0ahUKEwj6spb61tbaAhUFS7wKHfftDtIQ6AEIKDAA#v=onepage&q=%22increase%20in%20entropy%20accounts%20for%20the%20irreversibility%20of%20natural%20processes%20and%20the%20asymmetry%20between%20future%20and%20past.%22 111]}}</ref>

系统及其周围环境的总熵在理想情况下可以保持不变，在这种情况下，系统处于热力学平衡状态，或者正在经历一个假想的可逆过程。所有过程中，包括'''自发过程 spontaneous processes''',<ref>Atkins and de Paula, p.78</ref> 系统及其周围环境的总熵增加，这一过程在热力学意义上是不可逆的。熵的增加解释了自然过程的不可逆性，以及未来和过去之间的不对称性<ref>{{cite book|last=Zohuri|first=Bahman|title=Dimensional Analysis Beyond the Pi Theorem|url=https://books.google.com/books?id=pRVuDQAAQBAJ|year=2016|publisher=Springer|isbn=978-3-319-45726-0|page=[https://books.google.com.ph/books?id=pRVuDQAAQBAJ&pg=PA111&dq=%22increase+in+entropy+accounts+for+the+irreversibility+of+natural+processes+and+the+asymmetry+between+future+and+past.%22&hl=en&sa=X&ved=0ahUKEwj6spb61tbaAhUFS7wKHfftDtIQ6AEIKDAA#v=onepage&q=%22increase%20in%20entropy%20accounts%20for%20the%20irreversibility%20of%20natural%20processes%20and%20the%20asymmetry%20between%20future%20and%20past.%22 111]}}</ref>