为什么热机不可能把从高温热源吸收的热量全部转化为有用功？
Why can a heat engine never convert all the heat it absorbs from a hot reservoir into useful work?
卡诺分析表明，工质完成循环必须回到初态，而熵不会自行消失：若吸收的热全部变成功而无其他任何变化，宇宙总熵将减少，违反热力学第二定律。因此热机必须向低温热源放热以补偿熵增，极限效率即卡诺效率η=1-T_c/T_h，仅由两热源的温度决定。它为一切真实热机的性能设定了绝对上限。
Carnot's analysis shows that the working substance must return to its initial state after a cycle, yet entropy does not vanish on its own: if all absorbed heat became work with nothing else changing, the universe's total entropy would decrease, violating the second law of thermodynamics. An engine must therefore reject heat to a cold reservoir to pay this entropic cost, giving a limiting efficiency, the Carnot efficiency η=1-T_c/T_h, fixed only by the two reservoir temperatures. It sets an absolute ceiling on any real engine.
难度 进阶 · 适合 大众
CC BY 4.0 · 来源待核验 · Source pending review
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出处备注：此条未指向带版次的可引用著作（通识性内容、机构文件或未展开的英文书名），本库不提供独立书目出处；如需精确引用，需由编辑补全具体篇目与版次。
E. Fermi, Thermodynamics, 多佛出版社, 1956
E. Fermi, Thermodynamics, Dover Publications, 1956
#热力学第二定律 #卡诺效率 #熵
热量为何总是自发地从高温流向低温？熵增的统计学根源是什么？
Why does heat flow spontaneously from hot to cold, and what is the statistical origin of entropy increase?
玻尔兹曼把宏观状态的熵定义为 S = k ln W，W 是与之相容的微观方式数。“冷热混合后归于均匀”这类宏观态拥有的微观方式数以天文数字压倒其余分布，因此孤立系统几乎必然朝微观方式数更大的方向演化，宏观上表现为熵增。第二定律由此成为统计规律：熵减并非绝对不可能，只是概率低到实际时间尺度内观察不到。
Boltzmann defined the entropy of a macroscopic state as S = k ln W, where W counts the microscopic arrangements consistent with it. Macrostates such as hot and cold settling into uniform warmth command astronomically more microstates than any alternative, so an isolated system evolves almost inevitably toward larger W, seen macroscopically as rising entropy. The second law is thus statistical: decreasing entropy is not strictly impossible, merely too improbable to observe in any realistic span of time.
难度 进阶 · 适合 大众
CC BY 4.0 · 来源待核验 · Source pending review
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出处备注：著录仅给书名，无版次与年份；同一书名常对应多个版次，不臆测具体版本。
L. Boltzmann，《论机械式热理论第二定律与概率计算的关系》，维也纳帝国科学院院报 Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften (Wien) 第 76 卷（1877）第 373–435 页
L. Boltzmann, Über die Beziehung zwischen dem zweiten Hauptsatz der mechanischen Wärmetheorie und der Wahrscheinlichkeitsrechnung, Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften (Wien) 76 (1877) 373–435
#统计力学 #熵 #热力学第二定律
什么是熵，热力学第二定律说什么？
What is entropy and what does the second law of thermodynamics say?
熵粗略衡量一个系统的无序程度或微观状态数。热力学第二定律指出，孤立系统的熵不会减少，总会趋向增大，直至平衡。它解释了为何时间有方向、为什么热量不能自发从冷流向热。
Entropy roughly measures a system's disorder or number of microscopic states. The second law says the entropy of an isolated system never decreases and tends to increase until equilibrium. It explains why time has a direction and why heat cannot spontaneously flow from cold to hot.
难度 入门 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-08-23
来源 · Source
秦允豪,《热学》,高等教育出版社,2011
Qin Yunhao, Thermal Physics, Higher Education Press, 2011
#熵 #热力学第二定律 #无序