摘要
物理量的正负值性质不仅取决于它的零点选择,还通常与它的物理意义有关.一般约定用正值代表正面的、积极的事物,负值代表负面的、消极的事物.然而,状态量熵却是一个“奇怪”的物理量.它代表了一种负面的、消极的事物,但却一直被视为正值.基于平衡态热力学的对称性,我们建立了一个理想固体状态方程,该方程要求熵应具有负值性质;并从物理上规定了熵的零点应为熵的极大值点,即零压理想气体(理想气体的极限状态).分析表明,普朗克关于能斯特热定理的熵表述并不普适,它仅对单组分物质如完美晶体有效,“熵不可能为负”并不是一个绝对的事实.历史上选择零温完美晶体作为熵的零点是非物理操作,只涉及方便性的问题.以零压理想气体作为熵的零点,其他所有热力学状态的熵均为负值,实现了熵的零点与其宏观和微观物理意义的统一,熵的负值(即熵的绝对值)愈小,系统的不可用能愈大(可用能愈小),无序度愈大(有序度愈小).这与人们的认知习惯相符.与选择零温完美晶体作为熵的零点不同,零压理想气体作为熵的零点同时也是热力学对称性的要求,是一种基于物理本质的选择.此外,零压理想气体是理想气体状态方程和理想固体状态方程共同的熵零点,可以对两个极端方程之间的各种实际状态方程的适用范围作统一定位和划分.
The positive-and-negative property of a physical quantity is usually related to its physical meaning. The general convention is to use positive values to represent positive things and negative values to represent negative things. However, the state variable, entropy, is a strange physical quantity. From a macroscopic perspective, entropy represents the unavailable energy of the heat output of a thermodynamic system as it changes from an arbitrary state to a state in equilibrium with the environment, in short, the unavailable heat of the system. From a microscopic point of view, entropy represents the degree of disorder(chaos) in a system. Both the macroscopic and microscopic physical meanings of entropy indicate that it represents a negative thing. Therefore, entropy should be negative by its nature. However, historically, since Planck’s statement of Nernst’s heat theorem, entropy has always been taken for granted as a positive value. This is contrary to people’s cognitive habits, which adds to the mystery of entropy. Analyses indicate that Planck’s statement of Nernst’s heat theorem is not general and is valid only for single-component substances, such as perfect crystals. It is not an absolute fact that “entropy cannot be negative”. The Planck’s choice of a zero-temperature perfect crystal as the zero point of entropy is a non-physical operation and involves only a matter of convenience. Recently, based on the symmetry of reversible thermodynamics, a new equation of state for ideal solids has been established. Since entropy is explicitly included in the equation of state for ideal solids, the zero point of entropy cannot be chosen arbitrarily. The analysis from the thermodynamic stability and the verification of the property data show that entropy in the ideal solid equation of state must have a negative value. This physically specifies that the zero point of entropy should be the point of maximum entropy, i.e.,the zero-pressure ideal gas limiting state. In fact, the ideal gas equation of state has a zero point of volume. That is the ideal solid limiting state(zero-volume state). At this zero point, the volume always takes a positive value. In brief, the zero point of volume of the ideal gas equation of state is the zero-temperature ideal solid, while the zero point of the entropy of the ideal solid equation of state is the zero-pressure ideal gas. The ideal solid equation of state is a creation of thermodynamic symmetry. The zero point of entropy, i.e., zero-pressure ideal gas is likewise a requirement of thermodynamic symmetry.The smaller the absolute value of negative entropy, the greater the unavailable energy of the system(the smaller the available energy) and the higher the degree of disorder of the system(the lower the degree of order). This is consistent with people’s cognitive habits. In addition, the zero-pressure ideal gas can be used as the zero point of entropy for both the ideal gas and the ideal solid equations of state, thus allowing a uniform orientation and division of the applicability of various practical equations of state between these two extreme equations of state. Unlike the choice of zero-temperature perfect crystal as the zero point of entropy, when zero-pressure ideal gas is used as the zero point of entropy, entropy takes a negative value, which is in accordance with the physical meaning of entropy and is also the requirement of thermodynamic symmetry. Taking a negative value for entropy is the physical, essential choice.
作者
薛提微
吴晶
李阳
过增元
Tiwei Xue;Jing Wu;Yang Li;Zengyuan Guo(Key Laboratory for Thermal Science and Power Engineering of Ministry of Education,Deparment f Enginering Mechanics,Tsinghua University.,Beijing 100084.China;Department of Engineering Thermophysics,School of Energy and Power Engineering,Huazhong Universiy of Science and Technology,Wuhan 430074,China)
出处
《科学通报》
EI
CAS
CSCD
北大核心
2022年第25期3081-3087,共7页
Chinese Science Bulletin
基金
国家自然科学基金(51356001)资助。
关键词
熵
负值
零点
理想固体
理想气体
entropy
negative value
zero point
ideal solid
ideal gas
