📚 The Meaning of Temperature | 温度的含义
Temperature is one of the most familiar concepts in everyday life – we sense hot and cold – yet in physics it carries a precise, measurable meaning. It is a fundamental quantity that governs the direction of heat flow and determines the thermal state of matter. Understanding temperature from both macroscopic and microscopic perspectives is essential in thermodynamics and kinetic theory.
温度是日常生活中最熟悉的概念之一——我们能感知冷热，但在物理学中它具有精确、可测量的含义。它是一个基本物理量，决定着热量传递的方向并确定物质的热状态。从宏观和微观两个角度理解温度，对于热力学和分子动理论至关重要。
1. Introduction: What is Temperature? | 温度是什么？
In everyday language, temperature describes how hot or cold an object is. Scientifically, temperature is a physical property that indicates the direction of spontaneous heat transfer: energy flows from a region of higher temperature to a region of lower temperature. It is crucial to distinguish temperature from heat – heat is energy in transit, while temperature is a measure of the average internal kinetic energy of particles (in the microscopic picture).
在日常语言中，温度描述物体的冷热程度。在科学上，温度是表示自发传热方向的物理性质：能量从高温区域流向低温区域。必须区分温度和热量——热量是传递中的能量，而温度则是粒子平均内部动能的量度（微观图像）。
2. Thermal Equilibrium and the Zeroth Law | 热平衡与第零定律
When two objects at different temperatures are brought into contact, energy transfers until they reach the same temperature. At this point they are in thermal equilibrium and no net heat flows. The Zeroth Law of Thermodynamics formalises this: if system A is in thermal equilibrium with system C, and system B is also in thermal equilibrium with system C, then A and B are in thermal equilibrium with each other.
当两个温度不同的物体接触时，能量会传递直到它们达到相同的温度。此时它们处于热平衡，没有净热量流动。热力学第零定律对此做了规范化：如果系统A与系统C处于热平衡，系统B也与系统C处于热平衡，那么A和B彼此也处于热平衡。
The Zeroth Law justifies the use of a thermometer and establishes temperature as a transitive property that can be compared across systems without direct contact. It is the logical foundation of temperature measurement.
第零定律为使用温度计提供了依据，并确立了温度作为一种可传递的性质，可以在不直接接触的情况下跨系统比较。它是温度测量的逻辑基础。
3. Defining Temperature Empirically | 经验性定义温度
Before a deeper theoretical understanding was developed, temperature was defined operationally using thermometric properties – physical quantities that change predictably with hotness, such as the length of a liquid column, electrical resistance, or gas pressure at constant volume. An empirical scale (e.g., the Celsius scale) was established by fixing two reference points, such as the ice point (0 °C) and steam point (100 °C) at standard atmospheric pressure, and dividing the interval into 100 equal degrees.
在形成更深的理论认识之前，温度是通过测温属性来操作性定义的——这些物理量会随冷热程度有规律地变化，例如液柱长度、电阻或定容下气体的压强。经验温标（如摄氏温标）的建立是固定两个参考点，例如在标准大气压下的冰点（0 °C）和汽点（100 °C），然后将间隔等分为100度。
However, different thermometric substances give slightly different readings between fixed points because their properties do not change linearly with the 'degree of hotness'. This motivated the search for a substance-independent scale.
然而，不同的测温物质在固定点之间会给出略有不同的读数，因为它们的性质并不与“热程度”线性地变化。这就促使人们寻找一种与物质无关的温标。
4. Celsius and Kelvin Scales | 摄氏温标与开尔文温标
The Celsius scale (°C) is widely used in daily life and laboratory work. Its defining points are now no longer based on the ice–steam interval but on the absolute zero and the triple point of water (0.01 °C), consistent with the Kelvin scale. The Kelvin (K) is the SI base unit of temperature.
摄氏温标（°C）广泛用于日常生活和实验室工作。其定义点现在不再基于冰–汽间隔，而是基于绝对零度和水的三相点（0.01 °C），与开尔文温标一致。开尔文（K）是温度的国际单位制基本单位。
The relationship between the two scales is exact:
T (K) = θ (°C) + 273.15
两者之间的关系精确为：T (K) = θ (°C) + 273.15
A temperature difference of 1 K equals a difference of 1 °C. Thus, when quoting temperature intervals, both scales give the same numerical value. The Kelvin scale begins at absolute zero, the lowest possible temperature.
1 K的温差等于1 °C的温差。因此，在表示温度间隔时，两个标度给出相同的数值。开尔文温标始于绝对零度，即可能的最低温度。
5. Absolute Zero and Its Significance | 绝对零度及其意义
temperature at which the pressure of an ideal gas would extrapolate to zero at constant volume, and where the random kinetic energy of particles reaches its classical minimum. In quantum mechanics, particles still possess zero-point energy, but no thermal energy can be extracted. Absolute zero is unattainable in practice according to the third law of thermodynamics.
绝对零度（0 K = −273.15 °C
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