📚 IB Physics: Core Concepts and Formulas of Thermodynamics | IB物理：热力学核心概念与公式梳理
Thermodynamics is a central topic in IB Physics, linking microscopic particle behaviour with macroscopic measurable quantities. This article consolidates the core concepts and formulas you need for exams.
热力学是IB物理的核心主题，将微观粒子行为与宏观可测量量联系起来。本文旨在系统梳理考试所需的核心概念与公式。
1. Temperature and Heat | 温度与热
Temperature is a measure of the average kinetic energy of the particles in a substance. It is not the same as heat; heat is energy transferred from a hotter body to a colder body due to a temperature difference.
温度是物质粒子平均动能的量度。它与热不同；热是由于温差而从高温物体传递到低温物体的能量。
In IB Physics, thermodynamic temperature must be expressed in kelvin (K). To convert from Celsius:
在IB物理中，热力学温度必须以开尔文（K）为单位。从摄氏度转换时：
T(K) = T(°C) + 273.15
Note that a change of 1°C equals a change of 1 K, so temperature differences are identical in both scales.
注意1°C的变化等于1K的变化，因此两种温标的温度差相同。
2. Internal Energy | 内能
Internal energy U is the total kinetic and potential energy of the molecules in a system. For an ideal gas, intermolecular potential energy is negligible, so U depends only on temperature.
内能U是系统内所有分子动能与势能的总和。对于理想气体，分子间势能可忽略，因此U仅取决于温度。
For a monatomic ideal gas:
对于单原子理想气体：
U = (3/2)nRT
For a diatomic gas, U = (5/2)nRT. In general, the change in internal energy is given by ΔU = nCVΔT, where CV is the molar heat capacity at constant volume.
对于双原子气体，U = (5/2)nRT。一般而言，内能变化为ΔU = nCVΔT，其中CV是定容摩尔热容。
3. Work and P-V Diagrams | 功与P-V图
Work done by a gas is related to the area under the pressure-volume (P-V) curve. When the volume changes, the work done by the gas is given by:
气体做功与压强-体积（P-V）曲线下的面积相关。当体积变化时，气体对外做功为：
W’ = ∫ P dV
For an isobaric (constant pressure) process:
对于等压（定压）过程：
W’ = PΔV
If a gas expands, it does positive work on the surroundings; if compressed, negative work is done. In P-V diagrams, the work is the area between the curve and the volume axis. A cyclic process forms a closed loop, and the net work equals the area enclosed by the loop.
如果气体膨胀，它对外界做正功；如果被压缩，则做负功。在P-V图中，功等于曲线与体积轴之间的面积。循环过程形成闭合回路，净功等于回路所围的面积。
4. The First Law of Thermodynamics | 热力学第一定律
The first law is a statement of energy conservation. It relates the change in internal energy ΔU to heat Q absorbed by the system and work W done on the system:
第一定律是能量守恒的表述。它将内能变化ΔU与系统吸收的热量Q和外界对系统做的功W联系起来：
ΔU = Q + W
Here, W is positive when work is done on the system (compression), and Q is positive when heat enters the system. An equivalent form uses work done by the system:
这里，当外界对系统做功（压缩）时W为正，当热量进入系统时Q为正。等价形式使用系统对外做功：
ΔU = Q – W’
where W’ is the work done by the system. Always state your sign convention in exam solutions to avoid errors.
其中W’是系统对外做的功。在考试答题中务必说明你的符号约定以避免错误。
5. Ideal Gas Law and Kinetic Theory | 理想气体定律与分子动理论
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