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热力学第一定律

1850

Julius Robert von Mayer
James Prescott Joule
Hermann von Helmholtz
Rudolf Clausius

（图片仅供参考）

The First Law is a statement of the conservation of energy. It posits that the change in a closed system’s internal energy ([latex]\Delta U[/latex]) is equal to the heat supplied to the system ([latex]Q[/latex]) minus the work done by the system on its surroundings ([latex]W[/latex]). The governing equation is [latex]\Delta U = Q – W[/latex]. This law links heat, work, and internal energy, establishing heat as a form of energy transfer.

The First Law of Thermodynamics generalized the principle of conservation of energy, which was previously known in mechanics, to include heat. Its formulation was a major step in physics, as it definitively refuted the prevailing caloric theory, which considered heat a weightless fluid. Experiments by James Joule in the 1840s, which demonstrated the mechanical equivalent of heat, were crucial in establishing that heat and work are mutually convertible.

The law introduces internal energy ([latex]U[/latex]) as a state function, meaning its value depends only on the current state of the system, not on how it got there. In contrast, heat ([latex]Q[/latex]) and work ([latex]W[/latex]) are path-dependent process quantities. The law’s differential form is [latex]dU = \delta Q – \delta W[/latex]. For a cyclic process, where the system returns to its initial state, the change in internal energy is zero ([latex]\Delta U = 0[/latex]), so the net heat supplied equals the net work done. This principle is the basis for all heat engines. The law also implies the impossibility of a perpetual motion machine of the first kind—a machine that produces work without any energy input.

Closed-Loop Product Development, 活力, Enthalpy, Heat Treatment, 机械工业, 流程优化, 质量管理, 热力学

UNESCO Nomenclature: 2212

热力学、统计物理学和凝聚态物质

类型

抽象系统

中断

Revolutionary

用法

广泛使用

前体

Sadi Carnot’s analysis of heat engines and 热力学 cycles
James Joule’s experiments on the mechanical equivalent of heat
拒绝热质热理论
莱布尼茨等人提出的力学能量守恒定律

应用程序

热机（内燃机、蒸汽轮机）
冰箱、空调和热泵
化学反应分析（焓计算）
发电厂设计和效率分析
营养热量法用于计算食物能量（卡路里）

专利：

潜在创新理念

利用汽车尾气废热的热电发电机
由细胞代谢过程驱动的纳米机器人微尺度热机
先进的地热能系统，采用闭环能量循环为城市供电
利用热力学循环的太阳能大气水收集器
基于高效微型热泵的个人冷却和加热服装
集成热存储的电动汽车动能回收系统
低温 energy storage systems using liquid air as an energy vector
osmotic power generation plants at river estuaries, optimized by first-law principles
利用放射性衰变热的深空探测器直接热电转换器
通过精确管理热量和工作输出来优化微生物生长的生物反应器

Related to: first law, conservation of energy, internal energy, heat, work, thermodynamics, enthalpy, closed system, state function, perpetual motion.

历史背景

19th century laboratory with thermodynamic instruments and Mayer's relation equations.

Mayer’s Relation (thermodynamics)

Mayer's relation connects the specific heats of a perfect gas to the specific gas constant ([latex]R_s[/latex]). The relation is [latex]c_p - c_v = R_s[/latex]. For molar specific heats ([latex]C_p[/latex] and [latex]C_v[/latex]), the relation is [latex]C_p - C_v = R[/latex], where [latex]R[/latex] is the universal gas constant. This shows that [latex]c_p[/latex] is always greater than [latex]c_v[/latex].

19th-century laboratory with physicists studying energy conservation principles.

能量守恒

A fundamental principle stating that the total energy of an isolated system remains constant over time. Energy can neither be created nor destroyed, only transformed from one form to another, such as from potential to kinetic energy. In classical mechanics, for systems with only conservative forces, the total mechanical energy [latex]E = T + V[/latex] is conserved.

Laboratory setup for measuring viscosity of fluids at varying temperatures in thermodynamics.

粘度的温度依赖性

对于牛顿流体，粘度是温度和压力的函数，而非剪切速率的函数。在液体中，粘度会随着温度升高而显著降低，因为更高的热能使分子更容易克服分子间的内聚力。相反，在气体中，粘度会随着温度升高而增加，因为分子碰撞频率越高，速度越高，动量传递也越大。

热力学第一定律

Chemist conducting experiments with catalysts in a vintage laboratory setting, 1850.

化学平衡催化剂

催化剂通过提供活化能较低的替代反应途径，平等地提高正向和逆向反应的速率。虽然催化剂可以使体系更快地达到平衡，但它不会改变平衡本身的位置。反应物和产物的平衡浓度以及平衡常数K的值保持不变。

19th-century chemist measuring gas volumes illustrating the Ideal Gas Law in thermodynamics.

理想气体定律（摩尔形式）

The ideal gas law is the equation of state for a hypothetical ideal gas, approximating the behavior of many gases under various conditions. The molar form relates pressure ([latex]P[/latex]), volume ([latex]V[/latex]), amount of substance in moles ([latex]n[/latex]), and absolute temperature ([latex]T[/latex]) via the universal gas constant ([latex]R[/latex]): [latex]PV = nRT[/latex].

Laboratory apparatus for measuring vapor pressure and temperature in thermodynamics.

克劳修斯-克拉珀龙关系

The Clausius-Clapeyron relation describes the relationship between pressure and temperature for a substance at a phase transition, such as liquid and vapor. For water vapor, it shows that saturation vapor pressure increases exponentially with temperature. The approximate form is [latex]\frac{dp}{dT} = \frac{L}{T(V_v - V_l)} \approx \frac{L p}{R_v T^2}[/latex], where L is latent heat.

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Demonstration of Joule heating in a vintage laboratory with electric appliances.

焦耳热和电力

Joule heating, or ohmic heating, is the phenomenon where heat is produced by an electric current passing through a conductor. The rate of heat generation, or power dissipated ([latex]P[/latex]), is given by Joule's first law, [latex]P = VI[/latex]. By combining this with Ohm's law, the power can be expressed as [latex]P = I^2 R[/latex] or [latex]P = \frac{V^2}{R}[/latex].

Laboratory apparatus for measuring internal energy and enthalpy of a perfect gas in thermodynamics.

理想气体的内能和焓

For a perfect gas, the internal energy ([latex]U[/latex]) and enthalpy ([latex]H[/latex]) are functions of temperature only. Their changes are given by [latex]\Delta U = m c_v \Delta T[/latex] and [latex]\Delta H = m c_p \Delta T[/latex], where [latex]c_v[/latex] and [latex]c_p[/latex] are the specific heats at constant volume and pressure, respectively, and are assumed to be constant.

Researcher testing viscoelastic properties of synthetic polymer in a laboratory.

粘弹性

粘弹性是指材料在变形时同时表现出粘性和弹性的特性。粘性材料（例如蜂蜜）在施加应力时，能够抵抗剪切流，并且应变随时间呈线性变化。弹性材料（例如橡皮筋）在拉伸时会变形，一旦应力消除就会迅速恢复到原始状态。粘弹性材料兼具这两种特性。

Scientist measuring sublimation in a vintage laboratory, thermodynamics study.

升华焓

The enthalpy of sublimation, [latex]\Delta H_{\text{sub}}[/latex], is the heat required to convert one mole of a substance from solid to gas at a given temperature and pressure. According to Hess's Law, since enthalpy is a state function, this energy change is the sum of the enthalpy of fusion ([latex]\Delta H_{\text{fus}}[/latex]) and the enthalpy of vaporization ([latex]\Delta H_{\text{vap}}[/latex]).