Thermodynamic Properties: Fundamentals熱力學性質：基礎

What you'll be able to do本章學習成果

• Classify any property as intensive or extensive, and form specific (per-mass) properties.將任何性質分類為強度或廣延性質，並形成比（單位質量）性質。
• Define mass, density, specific volume, pressure, and temperature with their units.定義質量、密度、比體積、壓力與溫度及其單位。
• Define the energy properties — internal energy, enthalpy, and entropy — and the specific heats $c_v$, $c_p$.定義能量性質——內能、焓與熵——以及比熱 $c_v$、$c_p$。

Pairs with Basic Concepts (systems, state, the state postulate).與基本概念（系統、狀態、狀態假設）配對學習。

Key definitions關鍵定義

Intensive vs extensive強度性質與廣延性質

A property is any measurable characteristic of a system. Properties split into two kinds:性質是系統任何可量測的特徵。性質分為兩類：

• Intensive — independent of system size: temperature, pressure, density.強度性質——與系統大小無關：溫度、壓力、密度。
• Extensive — proportional to extent: mass, volume, total energy, entropy.廣延性質——與系統大小成比例：質量、體積、總能量、熵。

Divide an extensive property by mass and you get a specific (intensive) property. Quick test: split the system in two — whatever halves is extensive; whatever stays the same is intensive.廣延性質除以質量就得到「比」（強度）性質。快速測試：將系統一分為二——半掉的是廣延性質；不變的是強度性質。

The test — cut the system in half測試——將系統一分為二

Take a smaller portion of the same gas and watch what changes. Mass, volume, and total energy scale with the portion (extensive); temperature, pressure, density, and specific volume hold fast (intensive).取同一氣體的較小部分，觀察何者改變。質量、體積與總能量隨部分比例縮放（廣延）；溫度、壓力、密度與比體積不變（強度）。

Mass, density & specific volume質量、密度與比體積

We treat matter as a continuum. Density is mass per unit volume; specific volume is its reciprocal:我們將物質視為連續體。密度是单位體積的質量；比體積是其倒數：

Specific gravity is the ratio of a substance's density to that of water at 4 °C.比重是物質密度與 4 °C 水的密度之比。

Pressure壓力

Pressure is the normal force a fluid exerts per unit area (SI: $1\,\mathrm{Pa} = 1\,\mathrm{N/m^2}$). Gauges read relative to atmosphere:壓力是流體對各單位面積施加的法向力（SI：$1\,\mathrm{Pa}=1\,\mathrm{N/m^2}$）。壓力表讀數相對於大氣：

Pressures below atmospheric are vacuum pressures. Thermodynamic relations use absolute pressure. In a static fluid, $\Delta p = \rho g h$.低於大氣壓的為真空壓。熱力學關係式一律用絕對壓力。靜止流體中，$\Delta p = \rho g h$。

Temperature溫度

Temperature measures the tendency of a system to exchange heat; equality of temperature is the condition for thermal equilibrium (the zeroth law). The absolute Kelvin scale:溫度表徵系統交換熱量的傾向；溫度相等是熱平衡的條件（第零定律）。絕對克耳文溫標：

Thermodynamic relations involving temperature ratios require the absolute scale.涉及溫度比的熱力學關係式必須使用絕對溫標。

Internal energy內能

Internal energy $U$ is the energy stored in a system's molecular motion and interactions. Specific internal energy $u = U/m$ (kJ/kg). For an ideal gas, $u$ depends on temperature alone.內能 $U$ 是儲存於系統分子運動與互動中的能量。比內能 $u = U/m$（kJ/kg）。對理想氣體，$u$ 僅與溫度有關。

Enthalpy焓

The combination $u + pv$ appears so often it earns its own name, enthalpy:$u + pv$ 的組合極常出現，因此專門命名為焓：

It is the natural energy property for flowing streams and constant-pressure processes.焓是流動氣流與定壓過程的自然能量性質。

Entropy熵

Entropy $S$ is the extensive property that measures the dispersal of energy and the direction of spontaneous change. For an internally reversible process:熵 $S$ 是量度能量分散與自發變化方向的廣延性質。對內部可逆過程：

Unlike mass and energy, entropy can be produced by irreversibilities but never destroyed — the heart of the second law. (See Entropy.)與質量、能量不同，熵可由不可逆性產生，卻永不減少——這是第二定律的核心。（見熵）

Specific heats比熱

The specific heats quantify how much a property changes per degree of temperature — at constant volume or constant pressure:比熱量化性質每度溫度的變化——在定體積或定壓下：

For an ideal gas $c_p - c_v = R$; for an incompressible solid or liquid, $c_p = c_v = c$. Despite the name, they are properties — not heat transfer $Q$.對理想氣體，$c_p - c_v = R$；對不可壓縮圖體或液體，$c_p = c_v = c$。尽管名稱含「熱」，它們是性質——而非熱轉移量 $Q$ 本身。

Specific properties from totals由總量求比性質