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。
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。
Any extensive property can be made intensive by dividing by the amount of substance. Two amounts are in use, so two specific forms exist. Dividing by mass $m$ gives the lowercase specific property (per kg); dividing by the number of moles $n = m/M$ gives the molar property, written with an overbar (per kmol). They differ by the molar mass $M$ (kg/kmol):任何廣延性質除以物質的量即成為強度性質。由於「量」有兩種，因此有兩種比性質。除以質量 $m$ 得小寫的比性質（每 kg）；除以摩爾數 $n = m/M$ 得摩爾性質，以上橫線標記（每 kmol）。兩者相差一倍摩爾質量 $M$（kg/kmol）：
Uppercase = extensive (total), lowercase = per kg, overbar = per kmol. Steam and refrigerant tables are per kg; gas and combustion tables are usually per kmol.大寫＝廣延（總量），小寫＝每 kg，上橫線＝每 kmol。蒸汽表與冷媒表以每 kg 計；氣體表與燃燒表通常以每 kmol 計。
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
A system's total energy $E$ splits into what we can see from outside and what we cannot. Kinetic energy $KE$ and potential energy $PE$ belong to the system as a whole — its bulk velocity and its height in a gravity field. Everything else, all the energy hidden in the molecules themselves, is lumped into one property: the internal energy $U$.系統的總能量 $E$ 可分為從外部看得見與看不見的兩部分。動能 $KE$ 與位能 $PE$ 屬於系統整體——其整體速度與在重力場中的高度。其餘所有藏於分子內部的能量，統歸為一個性質：內能 $U$。
Microscopically, $U$ is the sum of every form of energy the molecules carry:從微觀看，$U$ 是分子所攜帶各種形式能量的總和：
- Sensible energy — the kinetic energy of molecular translation, rotation, and vibration (plus electron spin and orbital motion). This is the part that temperature measures: the higher $T$, the faster the molecules move.顯能——分子平移、轉動與振動的動能（及電子自旋與軌道運動）。這是溫度所量測的部分：$T$ 越高，分子運動越快。
- Latent energy — the energy of intermolecular forces that bind molecules into a liquid or solid. Supplying it breaks the bonds and changes phase without changing temperature.潛能——將分子束縛成液體或固體的分子間作用力能量。供給此能量會打斷鍵結、改變相，但溫度不變。
- Chemical energy — the energy of atomic bonds within molecules, released or absorbed in reactions such as combustion.化學能——分子內原子鍵結的能量，於燃燒等反應中釋放或吸收。
- Nuclear energy — the binding energy within the atomic nucleus.核能——原子核內的束縛能。
In classical thermodynamics without reactions, only the first two change — sensible energy through heating and cooling, latent energy through phase change. Chemical and nuclear energy ride along as constants and drop out of every $\Delta U$.在無反應的古典熱力學中，只有前兩者會改變——顯能經由加熱冷卻，潛能經由相變化。化學能與核能維持常數，在任何 $\Delta U$ 中都被消去。
Not by adding up molecular energies — we cannot count them. $U$ is defined macroscopically by the first law: for a closed system at rest, the energy transferred as heat and work between two equilibrium states is found to depend only on those end states, never on the path. That path-independent quantity is the change in a property, $\Delta U = Q - W$. Only changes are measurable, so tables assign $u = 0$ at an arbitrary reference state (water: saturated liquid at 0.01 °C).並非將分子能量逐一相加——我們無法計數。$U$ 是由第一定律巨觀地定義：對靜止的封閉系統，兩平衡狀態之間以熱與功傳遞的能量，實驗上只與端點狀態有關，與路徑無關。此路徑無關的量就是某一性質的變化，$\Delta U = Q - W$。因只有變化量可測，性質表在任意參考狀態設 $u = 0$（水：0.01 °C 的飽和液體）。
$U$ is extensive. Divide by mass for the specific internal energy $u$, or by moles for the molar internal energy $\bar u$ — both intensive:$U$ 為廣延性質。除以質量得比內能 $u$，除以摩爾數得摩爾內能 $\bar u$——兩者皆為強度性質：
The tables list $u$. Since it is a property, the state postulate applies: $u = u(T, v)$ for a simple compressible substance. For an ideal gas the volume dependence vanishes and $u = u(T)$ alone — the subject of Ideal Gas & Compressibility.性質表所列為 $u$。既然它是性質，狀態假設適用：對簡單可壓縮物質 $u = u(T, v)$。對理想氣體，體積相依性消失，僅 $u = u(T)$——詳見理想氣體與壓縮因子。
Enthalpy焓
Internal energy $u$ is the energy a closed system carries — a fixed lump of matter sitting in a container. But almost every device you will analyse has mass flowing through it: a boiler, a turbine, a nozzle, a condenser. Something extra happens there. To get a parcel of fluid into the device, the fluid behind it must push it across the inlet — and pushing takes work. That work enters with the fluid, on top of the $u$ it already carries.內能 $u$ 是封閉系統所攜帶的能量——一團固定不變的物質待在容器裡。但你將分析的裝置幾乎都有質量流經其中：鍋爐、渦輪機、噴嘴、冷凝器。這裡多出了一件事。要讓一團流體進入裝置，後方的流體必須把它推過入口——而推動需要作功。這份功隨流體一起進入，附加在它原本攜帶的 $u$ 之上。
How much? Nothing new is needed — it is the $W = F\,d$ you already know from statics and dynamics. Take a small element of fluid of mass $m$ and volume $V$, filling a pipe of cross-sectional area $A$ over a length $L$, so $V = AL$. The fluid upstream presses on its rear face at pressure $p$, giving a force $F = pA$. To push the whole element through the inlet, that force must act through the displacement $L$:有多少？不需要任何新的物理——就是你在靜力學與動力學學過的 $W = F\,d$。取一小團流體，質量 $m$、體積 $V$，填滿截面積 $A$、長度 $L$ 的管段，故 $V = AL$。上游流體以壓力 $p$ 壓在它的後端面上，產生力 $F = pA$。要把整團流體推過入口，此力必須作用於位移 $L$ 之上：
This $pv$ is called the flow work (or flow energy) — force times displacement, exactly as in mechanics. Notice that the areas cancel: the result depends only on the pressure and the volume the fluid occupies, not on the shape of the pipe.這個 $pv$ 稱為流動功（或流動能）——力乘以位移，與力學中完全相同。注意面積已相消：結果只取決於壓力與流體所占的體積，與管路形狀無關。
So a flowing stream always carries two things together: the internal energy $u$ stored in its molecules, and the flow work $pv$ it took to push it in. Because they are inseparable in flow problems, we give the sum a name — enthalpy:因此，流動的流體恆同時攜帶兩樣東西：分子內儲存的內能 $u$，以及把它推進來所需的流動功 $pv$。既然在流動問題中兩者形影不離，便為其和取一個名字——焓：
Enthalpy also turns out to be the natural property for constant-pressure processes: at fixed $p$, the heat added to a closed system equals $\Delta h$ per unit mass, so a boiler's or condenser's heat duty is read straight off an enthalpy difference. That result is derived in the First Law; for now, take it as the reason $h$ is tabulated everywhere.焓同時也是定壓過程的自然性質：在壓力固定下，加入封閉系統的熱量，每單位質量即等於 $\Delta h$，因此鍋爐或冷凝器的熱負荷可直接由焓差讀出。此結果將於第一定律推導；現在只需知道，這就是各處性質表都列出 $h$ 的理由。
The flow-work picture explains why the grouping $u + pv$ matters, but $h$ is defined for any state — it is built from properties alone, so it is a property itself. For gas sealed in a rigid tank nothing is flowing and no flow work is being done, yet $h = u + pv$ still has a perfectly good value. Use the story to remember the definition, not to restrict it.流動功的圖像解釋了 $u + pv$ 這個組合為何重要，但 $h$ 對任何狀態皆有定義——它完全由性質組成，故其本身即為性質。對密封於剛性容器中的氣體，沒有任何流動、也沒有流動功，然而 $h = u + pv$ 依然有明確的值。用這個故事來記住定義，而非限制它。
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：
Internal energy and enthalpy tell you how much energy a system holds. Entropy tells you how useful that energy is. Take 1 MJ of heat, once available at 40 °C (313 K) and once at 400 °C (673 K). The quantity is identical. But the entropy carried with it is $Q/T$: about 3.2 kJ/K at 40 °C versus 1.5 kJ/K at 400 °C. The hot megajoule can drive a turbine or heat almost anything; the warm one can barely warm a room, and no engine can extract much work from it. Same energy, less entropy, higher quality — entropy is the price tag on energy's usefulness.內能與焓告訴你系統擁有多少能量；熵則告訴你這些能量有多好用。取 1 MJ 的熱，一次在 40 °C（313 K）取得，一次在 400 °C（673 K）取得。數量完全相同，但隨之攜帶的熵為 $Q/T$：40 °C 時約 3.2 kJ/K，400 °C 時約 1.5 kJ/K。高溫的那 1 MJ 可推動渦輪機、幾乎能加熱任何東西；溫熱的那 1 MJ 頂多暖暖房間，任何引擎都難以從中取出多少功。能量相同、熵較少、品質較高——熵就是能量「好用程度」的標價。
Unlike mass and energy, entropy can be produced by irreversibilities but never destroyed — the heart of the second law. Every irreversibility adds entropy to the same energy, degrading its quality. (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。每一次不可逆都為同樣的能量添加熵，使其品質下降。（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
Example範例 Density, specific volume, enthalpy密度、比體積、焓 ›
Given: 2 kg of gas in 0.80 m³ at 150 kPa, $u=210$ kJ/kg.已知：2 kg 氣體在 150 kPa 下占據 0.80 m³，$u=210$ kJ/kg。
Find: density, specific volume, and specific enthalpy.求：密度、比體積與比焓。
Solution. $$\rho=2/0.80=2.5\ \tfrac{\text{kg}}{\text{m}^3},\quad v=0.40\ \tfrac{\text{m}^3}{\text{kg}},\quad h=210+150(0.40)=270\ \tfrac{\text{kJ}}{\text{kg}}$$解： $$\rho=2/0.80=2.5\ \tfrac{\text{kg}}{\text{m}^3},\quad v=0.40\ \tfrac{\text{m}^3}{\text{kg}},\quad h=210+150(0.40)=270\ \tfrac{\text{kJ}}{\text{kg}}$$
Thermodynamic vs transport properties熱力學性質與傳輸性質
Not every property of a fluid is a thermodynamic property. Viscosity $\mu$, thermal conductivity $k$, and mass diffusivity $D$ are properties too — they are fixed by the state and are tabulated alongside $v$, $h$, $s$ — but they belong to a different family.流體的性質並非全部都是熱力學性質。黏度 $\mu$、熱導係數 $k$ 與質量擴散係數 $D$ 也是性質——由狀態決定，並與 $v$、$h$、$s$ 一起列表——但屬於不同的家族。
Thermodynamic properties ($p, v, T, u, h, s, c_p, c_v, \ldots$) describe a system at equilibrium. They are linked by an equation of state and by the property relations, and they answer the question “how much?” — how much energy is stored, how much work a process can yield. Time never appears.熱力學性質（$p, v, T, u, h, s, c_p, c_v, \ldots$）描述平衡狀態下的系統。它們由狀態方程式與性質關係式相連，回答「多少？」——儲存多少能量、一個過程能產生多少功。時間從不出現。
Transport properties ($\mu, k, D$) describe how fast a system out of equilibrium returns to it. Each is the coefficient in a flux–gradient law — momentum flux driven by a velocity gradient (Newton), heat flux by a temperature gradient (Fourier), mass flux by a concentration gradient (Fick). They answer “how fast?” and carry time in their units.傳輸性質（$\mu, k, D$）描述偏離平衡的系統以多快的速率回到平衡。每一個都是通量–梯度定律中的係數——速度梯度驅動動量通量（牛頓）、溫度梯度驅動熱通量（傅立葉）、濃度梯度驅動質量通量（菲克）。它們回答「多快？」，單位中含有時間。
Classical thermodynamics uses only the first family: it compares end states and never asks how long the process takes or how large the equipment must be. The moment those questions arise — sizing a heat exchanger, estimating pressure drop — the transport properties enter and the subject becomes heat transfer and fluid mechanics. Both families are intensive, both depend on the state (for a gas, mainly on $T$), and both are found in the same property tables; the difference is what they are for.古典熱力學只用前一家族：它比較端點狀態，從不問過程需要多久、設備需要多大。一旦這些問題出現——熱交換器選型、壓降估算——傳輸性質便登場，學科也轉為熱傳與流體力學。兩家族皆為強度性質，皆由狀態決定（對氣體主要取決於 $T$），也都見於同一份性質表；差別在於它們用來做什麼。
Key definitions關鍵定義
Every property in this chapter is either measured directly (p, v, T) or defined from those and the laws (u, h, s, c). Know which is which, and know the three forms — total, per kg, per kmol — of each.本章每個性質要么直接量測（p、v、T），要么由這些與定律定義而來（u、h、s、c）。分清兩者，並熟悉每個性質的三種形式——總量、每 kg、每 kmol。