Basic Concepts: Systems, Properties & Equilibrium基本概念：系統、性質與平衡

What you'll be able to do本章學習成果

• Define system, surroundings, boundary, and distinguish closed systems from control volumes.定義系統、外界與邊界，並區分封閉系統與控制體積。
• Recognize what a property is and the kinds of equilibrium that define a state.辨識性質的意涵，以及定義狀態的各類平衡。
• State the state postulate and recognize equilibrium.陳述狀態假設並辨識平衡狀態。
• Define process, path, and cycle, and state the zeroth law and temperature scales.定義過程、路徑與循環，陳述第零定律與溫標。

Key terms關鍵術語

Systems & boundaries系統與邊界

Thermodynamics is the science of energy. The system is whatever we choose to study; the surroundings are everything external; the boundary separates them.熱力學是能量的科學。系統是我們選擇研究的對象；外界是一切外部環境；邊界將兩者分隔。

• Closed system (control mass): a fixed amount of mass. No mass crosses the boundary, though energy still can.封閉系統（控制質量）：固定的質量。質量不穿越邊界，但能量仍可。
• Open system (control volume): a region in space through which both mass and energy can flow.開放系統（控制體積）：空間中的一個區域，質量與能量皆可穿越。

We treat matter as a continuum, ignoring atomic graininess — valid when the system is large compared with the molecular mean free path (Knudsen number ≤ 0.01).我們將物質視為連續體，忽略原子尺度的不均勻性——在系統遠大於分子平均自由路徑時成立（努森數 ≤ 0.01）。

Properties性質

A property is any characteristic of a system — pressure, temperature, volume, mass. Properties are either intensive (independent of size) or extensive (proportional to extent); dividing extensive by mass gives a specific intensive property. See Thermodynamic Properties: Fundamentals for the full definitions and interactive lab.性質是系統的任何特徵——壓力、溫度、體積、質量。性質分為強度性質（與大小無關）與廣延性質（與大小成比例）；廣延性質除以質量得到比強度性質。完整定義與互動實驗請見熱力學性質：基礎。

State & equilibrium狀態與平衡

The state is the condition of a system described by its properties. Thermodynamics deals with equilibrium states — no unbalanced driving forces. Full equilibrium requires: thermal (uniform T), mechanical (uniform p), phase (stable phases), and chemical (no net reaction).狀態是由性質描述的系統條件。熱力學處理平衡狀態——無不平衡驅動力。完全熱力學平衡需同時滿足：熱平衡（溫度均勻）、力學平衡（壓力均勻）、相平衡（穩定相量）與化學平衡（無淨反應）。

The state postulate狀態假設

Fix any two independent intensive properties (say $T$ and $p$, or $T$ and $v$) and every other property is determined. This is the principle the psychrometric chart and property tables rely on. At saturation, $T$ and $p$ are not independent — a different second property is needed.固定任意兩個獨立強度性質（如 $T$ 與 $p$，或 $T$ 與 $v$），其他所有性質即被確定。這正是濕空氣線圖與性質表的依據。在飽和區，$T$ 與 $p$ 並非獨立——需另選第二個性質。

Process & path過程與路徑

A process is any change from one equilibrium state to another; the path is the series of states passed through. A quasi-equilibrium process stays infinitesimally close to equilibrium — the idealization that lets us draw a process as a line on a diagram. Iso- prefixes name constant-property processes: isothermal (T), isobaric (p), isochoric (v). A cycle returns to its initial state.過程是從一個平衡狀態到另一個平衡狀態的任何變化；路徑是所經歷的一系列狀態。準平衡過程在整個過程中始終無限接近平衡——這一理想化使我們能在性質圖上以線段表示過程。等字頭命名定性質過程：等溫（T）、等壓（p）、等容（v）。循環使系統回到初始狀態。

Temperature & the zeroth law溫度與第零定律

The absolute Kelvin scale: $T(\mathrm{K}) = T(°\mathrm{C}) + 273.15$. Pressures are absolute (from vacuum) or gage (relative to atmosphere): $p_{abs} = p_{atm} + p_{gage}$.絕對克耳文溫標：$T(\mathrm{K}) = T(°\mathrm{C}) + 273.15$。壓力分為絕對壓力（以真空為基準）或表壓（以大氣壓為基準）：$p_{abs} = p_{atm} + p_{gage}$。

Gauge to absolute pressure表壓換算絕對壓力