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Physics · 术语表

什么是 Supercooling, superheating？

Definition 25.14 University Physics — Year 1 · 第 25 章 — Phase Changes of a Pure Substance

A phase can persist beyond its equilibrium boundary: liquid water cooled below $0{}^{\circ}\mathrm{C}$ without freezing (supercooling), heated above its boiling point without boiling (superheating), vapour compressed beyond $P_s$ without condensing (supersaturation). Such metastable states require the absence of nucleation sites — dust, scratches, dissolved gas, ice crystals — on which the new phase can start; a disturbance tips them suddenly and irreversibly to equilibrium.

示例

Example 25.15 (The supercooled bottle)

Still water at $-8{}^{\circ}\mathrm{C}$ in a clean bottle, knocked: ice crystals shoot through it and the temperature jumps to $0{}^{\circ}\mathrm{C}$ . The process is adiabatic and fast, so the sensible heat $c\,\Delta T$ freezes a fraction $x = c\,\Delta T/L_f = 4.18 \times 8/334 = 0.10$ of the water; the entropy created, $c\ln(273/265) - xL_f/273 = 124 - 122 = +2\,\mathrm{J}/(\mathrm{kg}\,\mathrm{K})$ , is positive as it must be. Clouds are full of supercooled droplets down to $-40{}^{\circ}\mathrm{C}$ ; aircraft icing and the seeding of clouds both exploit that.

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