IGCSE Physics 热物理 比热容 气压 对流辐射
热物理是 CAIE IGCSE Physics 0625 六个主题里的第二个，粒子模型、比热容、传导对流辐射这些内容在选择题和理论卷里既出计算也出解释。下面按现行大纲逐条拆开：每条要求长什么样、题目怎么问、哪里最容易丢分。
Thermal physics is topic 2 of six in Cambridge IGCSE Physics 0625. Core candidates meet it in Paper 1 and Paper 3, Extended candidates in Paper 2 and Paper 4, and the same ideas return in the practical paper. What follows tracks the syllabus wording item by item, with the calculations and particle explanations examiners actually set.
1 热物理在 2026 至 2028 考纲里的位置与卷面结构
0625 适用于 2026 至 2028 年考试。Core 考生考 Paper 1（选择题，45 分钟，40 分，占 30%）与 Paper 3（理论卷，1 小时 15 分，80 分，占 50%）；Extended 考生考 Paper 2 与 Paper 4，时长与分值相同，内容在 Core 之外加上 Supplement；实践部分从 Paper 5（实验操作，1 小时 15 分）或 Paper 6（实验替代卷，1 小时）里选一份，占 20%。官方大纲给出的评价目标权重是知识理解 50%、信息处理与解题 30%、实验技能 20%（Cambridge 0625 syllabus，2026 至 2028）。
The 0625 syllabus runs for the 2026, 2027 and 2028 exam series. Core candidates sit Paper 1, a multiple-choice paper of 45 minutes and 40 marks worth 30%, together with Paper 3, a theory paper of 1 hour 15 minutes and 80 marks worth 50%. Extended candidates sit Paper 2 and Paper 4, which carry the same time and marks but include the Supplement content. One practical paper, Paper 5 or Paper 6, makes up the remaining 20%. Cambridge weights the assessment objectives as 50% knowledge, 30% handling information and problem solving, 20% experimental skills.
2 粒子模型与三态的排列、间距和运动
2.1.1 与 2.1.2 要求用粒子的排列、间距和运动描述固体、液体和气体，并画出简单粒子图。固体粒子规则排列、靠得很近、只在固定位置振动；液体粒子间距略大，可以互相滑动，所以有固定体积但没有固定形状；气体粒子间距很大，粒子间作用力几乎为零，随机高速运动，既没有固定体积也没有固定形状。常见的错误是解释可压缩性时写气体粒子本身更小，粒子的体积并没有变小，变的是粒子之间的空隙。
Points 2.1.1 and 2.1.2 ask for the arrangement, separation and motion of particles in the three states, drawn as simple particle diagrams. In a solid the particles sit in a regular lattice, close together, vibrating about fixed positions. In a liquid they stay close but slide past one another, which is why a liquid keeps its volume and takes the shape of its container. In a gas the separation is large, the forces between particles are negligible, and the motion is fast and random. When you explain compressibility, the particles themselves do not shrink; the gaps between them do.
3 温度、平均动能与绝对零度
温度衡量的是粒子平均动能的大小，温度升高，粒子平均动能随之增大。大纲要求知道存在最低温度 -273 °C，也就是绝对零度，此时粒子的动能最小；补充内容进一步要求把温度升高描述成物体内所有粒子平均动能的增大，并知道温度升高意味着内能增加。很多学生把 -273 °C 写成粒子完全静止，碰上一个要求解释的题就会多出一句站不住的表述。
Temperature measures the average kinetic energy of the particles, and that average rises when the temperature rises. The syllabus sets -273 °C as absolute zero, the lowest possible temperature, where the particles have least kinetic energy. The Supplement wording goes further: a rise in temperature is described as an increase in the average kinetic energy of all the particles in the object, and internal energy rises with it. Writing that the particles stop moving at absolute zero adds a claim the mark scheme does not want.
4 气体压强与开尔文温标 T = θ + 273
气体压强来自粒子撞击容器壁，按力除以面积来理解。开尔文温标用 T = θ + 273 换算：27 °C 是 300 K，127 °C 是 400 K，0 K 对应 -273 °C。固定体积下升温，粒子平均动能变大，撞击容器壁更频繁也更猛烈，压强升高；温度不变而压缩体积，单位面积上的撞击次数增加，压强同样升高。很多学生把 27 °C 和 127 °C 直接按比例比较，忘了先换算成 300 K 和 400 K，这是这一节最常见的失分。
Gas pressure comes from particles colliding with the walls of the container, taken as force divided by area. The kelvin scale uses T = theta + 273, so 27 °C is 300 K, 127 °C is 400 K, and 0 K sits at -273 °C. Heat a fixed mass at constant volume and the particles move faster, hit the walls more often and more violently, and the pressure rises. Squeeze the same gas at constant temperature and the collisions per unit area increase instead. Comparing temperatures in degrees Celsius is wrong; convert to kelvin first.
5 pV 为常数时的等温压缩计算
补充内容要求对固定质量的气体在恒定温度下使用 pV = 常数，并能读懂对应的图像。压强与体积成反比：100 kPa 的气体由 200 cm³ 压缩到 50 cm³，压强变为 100 × 200 ÷ 50 = 400 kPa。以 p 对 V 作图得到一条曲线，改画 p 对 1/V 则是一条过原点的直线。使用这个关系前先确认质量不变、温度不变两个条件，缺一个都不能直接套公式。
The Supplement point in 2.1.3 is pV = constant for a fixed mass of gas at constant temperature, together with a graphical representation of the relationship. Pressure and volume are inversely proportional: 200 cm3 of gas at 100 kPa compressed to 50 cm3 reaches 100 x 200 / 50 = 400 kPa. A graph of p against V is a curve, while p against 1/V is a straight line through the origin. Check that the mass is fixed and the temperature is constant before you use the relationship; either condition failing rules the equation out.
6 热膨胀的量级比较与日常后果
在同样的温升下，气体的膨胀量最大，液体次之，固体最小，原因仍然是粒子的自由度：气体粒子之间几乎没有约束，小幅升温就能让间距明显变大；固体粒子被晶格束缚，只能多振动一点。大纲要的是定性描述加日常例子：铁轨之间留缝、桥梁的伸缩接头、双金属片做的温控开关、用热水冲开拧不动的金属瓶盖。答题时写清楚是哪一种材料在膨胀、往哪个方向膨胀，比只写热胀冷缩四个字更容易拿分。
For the same temperature rise, gases expand most, liquids less, and solids least, and the reason is how free the particles are. Gas particles are barely constrained, so a small rise in temperature moves them much further apart; particles in a solid are held by the lattice and can only vibrate a little harder. The syllabus asks for a qualitative description with everyday examples: gaps between railway rails, expansion joints on bridges, the bimetallic strip in a thermostat, and a metal lid loosened with hot water. Name the material that expands rather than writing expansion and contraction on its own.
7 比热容的定义与 c = ΔE / (mΔθ)
比热容定义为单位质量升高单位温度所需的能量，公式写作 c = ΔE / (mΔθ)，单位是 J/(kg °C)。常见值是水 4200、铝约 900、铜约 385，考试题目一般会把需要的值直接给出。0.5 kg 的水从 20 °C 加热到 80 °C，ΔE = 0.5 × 4200 × 60 = 126000 J，也就是 126 kJ。代入前先算出温度变化 Δθ，把两个温度相加当成温度变化是这一节最典型的算式失误。
Specific heat capacity is defined as the energy required per unit mass per unit temperature increase, written c = dE / (m dtheta) and measured in J/(kg °C). Typical values are 4200 for water, about 900 for aluminium and 385 for copper, and questions normally supply whatever you need. Heating 0.5 kg of water from 20 °C to 80 °C takes dE = 0.5 x 4200 x 60 = 126000 J, or 126 kJ. Work out the temperature change first; adding the two temperatures together instead of subtracting them is the classic slip here.
8 比热容实验与散热造成的偏差方向
大纲要求描述测量固体和液体比热容的实验。电加热法最常考：先称出质量 m，把加热器和温度计插进金属块，通电后记录电流 I、电压 V 和时间 t，用 ΔE = VIt 算出供给的能量，再读出温度变化。散热会让测出的比热容偏大，因为一部分能量跑到环境里，温度上升得比理论值少，而算式里用的还是全部供给的能量。用保温材料包住金属块，或者提高功率缩短加热时间，都能把这项误差压小。常见的错误是把方向写反，说散热使结果偏小。
Syllabus point 2.2.2 also asks you to describe experiments to measure the specific heat capacity of a solid and of a liquid. The electrical method is the standard answer: weigh the block, insert the heater and a thermometer, run a current and record current, voltage and time, so the energy supplied is VIt. Heat lost to the surroundings makes the calculated value too large, because the measured temperature rise is smaller than the energy supplied would produce if nothing escaped. Insulation, or a higher power for a shorter time, keeps that error small. Many candidates state the direction of this error the wrong way round.
9 熔化与沸腾：持续输入能量而温度不变
标准大气压下水的熔点是 0 °C，沸点是 100 °C。熔化与沸腾期间能量持续输入，温度却停在熔点或沸点不动，这些能量用于改变粒子的间距与排列，而不是提高平均动能，加热曲线上的水平段对应的就是这一段。凝结与凝固要用粒子模型解释：粒子失去能量后重新靠拢、重新排列。现行 0625 大纲（2026 至 2028 年考试）在这一节只要求描述熔化与沸腾过程中能量输入而温度不变，没有列入比潜热的计算公式，不必把 A Level 的 E = mL 搬进来。
At standard atmospheric pressure water melts at 0 °C and boils at 100 °C. During melting and boiling, energy keeps flowing in while the temperature stays on the melting point or boiling point, because that energy changes the spacing and arrangement of the particles rather than their average kinetic energy. That is the flat section of a heating curve. Condensation and solidification are explained the other way round: particles lose energy, move closer together and settle into a new arrangement. The current 0625 syllabus, examined in 2026 to 2028, asks for this description only; a specific latent heat calculation is not on the content list.
10 蒸发与沸腾的差别及影响因素
蒸发在任何温度下都会发生，而且只发生在液体表面：能量较高的粒子挣脱表面离开，留下的粒子平均动能下降，液体温度因此降低，出汗和湿手吹风觉得凉就是这个过程。蒸发的快慢取决于液体的温度、表面积和表面空气的流动，这三个因素都要能配上具体例子。沸腾是整团液体在沸点同时发生的变化，需要持续供给能量，温度保持不变。两者的区别落在发生位置、温度和是否需要持续供能上。
Evaporation happens at any temperature and only at the surface of a liquid. The more energetic particles escape, the average kinetic energy of those left behind falls, and the liquid cools, which is why sweating and wet hands in a draught feel cold. The rate depends on the temperature of the liquid, its surface area and the air movement over that surface. Boiling is a change through the whole liquid at its boiling point, with energy supplied continuously and the temperature steady. Location, temperature and the need for a supply of energy separate the two.
11 传导：晶格振动与自由电子
传导在固体里靠两种机制完成：晶格上的原子或分子振动把能量传给相邻粒子，金属里还有自由电子在快速移动中搬运能量，金属通常因此成为良导体。气体和大部分液体传导很差，原因是粒子间距大，靠碰撞传能量的效率低。演示实验常把不同金属棒的一端同时加热，看另一端的蜡谁先熔化，考查的其实是导热快慢而不是比热容。非金属固体里也存在导热好于绝缘体、但不如良导体的材料。
Thermal conduction in solids works in two ways. Lattice vibrations pass energy from particle to particle, and in metals free electrons carry energy as they move through the structure, which is why metals are good conductors. Gases and most liquids conduct badly because their particles are far apart, so collisions transfer energy slowly. A common demonstration heats one end of rods of different metals and waits for wax to melt at the other end; that compares conduction, not specific heat capacity. Some solids sit between thermal insulators and good conductors.
12 对流：密度差驱动的循环
对流只发生在液体和气体中。受热部分的粒子动能变大、间距变大、密度变小，于是上升，较冷较密的流体下沉补位，形成循环，热就随流体被搬走。实验中往水里放一颗高锰酸钾晶体加热，可以看见紫色水流上升，用烟雾箱也能让气流变得可见。应用上，房间的暖气片装在低处、制冷用的空调装在高处，都是顺着密度差来布置的。液体的传导能力差，对流却可以把能量搬得很快。
Convection happens only in liquids and gases. The heated region gains kinetic energy, its particles spread out, its density falls, and it rises while cooler, denser fluid sinks to replace it. A crystal of potassium manganate(VII) dropped into heated water makes that flow visible, and a smoke box does the same job for air. Radiators sit low in a room and air conditioners sit high because of the same density difference. A liquid may be a poor conductor and still move energy quickly by convection.
13 红外辐射、表面属性与地球的热平衡
所有物体都在发射红外辐射，辐射是唯一不需要介质的传热方式，太阳的能量穿过真空到达地球就是证据。表面越黑、越粗糙，吸收和发射红外辐射越强；越白、越光亮，反射越强而发射越弱。发射的速率随表面温度和表面积增大而提高。地球的温度由入射辐射与地表向外辐射的收支决定，大气里的温室气体减少了逃逸出去的那部分辐射，地表平均温度因此上升。
Every object emits infrared radiation, and radiation is the only transfer mechanism that needs no medium, which is how energy from the Sun crosses space to reach the Earth. A black, dull surface is a good absorber and emitter, while a white, shiny one reflects well and emits poorly. The rate of emission rises with the surface temperature and the surface area. The Earth's temperature depends on the balance between incoming radiation and the radiation emitted from the surface, and greenhouse gases in the atmosphere reduce the outgoing share.
复习时把每个知识点对应回大纲的 2.1、2.2、2.3 三段，再拿近几年真题检验自己能不能用粒子模型把现象讲清楚，做计算题时重点检查温度与体积的单位是否换到位。
When you revise, map each idea back to its syllabus number in sections 2.1 to 2.3, check that you can explain the phenomena with the particle model, and work through recent past papers watching the units on temperature and volume.
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