Thermal Physics
IGCSE Physics Topic 2 11:53 English narration · English + 中文 subtitles burned in
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What is heat, really?
热到底是什么？
Zoom in far enough, and everything is made of tiny particles, always moving.
放大到足够小，你会发现万物都由不停运动的微小粒子组成。
Heat something, and its particles move faster. Cool it, and they slow down.
加热物体，它的粒子就运动得更快；冷却它，粒子就慢下来。
That one idea — moving particles — explains temperature, pressure, melting, boiling, and how heat travels.
运动的粒子这一个想法，就能解释温度、压强、熔化、沸腾，以及热是怎样传递的。
Welcome to thermal physics — the science of heat and temperature.
欢迎来到热学——研究热和温度的科学。
It all starts with particles in motion.
它的一切都始于运动中的粒子。
Let's begin.
让我们开始吧。
All matter comes in three states.
所有物质都有三种状态。
In a solid, the particles are packed close in a fixed, regular pattern, only vibrating in place — so a solid keeps its shape.
在固体中，粒子紧密排列成固定、规则的图案，只在原位振动—— 所以固体保持自己的形状。
In a liquid, the particles are still close, but they can slide past each other, so a liquid flows and takes the shape of its container.
在液体中，粒子仍然靠得很近，但能彼此滑过， 所以液体会流动，并呈现容器的形状。
In a gas, the particles are far apart and rush around freely, filling all the space they can.
在气体中，粒子相距很远，自由地四处飞奔， 填满它们能到达的全部空间。
Look at the same particles in three boxes.
看同样的粒子装在三个盒子里。
In a solid, shape and volume are both fixed: the particles sit close together in a regular pattern and only vibrate.
在固体中，形状和体积都固定：粒子紧密排成规则图案，只振动。
In a liquid, volume stays fixed but the shape is not — the particles are still close, with no fixed pattern, so they slide and the liquid fills the bottom of its container.
在液体中，体积固定但形状不固定——粒子仍然靠得很近，没有固定图案，所以会滑动， 液体填满容器底部。
In a gas, neither shape nor volume is fixed: particles are far apart, move fast and at random, and fill every space they can reach.
在气体中，形状和体积都不固定：粒子相距很远，快速随机运动， 填满它们能到达的每一处空间。
Matter can change from one state to another.
物质可以从一种状态变成另一种。
Melting turns a solid into a liquid.
熔化把固体变成液体。
Boiling or evaporating turns a liquid into a gas.
沸腾或蒸发把液体变成气体。
Condensation is the reverse: a gas cools into a liquid.
凝结是反过来：气体冷却成液体。
Solidification freezes a liquid back into a solid.
凝固把液体冻回固体。
Each change needs energy in one direction, or releases energy in the other, because the forces between particles are being broken or remade.
一个方向的变化需要能量，另一个方向则释放能量，因为粒子之间的力在被破坏或重新形成。
Temperature measures the average kinetic energy of the particles — how fast, on average, they are moving.
温度衡量粒子的平均动能——也就是它们平均运动得有多快。
Heat a substance and the particles speed up, so the temperature rises.
加热物质，粒子加速， 温度就升高。
But going down, there is a limit.
但往下降是有极限的。
At minus two hundred and seventy-three degrees Celsius — absolute zero — the particles have the least energy possible.
在零下二百七十三摄氏度——绝对零度—— 粒子拥有尽可能少的能量。
Scientists count from there, on the kelvin scale.
科学家从这里开始计数，用开尔文标度。
To convert, just add two hundred and seventy-three.
换算时，只要加上二百七十三。
So twenty-five degrees Celsius becomes two hundred and ninety-eight kelvin.
所以二十五摄氏度就是二百九十八开尔文。
The kelvin scale starts at absolute zero, so zero degrees Celsius is two hundred and seventy-three kelvin.
开尔文标度从绝对零度开始，所以零摄氏度就是二百七十三开尔文。
To go the other way, subtract two hundred and seventy-three.
反过来换算，要减去二百七十三。
Two hundred kelvin minus two hundred and seventy-three is minus seventy-three degrees Celsius.
二百开尔文减去二百七十三，等于零下七十三摄氏度。
Always check which way the conversion runs — add for Celsius to kelvin, subtract for kelvin to Celsius.
一定要分清方向——摄氏度到开尔文就加，开尔文到摄氏度就减。
Why does a gas push on its container?
气体为什么会推它的容器？
Because its particles are constantly hitting the walls.
因为它的粒子不停地撞击容器壁。
Each hit is a tiny push, and together they make the gas pressure.
每一次撞击都是一个小小的推力， 合起来就形成气体压强。
Now squeeze the gas into half the space, keeping the temperature the same.
现在把气体压进一半的空间，温度保持不变。
The particles hit twice as often, so the pressure doubles.
粒子撞击的次数变成两倍，所以压强也翻倍。
Pressure times volume stays constant.
压强乘以体积保持不变。
Start at two hundred cubic centimetres and one hundred kilopascals, squeeze to fifty, and the pressure rises to four hundred kilopascals.
从二百立方厘米、一百千帕开始，压到五十，压强就升到四百千帕。
Pressure is not only about volume.
压强不只跟体积有关。
Heat a gas while you keep the volume fixed, and the particles move faster.
保持体积不变加热气体，粒子会运动得更快。
They hit the walls harder and more often, so the pressure rises.
它们更用力、更频繁地撞壁，所以压强升高。
Cool the gas, and the pressure falls.
冷却气体，压强就下降。
Same box, fewer hard hits.
同一只盒子，撞击变少变轻。
Remember: particles themselves do not grow when you heat them — they just move more vigorously.
记住：加热时粒子本身并没有变大——它们只是运动得更剧烈。
At constant temperature, the pressure–volume graph for a gas is a curve that falls steeply then levels off.
温度不变时，气体的压强体积图是一条先陡降再变缓的曲线。
Halve the volume and the pressure doubles; double the volume and the pressure halves.
体积减半，压强加倍； 体积加倍，压强减半。
That is why pressure times volume stays constant.
这就是为什么压强乘以体积保持不变。
On the graph you can pick any two points on the curve and check that p times V gives the same product.
在图上你可以任选曲线上的两个点，检查压强乘以体积是否得到同一个乘积。
Here is the proof that particles are really there, and really moving.
这就是粒子真实存在、并且真的在运动的证据。
Look at a speck of smoke under a microscope.
在显微镜下看一粒烟尘。
It jitters along a jerky, random path.
它沿着一条颠簸、随机的路径抖动。
Nothing visible is pushing it.
没有任何看得见的东西在推它。
It is being knocked about by fast, invisible air particles, striking it from all sides.
它是被快速、看不见的空气粒子从四面八方撞来撞去。
This jittering is called Brownian motion, and it is strong evidence for the kinetic particle model.
这种抖动叫做布朗运动， 是分子动理论的有力证据。
Under the microscope the smoke grain zig-zags because invisible air molecules strike it from every side.
在显微镜下，烟粒之所以曲折前进，是因为看不见的空气分子从四面八方撞击它。
Each hit is tiny, but they do not cancel perfectly, so the grain is shoved one way then another.
每一次撞击都很微小，但它们并不能完全抵消，所以烟粒被一会儿推到这边，一会儿推到那边。
Brownian motion does not prove what the air particles look like — it proves that something small and fast is really there, knocking the grain about.
布朗运动并不能证明空气粒子长什么样——它证明的是：真有又小又快的东西在撞这颗烟粒。
When matter is heated, its particles move more vigorously and take up more space, so the material expands.
物质被加热时，粒子运动更剧烈，占据更多空间，所以材料会膨胀。
Gases expand the most, then liquids, then solids.
气体膨胀最多，其次是液体，再是固体。
Everyday design uses this.
日常设计会用到这一点。
Gaps are left between railway rails so hot weather does not buckle the track.
铁轨之间留下缝隙，热天才不会把轨道顶弯。
Bridges sit on rollers so the deck can grow a little in summer.
桥梁放在滚轴上，桥面夏天可以稍微伸长。
A tight metal lid loosens under hot water because the metal expands.
拧得很紧的金属盖用热水烫一下会松，因为金属膨胀了。
The internal energy of an object is the total energy of all its particles — both how fast they move and how they are held together.
物体的内能是它全部粒子的总能量——既包括粒子运动有多快，也包括它们怎样被束缚在一起。
Heating an object raises its internal energy, and usually its temperature rises too.
加热物体会提高它的内能，通常温度也会升高。
Specific heat capacity tells you how much energy you must supply for each kilogram and each degree of temperature rise.
比热容告诉你：每千克、每升高一度，需要供给多少能量。
Water has a high value, so it stores a lot of thermal energy.
水的比热容很高，所以它能储存很多热能。
Some things heat up easily; others need a lot of energy.
有些东西容易升温，有些则需要很多能量。
The specific heat capacity is the energy needed to raise one kilogram of a material by one degree.
比热容是把一千克某种材料升高一度所需的能量。
Water's is high — four thousand two hundred joules — which is why the sea warms slowly and holds its heat.
水的比热容很高——四千二百焦耳——这就是为什么海洋升温缓慢、又能长时间保温。
To warm two kilograms of water by fifty degrees, multiply the mass, by the specific heat capacity, by the temperature change.
要把两千克水升高五十度，就把质量、乘以比热容、再乘以温度变化。
That comes to four hundred and twenty thousand joules.
结果是四十二万焦耳。
In symbols, specific heat capacity c equals energy change divided by mass times temperature change.
用符号写，比热容等于能量变化除以质量再除以温度变化。
Rearrange to find energy: delta E equals m c delta theta.
改写成求能量：能量变化等于质量乘比热容再乘温度变化。
The temperature change is the final temperature minus the starting temperature — never use the final temperature alone.
温度变化是末温减去初温——绝不要单独用末温。
For water from twenty to seventy degrees, the change is fifty, not seventy.
对从二十度升到七十度的水，变化是五十度，不是七十度。
Keep heating a solid and something surprising happens.
持续加热一块固体，会发生一件出人意料的事。
As it melts, and later as it boils, the temperature stops rising — even though energy is still pouring in.
在它熔化时、以及后来沸腾时， 温度停止上升——尽管能量还在不断注入。
See the flat steps on the graph.
看图上那些水平的台阶。
During these steps, the energy is not making particles faster; it is breaking the forces that hold them together.
在这些台阶期间，能量不是让粒子变快，而是在破坏把粒子束缚在一起的力。
Only once the state has fully changed does the temperature climb again.
只有当状态完全改变之后，温度才会再次升高。
For water at normal air pressure, melting is at zero degrees Celsius and boiling at one hundred.
在正常气压下，水在零摄氏度熔化，在一百摄氏度沸腾。
While ice melts, and while water boils, the temperature graph stays flat even though energy keeps flowing in.
冰熔化时，以及水沸腾时， 温度图保持水平，尽管能量还在不断输入。
That energy is used to break the forces between particles, not to raise the average kinetic energy.
这些能量用来破坏粒子间的力， 而不是提高平均动能。
Once melting or boiling is complete, the line climbs again.
熔化或沸腾一完成，曲线才会再次上升。
Evaporation is different from boiling.
蒸发和沸腾不一样。
It happens only at the surface, and at any temperature.
它只发生在表面，而且在任何温度下都能发生。
The fastest particles near the surface break free and escape into the air.
表面附近最快的粒子挣脱出来，逃进空气中。
And here is the clever part.
巧妙的地方就在这里。
Because the fastest, most energetic particles leave, the ones left behind are slower on average, so the liquid cools down.
因为最快、能量最高的粒子离开了，留下来的粒子平均而言更慢，所以液体就冷却下来。
That is why sweat cools your skin.
这就是为什么汗水能让你的皮肤变凉。
Evaporation is faster when three things rise.
有三样东西升高时，蒸发会加快。
First, a higher temperature — more particles have enough energy to escape.
第一，温度更高——更多粒子有足够能量逃出。
Second, a larger surface area — more of the liquid sits at the free surface.
第二，表面积更大——更多液体处在自由表面上。
Third, more air movement over the surface — moving air carries away the vapour so it does not linger and re-condense.
第三，表面上方空气流动更强—— 流动的空气带走蒸气，使它不会停留并重新凝结。
Warm, shallow, windy: that is the fastest dry-out.
又暖、又浅、又有风：干得最快。
Finally, how does heat travel?
最后，热是怎么传递的？
Three ways.
有三种方式。
Conduction: in a solid, vibrating particles pass energy to their neighbours, and in metals free electrons carry it fast — which is why metals feel cold, pulling heat from your hand.
传导：在固体中，振动的粒子把能量传给邻居， 在金属里自由电子快速地携带能量——这就是为什么金属摸起来凉，它在从你手上带走热。
Convection: in a liquid or gas, warm fluid expands, becomes lighter, and rises, while cooler fluid sinks to replace it — a moving loop.
对流：在液体或气体中，温暖的流体膨胀、变轻、上升，而较冷的流体下沉来补位—— 一个循环流动的圈。
Radiation: infrared waves that need no material at all, crossing empty space from the Sun to you.
辐射：红外线，完全不需要介质，能穿过真空，从太阳来到你身上。
Dull black surfaces emit and absorb it best.
暗淡的黑色表面发射和吸收它的效果最好。
Conduction transfers thermal energy through a material without the material itself flowing.
热传导是通过材料传递热能，而材料本身并不流动。
Heated particles vibrate more and pass energy to their neighbours.
受热的粒子振动更强，把能量传给邻居。
In metals, free delocalised electrons carry energy quickly, so metals are good thermal conductors.
在金属中，自由的离域电子快速携带能量，所以金属是优良的热导体。
Air, wood and plastic conduct badly — they are thermal insulators.
空气、木头和塑料导热很差——它们是热绝缘体。
That is why pan handles are plastic and loft insulation traps air.
所以锅柄用塑料，而屋顶保温层要困住空气。
Convection needs a fluid that can flow — a liquid or a gas.
对流需要能流动的流体——液体或气体。
Heated fluid expands, becomes less dense, and rises; cooler, denser fluid sinks to take its place.
受热的流体膨胀、密度变小并上升； 更冷、更密的流体下沉来占据它的位置。
The loop is a convection current.
这个循环就是对流。
Solids cannot do this, because their particles are locked in place and cannot stream from hot to cold.
固体做不到这一点， 因为粒子被锁在原位，不能从热处流到冷处。
So double-glazing slows convection by trapping a thin layer of still air.
所以双层玻璃通过困住薄层静止空气来减缓对流。
Thermal radiation is energy carried by infrared waves.
热辐射是由红外线携带的能量。
All objects emit it, and it needs no material to travel — it crosses empty space from the Sun to Earth.
所有物体都发射它，而且不需要介质就能传播—— 它穿过真空从太阳来到地球。
A dull black surface is a good emitter and a good absorber of infrared.
暗淡的黑色表面是红外线的优良发射体和吸收体。
A shiny white surface is a poor emitter and a good reflector.
光亮的白色表面是差的发射体、好的反射体。
Choose black to warm up fast in the sun; choose shiny to stay cool.
想在阳光下快点变热就选黑色；想保持凉爽就选光亮的。
A thermal camera turns invisible infrared into a picture.
热成像相机把看不见的红外线变成画面。
Brighter regions mean the surface is hotter and emitting more.
更亮的区域表示表面更热、发射得更多。
An object stays at a constant temperature only when it emits energy at the same rate as it absorbs energy.
只有当物体发射能量的速率等于吸收能量的速率时，温度才保持恒定。
Hotter and larger surfaces emit faster.
更热、更大的表面发射得更快。
That is why a hot cup cools until its emission matches the energy it gains from the room.
所以一杯热饮会冷却，直到它的发射与从房间获得的能量相匹配。
Three marks to lock in.
锁住三个分。
First, heating a gas makes its particles move faster and hit harder — the particles themselves do not grow.
第一，加热气体让它的粒子运动更快、撞得更狠——粒子本身并没有变大。
Second, evaporation happens at the surface, at any temperature, and cools the liquid left behind; boiling happens throughout, at one fixed temperature.
第二，蒸发发生在表面，在任何温度下都能发生，并使留下的液体变凉； 沸腾发生在整个液体中，在一个固定的温度下。
Third, during melting and boiling the temperature stays constant, because the energy is breaking bonds, not speeding particles up.
第三，在熔化和沸腾时温度保持不变， 因为能量在破坏粒子间的键，而不是让粒子变快。
Nail these, and thermal physics is yours.
把这些掌握好，热学就是你的了。
Two more exam traps.
再锁两个考试陷阱。
For specific heat capacity, always use the change in temperature, delta theta — not the final reading on the thermometer.
对比热容，永远用温度的变化量，不要用温度计上的末温读数。
And remember surface colour: dull black wins for both emitting and absorbing infrared; shiny light surfaces reflect best.
还要记住表面颜色：暗淡黑色在发射和吸收红外线上都最好；光亮浅色表面反射最好。
Convection needs a fluid that can flow, so it cannot happen inside a solid.
对流需要能流动的流体，所以在固体内部不可能发生。