The kinetic particle model
| English | Chinese | Pinyin |
|---|---|---|
| particles | 粒子 | lì zi |
| kinetic particle model | 分子动理论 | fèn zǐ dòng lǐ lùn |
| solid | 固体 | gù tǐ |
| liquid | 液体 | yè tǐ |
| gas | 气体 | qì tǐ |
| temperature | 温度 | wēn dù |
| absolute zero | 绝对零度 | jué duì líng dù |
| kelvin | 开尔文 | kāi ěr wén |
| Brownian motion | 布朗运动 | bù lǎng yùn dòng |
Everything is made of particles 粒子
- Look closely enough and all matter is tiny particles that never stop moving.
- This one idea — the kinetic particle model 分子动理论 — explains solids 固体, liquids 液体, gases 气体, heat and pressure.
- Let's see what the particles are doing.
The three states of matter
- Solid — particles packed in a regular pattern, only vibrating. Fixed shape and volume.
- Liquid — particles still close but jumbled; they slide past each other. Fixed volume, takes the container's shape.
- Gas — particles far apart, fast and random. Fills the whole container.
Squeeze a gas (Boyle's law)
Slide the piston in to shrink the volume. The same particles get crammed into less space, so they hit the walls more often and the pressure climbs — while pressure × volume stays constant.
In which state are the particles far apart, fast, and moving in random directions?
Gas particles are far apart and move quickly in random directions, filling the container. Solids only vibrate; liquids slide past each other.
Temperature 温度 and particle energy
- Heat a substance and its particles move faster — they gain kinetic energy.
- Temperature is a measure of the average kinetic energy of the particles.
- The lowest possible temperature is absolute zero 绝对零度, $-273\ {}^{\circ}\text{C}$ — particles have the least energy.
The particle model of solids, liquids and gases
Temperature is a measure of:
Hotter means the particles move faster, so temperature measures their average kinetic energy.
The kelvin 开尔文 scale
- Scientists often measure temperature in kelvin (K), starting from absolute zero.
- To convert:
- So $0\ {}^{\circ}\text{C} = 273\ \text{K}$ and $27\ {}^{\circ}\text{C} = 300\ \text{K}$.
Convert $27\ {}^{\circ}\text{C}$ to kelvin.
$T = \theta + 273 = 27 + 273 = 300\ \text{K}$.
Gas pressure
- Gas particles constantly hit the walls of their container. Each hit is a tiny push.
- The pressure is the total force of these hits on each unit of area.
- Heat the gas (fixed volume) → faster, harder, more frequent hits → higher pressure.
- Squeeze it smaller (fixed temperature) → more hits per second on each area → higher pressure.
- For a fixed mass of gas at constant temperature, $pV = \text{constant}$ (halve the volume → double the pressure).
A gas has a volume of $200\ \text{cm}^3$ at $100\ \text{kPa}$. It is squeezed to $100\ \text{cm}^3$ at constant temperature. What is the new pressure, in kPa?
$pV$ is constant: $100 \times 200 = p \times 100$, so $p = 200\ \text{kPa}$. Halving the volume doubles the pressure.
Heating a gas in a sealed, fixed-volume can increases its pressure.
Hotter particles move faster and hit the walls harder and more often, so the pressure rises.
Brownian motion 布朗运动
- Under a microscope, smoke specks in air jiggle along jerky, random paths.
- This is Brownian motion: the big specks are knocked about by fast, invisible air particles.
- It is strong evidence that matter really is made of moving particles.
Random hits from fast, invisible air particles push a smoke grain along a jerky, random path
Smoke specks seen under a microscope jiggle randomly. This is because:
Brownian motion: tiny fast air particles knock the larger smoke specks about, evidence for moving particles.
You've got it
- solid (fixed, vibrating) · liquid (close, sliding) · gas (far apart, fast)
- temperature = average kinetic energy of particles; absolute zero $= -273\ {}^{\circ}\text{C}$
- $T\,(\text{K}) = \theta\,({}^{\circ}\text{C}) + 273$
- gas pressure = particle hits per unit area; at constant temperature $pV = \text{constant}$