What are the laws of thermodynamics?
Heat flows downhill, energy always balances, no engine is perfect and absolute zero is out of reach. Watch hot and cold molecules share their speed, push a piston with a heater, run Carnot's perfect engine, pump heat uphill with an AC, and meet Maxwell's demon.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Temperature is jiggling, and heat flows downhill. Air molecules at 20 °C fly at about 500 m/s. Put a hot box against a cold one and the fast molecules share their energy until both reach one temperature: the zeroth law. Heat never flows back from cold to hot by itself: the second law.
- The energy books: ΔU = Q − W. Heat a gas under a piston and part of the heat raises its internal energy while the rest pushes the piston as work. For air at steady pressure, 2/7 of the heat becomes work. Lock the piston and all of it warms the gas.
- No engine can be perfect. Heat does work only while flowing from hot to cold, and some must always be dumped. Carnot's limit is 1 − T_cold ÷ T_hot in kelvin: 88% for a car's burning gas, but real engines reach about 30%.
- Pumping heat uphill costs work. An AC or fridge moves heat from cold to hot and pays in electricity. Heat out = heat taken + electricity. The smaller the temperature gap, the more heat each unit moves, up to Carnot's T_cold ÷ (T_hot − T_cold).
- Entropy, the demon and absolute zero. Spread-out arrangements vastly outnumber tidy ones, so entropy grows. Maxwell's demon can sort molecules, but wiping its memory costs more entropy than it saves. And every cooling step leaves something: 0 K is never reached.
| Term | Meaning |
|---|---|
| Temperature | How fast molecules jiggle on average. In kelvin it starts at absolute zero, −273.15 °C. |
| Heat (Q) | Energy that flows because of a temperature difference, by itself always from hotter to colder. |
| Work (W) | Energy passed on by a push through a distance, like gas pushing a piston. |
| Internal energy (U) | All the jiggling and spinning energy of the molecules inside something. |
| Thermal equilibrium | Things in touch at the same temperature, with no more heat flowing between them. |
| Entropy (S) | A count of the ways energy and molecules can be arranged, S = k ln W. Left alone, it only grows. |
| Carnot limit | The best efficiency any heat engine can have: 1 − T_cold ÷ T_hot, in kelvin. |
| Absolute zero | 0 K, the coldest possible temperature. You can get ever closer but never reach it. |
From a warm glass bulb in Padua to atoms a few trillionths of a degree above absolute zero: 400 years of learning what heat is and where it can go.
- 1593 · A glass bulb that shows heat (Galileo Galilei, Padua, Republic of Venice)
- 1824 · The perfect engine on paper (Sadi Carnot, Paris)
- 1845 · Paddle wheels and the price of heat (James Prescott Joule, Manchester, England)
- 1850 · The first and second laws, written down (Rudolf Clausius, Berlin, Prussia)
- 1865 · A new word: entropy (Rudolf Clausius, Zürich)
- 1877 · Entropy is counting (Ludwig Boltzmann, Graz, Austria)
- 1906 · The third law: you can never reach absolute zero (Walther Nernst, Berlin)
The full story, with 20 moments, charts, people and 30 sources: glassbox.how/e/thermoclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
| File | What | Size |
|---|---|---|
glassbox/reel.mp4 |
Reel / Short, with captions and soundtrack | 1080×1920 |
glassbox/video.mp4 |
YouTube video, with captions and soundtrack | 1920×1080 |
glassbox/slide-1…10.jpg |
Instagram carousel | 1080×1350 |
glassbox/thumb.jpg |
YouTube thumbnail | 1280×720 |
glassbox/cover.jpg |
Share card and repo social preview | 1200×630 |
glassbox/history-reel.mp4 |
“History in 10 moments” Reel / Short | 1080×1920 |
glassbox/history-slide-*.jpg |
History carousel | 1080×1350 |
glassbox/post.json |
Post copy and schedule used by the publish kit |
This box has no accounts and no ads, and it ships its own fonts and libraries. When you run it yourself it sends nothing anywhere. On glassbox.how, the site's /bar.js also loads Glassbox's analytics: Google Analytics to count visits (it asks first in the EU, UK and Switzerland, and stays off when your browser sends Global Privacy Control or Do Not Track) and ClickTrust to detect bots.
It remembers a few things in your own browser only, and never sends them anywhere:
| Browser storage key | What it holds |
|---|---|
thermoclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
Exactly what each one sees is at glassbox.how/privacy.
- Code: MIT. Use it, change it, ship it.
- Explanations, text, images and videos (
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python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
The page and its styles |
js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
glassbox.json |
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glassbox/ |
The published video, slides, thumbnail and post copy |
fonts/, vendor/three/ |
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