What is conservation of energy?
Energy is never made and never destroyed, only changed. Drop a skater down a ramp and watch height become speed and heat, brake a car, boil a cup in a microwave, run an AC that seems to beat the rules, and bust two perpetual-motion machines.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Energy only changes form. A skater at the top of a ramp has height energy, m g h. Dropping, it becomes motion energy, ½ m v², and climbing turns it back. Friction turns a little into heat each time, but add up height, motion and heat and the total never changes.
- Fuel, food and brakes. A car turns only about a fifth of its petrol into motion; the rest leaves as heat. Every stop turns motion into hot brake discs, unless an electric car's motor turns it back into electricity. A bike rolling downhill trades height for speed, warm air and hot brakes.
- Electricity in, heat out. Warming water takes Q = m c ΔT: 2.2 million joules for a 15 L geyser. The element turns every joule into heat; the tank then leaks some away. A microwave gets about two thirds of its power into the food and warms the kitchen with the rest.
- Stores and heat pumps. An inverter battery gives back about 58 of every 100 units put in; the rest is heat. An AC seems to cheat, moving 5.3 kW of heat with 1.5 kW, but it only moves heat, and blows out 6.8 kW: the exact sum.
- No free energy, and mass counts. Overbalanced wheels and magnet ramps always stop, because gravity and magnets give back only what they take. Einstein showed mass is energy too, E = mc², which is how the Sun shines. Noether showed why the law holds: physics is the same today as yesterday.
| Term | Meaning |
|---|---|
| Energy | The ability to make something happen. Measured in joules, it changes form but is never made or destroyed. |
| Kinetic energy | Energy of motion, ½ × m × v². Double the speed and it becomes four times as big. |
| Potential energy | Energy stored by position or shape, like height (m × g × h) or a stretched string. |
| Heat | Energy in the jiggling of atoms. Friction, electricity and burning all end up here in the end. |
| First law of thermodynamics | Energy in = energy out + change in energy stored. It includes heat, and it always balances. |
| Efficiency | Useful energy out ÷ energy in. The rest isn't lost, just not useful: usually heat. |
| Perpetual motion | A machine that runs for ever or makes energy from nothing. Conservation of energy rules it out. |
| E = mc² | Mass is a very concentrated form of energy: 1 gram is worth about 90 trillion joules. |
From a pendulum that always swings back to its starting height to a ghostly particle caught at a nuclear reactor: 300 years of learning that energy is never made or destroyed, only moved and changed.
- 1638 · The pendulum that remembers its height (Galileo Galilei, Published in Leiden, Dutch Republic)
- 1686 · Living force (Gottfried Wilhelm Leibniz, Published in Acta Eruditorum, Leipzig)
- 1740 · Balls dropped into clay (Émilie du Châtelet, building on Willem 's Gravesande, Paris)
- 1798 · Boiling water with a cannon borer (Benjamin Thompson, Count Rumford, Munich arsenal, Bavaria)
- 1842 · The ship's doctor and the red blood (Julius Robert Mayer, Voyage to the Dutch East Indies, then Heilbronn)
- 1845 · The falling weight and the paddle wheel (James Prescott Joule, Manchester)
- 1847 · On the conservation of force (Hermann von Helmholtz, Physical Society of Berlin)
- 1905 · Mass is energy (Albert Einstein, Bern)
The full story, with 20 moments, charts, people and 30 sources: glassbox.how/e/energyclear/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 |
|---|---|
energyclear.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 (
glassbox.json,glassbox/): CC BY 4.0. Credit “Glassbox, glassbox.how/e/energyclear”. - Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
- The Glassbox name and logo aren't covered by either licence. See the terms.
Found a mistake? Open an issue. Corrections happen in public.
It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.
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 |
js/energy.js |
Shared parts and physics: energy colours, chart boards, the Sankey (energy-flow) drawer, a slide-along-a-path engine that keeps KE + PE + heat exactly constant, and simple models of a person, a car and a bicycle |
glassbox.json |
Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how |
reel in each chapter |
The storyboard the Glassbox studio records into short videos |
glassbox/ |
The published video, slides, thumbnail and post copy |
fonts/, vendor/three/ |
Self-hosted Geist and Instrument Serif (SIL OFL 1.1) and three.js (MIT) |