Lesson 4 of 6 · Energy
Efficiency and the Sankey Picture
⚡
🎯 Your mission
Set it up. Test it. Be amazed.
⚡ The twist
Speed isn't the same as force. Don't mix them up.
What you'll learn
Efficiency is useful output energy divided by total input energy, and it can never exceed one, because the output cannot contain joules the input did not supply. An LED converting 45 J of every 50 J into light is 90% efficient; a filament lamp managing 5 J of the same 50 J is 10%, with the other 45 J dissipated as heat. A Sankey diagram draws every branch with a width proportional to its energy, so the wasted branch is seen rather than merely admitted.
Key words
Sankey diagram
An energy flow diagram in which the width of each arrow is proportional to the energy it carries.
useful output
The part of the energy transferred that does the job the device was built for.
Make a hypothesis first
Guess: where does the energy go?
Say your guess out loud — then find out if you were right.
Get ready
🧰 What you need
- graph paper
- ruler
- coloured pencils
- filament lamp
- LED lamp
- infrared thermometer
Get ready
⚠️ Safety first
- A filament lamp gets hot enough to burn — measure without touching the glass.
Step 1 of 4 · Read
Think about what you just learned: say what "Sankey diagram" and "useful output" mean in your own words before you start.
Step 2 of 4 · Do
Draw Sankey diagrams to scale for a filament lamp (10% useful) and an LED (90% useful), using 1 cm of arrow width for every 10 J from a 50 J input. Then measure the surface temperature of each lamp after two minutes and connect what you felt to the width you drew.
Step 3 of 4 · Observe
Record what actually happened, including anything that did not match what you expected.
Step 4 of 4 · Record
Write a conclusion that answers the question this lesson opened with, and state your uncertainty.
Question 1 of 3
Question 2 of 3
Question 3 of 3
Mind = blown
🔥 A bolt of lightning is 5 times hotter than the surface of the sun.
Nice work!
You got 0 of 3 right.
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✨ More to explore
🌍 Where you see this in real life: You use this every time you flip a light switch, ride a bike, or kick a ball.
🏆 Stretch challenge (try it this week): Time how long a small ball rolls down something, then change the slope and try again.
👨👩👧 For the dinner table: “Show me something at home that uses this kind of energy.”
Share this experiment
Send it to a parent who's looking for a 10-minute kitchen science win.
Ready?
Efficiency and the Sankey Picture
Set it up. Test it. Be amazed.
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Print the whole Energy track
All 8th Grade Energy lessons as a printable PDF, with worksheets and answer keys.