TwiddleLab / Physics / Energy: The Universe's Currency

Efficiency: Where Your Energy Goes

Lesson 9 of 10 · 7 min · Free, no account needed

What you will learn

• A Sankey diagram draws energy as arrows whose width is the amount. What goes in always equals what comes out.
• Efficiency = useful energy out ÷ total energy in, often written as a percentage.
• An old filament bulb is about 5% efficient, an LED several times better: the same light for far less energy.

Start the lesson

How the lesson goes

1. An old bulb and an LED make the same light. Why does only one burn your fingers?

   First, tap the bulb you think wastes more energy.

2. Switch both bulbs on. Watch the colours and the arrows.

3. A car engine burns 100 J of fuel. Drag the heat arrow until nothing is missing.

4. How much of it is useful has a name.

   A Sankey diagram shows energy as arrows: width means amount. In = out, always.

   Efficiency = useful energy out ÷ total energy in. Multiply by 100 for a percentage. It can never be more than 100%.

   Engines, bulbs and muscles all waste some energy, mostly as heat. Better designs waste less.

5. Build the cyclist's Sankey diagram from the arrow pieces.

   400 J of food energy goes in.

6. Game: Upgrade the house. Cut its yearly energy use by 30% with 13 coins.

   Tap upgrades to buy them, tap again to undo.

7. Math: 1000 J goes into a bulb and 50 J comes out as light. What percentage is useful?

   Drag the marker, then Check.

8. A motor takes in 400 J and gives out 300 J of useful motion. What is its efficiency, in %?

9. Your house before and after. Press Play.

Previous lesson: Springs Store Energy · Next lesson: Project: Design a Roller Coaster · Part of the course: Energy: The Universe's Currency

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