Maths for Electronics L14: Efficiency — where does the lost energy go?
// WHAT YOU WILL LEARN
Efficiency = power out ÷ power in × 100 %: a charger taking 20 W and giving 18 W is 90 % efficient; the missing 2 W becomes heat (an old filament bulb makes only about 5 % light).
// LESSON NOTES
Where does the LOST energy go? 🤔⚡
📐 Efficiency = power OUT ÷ power IN × 100 %
🔌 A phone charger: 20 W in, 18 W out → 18 ÷ 20 = 90 % efficient.
🔥 The missing 2 W? It turns into heat — that's why the charger brick feels warm.
💡 An old filament bulb is much worse: only about 5 % of its power becomes light. The other 95 %? All heat!
Energy never vanishes — it just turns into heat.
Read the voiceover (transcript)
- Where does the lost energy go?
- Efficiency is power out, divided by power in!
- A charger: twenty watts in, eighteen watts out.
- That's ninety percent. The lost two watts become heat!
- Guess: how much of it is light?
- Just five percent!
- The other ninety-five? All heat!
- Next: why resistors burn. Follow for lesson fifteen!
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