S3.8.3 For the adult
Energy is never used up
A bicycle rolls to a stop. Where did its energy go?
What this chapter teaches
Energy is never made or destroyed. In real machines some energy always spreads into the surroundings as heat. It is wasted, not used up. Efficiency compares useful energy out with energy in.
Three questions to ask
There is no one right wording. Listen for the idea.
- Where did the energy of a bicycle go when it stopped?
- What does it mean to say energy is wasted?
- If a machine takes in 100 joules and gives 40 as useful energy, what is its efficiency?
After the chapter, the learner can
- Explain that energy is not made or destroyed, only moved
- Explain why some energy always ends up as wasted heat
What they copy into their notebook
Energy is never made or destroyed. In every transfer some spreads into the surroundings as wasted heat. Efficiency = useful out ÷ energy in × 100.