S4.9.5 For the adult
Efficiency
A lamp takes in 100 J of electrical energy and gives out 20 J of light. What is its efficiency?
What this chapter teaches
Efficiency is the useful energy out divided by the total energy in, never above 1 or 100%. Wasted energy is not destroyed but spreads out, mostly as heat. A Sankey diagram draws each energy transfer as an arrow as wide as its energy.
Three questions to ask
There is no one right wording. Listen for the idea.
- How do you work out the efficiency of a lamp that takes in 100 J and gives out 20 J of light?
- Why can efficiency never be 120%?
- What do the arrows in a Sankey diagram always add up to?
After the chapter, the learner can
- Calculate efficiency as useful output over total input
- Draw a Sankey diagram for a lamp or a motor
What they copy into their notebook
Efficiency = useful output ÷ total input. At most 1, or 100%. Wasted = input − useful, and it spreads out as heat. A Sankey diagram: output arrows add up to the input arrow.