Efficiency is the share of the energy put in that comes out as useful energy: efficiency = useful energy out ÷ total energy in × 100%. No real device is 100% efficient, because some energy always becomes by-products, usually heat. A device can give out more useful energy and still be less efficient.
Old filament light bulbs were banned in Australia from 2009. They worked fine. So why ban them?
Energy is never lost. This lesson asks how much of it does what you want.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: two bulbs
ExploreTry it on the board and compare what happens with your prediction.
Explore: build the diagrams
Use the sliders or + and −. The board tells you when the outputs add up to the input.
Try it yourselfYour edits are saved in this browser.
A filament bulb takes in 100 J and gives out 10 J of light. Set the outputs so every joule is accounted for.
Use the board. This ticks when you get there.
An LED bulb takes in 100 J and gives out 40 J of light. Set its outputs.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: calculating efficiency
In your book: work out the efficiency of each bulb you built. Then write one sentence explaining why a bulb that gives out more light can still be less efficient.
A kettle takes in 2000 J and 1800 J goes into heating the water. What is its efficiency (%)?
An electric motor takes in 500 J and does 150 J of useful work lifting a load. What is its efficiency (%)?
Elaborate TypicalUse the same idea in a new context.
Elaborate: a car engine
Try it yourselfYour edits are saved in this browser.
A car engine: 100 J of chemical energy in fuel gives 25 J of movement, 65 J of heat and 10 J of sound. Build its diagram.
Use the board. This ticks when you get there.
What is the engine's efficiency (%)?
Elaborate WiderUse the same idea in a new context.
Elaborate: energy star ratings
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Efficiency = useful energy out ÷ total energy in × 100%.
It is a proportion: a device that gives out more useful energy can still be less efficient.
No real device is 100% efficient: some energy always becomes by-products, usually heat.
More efficient devices need less energy input for the same job.
For teachers
Students define efficiency, calculate it as useful output over total input, and explain why no real system reaches 100% (AC9S9U05, node 2, a hinge node). About 25 minutes.
I can explain why more useful output doesn't always mean more efficient.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider. It follows Where does the energy go? and comes before What does a Sankey diagram show?
The hinge: efficiency is a ratio. Students who compare useful outputs instead of proportions can't compare real systems. The two-bulbs prediction targets it.
Board notes: the Sankey board's band widths are drawn to scale; outputs start at 0 for students to set, and the board says when they add up to the input. Figures are rounded and typical: filament bulbs about 10%, LEDs about 40% (better ones more), car petrol engines 20 to 30%.
Running the lesson
Engage: tally the predictions. 'Bulb A' compares amounts, not proportions.
Explore: ask what had to be true before the efficiency appeared (outputs = input).
Explain: the two calculations and 'never 100%' are the success criteria.
In your own words: both bulbs' efficiencies and the sentence about proportion.
The hard part (on purpose)
Efficiency as a proportion. When a student says the bigger device is more efficient, ask them to work out the useful share of each before deciding.
When students get stuck
A student divides input by output. Ask: Can a device give out more useful energy than goes in? Fix: No, so efficiency can't be more than 100%. Useful out goes on top.
A student forgets to multiply by 100. Ask: What does 0.1 mean as a percentage? Fix: 10%. Multiply the fraction by 100.
A student thinks wasted energy is destroyed. Ask: Where did the 90 J go? Fix: It became heat in the room: transferred, not destroyed.
Stretch and support
Stretch: A device is 40% efficient and gives out 60 J of useful energy. How much energy went in? (150 J.)
Support: Give the formula as a fraction with labelled boxes: useful out on top, total in underneath, then × 100%.
Where it leads
Next: What does a Sankey diagram show? (nodes 3 and 4).
In class: compare a filament and an LED lamp with a thermometer or by touch; read energy ratings on appliance labels.
What each step is for
Engage: two bulbs: Students who judge efficiency by the amount of useful output, not the proportion, pick the big bulb; the ratio is the hinge.
Explore: build the diagrams: Setting each output so the diagram balances makes students account for all the energy before the efficiency appears.
Explain: calculating efficiency: Applying the formula in several contexts, then explaining why it can't reach 100% and why it's a proportion, is the node.
Elaborate: a car engine: A device with two by-products checks that every output is counted and that efficiency uses only the useful one.
Elaborate: energy star ratings: Energy ratings turn efficiency into a buying decision and connect it to cost and emissions.
Now you can
Work out the efficiency of a device.
Explain why no device is 100% efficient.
Explain why more useful output doesn't always mean more efficient.