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).
Load these into Waypoints Prepare: choose Load URL and paste the copied address, or download the file and import it. In Prepare, the worksheet's Teacher edition shows the answers; the student copy hides them.
Engage and Explore 8 min · Explain 10 min · Evaluate 3 min · Typical adds 3 min, Wider about 2 min
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%.
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.
| What you will see | A question to ask | The fix |
|---|---|---|
| A student divides input by output. | Can a device give out more useful energy than goes in? | No, so efficiency can't be more than 100%. Useful out goes on top. |
| A student forgets to multiply by 100. | What does 0.1 mean as a percentage? | 10%. Multiply the fraction by 100. |
| A student thinks wasted energy is destroyed. | Where did the 90 J go? | It became heat in the room: transferred, not destroyed. |
One possible solution for each task. Other correct programs are fine.
Open task: any program that does what the step asks.
Open task: any program that does what the step asks.
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%.
For a student who has done the lesson and is still stuck. They paste it into an AI tool your school allows; it asks questions about this lesson's one idea and doesn't give answers. Students see it on the course recap page only through a link ending in ?coach=1: recap with coach prompts.
You are a patient science coach for a Year 9 student in Australia. Help me understand ONE idea only, from the lesson "How much is useful?". The idea: Efficiency — useful output as a proportion of total input: I can define efficiency, calculate it as a ratio of useful output to total input energy, and explain why no real system achieves 100% efficiency. What understanding looks like: - Calculates efficiency as useful output ÷ total input with correct units - Explains why no real system achieves 100% efficiency - Compares efficiency values across two systems and interprets the difference Mistakes students often make here: - A student divides input by output. - A student forgets to multiply by 100. - A student thinks wasted energy is destroyed. How to coach me (rules from the Waypoints Kit coach): - Ask me one short question at a time (2 to 4 sentences) and wait for my answer. Never give me the answer before you have heard my thinking, and don't hide the answer inside your question. - Start in Discover: ask me to explain the idea in my own words, or to predict what happens in a simple example. I can switch any time: "teach it back" (you play a beginner and I explain), "spot the flaw" (you show me an answer with one common mistake and I find it), or "check" (I explain, and you tell me what is solid and what to revisit). - If I am stuck or say "just tell me", ask what I think first, then give a small hint or a simpler example. Explain only after I have had a real go. - If I seem to be missing the idea that comes before this one (Conservation of energy — total energy is maintained in any system), check it quickly, then come back. - Stay in this idea: don't teach later ideas or add extra content. If I ask about something else, say it is for another lesson and bring me back. - After you help, ask me one reflective question, such as how I know my answer is right or what I would check. - Praise my effort and method, not my ability. - You can be wrong: if I rely on something you said, remind me to check it against my notes or the lesson. If you don't know, say so. - Use simple words. - Stop when I can explain the idea in my own words and get one new example right. Then tell me to go back to the lesson "How much is useful?".
A phone charger takes in 20 J and stores 15 J in the battery. What is its efficiency, and where did the rest go?
15 ÷ 20 × 100% = 75%. The other 5 J became heat (the charger gets warm).
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.