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# How much is useful?

Year 9 Physics · Conservation of energy and efficiency

<!-- Speaker note: 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 (Engage and Explore 8 min · Explain 10 min · Evaluate 3 min · Typical adds 3 min, Wider about 2 min). -->

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## Today

**We are learning to** 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.

- I can work out the efficiency of a device.
- I can explain why no device is 100% efficient.
- I can explain why more useful output doesn't always mean more efficient.

<!-- Speaker note: 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%. Teacher: In your own words: both bulbs' efficiencies and the sentence about proportion. -->

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## Why it matters

> Old filament light bulbs were banned in Australia from 2009. They worked fine. So why ban them?

<!-- Speaker note: Energy is never lost. This lesson asks how much of it does what you want. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**Bulb A takes in 100 J and gives out 10 J of light. Bulb B takes in 20 J and gives out 8 J of light. Which is more efficient?**

- Bulb B: 40% of its energy becomes light, against 10% for A
- Bulb A: it gives out more light
- They are the same
- You can't tell without knowing the heat

<!-- Speaker note: What happens: Bulb B. Efficiency is a proportion: 8 ÷ 20 = 40% for B, 10 ÷ 100 = 10% for A. A gives out more light but wastes far more energy doing it. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. 'Bulb A' compares amounts, not proportions. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-9%2Fefficiency%2Fstep-2&code=widget%3A+energy%0Amode%3A+sankey%0Adevice%3A+Filament+bulb%0Ainput%3A+100%2C+electrical%0Aoutput%3A+Light%2C+10%2C+useful%0Aoutput%3A+Heat%2C+90%0Abuild%3A+on%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-9/efficiency/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask what had to be true before the efficiency appeared (outputs = input). -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* Bulb B. Efficiency is a proportion: 8 ÷ 20 = 40% for B, 10 ÷ 100 = 10% for A. A gives out more light but wastes far more energy doing it.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The formula

**Which is the formula for efficiency?**

- Useful energy out ÷ total energy in × 100%
- Useful energy out − wasted energy
- Total energy in ÷ useful energy out

* Answer: Useful energy out ÷ total energy in × 100%

<!-- Speaker note: Efficiency is the useful share of what goes in. -->

<!-- Speaker note: Teacher: the two calculations and 'never 100%' are the success criteria. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The kettle

**A kettle takes in 2000 J and 1800 J goes into heating the water. What is its efficiency (%)?**

* Answer: 90

<!-- Speaker note: 1800 ÷ 2000 × 100% = 90%. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The motor

**An electric motor takes in 500 J and does 150 J of useful work lifting a load. What is its efficiency (%)?**

* Answer: 30

<!-- Speaker note: 150 ÷ 500 × 100% = 30%. The other 350 J becomes heat and sound. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Never 100%

**Why can no real device be 100% efficient?**

- Devices wear out
- Some energy always becomes by-products, usually heat from friction and resistance
- Energy is destroyed in every device

* Answer: Some energy always becomes by-products, usually heat from friction and resistance

<!-- Speaker note: Every moving part and every wire turns a little energy into heat. Better design reduces it, but never to zero. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Two motors

**Motor A takes in 500 J and does 150 J of useful work (30%). Motor B takes in 200 J and does 100 J (50%). What does the difference mean?**

- A is more efficient, because it does more useful work
- B turns a bigger share of its input into useful work, so for the same job it needs less energy in and wastes less
- They are equally good, because both obey conservation of energy

* Answer: B turns a bigger share of its input into useful work, so for the same job it needs less energy in and wastes less

<!-- Speaker note: For every 100 J in, B gives 50 J of useful work and A only 30 J. To do the same job, B needs less energy in. -->

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## Elaborate: a car engine

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-9%2Fefficiency%2Fstep-4&code=widget%3A+energy%0Amode%3A+sankey%0Adevice%3A+Car+engine%0Ainput%3A+100%2C+chemical%0Aoutput%3A+Movement%2C+25%2C+useful%0Aoutput%3A+Heat%2C+65%0Aoutput%3A+Sound%2C+10%0Abuild%3A+on%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-9/efficiency/?full=step-4#step-4)

<!-- Speaker note: What this step is for: A device with two by-products checks that every output is counted and that efficiency uses only the useful one. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Its efficiency

**What is the engine's efficiency (%)?**

* Answer: 25

<!-- Speaker note: 25 ÷ 100 × 100% = 25%. Three quarters of the fuel's energy heats the engine, the exhaust and the air. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: energy star ratings

**Two fridges keep food equally cold. One has a 6-star energy rating and the other 3 stars. What does the extra efficiency mean?**

- The 6-star fridge makes food colder
- The 6-star fridge needs less electrical energy for the same cooling, so it costs less to run and needs less electricity generated
- Nothing: stars are about size

* Answer: The 6-star fridge needs less electrical energy for the same cooling, so it costs less to run and needs less electricity generated

<!-- Speaker note: More efficient devices waste less energy as heat, so less energy has to be generated in the first place. What this step is for: Energy ratings turn efficiency into a buying decision and connect it to cost and emissions. -->

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## 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.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**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).

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this 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. -->