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# Follow the energy

Year 8 Physics · Energy transfer and transformation

<!-- Speaker note: Students trace energy through multi-step systems, naming each stage's energy and every transfer, transformation and by-product, and construct and critique flow diagrams (AC9S8U05, nodes 5 and 6). About 26 minutes (Engage and Explore 9 min · Explain 9 min · Evaluate 3 min · Typical adds 3 min, Wider about 2 min). -->

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## Today

**We are learning to** real systems pass energy through many steps. A flow diagram follows it from start to end: a box for each stage, an arrow for each transfer or transformation, and side arrows for every by-product. Tracing all the outputs, useful and not, is how you find out where energy goes.

- I can trace energy through a system with several steps.
- I can draw a flow diagram with every by-product.
- I can spot what is missing from someone else's flow diagram.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. It closes Year 8 energy and sets up Year 9 conservation and efficiency. Not hinge nodes; tracing every output is the skill Year 9 needs. The prediction (the lower second hill) shows why by-products matter. Board notes: the coaster's friction is a steady 400 N while it moves, so the heat bar grows with distance. The fire-drill chain is simplified to four stages. Fire drills (a stick spun against a hearth board) have been used by many peoples, including many First Peoples of Australia; for local practice and knowledge, use local resources and community guidance. Teacher: In your own words: the fire-drill flow diagram with side arrows. -->

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## Why it matters

> A coaster's second hill is always lower than its first. Designers have no choice. Why?

<!-- Speaker note: You can tell transfers from transformations. This lesson follows energy through whole systems. -->

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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

**A real coaster car starts at the top of a 30 m hill with no engine. Could the next hill also be 30 m tall?**

- No: energy is used up by going downhill
- Yes, if it goes fast enough
- Yes: it has the same energy
- No: some energy becomes heat and sound along the track, so it can't climb as high

<!-- Speaker note: What happens: No. Friction and air resistance turn some of the car's energy into heat and sound on the way, so it has less to turn back into height. The energy isn't used up: it's in the surroundings. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. 'Yes, same energy' ignores by-products; 'used up' calls them destroyed. -->

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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-8%2Fflows%2Fstep-2&code=widget%3A+energy%0Amode%3A+coaster%0Afriction%3A+on%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-8/flows/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask what the red bar would show if the ride went on for longer. -->

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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?

* No. Friction and air resistance turn some of the car's energy into heat and sound on the way, so it has less to turn back into height. The energy isn't used up: it's in the surroundings.

<!-- 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 red bar

**On the coaster, what did the red bar show?**

- Energy that was destroyed
- The car's speed
- Energy that became heat and sound because of friction and air resistance

* Answer: Energy that became heat and sound because of friction and air resistance

<!-- Speaker note: By-product energy builds up as the car moves. It is still there, in the track, wheels and air. -->

<!-- Speaker note: Teacher: the 'what's missing' question is the critique skill; the sort checks useful versus by-product. -->

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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>

## Useful or by-product

**In a torch (battery, wires, bulb), useful output or by-product?**

Groups: Useful output · By-product

Light from the bulb · Heat from the bulb · Heat in the battery and wires

* Answer: Light from the bulb → Useful output; Heat from the bulb → By-product; Heat in the battery and wires → By-product

<!-- Speaker note:  -->

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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's missing

**A student's flow diagram for a car: fuel (chemical) → engine → moving car (kinetic). What is missing?**

- By-products: heat from the engine and sound
- Nothing: it's complete
- A transfer arrow from the car to the fuel

* Answer: By-products: heat from the engine and sound

<!-- Speaker note: A complete flow diagram accounts for every output. In a car engine most of the energy becomes heat. -->

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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>

## Elaborate: from a dam to your lights

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-8%2Fflows%2Fstep-4&code=widget%3A+energy%0Amode%3A+chain%0Astage%3A+Water+behind+a+dam+%7C+gravitational%0Astage%3A+Falling+water+%7C+kinetic+%7C+sound%0Astage%3A+Generator+%7C+electrical+%7C+heat+%7C+sound%0Astage%3A+Lights+at+home+%7C+light+%7C+heat%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-8/flows/?full=step-4#step-4)

<!-- Speaker note: What this step is for: A hydro-electric system links four stages and two by-products, the full multi-step trace. -->

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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>

## Where the heat goes

**Where in this system does energy end up as by-product heat?**

- Spread across every stage: the generator, the wires and the lights all give some off
- Nowhere: hydro power is perfect
- Only in the falling water

* Answer: Spread across every stage: the generator, the wires and the lights all give some off

<!-- Speaker note: Every stage gives off some, and the lights often give off the most. Year 9 measures how much with efficiency and Sankey diagrams. -->

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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: the coaster that stops

**If a coaster had no brakes and no engine and just kept rolling back and forth, where would all its energy end up?**

- As heat (and sound) in the track, the wheels and the air: the car stops in a dip
- Back as potential energy at the top
- Destroyed by friction

* Answer: As heat (and sound) in the track, the wheels and the air: the car stops in a dip

<!-- Speaker note: Each pass turns more into heat and sound until the car rests in the lowest dip. The energy is all still there, spread out. What this step is for: Following a system to the end shows where all the energy finally goes: heat in the surroundings. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- Real systems pass energy through many stages: trace it one step at a time.
- A **flow diagram** has a box for each stage, an arrow for each transfer or transformation, and side arrows for **by-products**.
- Count every output, useful and by-product. By-products (mostly heat and sound) go to the surroundings; they are never destroyed.

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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

**Draw a flow diagram for a battery-powered fan, with one by-product.**

* Battery (chemical) → wires (electrical) → spinning blades (kinetic), with a by-product of heat (and sound) from the motor.

<!-- 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: Year 9, Where does the energy go? (conservation of energy, AC9S9U05). In class: trace energy in a hand-crank torch or a bouncing ball (measure each bounce's height). -->