--- title: "Follow the energy" subtitle: "Year 8 Physics · Energy transfer and transformation · Lesson 3" --- # Follow the energy **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A coaster's second hill is always lower than its first. Designers have no choice. Why? :::teacher-note **In one sentence:** 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). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **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 My prediction, and why: {{lines-3}} ::: :::teacher-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. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: tracing systems :::keep-together ### 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 ::: :::teacher-note **Answer:** Energy that became heat and sound because of friction and air resistance By-product energy builds up as the car moves. It is still there, in the track, wheels and air. ::: :::keep-together ### Useful or by-product **In a torch (battery, wires, bulb), useful output or by-product?** Groups: Useful output, By-product - Light from the bulb: {{field-15}} - Heat from the bulb: {{field-15}} - Heat in the battery and wires: {{field-15}} ::: :::teacher-note **Answer:** Light from the bulb → Useful output; Heat from the bulb → By-product; Heat in the battery and wires → By-product ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** By-products: heat from the engine and sound A complete flow diagram accounts for every output. In a car engine most of the energy becomes heat. ::: :::typical ## Elaborate: from a dam to your lights :::keep-together ### 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 ::: :::teacher-note **Answer:** Spread across every stage: the generator, the wires and the lights all give some off Every stage gives off some, and the lights often give off the most. Year 9 measures how much with efficiency and Sankey diagrams. ::: ::: :::wider ## Elaborate: the coaster that stops :::keep-together **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 ::: :::teacher-note **Answer:** As heat (and sound) in the track, the wheels and the air: the car stops in a dip Each pass turns more into heat and sound until the car rests in the lowest dip. The energy is all still there, spread out. ::: ::: ## 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. ## Exit ticket :::keep-together **Draw a flow diagram for a battery-powered fan, with one by-product.** {{lines-3}} ::: :::teacher-note **Answer:** Battery (chemical) → wires (electrical) → spinning blades (kinetic), with a by-product of heat (and sound) from the motor. :::