--- title: "How much is useful?" subtitle: "Year 9 Physics · Conservation of energy and efficiency · Lesson 9" --- # How much is useful? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Old filament light bulbs were banned in Australia from 2009. They worked fine. So why ban them? :::teacher-note **In one sentence:** 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). **Students should be able to say:** 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. **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 **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 My prediction, and why: {{lines-3}} ::: :::teacher-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. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: calculating efficiency :::keep-together ### 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 ::: :::teacher-note **Answer:** Useful energy out ÷ total energy in × 100% Efficiency is the useful share of what goes in. ::: :::keep-together ### The kettle **A kettle takes in 2000 J and 1800 J goes into heating the water. What is its efficiency (%)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 90 1800 ÷ 2000 × 100% = 90%. ::: :::keep-together ### The motor **An electric motor takes in 500 J and does 150 J of useful work lifting a load. What is its efficiency (%)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 30 150 ÷ 500 × 100% = 30%. The other 350 J becomes heat and sound. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** Some energy always becomes by-products, usually heat from friction and resistance Every moving part and every wire turns a little energy into heat. Better design reduces it, but never to zero. ::: :::keep-together ### 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 ::: :::teacher-note **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 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. ::: :::typical ## Elaborate: a car engine :::keep-together ### Its efficiency **What is the engine's efficiency (%)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 25 25 ÷ 100 × 100% = 25%. Three quarters of the fuel's energy heats the engine, the exhaust and the air. ::: ::: :::wider ## Elaborate: energy star ratings :::keep-together **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 ::: :::teacher-note **Answer:** The 6-star fridge needs less electrical energy for the same cooling, so it costs less to run and needs less electricity generated More efficient devices waste less energy as heat, so less energy has to be generated in the first place. ::: ::: ## 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. ## Exit ticket :::keep-together **A phone charger takes in 20 J and stores 15 J in the battery. What is its efficiency, and where did the rest go?** {{lines-3}} ::: :::teacher-note **Answer:** 15 ÷ 20 × 100% = 75%. The other 5 J became heat (the charger gets warm). :::