--- title: "What does a Sankey diagram show?" subtitle: "Year 9 Physics · Conservation of energy and efficiency · Lesson 10" --- # What does a Sankey diagram show? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** About two thirds of the energy in the coal burned at a power station never becomes electricity. You can see exactly where it goes in one picture. :::teacher-note **In one sentence:** Students construct and interpret Sankey diagrams, read efficiency and the largest waste from them, and apply efficiency and conservation to compare real energy systems (AC9S9U05, nodes 3 and 4). **Students should be able to say:** I can build a Sankey diagram from energy data. I can read the efficiency and the biggest waste from a Sankey diagram. I can say what a Sankey diagram doesn't show. **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 **In a Sankey diagram, the input band is 1000 J wide. The useful band is about a third as wide. Roughly what is the efficiency?** ☐ About 3% ☐ About 1000% ☐ About 33% ☐ You can't tell from widths My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** About 33%. Band widths are proportional to energy, so a band a third as wide as the input carries about a third of the energy: an efficiency of about a third. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: reading and using Sankey diagrams :::keep-together ### What widths mean **What do the widths of the bands in a Sankey diagram show?** ☐ How important each output is ☐ The amount of energy in each flow, to scale ☐ How fast the energy moves ::: :::teacher-note **Answer:** The amount of energy in each flow, to scale Band widths are proportional to energy, and the output bands always add up to the input band (conservation of energy). ::: :::keep-together ### Biggest waste **In the coal station's diagram, which output is the biggest waste?** ☐ Heat in the cooling water (550 J) ☐ Heat up the chimney (100 J) ☐ Electricity (350 J) ::: :::teacher-note **Answer:** Heat in the cooling water (550 J) The widest wasted band shows where improvements would save the most: here, the heat carried away by cooling water. ::: :::keep-together ### Solar panel **A solar panel's Sankey diagram shows 1000 J of light in and 200 J of electricity out. What is its efficiency (%)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 20 200 ÷ 1000 × 100% = 20%. ::: :::keep-together ### What it hides **What does a Sankey diagram NOT show?** ☐ How much the system costs, or the pollution it makes ☐ The efficiency ☐ Where energy is wasted ::: :::teacher-note **Answer:** How much the system costs, or the pollution it makes It shows energy only. Comparing real systems also needs cost, emissions and whether the energy source runs out. ::: :::keep-together ### Make it better **The coal station's biggest waste is the 550 J of heat carried off by the cooling water. Which change would make the most use of it?** ☐ Narrow the chimney so less heat escapes up it ☐ Burn more coal so more electricity comes out ☐ Pipe the warm cooling water to heat nearby homes or greenhouses ::: :::teacher-note **Answer:** Pipe the warm cooling water to heat nearby homes or greenhouses Target the widest wasted band first. Combined heat and power stations pipe their warm water to nearby buildings, so much of that energy becomes useful. ::: :::typical ## Elaborate: a kettle :::keep-together ### Useful depends on the job **In a kettle, heat is the useful output. In a light bulb, heat is waste. Why?** ☐ Kettles make a different kind of heat ☐ 'Useful' means what the device is for: a kettle's job is heating water ☐ Light bulbs don't really make heat ::: :::teacher-note **Answer:** 'Useful' means what the device is for: a kettle's job is heating water The same type of energy can be useful in one device and wasted in another. ::: ::: :::wider ## Elaborate: choosing an energy source :::keep-together **A hydro station is about 90% efficient and a coal station about 35%. Why doesn't every country just build hydro stations?** ☐ Hydro stations make no electricity in winter ☐ Hydro needs the right rivers and valleys, and dams flood land; efficiency is only one factor ☐ Coal is more efficient really ::: :::teacher-note **Answer:** Hydro needs the right rivers and valleys, and dams flood land; efficiency is only one factor Real choices weigh efficiency with cost, location, pollution and whether the source is renewable. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - A **Sankey diagram** shows energy flows as bands whose widths are proportional to the amounts. - The input band splits into the useful output (straight on) and wasted outputs (bending away); outputs always add up to the input. - Read the efficiency as useful ÷ input, and find the biggest waste by the widest wasted band. - Sankey diagrams show energy only, not cost or pollution. ## Exit ticket :::keep-together **A wind turbine takes in 500 J of kinetic energy and gives out 200 J of electricity. What is its efficiency, and how would its Sankey diagram show the rest?** {{lines-3}} ::: :::teacher-note **Answer:** 200 ÷ 500 × 100% = 40%. The other 300 J would bend away as wasted bands (heat and sound), together as wide as 300 J. :::