Year 9 Science · Unit 3 · Lesson 4
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Learning Goals
1. Why is the waste arrow usually drawn bending downward? What does the waste energy usually become?
Order the steps
Number the steps from 1 to 6 to show how to draw a Sankey diagram. Step 1 comes first.
| Order | Step |
|---|---|
| Calculate the width of each arrow: energy ÷ scale. | |
| Add a title, label each arrow with its energy form and value, and state the scale. | |
| Work out the input, useful and waste energy values. | |
| Draw the output arrows: the useful arrow straight on, and the waste arrow bending down. | |
| Choose a scale, such as 1 cm = 100 J. | |
| Draw the input arrow on the left. |
2. A student’s diagram for an incandescent globe shows light as the widest arrow. Identify the error, and describe what the diagram should look like.
3. A pool pump’s old motor was 50% efficient. Its new motor is 80% efficient. Both give the pool water 400 J of kinetic energy every second. Calculate the electrical energy each motor needs every second, and the percentage of energy saved by the new motor.
Old motor J
New motor J
Saving %
Cause and effect
Complete the chain. Each box is caused by the one before it.
Investigation: kettle or gas burner?
A class heated the same mass of water through the same temperature rise twice: once in an electric kettle and once in a pot on a gas burner. A plug-in energy meter measured the energy supplied to the kettle. For the burner, they weighed the gas canister before and after, and used the energy stored in each gram of gas. Both times, 240 kJ of energy went into the water.
| Heater | Energy supplied (kJ) | Energy into the water (kJ) | Waste energy (kJ) | Efficiency (%) |
|---|---|---|---|---|
| Electric kettle | 300 | 240 | ||
| Gas burner | 600 | 240 |
Source: sample class results.
4. Complete Table 1.
5. At a scale of 1 cm = 60 kJ, how wide should each arrow be for the kettle and for the burner? Fill in the table.
| Arrow | Kettle width (cm) | Burner width (cm) |
|---|---|---|
| Input | ||
| Useful (into the water) | ||
| Waste |
6. Now draw both heaters’ diagrams, one above the other, using your widths. Name what each stream of energy is, and how many kilojoules it carries.
7. Look at the useful-energy arrows for the kettle and the burner. Why are they equally wide, and what do the other arrows show?
8. The school staffroom boils water on a gas burner. Write a recommendation on whether to switch to an electric kettle. Use your two Sankey diagrams.