1. Sankey diagrams
These provide a visual representation of energy transfers in a closed system. They are always drawn to scale.

2. Sankey Diagrams
Introduce this as a representation of energy transfers, and explain that we will take a step by step approach for drawing a sankey diagram.

3. Sankey Diagrams
Step 1) start with a blank piece of graph paper.

4. Sankey Diagrams
The height of the box represents the amount of energy that is put in.
Step 2) Draw a box to represent the input energy. Emphasise that it doesn’t matter how long the box is.

5. Sankey Diagrams
The height of the next box represents the amount of useful energy.
Step 3) Draw a second box to represent the useful energy.

6. Sankey Diagrams
The height of the final box represents the amount of wasted energy.
Step 4) We draw another box to represent the wasted energy, however, we rotate it around to show it is wasted. I liken this to a toilet, our waste goes downwards.
But we rotate it to show that it is wasted energy.

7. Sankey Diagrams Useful Output Energy Input Energy
Finally we add arrows to our boxes and label the diagram.
Step 5) Complete the arrows and add labels.
Wasted Output Energy

8. Sankey Diagrams 8 x £1 Coins £ 10 Note 1 x £2 Coin
Show for the money example, asking students where they think the boxes should go.
1 x £2 Coin

9. Energy Transfers Light Energy 8 Joules Input Useful Wasted
Now for an example, an energy efficient light bulb.
Electrical Energy
10 Joules
Light
Energy
8 Joules
Heat Energy
2 Joules
Input
Useful
Wasted
Now compare with a real example.
There is still 10 joules of energy available, but it now appears differently.

10. What about energy? Light Energy Electrical Energy 8 joules 10 joules
An energy efficient light bulb with an input energy of 10 joules, useful output of 8 joules and a wasted output of 2 joules.
Again I asked students where they think the boxes should go.
Heat Energy
2 joules

11. What makes a good sankey?
Arrows are labelled with input and output energies.
Light Energy
8 joules
Electrical Energy
10 joules
Boxes are the correct width.
Straight lines, drawn with a ruler.
Emphasise what makes a good sankey and what you would expect from their work.
The wasted energy is labelled.
Heat Energy
2 joules

12. Draw a sankey diagram to show the energy change for:
A1) An inefficient light bulb. The input energy is 10 joules. The useful output energy is light 4 joules and the wasted energy is heat 6 joules. 2) A candle. The input energy is 200 joules. The useful output energy is heat 140 joules and wasted energy is sound 60 joules. 3) An energy efficient light bulb. The input energy is 10 joules. The useful output energy is light 8 joules. The wasted energy is heat 2 joules. 4) An electric kettle. The input energy is 10 joules. The useful output energy is heat 9 joules. Wasted energy is sound 1 joules. B) Now work out the efficiency of each item.