A Sankey diagram illustrates an input/output situation. It is drawn to scale - there are lots of variations as to how they are drawn - only thing they have in common is that the width of the 'arms' represents the energy transferred but the length of the 'arms' does not!

Example - Energy losses from a typical domestic house

You can look at this table and work out the proportions of energy loss. However a Sankey diagram shows this more clearly.

Sankey diagrams allow us to visualize flow through a process or system more easily that numerical data can. They show not only the order of changes but also the quantitative distribution of values in the transfers.

Sankey diagrams do add an indisputable expressive power to mathematical rendering of a system. When professionally constructed, Sankey diagrams represent flow in a manner that can be understood by anyone, instantly.

However, Sankey diagrams can be difficult, time-consuming, and uninteresting to produce by hand - very tedious to draw! The benefits of being able to generate these diagrams automatically, anytime, are obvious to anyone who has tried to draw one and commercial computer packages for their production are available. They are used not only in physics and engineering to demonstate how energy is distributed but also for cash flow in businesses.

In the AQA GCSE there is a specific way in which the Sankey diagrams as drawn. The input is from the left of the diagram. The wanted (useful) output is to the right. All unwanted (wasted) output is made to go vertically down. Remember the total input always equals the total output - but an efficient system will have a high percentage of useful output.

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