The Sankey diagram

Energy shifting: tracking power

One way of starting to chart the quantity of energy shifting per second, the power, is to use a Sankey diagram. This kind of diagram is often used by power engineers and those concerned with power supply and demand. Sankey diagrams built of stylised arrows, where the thickness of arrow shows the the power in a particular pathway.

To represent the conservation of energy correctly the total thickness of the arrows must be constant.

Here, we'll estimate (rather than precisely calculate) the quantities of power, so supporting the semi-quantitative approach suitable for 11-14 pupils.

Drawing these diagrams provides an excellent starting point for thinking about power in a process, quantifying the pathways description.

Sankey diagrams are diagrams of processes in action, so complement the descriptions developed in terms of energy, which is always from snapshot to snapshot, so looking at the start and end points of a process.

Three examples

A coal fired power station connected to a domestic cooker.

There's not enough information to decide on the precise relative thickness of the arrows, so these diagrams are really plausible illustrative sketches.

Another pair of diagrams show how you can deal with an increased rate of dissipation, for a commonly used example.

Two cars at different speeds.

A car accelerating on the level.

You can chose to develop either power or energy descriptions: to describe the changes from snapshot to snapshot, or to describe the process happening, as here.