Draw and interpret diagrams to represent energy transfers

The idea

Energy transfer diagrams show how energy moves between stores. A Sankey diagram uses an input arrow whose width represents the total energy supplied, then splits into output arrows whose widths represent useful and wasted transfers; the widths must be proportional to the energy values. A simple flow diagram uses labelled boxes joined by arrows, for example a lamp: electrical input → light (useful) and thermal (wasted). To interpret a diagram, read the arrow widths or labels, identify the useful output, identify the wasted output, and check that the outputs add to the input because energy is conserved. A Sankey diagram for a 100 J input with 60 J useful and 40 J wasted has a 60:40 width ratio.

What earns the marks

• Draws arrows whose widths are proportional to the energy values transferred.
• Labels each arrow with the energy store or transfer and, where given, the value in joules.
• Identifies the useful output and the wasted output from a diagram.
• Checks that output energies sum to the input energy, showing conservation.
• Interprets a Sankey or flow diagram to describe the energy transfer for a device.

Common mistakes and how to put them right

• Draws arrows of equal width regardless of energy.

  Put it right: Make widths proportional to values.

• Omits the wasted output.

  Put it right: Include all outputs so they sum to the input.

• Confuses the input with an output.

  Put it right: Place the total supply as the single input arrow.

• Fails to label stores.

  Put it right: Label each arrow with the store or transfer it represents.

Exam tips

• Use a ruler so arrow widths are clearly proportional.
• Add the output values and compare with the input to check conservation.
• Label useful and wasted outputs explicitly.
• Read the question to see whether a Sankey or simple flow diagram is expected.