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2.2.5Atoms, Elements and Compounds

Sankey Diagrams — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 9 of 26 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Reading a Sankey Diagram

2.2.5 Sankey Diagrams
Input arrow
  • The flat arrow on the left represents the total energy supplied to the system.
Useful output
  • The straight arrow pointing to the right represents the energy that ends up in the desired store.
Wasted arrows
  • Arrows bending away from the main flow show energy dissipated to the surroundings, usually as thermal energy.
Proportional widths
  • The width of each arrow is proportional to the amount of energy it carries, so a wider arrow means more energy.

Part 2 of 5Drawing an Accurate Sankey Diagram

1
Calculate the energy in every arrow and choose a single scale, for example 1 mm per joule.
2
Draw the input arrow on the left using the scaled width.
3
Draw the useful output arrow to the right, with a width that matches its energy.
4
Draw the wasted arrow bending away, and check that the input width equals the useful output plus the wasted energy.

Part 3 of 5Comparing Devices

Comparing Devices
Filament bulb
  • Most of its input energy is wasted as thermal energy, so its wasted arrow is very wide.
LED bulb
  • A much smaller proportion of input energy is wasted, so its downward arrow is narrow.
Conservation
  • For each diagram, the total input must equal the useful output plus the wasted energy.
Scale
  • Drawing to a common scale allows two diagrams to be compared directly through their arrow widths.

Part 4 of 5Worked Example

Question

  • A device has a total energy input of 800 J and a useful energy output of 260 J.
  • Calculate the energy wasted.

  1. Conservation of energytotal in = useful out + wasted
  2. Rearrangewasted = total in − useful out
  3. Substitutewasted = 800 − 260 = 540 J

Answer 540 J of energy is wasted.

Part 5 of 5Exam Checklist & Tips

2.2.5 – Key Points for the Exam

  • Sankey diagram A diagram in which arrow widths are drawn proportional to the energy in each transfer.
  • Energy input The total energy supplied to a system at the start of the transfer.
  • Useful output The energy that reaches the store the device was designed to fill.
  • Wasted energy Energy dissipated to the surroundings, which cannot be used for the intended purpose.
  • Conservation of energy Total energy in equals total energy out, including all wasted energy.

Common Mistakes

  • Drawing arrow widths that are not proportional to the energy they represent.
  • Making the useful output arrow larger than the input arrow.
  • Placing wasted energy on the input side of the diagram.
  • Leaving out the wasted arrows, so that the diagram does not balance.

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