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2.2.4Energy

Sankey Diagrams — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions

5 min read 7 parts Lesson 10 of 12

📚 Oxford AQA International GCSE Combined Science - Physics Hub / Topical Revision
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Part 1 of 5What a Sankey diagram shows

Energy flow as an arrow
  • Energy flow as an arrow A Sankey diagram uses a wide arrow that splits into branches to show how the input energy is shared out.
  • The width of each arrow is proportional to the amount of energy it carries.
Input arrow
  • Input arrow The flat left-hand end of the diagram represents the total energy transferred into the system.
Useful output
  • Useful output The arrow that carries straight on to the right shows energy transferred to the store that we want.
Wasted energy
  • Wasted energy Arrows that bend away, usually downwards, show energy that ends up in stores that are not useful, often the thermal store of the surroundings.

Part 2 of 5Conservation of energy

IdeaEquation or statement
Energy is never created or destroyedTotal energy in = total energy out
Split of the outputTotal energy in = useful energy out + wasted energy
Finding the wasteWasted energy = total energy in − useful energy out
Unit for all energy valuesJoules (J)

Part 3 of 5Worked example

1
An electric motor lifts a weight. 500 J of electrical energy is supplied and the diagram shows 120 J going to the gravitational potential store of the weight.
2
Write the conservation equation: total energy in = useful energy out + wasted energy.
3
Rearrange it: wasted energy = total energy in − useful energy out.
4
Substitute the values: wasted energy = 500 − 120 = 380 J.
5
Check by adding: 120 J + 380 J = 500 J, which matches the input.

Part 4 of 5Comparing two light bulbs

Filament lamp and energy-efficient lamp
Filament lamp
  • Filament lamp Most of the input energy is transferred to the thermal store of the surroundings,
    • so the wasted arrow is very wide and the useful light arrow is thin.
Efficient lamp
  • Efficient lamp Much less energy is wasted by heating, so the downward arrow is narrow and the light arrow is wider.
Same input, different outputs
  • Same input, different outputs Both lamps may take in the same total energy, so the input arrows are equally wide.
    • Only the split differs.

Part 5 of 5Drawing one to scale

1Plan the widths
  • Plan the widths Choose a scale on graph paper, for example 1 square for 10 J, and work out the width of the input, useful and wasted arrows.
2Draw the input
  • Draw the input Draw the flat left end and the line along the top using a ruler.
3Add useful output
  • Add useful output Draw the straight arrow to the right with exactly the planned width.
4Add wasted energy
  • Add wasted energy Mark the start and end of the bent arrow the correct distance apart, then join the marks and add the arrowhead.

Key terms

TermMeaning
Sankey diagramA diagram in which arrow width shows the amount of energy in each transfer.
Useful energy outputEnergy transferred to the store that we want it to reach.
Wasted energyEnergy transferred to stores that are not useful, usually dissipated to the surroundings by heating.
Conservation of energyThe principle that the total energy in a closed system stays the same.
DissipatedSpread out into the surroundings, mainly in the thermal store, so it is hard to use again.

Avoid these mistakes

1
Drawing arrows whose widths are not in proportion to the energy values. Use a scale and measure carefully.
2
Forgetting units. Energy values are in joules (J).
3
Making the total output larger or smaller than the input. Useful plus wasted must equal the total input.
4
Describing wasted energy as 'lost'. It is transferred to the surroundings, so say it is dissipated.

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