GCSE-ready- Home
- Lessons
- GCSE & IGCSE Physics
- Efficiency and Sankey diagrams
Efficiency and Sankey diagrams
🎬 The doodle video for this lesson is coming soon. Subscribe on YouTube to see it first.
Energy is always conserved, but not all of it ends up where we want it. Sankey diagrams show where it goes.
Conservation of energy
Energy cannot be created or destroyed, only transferred between stores.
Total energy in = useful energy out + wasted energy out.
Wasted energy usually ends up heating the surroundings.
Total energy in = useful energy out + wasted energy out.
Wasted energy usually ends up heating the surroundings.
Efficiency
efficiency = (useful energy output ÷ total energy input) × 100%
A light bulb that uses 100 J of electrical energy and gives out 20 J of light is 20% efficient. The other 80 J is wasted as thermal energy.
A light bulb that uses 100 J of electrical energy and gives out 20 J of light is 20% efficient. The other 80 J is wasted as thermal energy.
Sankey diagrams
A Sankey diagram shows energy transfers as arrows.
The width of each arrow is proportional to the amount of energy.
The useful output goes straight on; wasted energy branches off, usually downwards.
The input width always equals the total of the output widths.
The width of each arrow is proportional to the amount of energy.
The useful output goes straight on; wasted energy branches off, usually downwards.
The input width always equals the total of the output widths.
A motor is supplied with 500 J and does 350 J of useful work. Find the wasted energy and the efficiency.
- Wasted = 500 − 350 = 150 J
- Efficiency = 350 ÷ 500 × 100
Answer: 150 J wasted; 70% efficient
Energy is conserved: input = useful output + wasted output. Efficiency = useful output ÷ total input × 100%. In a Sankey diagram, arrow width shows the amount of energy; wasted energy branches off.
Check you have got it
Answer 6 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.