🧰 You need: pen, pencil, ruler, calculator, squared paper (1 cm squares) and coloured pencils if you have them. Time: 45 minutes. Course: Combined and Separate.
Information – read this first
- Energy is never created or destroyed. It is only transferred. So total energy in = total energy out.
- The energy that goes where we want it is useful. The rest is wasted – usually heating the surroundings.
- A Sankey diagram shows energy transfers as arrows. The width of each arrow shows how much energy it carries. Useful energy goes straight on; wasted energy bends downwards.
- Efficiency = useful energy output ÷ total energy input. Multiply by 100 to get a percentage.
- Efficiency can never be more than 1 (100%).
The data
| Device | Energy in | Useful out | Wasted out |
|---|---|---|---|
| A. Filament bulb | 100 J electrical | 10 J light | 90 J heating surroundings |
| B. LED bulb | 100 J electrical | 40 J light | 60 J heating surroundings |
| C. Kettle | 2000 J electrical | 1800 J heating water | 200 J heating surroundings and sound |
| D. Car engine | 1000 J chemical | 250 J kinetic | 700 J heating, 50 J sound |
What to do
- (2 min) Write the title Sankey Diagrams and Efficiency and today’s date. Underline both with a ruler.
- (8 min) Copy the five bullet points from the information box into your book.
- (10 min) Draw a Sankey diagram for A, the filament bulb. Use the scale 1 square = 10 J:
• Draw an arrow 10 squares wide coming in from the left. Label it “100 J electrical”.
• Split it: a strip 1 square wide carries straight on to the right – label it “10 J light (useful)”.
• The other 9 squares bend downwards – label it “90 J heating surroundings (wasted)”.
• Check: 1 + 9 = 10 squares, so energy is conserved. - (10 min) Now draw Sankey diagrams for B (1 square = 10 J) and C (1 square = 200 J). If you have time, draw D (1 square = 100 J; the 50 J sound arrow is half a square).
- (10 min) Write the heading Calculations. Copy the worked example below, then do Q1–Q5. Show the equation, the numbers and the answer every time.
- (5 min) Check your answers, correct any mistakes in a different colour, then try the challenge.
Calculations
Worked example – filament bulb: efficiency = useful output ÷ total input = 10 ÷ 100 = 0.1. As a percentage: 0.1 × 100 = 10%.
Q1 Calculate the efficiency of the LED bulb as a percentage.
Show answer
40 ÷ 100 = 0.4 = 40%
Q2 Calculate the efficiency of the kettle.
Show answer
1800 ÷ 2000 = 0.9 = 90%
Q3 Calculate the efficiency of the car engine.
Show answer
250 ÷ 1000 = 0.25 = 25%
Q4 A motor is supplied with 500 J. Its efficiency is 0.6. How much useful energy does it transfer?
Show answer
Useful output = efficiency × total input = 0.6 × 500 = 300 J
Q5 A TV wastes 120 J for every 200 J supplied. Calculate its efficiency.
Show answer
Useful = 200 − 120 = 80 J. Efficiency = 80 ÷ 200 = 0.4 = 40%
⭐ Challenge
Use your Sankey diagrams to explain why people are encouraged to replace filament bulbs with LED bulbs. Then suggest one way to reduce the energy wasted by a kettle.
Show answer
The LED transfers more of its input energy usefully as light (40% compared with 10%), so less energy is wasted heating the surroundings. To give the same light it needs less electrical energy, so it costs less to run. A kettle wastes less energy if it is insulated (e.g. a double-walled kettle) and if you only boil the water you need.
