Year 9 Science · Unit 3 · Lesson 4

Energy Efficiency and Sankey Diagrams

Foundation Worksheet

Name

Date

Class

Learning Goals

Name the parts of a Sankey diagram.
Sort energy flows into input, useful output and waste output.
Read energy values and efficiency from Sankey arrow widths.

Warm Up

Match each Sankey diagram feature to its description

Write the letter of the matching description in the middle column. Each letter is used once.

Sankey diagram feature	Letter	Description
Input arrow		A. The rule that a wider arrow means more energy
Useful output arrow		B. The arrow that bends downward, showing energy that spreads into the surroundings as heat
Waste output arrow		C. The arrow entering from the left, showing all the energy supplied
Width proportional to energy		D. The point where the main arrow divides into separate streams
Split point		E. Useful output ÷ total input × 100, written as a percentage
Efficiency		F. The arrow that carries straight on, showing the energy that does the job we want

1. Explain why the widths of all the arrows leaving a Sankey diagram must add up to the width of the input arrow.

Recall 2 marks

Your Turn

Sort it: input, useful or waste?

These energy flows come from a petrol lawnmower, an LED globe and a television. Write each one in the correct box.

Chemical energy in the petrol (lawnmower) Light from the LED globe Heat from the back of the television Kinetic energy of the spinning blade (lawnmower) Electrical energy into the television Heat from the LED globe’s circuit Sound from the television’s speakers Electrical energy into the LED globe Noise and exhaust heat from the engine (lawnmower) Light from the television screen

Energy input

Useful output

Waste output

2. In a Sankey diagram, the input arrow is 20 mm wide and the useful output arrow is 5 mm wide. What is the efficiency of the device? How wide is the waste arrow?

Apply 3 marks

Efficiency %

Waste arrow width mm

Go Deeper

Data: a student’s Sankey diagram for a ceiling fan

A student drew a Sankey diagram, to scale, for a ceiling fan. The scale is 1 cm = 10 J. Table 1 lists the arrows. The student forgot to measure one width.

Table 1. Arrows in the ceiling fan’s Sankey diagram (scale 1 cm = 10 J)

Arrow	Width (cm)	Energy (J)
Input: electrical energy	10
Useful: kinetic energy of the moving air	7
Waste: thermal energy in the motor	2.5
Waste: sound

Source: sample class results.

3. Complete Table 1. Show how you found the width of the sound arrow.

Apply 4 marks

4. Calculate the efficiency of the ceiling fan. What does the thin sound arrow tell you?

Analyse 2 marks

Efficiency %

Show What You Know

5. An LED bike light takes in 600 J of electrical energy each minute and gives out 150 J of light. Use a scale of 1 cm = 50 J to draw its Sankey diagram. Label each arrow with its energy form and value, and write the scale.

Apply 3 marks

6. Describe, step by step, how to draw a Sankey diagram for an electric motor that takes in 400 J and is 90% efficient. Include your calculations for every arrow.

Apply 5 marks