Year 9 Science · Unit 3 · Lesson 4
Challenge Worksheet
Learning Goals
1. A school’s gas hot-water system is 80% efficient. For every 1,000 J of chemical energy in the gas, how much energy is wasted, and where does it go?
Wasted J
Find the mistakes: a Sankey diagram for a diesel generator
A student drew a Sankey diagram for a diesel generator on a remote cattle station. The student chose a scale of 1 cm = 200 J. The generator takes in 2,000 J of chemical energy and gives out 600 J of electrical energy.
| Arrow | Label written by the student | Width drawn |
|---|---|---|
| Input | Chemical energy, 2,000 J | 10 cm |
| Useful output | Electrical energy, 600 J | 6 cm |
| Waste output | Thermal energy, 1,200 J | 6 cm |
Under the diagram the student wrote: “Efficiency = 1,400 ÷ 2,000 × 100 = 70%.”
2. Find the three errors in the student’s diagram and working. Correct each one.
| Error | What is wrong | Correction |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 |
Plan and analyse an investigation
Question: does the mass lifted affect the efficiency of a small electric motor? The motor winds up a string that lifts a mass through 0.50 m. A joulemeter measures the electrical energy supplied.
Hypothesis
If the mass lifted increases, then the efficiency will …, because …
Variables
Results from the class
The class tested four masses. Each value in Table 2 is the mean of three trials.
| Mass lifted (kg) | Electrical energy in (J) | GPE gained (J) | Efficiency (%) |
|---|---|---|---|
| 0.10 | 2.5 | 0.5 | 20 |
| 0.20 | 4.0 | ||
| 0.30 | 5.0 | ||
| 0.40 | 6.4 |
Source: sample class results.
3. Complete Table 2, using GPE = mgh with g = 10 m/s² and h = 0.50 m. Give efficiencies to the nearest per cent.
4. Plot efficiency against mass lifted, and draw a smooth curve of best fit.
5. Describe the trend and say whether it supports your hypothesis. Suggest one reason why the GPE gained may not be all of the useful energy the motor gave out.
6. Using a scale of 1 cm = 1 J, give the input, useful and waste widths for the 0.40 kg trial.
Input cm
Useful cm
Waste cm
7. A student says: “Any device whose waste arrow is wider than its useful arrow is badly designed and should be replaced.” Evaluate this claim, using Sankey diagram reasoning and at least two devices from this sheet or the lesson.