Year 9 Science · Unit 3 · Lesson 10
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Learning Goals
1. For the same temperature rise, gases expand the most and solids the least. Use particle theory to explain why.
Order the steps
A heater is controlled by a bimetallic strip made of brass and steel. Brass expands more than steel. Number the steps from 1 to 6. Step 1 comes first.
| Order | Step |
|---|---|
| The brass expands more than the steel, so the strip bends away from the contact. | |
| The room loses heat through the walls and windows, and the strip cools. | |
| The room is cold. The strip is straight and touches the contact, so the heater is on. | |
| The strip straightens, touches the contact again, and the heater turns back on. | |
| The heater warms the room, and the strip warms with it. | |
| The circuit is broken and the heater turns off. |
2. When the strip bends, which metal is on the outside of the curve? Explain why.
Real-world context: the Sydney Harbour Bridge
Including its approach spans, the Sydney Harbour Bridge is 1,149 m long. It is made mostly of steel, and it has expansion joints along its length. Use the steel expansion value of 0.012 mm per metre per °C.
Source for the length: “Sydney Harbour Bridge”, Wikipedia.
3. Why are expansion joints essential in the bridge? What would happen if it were one continuous, rigid structure?
4. (a) Calculate how much longer a 1,149 m steel structure becomes when it warms from 10 °C to 45 °C. Give your answer in mm and in cm. (b) Predict the expansion of a steel bridge twice as long over the same temperature change, and explain.
(a) Expansion mm
(a) Expansion cm
(b) Expansion cm
5. Express your answer to 4(a) as a percentage of the bridge’s length. Use it to explain why people walking across the bridge never notice it grow.
Percentage %
| Material | Growth per metre for each 1 °C (mm) | Growth of a 2 m rod heated by 50 °C (mm) |
|---|---|---|
| Aluminium | 0.024 | |
| Copper | 0.017 | |
| Steel | 0.012 | 1.2 |
| Glass | 0.009 | |
| Pyrex glass | 0.0033 |
Source: values from Lesson 10.
6. Complete Table 1. Steel is done for you: 2 × 0.012 × 50 = 1.2 mm.
7. (a) Which TWO metals in Table 1 would make the bimetallic strip that bends the most? Explain. (b) Use Table 1 to explain why a Pyrex dish is less likely to crack than an ordinary glass dish when hot water is poured into it.
8. Power lines are made of aluminium. A line crew hangs a 300 m length of line between two towers on a 5 °C winter morning. On a 45 °C summer afternoon the line is much hotter. Using a calculation, explain why the crew must leave slack in the line, and what would happen if they pulled it tight in summer instead.
Extra length of line mm