Year 9 Science · Unit 3 · Lesson 10

Thermal Expansion and Contraction

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Learning Goals

Trace the cause-and-effect chain of thermal expansion in a railway track.
Apply the concept of thermal expansion to explain engineering decisions on the Sydney Harbour Bridge.
Predict what happens to structures when thermal expansion is not accounted for.

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Because… chain

Fill in the missing effects. Each cause leads to the next step. The chain shows what happens to a railway track across the seasons.

Cold winter morning: air temperature drops to 5 °C

→

The metal rail particles slow down and move closer together

→

Hot summer day: temperature rises to 45 °C

→

The gaps in the track close as rails expand toward each other

→

Overall outcome (what happens without expansion gaps):

Your Turn

Real-world context

The Sydney Harbour Bridge is 1149 metres long and is made of steel. Engineers have measured that the bridge expands by approximately 18 cm between a cold winter day (10 °C) and a hot summer day (45 °C). This 35 °C temperature difference causes steel to expand at a rate of about 12 parts per million per degree Celsius. The bridge contains multiple expansion joints along its length to manage this movement.

(a) Why are expansion joints essential in the Sydney Harbour Bridge? What would happen if the bridge were one continuous rigid structure with no gaps?

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(b) The bridge expands by 18 cm over summer, yet a person walking across it would not notice any change in its length. Why is 18 cm of expansion invisible over a 1149 m bridge?

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(c) If a bridge were built twice as long (2298 m) using the same steel, predict how much it would expand over the same 35 °C temperature change. Explain your reasoning.

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Wrap Up

In one sentence, what was the main idea of this lesson?