Thermal expansion of solids – metal rods and a bimetallic strip
Integrated ScienceMatter & MaterialsAges 13–14Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Matter & Materials
Three rods of aluminium, copper and steel with the same length are heated equally; a dial gauge at the end of each rod measures the increase in length ΔL = α·L₀·Δt to a hundredth of a millimetre. Students record a data table and see that aluminium expands the most and steel the least. In bimetallic strip mode, a copper–steel strip bends towards the steel (the metal that expands less) and breaks an electric circuit, like the thermostat in an electric iron. Further experiments show area and volume expansion of a plate and a cube (β ≈ 2α, γ ≈ 3α), a liquid in a flask with a narrow tube and air pushing a bead (gases expand most, then liquids, solids least), and a lake that freezes from the top because water is densest at 4 °C.
Lesson: Thermal expansion
What it shows
When a substance is heated its particles move more and take up more space, so it expands; on cooling it contracts. A rod lengthens by ΔL = α·L₀·Δt, an area grows with β ≈ 2α and a volume with γ ≈ 3α. Liquids expand far more than solids, and gases more still, because their particles are held less tightly. A liquid in a glass flask shows its apparent expansion: its real expansion minus that of the glass. Water is unusual: it is densest at about 4 °C, so lakes freeze from the top. Bimetallic strips and expansion gaps in rails are everyday applications.
How to use
Choose an experiment with the tabs. With Three metal rods, light the burner and press Record data at several temperatures. In Area & volume, choose the Metal and compare ΔA and ΔV with α. In Liquids & gases, press Light the burner and watch the level in the narrow tube dip and then rise; when it is steady, press Record data, change the Water-bath temperature and repeat. In Water at 4 °C, set the Winter air temperature and watch the lake freeze.
Parameters you can change
- Experiment Three metal rods and dial gauges, Copper–steel bimetallic strip, Plate and cube: area and volume expansion, Liquids and gases in flasks, Water at 4 °C and a freezing lake
- Initial length of each rod 200–1000 mm
- Room temperature 10–35 °C
- Maximum heating temperature 60–300 °C
- Metal (plate and cube) Aluminium, Copper, Steel
- Liquid in the flask Coloured water, Coloured ethanol, Mercury
- Water-bath temperature 30–75 °C
- Winter air temperature -20–10 °C
Questions to explore
- For the same temperature rise, which rod lengthens the most and which the least?
- If you double the initial length L₀, what happens to ΔL for the same temperature rise?
- Why does the copper–steel strip bend toward the steel when heated, and how does this switch off the lamp?
