Thermal expansion · Grade 6
Most materials expand when they are heated and contract when they cool, because their particles vibrate more and move slightly further apart. Engineers leave gaps in railways and bridges for this, and thermometers and thermostats rely on it.
Key facts
- Heating makes particles vibrate more and take up more space; the particles themselves do not grow.
- Mass stays the same when something expands, so its density decreases.
- Linear expansion: ΔL = αLΔT (α = linear expansivity, per °C).
- Volume expansion: ΔV = βVΔT; liquids expand more than solids.
- A bimetallic strip bends when heated: the metal that expands more ends up on the outside of the curve.
- Water is unusual: it is densest at 4°C and expands when it freezes, so ice floats.
Worked examples
Example 1
A 30 m steel rail (α = 12 × 10⁻⁶ /°C) warms by 40°C. By how much does it expand?
Answer: 14.4 mm
Example 2
A steel ring of diameter 50.00 mm must fit over a 50.06 mm shaft. By how much must it be heated? (α = 12 × 10⁻⁶ /°C)
Answer: 100°C