Thermal expansion
Most materials expand when heated because their particles move faster and take up more space on average.
Practise this conceptHeat a material and it swells, on average, because its particles jiggle around more energetically and each one needs more room to do so. The effect is usually small per degree, but it adds up, and engineers design around it constantly.
That's why bridges and railway tracks include small gaps between sections — without them, a metal beam expanding on a hot summer day would have nowhere to go and could buckle. It's also why a stuck metal jar lid loosens after running the lid under hot water: the metal expands slightly faster and more than the glass beneath it, briefly opening a bit of clearance.
Different materials expand by different amounts for the same temperature change, which is precisely why bimetallic strips — two different metals bonded together — bend when heated: one side expands more than the other, forcing the whole strip to curve. That simple bending is the working principle behind many old-fashioned mechanical thermostats.
Key ideas
Requires: Temperature and heat
Formulas
| Symbol | Name | Unit |
|---|---|---|
| distance | m (metre) | |
| expansion coefficient | 1/K (per kelvin) | |
| distance | m (metre) | |
| temperature | K (kelvin) |
Common misconceptions
- All materials expand by the same amount for the same temperature increase.
- Thermal expansion only happens in solids, not in liquids or gases.
- Thermal expansion means a material's mass increases when it is heated.