Expansion of solids, liquids and gases
Most substances expand when heated and contract when cooled. As the temperature rises the particles vibrate more and push further apart, so the object gets bigger. Gases expand most, liquids less and solids least.
- coefficient of linear expansion (per K)
| Material | α (× 10⁻⁶ per K) |
|---|---|
| Glass (ordinary) | 9 |
| Concrete | 12 |
| Steel | 12 |
| Copper | 17 |
| Aluminium | 23 |
| Brass | 19 |
Choose the length of the rod, the temperature rise and the material (α). Find how much longer it becomes.
A steel bridge girder is 50 m long at 20 °C. How much does it expand when it reaches 50 °C? (α = 12 × 10⁻⁶ per K)
- ΔT = 30 K.
Answer: The girder expands by 18 mm: the bridge needs expansion joints.
Applications and problems
| Situation | Allowance or use |
|---|---|
| Railway tracks and bridges | Small gaps or expansion joints; rollers under bridges |
| Power lines | Hung slackly in summer so they do not snap in cold weather |
| Fitting a metal tyre on a wheel | The hot tyre expands, slips on and shrinks tight |
| Pyrex glass | Expands little, so it does not crack when heated |
| Concrete reinforced with steel | Both expand by almost the same amount |
| Bimetallic strip | Two metals expand differently and the strip bends: thermostats and fire alarms |
Expansion of liquids and the unusual behaviour of water
Liquids expand more than solids, which is why a thermometer works. Water is unusual: between 0 °C and 4 °C it contracts when heated. Its density is greatest at 4 °C. When a lake cools, water at 4 °C sinks to the bottom, so ice forms at the top and fish survive underneath. Ice is less dense than water, so it floats.
Key terms
- Thermal expansion
- The increase in size of a substance when its temperature rises.
- Coefficient of linear expansion
- The fractional increase in length per kelvin rise in temperature.
- Bimetallic strip
- Two metals joined together that bend when heated.
- Anomalous expansion
- The unusual contraction of water between 0 °C and 4 °C.
Summary
- Solids, liquids and gases expand when heated; ΔL = α L₀ ΔT.
- Allow for expansion in bridges, rails and power lines.
- Bimetallic strips make thermostats and fire alarms.
- Water is densest at 4 °C, so lakes freeze from the top.