Thermal expansion
Definition
Thermal expansion is the tendency of matter to grow slightly larger when heated — the reason bridges have expansion joints and power lines sag in summer.
Technical definition
For solids, ΔL = αL₀ΔT, where α is the linear expansion coefficient. Liquids and gases expand more; the bimetallic strip in thermostats exploits two metals with different α.
Example
A 100 m steel bridge (α ≈ 12×10⁻⁶/°C) warming 40 °C grows by 100×12×10⁻⁶×40 = 0.048 m — nearly 5 cm, enough to buckle the deck without expansion joints.
Common confusion
Not everything expands when heated. Water between 0 °C and 4 °C contracts as it warms — the famous density anomaly that lets ice float and lakes freeze from the top down.