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Why do engineers leave gaps between sections of the Eiffel Tower's iron frame — and why does the tower actually measure 15 cm taller in summer than in winter? It's not sinking, it's not tilting. The iron itself is growing. How does heating a solid object make it physically bigger?
Why do engineers leave gaps between sections of the Eiffel Tower's iron frame — and why does the tower actually measure 15 cm taller in summer than in winter? It's not sinking, it's not tilting. The iron itself is growing. How does heating a solid object make it physically bigger?
When you heat a solid, you give its particles more kinetic energy — they vibrate more vigorously and push each other slightly farther apart. The object grows. Cool it down and the particles settle closer together — the object shrinks. This is thermal expansion, and it's predictable: the change in length depends on the material, the original length, and the temperature change.
Every material has a characteristic coefficient of linear expansion α (alpha), measured in /°C or /K. It tells you what fraction of its length a material expands per degree. Steel: α = 12×10⁻⁶ /°C. Aluminium: α = 23×10⁻⁶ /°C. Aluminium expands nearly twice as much as steel for the same temperature change. The total change in length ΔL depends on three things: how big α is, how long the object started (L₀), and how much the temperature changed (ΔT).
The formula ΔL = αL₀ΔT has a beautiful logic: longer objects expand more in absolute terms (a 2 m rod expands twice as much as a 1 m rod of the same material at the same temperature). The material property α captures how 'eager' the atoms are to spread apart when heated. Engineers use this constantly: bridges have expansion joints, power lines are strung with some sag, and the gaps between railway tracks are carefully sized for the local temperature range.