• word_count: 200

Thermal expansion occurs because increased temperature increases atomic vibrations, pushing atoms apart. Three types: linear ($\Delta L = L_0\alpha\Delta T$), area ($\Delta A = A_0\beta\Delta T$, $\beta = 2\alpha$), and volume ($\Delta V = V_0\gamma\Delta T$, $\gamma = 3\alpha$). The ratio $\alpha:\beta:\gamma = 1:2:3$ is fundamental.

Key materials: Invar ($\alpha \approx 0.9 \times 10^{-6}$ K$^{-1}$) for precision instruments, Pyrex glass ($\alpha \approx 3.3 \times 10^{-6}$) for thermal shock resistance, steel ($\alpha \approx 12 \times 10^{-6}$) as a reference standard. Holes and cavities expand as if filled with the same material — a heated ring has a larger inner diameter.

Water exhibits anomalous expansion between 0 and 4 degrees C — it contracts on heating, reaching maximum density at 4 degrees C. This causes lakes to freeze from the top, with the bottom at 4 degrees C supporting aquatic life.

Bimetallic strips exploit differential expansion — two metals bonded together bend toward the less-expanding metal, used in thermostats. When expansion is constrained, thermal stress $\sigma = Y\alpha\Delta T$ develops, independent of length.