Thermal Properties of Matter | Class 11 Physics Notes

10.5 Thermal Expansion

Thermal expansion is the increase in dimensions of a body due to a rise in temperature. Most substances expand on heating and contract on cooling. This phenomenon is observed in daily life, such as when a tightly screwed metallic lid loosens after heating, or mercury rises in a thermometer when warmed. Expansion can be linear (length), area, or volume expansion. For a rod, the fractional change in length Δl/l is proportional to the temperature change ΔT, expressed as Δl/l = α₁ ΔT, where α₁ is the coefficient of linear expansion characteristic of the material. Typical values of α₁ for various materials are given in Table 10.1, showing metals generally have higher values than glass. Volume expansion is similarly defined with coefficient α_V = (ΔV/V)/ΔT. For isotropic solids, α_V ≈ 3α₁. Water exhibits anomalous behavior, contracting between 0°C and 4°C, reaching maximum density at 4°C, which has environmental significance for aquatic life. Gases expand more than solids and liquids, with α_V inversely proportional to temperature for ideal gases. Preventing thermal expansion in constrained bodies leads to thermal stress, which can be calculated using Young's modulus. The section includes examples demonstrating calculation of area expansion and thermal expansion in practical scenarios.

📊 Diagram: Fig. 10.5 Thermal Expansion; Fig. 10.8; Tables 10.1 and 10.2 showing coefficients of expansion for various materials; Fig. 10.6 Coefficient of volume expansion of copper as a function of temperature; Fig. 10.7 Thermal expansion of water.

🔗 Connection: This section connects to the next one on specific heat capacity, which explains how substances absorb heat.

Table on page 4 (9×2)

Table on page 5 (12×2)

Table on page 21 (9×5)

Table on page 23 (3×3)

Table on page 24 (6×2)