Thermal expansion in mechanics is usually expressed as which of the following?

• A. The amount of heat energy stored in a material.
• B. It is a fractional change in length or volume per unit temperature change.
• C. The maximum stress before yielding.
• D. The speed at which heat transfers.

Answer: (B)

Thermal expansion is about how a material’s size changes as temperature changes. It’s expressed as a fractional change in a dimension per degree of temperature change. For a solid rod, the linear expansion is ΔL/L0 = α ΔT, where α is the linear coefficient of thermal expansion. For volume, the analogous relation is ΔV/V0 = β ΔT, with β ≈ 3α for isotropic materials. This is why the correct description is the fractional change in length or volume per unit temperature change. The other options describe heat energy stored, yield strength, or heat transfer speed, which are different properties.