Which of the following describes thermal expansion?

• A. The elastic response of a material to load.
• B. The fractional change in length or volume per unit temperature change.
• C. The rate of heat transfer through a material.
• D. The resistance to deformation under load.

Answer: (B)

Thermal expansion is how a material’s size changes as temperature changes. The best description is the fractional change in length or volume per unit temperature change. In practice, a small temperature rise causes the dimensions to increase by a tiny amount: for a rod, ΔL ≈ α L0 ΔT, and for a solid, ΔV ≈ β V0 ΔT, where α and β are the linear and volumetric expansion coefficients.

This happens because higher temperatures make atoms vibrate more and occupy more average space, expanding the lattice. The other ideas describe different properties: elasticity is how a material deforms under mechanical load, heat transfer rate is how quickly heat moves through a material, and resistance to deformation under load is related to stiffness. So the statement about fractional size change per temperature change directly captures thermal expansion.