Which heat transfer mode is the only one that works within opaque (non-transparent) solids?

• A. Convection
• B. Conduction
• C. Radiation
• D. Mass Transfer

Answer: (B)

Heat moves through opaque solids mainly by conduction. In solids, energy is passed from one particle to the next through lattice vibrations (phonons) and, in metals, by free electrons that rapidly transfer energy to neighboring particles. This happens without any bulk movement of the material itself. Convection needs a fluid that can circulate, which isn’t present inside a solid. Radiation involves electromagnetic waves traveling through a medium; in opaque solids these waves are absorbed and do not propagate efficiently through the interior. Mass transfer is about moving material, not transferring heat through the solid. So conduction is the mechanism that carries heat inside opaque solids.