In metals, heat is primarily transported by

• A. Impurity diffusion
• B. Lattice vibrations
• C. Free electrons
• D. Nuclear lattice excitations

Answer: (C)

Heat in metals is carried primarily by free electrons. The electrons are delocalized and highly mobile, so when there is a temperature difference, energetic electrons on the hot side move toward the cooler side, transporting energy with them. Because these electrons have long mean free paths and interact often with lattice vibrations and impurities rather than being trapped, they sweep heat through the metal far more effectively than the lattice can through vibrations alone. Lattice vibrations (phonons) do contribute to heat transfer, but their role is smaller in metals because the electronic contribution dominates the overall thermal conductivity. Impurity diffusion is a mechanism for moving atoms, not heat, and nuclear lattice excitations are negligible at typical temperatures, so they don’t meaningfully affect heat transport. The link between electrical and thermal conductivity in metals (Wiedemann-Franz law) also supports why free electrons are the main carriers of heat.