Which of the following is not a mode of heat transfer?

• A. Conduction
• B. Expansion
• C. Convection
• D. Radiation

Answer: (B)

Heat moves from hotter regions to cooler ones mainly through three ways: conduction, convection, and radiation. Conduction is energy transfer via direct contact and interactions between particles in a solid; convection involves the bulk movement of a fluid that carries energy with it; radiation is energy transfer by electromagnetic waves that can occur through vacuum or space.

Expansion, or thermal expansion, is the change in a material’s size with temperature. It is an effect of heating, not a mechanism for transferring energy. The energy still travels from hot to cold through one of the three true modes, while the material simply changes dimensions in response to that heat. For example, a metal bar heats and lengthens, but the actual energy movement from the hot end to the cold end happens via conduction. That makes expansion not a mode of heat transfer.