Which of the following is an example of heat transfer by conduction?

• A. The heating of the Earth's surface by the sun.
• B. The uniform heating of the air in a room by a floor heater.
• C. Removal of heat from an engine cylinder by air flowing over its surface.
• D. Removal of heat from an engine cylinder by air flowing over its surface.

Answer: (D)

Heat transfer by conduction happens when heat moves through a solid (or between solids in direct contact) due to a temperature difference, without the bulk movement of air or fluid. In the given scenarios, the sun’s heat reaching the ground is radiation, not conduction. Heating the room air with a floor heater is convection—the warm air circulates and carries heat. Removing heat from an engine cylinder with air flowing over its surface is also convection—the moving air carries heat away from the surface. None of the options show conduction directly. A true conduction example would be heat moving through the metal of the engine block to a cooler part in contact with it (or heat passing through a solid rod from hot to cold).