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

• A. Removal of heat from an engine cylinder by air flowing over its surface.
• B. The uniform heating of the air in a room by a floor heater.
• C. The heating of the Earth's surface by heat transmitted through space from the sun.
• D. Heat transfer by conduction through a metal rod from hot end to cold end.

Answer: (D)

Conduction is heat transfer that occurs through direct contact within a material or between touching objects, moving energy from regions of higher temperature to lower temperature by particle interactions and electron movement in solids.

The example of heat moving along a metal rod from its hot end to its cold end demonstrates this clearly: energy is transferred along the lattice as vibrating particles collide with neighbors, passing the energy along without any bulk motion of the rod itself.

The other scenarios involve different mechanisms. Heat moving from an engine cylinder to the surrounding air by wind over the surface is mainly convection, as moving air transports the energy away. Uniform heating of room air by a floor heater is also primarily convection, with warm air circulating. Heating the Earth’s surface by sunlight traveling through space is radiation, heat moving via electromagnetic waves through vacuum.