Which example illustrates heat transfer by radiation?

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

Answer: (A)

Radiation is heat transfer by electromagnetic waves that can travel through space without a medium. The heating of the Earth’s surface by energy traveling through space from the sun is radiation, because the sun’s energy reaches Earth across the vacuum of space without needing air or another substance to carry it.

The other scenarios involve conduction or convection. Heat removal by air flowing over an engine cylinder happens as convection, with moving air carrying heat away. Uniform heating of the room’s air by a floor heater is also primarily convection, as the warm air circulates to distribute heat. Conduction through a solid is direct transfer of heat through contact between solid particles.