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

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

Answer: (B)

Radiation is heat transfer that occurs through electromagnetic waves and does not require a medium to travel through. The sun’s energy reaching the Earth is the classic example: sunlight travels through the vacuum of space and heats the planet when it is absorbed by the surface and objects, even though there’s no air or other medium in between.

The other scenarios involve heat moving through a medium or by contact. Removing heat from an engine cylinder by air flowing over it is convection—heat carried away by moving air. Uniformly heating the air in a room with a floor heater is also convection, driven by warm air circulating. Conductive heat transfer occurs through direct contact within a solid, such as a metal rod transmitting heat along its length.