Which of the following demonstrates convection as heat transfer?

• A. A metal spoon warming liquid
• B. Ice cooling a drink
• C. Sun heating the earth
• D. Warm air rising in a room

Answer: (D)

Convection is heat transfer through the movement of a fluid. When air in a room is warmed near a heater, it becomes lighter and rises toward the ceiling. As it rises, cooler air moves in to take its place, and this continual up-and-down flow creates convection currents that carry heat through the room. The other scenarios involve different methods: a metal spoon warming liquid is heat transfer by conduction through direct contact, the sun heating the earth is radiation (heat traveling as waves), and ice cooling a drink is mainly conduction as heat moves from the drink to the cooler surface. The rising warm air in a room best demonstrates convection.