Which heat transfer mechanism is demonstrated when warm air rises and circulates within a room?

• A. Conduction
• B. Radiation
• C. Evaporation
• D. Convection

Answer: (D)

The idea being tested is heat transfer by the movement of a fluid, which is convection. When air near a heat source is warmed, it becomes less dense and rises. As it rises, cooler air from the rest of the room moves in to take its place, and that air then heats up and rises again, creating a circulating pattern called a convection current. This differs from conduction, which requires heat to move through direct contact and through materials or still air by molecular interactions, and from radiation, which heats objects by infrared waves without needing air flow. Evaporation involves a liquid changing to a gas and is not the mechanism by which room air is warmed and circulated. So the warm air rising and circulating is convection.