Which is an example of convection?

• A. Cold water flowing through hot tubes of a heat exchanger
• B. A metal rod warming up from one end
• C. Heat transfer from the sun to the Earth across space
• D. Evaporation at a liquid surface

Answer: (A)

Convection is heat transfer by the bulk motion of a fluid. In this scenario, cold water flows through hot tubes, so the liquid itself moves and carries heat from the hot tube walls into the flowing coolant. This movement of the fluid is the essence of convection, and when it’s driven by a pump or flow, it’s called forced convection.

The other options don’t show that bulk fluid motion transferring heat. Warming a metal rod from one end happens by conduction through the solid; heat moves by molecular interactions inside the solid, not by moving fluid. Heat transfer from the sun to the Earth across space is radiation, energy carried by electromagnetic waves through vacuum. Evaporation at a liquid surface involves phase change and mass transfer; while air around the surface may circulate, the primary mechanism described is evaporation (not convection of a bulk fluid transferring heat).