Which heat transfer mode moves heat with fluids or gases?

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

Answer: (D)

Convection is the heat transfer that moves heat with fluids or gases. It happens when a fluid itself carries thermal energy as it flows, creating currents that transfer heat from one region to another. Hot, less dense fluid rises while cooler, denser fluid sinks, setting up a circulation that can be natural (buoyancy-driven) or forced by something like a fan or pump. This is why a room warms faster when air is circulated or why a radiator heats nearby air that then circulates.

Conduction is heat transfer through direct contact, including in fluids, but it doesn’t rely on the bulk movement of the fluid. Radiation transfers heat via electromagnetic waves and doesn’t require a medium. Evaporation involves a phase change and energy exchange during that process, not the general transport mechanism by moving fluids.