Which energy transfer mode is driven by the movement of a fluid and transfers heat via the fluid's motion?

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

Answer: (B)

Convection is the heat transfer mode driven by the movement of a fluid. When part of the fluid is heated, it becomes less dense and rises, while cooler fluid moves in to take its place, creating circulating currents that carry thermal energy with the fluid’s motion. This is how heat moves through air or water in a room or a fire scene—hot gases rise and heat spreads as the fluid flows. Natural convection relies on buoyancy forces from temperature differences, while forced convection uses fans or pumps to push the fluid and boost heat transfer. Conduction, by contrast, transfers heat through direct contact without bulk fluid motion, and radiation transfers heat by emitting electromagnetic waves without relying on fluid movement. Absorption isn’t a separate heat-transfer mode; it describes energy uptake by a material rather than the mechanism that moves heat through a fluid.