Which mechanism transfers heat mainly via fluid motion?

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

Answer: (A)

Convection is heat transfer caused by the bulk motion of a fluid. When part of a liquid or gas is heated, it becomes less dense and rises, while cooler, denser fluid sinks, creating a circulating current that carries thermal energy with it. This movement of the fluid itself is what transports heat from warmer regions to cooler ones, more effectively than if the fluid were just sitting still.

Conduction, by contrast, transfers heat through direct molecular interactions within a substance or across a boundary without relying on large-scale fluid flow. Radiation moves heat through electromagnetic waves and can occur even in a vacuum, so it doesn’t depend on a moving fluid. Evaporation involves a surface phase change and latent heat; it can carry energy away, but the main mechanism isn’t sustained bulk fluid motion.

So the mechanism that transfers heat mainly via fluid motion is convection.