Which term describes the transfer of heat through a fluid due to density differences caused by temperature changes?

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

Answer: (D)

Convection is the transfer of heat through a fluid by the movement of the fluid itself, driven by density differences that come from temperature changes. When part of the fluid gets warmer, it becomes lighter and rises; cooler, denser fluid nearby sinks to take its place. This creates a circulating flow that moves heat from warmer regions to cooler ones. You can see this in atmospheric circulation, ocean currents, or a pot of heated water.

Radiation would move heat through space or any medium without needing fluid flow or density differences. Conduction involves heat transfer through a substance by direct contact and molecular interactions without bulk fluid movement. Evaporation is a phase change where liquid becomes vapor at the surface, which can involve energy transfer but doesn’t describe heat moving through a fluid via buoyant circulation.