Which heat transfer occurs primarily in fluids like air and water?

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

Answer: (B)

Convection is the main way heat moves through fluids like air and water. When part of the fluid is heated, it becomes less dense and rises, while cooler, denser fluid sinks to take its place. This sets up circulating currents that carry thermal energy through the liquid or gas, making heat transfer much more efficient than by direct molecular touch alone. Conduction does occur in fluids, but the circulation of warm and cool regions dominates the process. Radiation can transfer heat across a distance without a medium, but inside the fluid the bulk transfer happens via these moving currents. Evaporation involves changing a liquid to a gas and uses energy for the phase change, not the continuous transfer of heat through the fluid itself. So convection is the primary heat transfer mechanism in fluids.