The transfer of heat by the movement of air or liquid is best defined as

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

Answer: (A)

Convection is heat transfer by the movement of air or liquid. When a part of the fluid is heated, it becomes less dense and rises, while surrounding cooler fluid moves in to replace it. This creates a circulating flow that transports thermal energy from warmer areas to cooler ones. In a room with a heater, warm air rises and circulates, warming the space. In a pot of soup, heat is carried from the bottom to the top by rising convection currents.

Conduction, by contrast, is heat transfer through direct contact without bulk fluid motion, and radiation is heat transferred by electromagnetic waves (which can happen through empty space). Evaporation involves a phase change and is not the mechanism for moving heat through a fluid.