Which mechanism involves heat transfer by circulation within a medium such as a gas or a liquid?

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

Answer: (A)

Heat transfer by circulation within a fluid is convection. When part of a gas or liquid is heated, it becomes less dense and rises, while cooler, denser fluid moves in to take its place. This creates a continuous loop that moves thermal energy from hotter regions to cooler ones through the actual movement of the fluid itself. This is different from conduction, which is energy transfer through direct molecular contact without bulk fluid motion, and from radiation, which transfers energy via electromagnetic waves and can occur through empty space. Absorption describes energy being taken up by material, not the mechanism that moves heat through the fluid.