What is convection in heat transfer?

Convection in heat transfer refers to the bulk movement of fluid molecules, whether they are in liquid or gas form, that facilitates the transfer of heat. This process occurs when warmer, less dense areas of a fluid rise, while cooler, denser areas descend. As a result, heat moves from regions of higher temperature to regions of lower temperature, establishing a continuous circulation that facilitates the overall transfer of heat throughout the fluid.

This process is distinct from conduction, which involves the transfer of heat through solid materials due to direct contact. In conduction, heat is passed from molecule to molecule without any bulk movement of the material itself.

Moreover, convection cannot occur in solids since it relies on the movement of liquid or gas molecules, which is why options referring to heat transfer without a medium or to the transfer between solids do not accurately describe convection. Thus, the correct answer captures the essence of how heat is transferred within fluids through the motion of molecules.