Which term describes heat transfer by circulation within a medium?

The correct choice, which describes heat transfer by circulation within a medium, is convection. This process involves the movement of warmer, less dense fluid rising and cooler, denser fluid sinking. As the warmer fluid rises, it transfers heat to the surrounding cooler fluid, creating a continuous circulation pattern. This is commonly observed in liquids and gases where the molecules can move freely, allowing for efficient heat distribution.

Convection plays a significant role in many natural processes, such as atmospheric weather patterns and ocean currents, as well as in engineered systems like heating and cooling in buildings. Understanding convection is crucial in fire investigations as it influences fire behavior and the spread of heat, smoke, and flames within a structure.

The other terms refer to different methods of heat transfer. Conduction involves direct contact between materials, where heat travels through solid materials without the movement of the material itself. Radiation is the transfer of heat through electromagnetic waves and can occur even in a vacuum without the need for a medium. Evaporation is a phase change process where liquid turns to gas, which can involve heat transfer but is not characterized by circulation within a medium. Therefore, convection is distinctly characterized by the movement of fluids and the circulation pattern that arises from differences in temperature and density.