Which term refers to the transfer of thermal energy by the circulation or movement of a liquid or gas?

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

Answer: (B)

Convection is heat transfer that happens through the movement of a fluid. When a part of a liquid or gas is heated, it becomes less dense and rises, while cooler, denser portions sink, creating a circulating flow that carries thermal energy with it. You can feel this in a pot of boiling soup where bubbles mix heat upward, in warm room air circulating from a heater, or in large-scale patterns like ocean currents and atmospheric circulation. This is different from conduction, which transfers heat by direct contact and mostly involves solids, where energy moves from molecule to molecule. Radiation transfers heat through invisible waves and can occur through empty space, not requiring a fluid to move. Evaporation is a phase change at the surface of a liquid and doesn’t describe the bulk movement of a liquid or gas carrying heat through a system.