Convection is defined as?

• A. The transfer of heat by the movement of gas or a liquid due to pressure difference
• B. The transfer of heat by electromagnetic waves emitted by a hot object
• C. The transfer of heat through direct contact between solids
• D. The change of phase of a substance to absorb heat

Answer: (A)

Convection is heat transfer through the bulk motion of a gas or liquid. When part of the fluid is heated, it becomes less dense and rises, while cooler, denser portions sink. This creates a circulating flow that carries thermal energy with it, moving heat from the hotter region to the cooler region. Pressure differences can arise as the fluid moves, but the key driver is buoyancy from temperature-induced density changes. The other modes are different: radiation is heat transfer by electromagnetic waves, conduction is heat transfer through direct contact without bulk fluid movement, and a phase change involves latent heat rather than fluid motion.