Which statement best defines convection as a heat transfer mode?

• A. Heat transfer by movement of gas or liquid from one place to another due to pressure difference.
• B. Heat passes through a solid material.
• C. Heat transfer by electromagnetic waves.
• D. Heat transfer only in vacuum.

Answer: (B)

Convection transfers heat through the bulk movement of a fluid (gas or liquid) driven by buoyancy: when a part of the fluid is heated, it becomes less dense and rises while cooler, denser fluid sinks, setting up a circulating flow that carries heat with it. This is different from conduction, which is heat transfer through a stationary substance by molecular interactions, and from radiation, which uses electromagnetic waves. Convection requires a medium to move, so in vacuum there is no convection. While pressure differences can drive fluid flow in some situations (forced convection), the defining mechanism is temperature-induced density differences causing the fluid to circulate and transport heat.