Natural convection: When a medium's density changes due to temperature change (Warmed air becomes less dense, and rises).

Natural convection: When a medium's density changes due to temperature change (Warmed air becomes less dense, and rises).

• A. Forced convection: Is controlled by humans (Hot air systems, circulated hot water systems).
• B. Forced convection: Density changes cause warm air to rise.
• C. Unwanted convection: Caused by temperature differences (Stack effect inside a wall cavity or air leakage from poor insulation, leaky windows or doors).
• D. Natural convection: When a medium's density changes due to temperature change (Warmed air becomes less dense, and rises).

Answer: (D)

Natural convection is the buoyancy-driven movement that occurs when a fluid’s density changes with temperature. When air is heated, its density drops, so it becomes lighter and rises. Cooler air, being denser, moves in to take its place, creating a circulating flow without any mechanical aid. This is exactly what the statement describes: density changing with temperature and the resulting rise of warmer air. The other descriptions refer to movement driven by external devices (like fans or pumps) or to a situational label rather than the basic mechanism. Forced convection involves external means driving the flow, not buoyancy alone. The “unwanted convection” wording points to outcomes or particular conditions, not the fundamental natural convection process itself.

Natural convection is the buoyancy-driven movement that occurs when a fluid’s density changes with temperature. When air is heated, its density drops, so it becomes lighter and rises. Cooler air, being denser, moves in to take its place, creating a circulating flow without any mechanical aid. This is exactly what the statement describes: density changing with temperature and the resulting rise of warmer air.

The other descriptions refer to movement driven by external devices (like fans or pumps) or to a situational label rather than the basic mechanism. Forced convection involves external means driving the flow, not buoyancy alone. The “unwanted convection” wording points to outcomes or particular conditions, not the fundamental natural convection process itself.