Which statement about natural convection is true?

Which statement about natural convection is true?

• A. It is always caused by external fans.
• B. It requires contact between solids.
• C. It occurs when a medium's density changes due to temperature, causing warm air to rise.
• D. It only occurs in liquids, not gases.

Answer: (C)

Natural convection is driven by buoyancy forces that come from temperature-induced density differences inside a fluid. When part of the fluid is heated, it expands and becomes less dense, so gravity makes it rise. Cooler, denser fluid then moves in to take its place, creating a circulating current. This mechanism doesn’t rely on external fans or other mechanical pumping, and it doesn’t require solids to touch; the movement occurs entirely within the fluid. It can happen in both liquids and gases, which is why you see warm air rising in a room as well as hot water circulating in a pot. The statement describing density changes due to temperature and the resulting rise of warm air correctly captures how natural convection works.

Natural convection is driven by buoyancy forces that come from temperature-induced density differences inside a fluid. When part of the fluid is heated, it expands and becomes less dense, so gravity makes it rise. Cooler, denser fluid then moves in to take its place, creating a circulating current. This mechanism doesn’t rely on external fans or other mechanical pumping, and it doesn’t require solids to touch; the movement occurs entirely within the fluid. It can happen in both liquids and gases, which is why you see warm air rising in a room as well as hot water circulating in a pot. The statement describing density changes due to temperature and the resulting rise of warm air correctly captures how natural convection works.