Which statement best describes when convection occurs?

Which statement best describes when convection occurs?

• A. It only occurs in solids.
• B. It only occurs when there is a heat source.
• C. It occurs when a fluid is heated and becomes less dense than surrounding fluid.
• D. It only occurs in outer space.

Answer: (C)

Convection is heat transfer driven by the movement of a fluid that results from density differences created when the fluid is heated. When a portion of a fluid is warmed, it expands and becomes less dense than the surrounding cooler fluid, so gravity causes the lighter, warmer part to rise while the cooler, denser part sinks. This sets up a circulating flow that carries heat with it. That’s why you see warm air rising from a heater or hot water circulating in a kettle. The idea that convection only happens in solids isn’t right, because solids don’t have the bulk fluid movement needed for convection (they mostly transfer heat by conduction). Convection also isn’t limited to environments with a heat source in the sense of a single spark; what matters is a temperature (and density) difference that provides buoyant movement of the fluid. And convection requires a fluid; in outer space there isn’t a medium to circulate, so heat transfer there is mainly by radiation.

Convection is heat transfer driven by the movement of a fluid that results from density differences created when the fluid is heated. When a portion of a fluid is warmed, it expands and becomes less dense than the surrounding cooler fluid, so gravity causes the lighter, warmer part to rise while the cooler, denser part sinks. This sets up a circulating flow that carries heat with it. That’s why you see warm air rising from a heater or hot water circulating in a kettle.

The idea that convection only happens in solids isn’t right, because solids don’t have the bulk fluid movement needed for convection (they mostly transfer heat by conduction). Convection also isn’t limited to environments with a heat source in the sense of a single spark; what matters is a temperature (and density) difference that provides buoyant movement of the fluid. And convection requires a fluid; in outer space there isn’t a medium to circulate, so heat transfer there is mainly by radiation.