What causes natural convection flows in a fluid?

• A. Density differences due to temperature differences
• B. External impedance
• C. Chemical reactions
• D. Magnetic fields

Answer: (A)

Natural convection flows arise when temperature-induced density differences create buoyancy forces in a gravitational field. Heating a portion of the fluid lowers its density, causing it to rise, while cooler surrounding fluid is denser and sinks, setting up a circulating flow. This buoyancy-driven motion is the essence of natural convection. External impedance, chemical reactions, or magnetic fields don’t establish the primary buoyancy mechanism in the typical natural convection scenario, though they can modify the flow once convection is underway.