Which statement best describes convection?

• A. It transfers heat through direct contact.
• B. It transfers heat via the movement of a fluid due to density differences.
• C. It transfers heat by electromagnetic radiation.
• D. It requires a solid medium to work.

Answer: (B)

Convection occurs when heat is transferred by the movement of a fluid that changes density with temperature. When part of the fluid is heated, it becomes less dense and rises; cooler fluid, being denser, sinks, creating a continuous loop. This circulation carries heat with the moving fluid—as seen in warm air rising to heat a room, or in boiling water where hot fluid rises and cooler fluid moves downward. Convection requires a fluid (liquids or gases) and relies on the fluid’s motion, not just contact between particles. By contrast, heat moving through direct contact is conduction, and heat moving as waves through space is radiation, which can occur without a medium. So the description that heat transfer happens via the movement of a fluid due to density differences best captures convection.