convection

Meaning: Convection is heat transfer by the bulk movement of a fluid, a liquid or a gas. Fluid warmed at a hot surface typically expands and becomes less dense, so it rises; cooler, denser fluid sinks to replace it, creating a circulating current (natural convection). A fan or pump gives forced convection, which usually moves heat faster. Convection needs a fluid that can flow, so it does not occur through a solid or a vacuum.

On the MCAT: Passages compare conduction, convection and radiation. Typical questions ask why a heater placed low warms a room well, why wind increases heat loss from skin, or why only radiation carries energy from the Sun across a vacuum. You predict whether a change, such as adding a fan or stirring, increases or decreases heat transfer.

Common confusion: Convection is not conduction. In conduction, energy passes between neighboring particles by collisions while they stay roughly in place; in convection the heated matter itself moves and carries energy with it. Also, warm fluid rises because it is less dense, not because heat itself rises.