convection

Convection is the transfer of heat energy in a liquid or a gas. The fluid near the heat source is heated, becomes less dense, and rises upwards. Once away from the heat source, the fluid starts to cool, becomes denser, and starts to sink. A kettle uses the principle of convection currents. As the water at the bottom is heated by the element, it rises and brings cool water closer to the element.

Transfer of heat energy that involves the movement of a fluid (gas or liquid). Fluid in contact with the source of heat expands and tends to rise within the bulk of the fluid. Cooler fluid sinks to take its place, setting up a convection current. This is the principle of natural convection in many domestic hot-water systems and space heaters.

Hot-air balloons use convection in order to rise into the air. The air inside the balloon is heated. As the hot air rises, so does the balloon. For the balloon to descend, the air in the balloon is cooled or allowed to escape.

Convection currents in the air allow gliders to fly. The glider gains height from rising, warm currents of air known as thermals. These currents are formed by air being heated by the ground; the heated air becomes less dense and rises.