Energy in the form of heat can also be transferred through the movement of gases or liquids. Convection (kuhn-VEK•shuhn) is the transfer of energy as heat by the movement of liquid or gas, In most substances, as temperature increases, the density of the liquid or gas decreases. Convection occurs when a cooler, denser mass of gas or liquid replaces a warmer, less dense mass of gas or liquid by pushing it opward.
When you boil water in a pot, the water moves in roughly circular patterns because of convection. The water at the bottom of the pot gets hot because there is a source of heat at the bottom As the water heats, it becomes less dense. The warmer water rises through the denser, cooler water above it. At the surface, the warm water begins to cool. The particles move closer together, making the water denser. The cooler water then sinks back to the bottom is heated again, and the cycle repeats. This cycle causes a circular motion of liquids or gases. The motion is due to density differences that result from temperature differences. The motion is called a convection current.