10.3: CONVECTION

Convection is the transfer of thermal energy due to the motion of the substance that contains the thermal energy. Although the conduction process can occur in liquids and gases as well as solids, convection may have a much larger effect in liquids and gases where the molecules are free to migrate.A hot cup of coffee transfers thermal energy as heat to the surrounding air by convection. The heated air over the coffee expands and rises. The rising heated air is replaced by cooler, denser air, which in turn, expands and rises. This circulation of air is an example of convection.

Convection occurs because as the temperature is increased, most materials expand. This means that they become less dense. Thus, the warmer air near the hot coffee is floated up to the ceiling just like a piece of cork, which is less dense than water, rises to
the surface of the water.

In liquids and gases, convection is usually the most efficient way to transfer heat. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. As this happens, cooler liquid or gas takes the place of the warmer areas which have risen higher. This cycle results in a continous circulation pattern and heat is transfered to cooler areas. You see convection when you boil water in a pan. The bubbles of water that rise are the hotter parts of the water rising to the cooler area of water at the top of the pan. You have probably heard the expression "Hot air rises and cool air falls to take its place" - this is a description of convection in our atmosphere. Heat energy is transfered by the circulation of the air.

So, if you have a hot bowl of soup on the table, it heats a layer of air surrounding the bowl. That layer then rises because it is hotter than the surrounding air. Cold air fills in the space left by the rising hot air. This new cold air then heats up and rises, and the cycle repeats. It is possible to speed up convection -- that is why you blow on hot soup to cool it down. If it weren't for convection your soup would stay hot a lot longer, because it turns out that air is a pretty poor heat conductor.