Convection

Convection is the flow of heat through a bulk, macroscopic movement of matter from a hot region to a cool region, as opposed to the microscopic transfer of heat between atoms involved with conduction. Suppose we consider heating up a local region of air. As this air heats, the molecules spread out, causing this region to become less dense than the surrounding, unheated air. For reasons discussed in the previous section, being less dense than the surrounding cooler air, the hot air will subsequently rise due to buoyant forces - this movement of hot air into a cooler region is then said to transfer heat by convection.

Heating a pot of water on a stove is a good example of the transfer of heat by convection. When the stove is first turned on heat is transferred first by conduction between the element through the bottom of the pot to the water. However, eventually the water starts bubbling - these bubbles are actually local regions of hot water rising to the surface, thereby transferring heat from the hot water at the bottom to the cooler water at the top by convection. At the same time, the cooler, more dense water at the top will sink to the bottom, where it is subsequently heated. These convection currents are illustrated in the following figure.

 

Figure 8.3: Convection currents in boiling water

Consider now two regions separated by a barrier, one at a higher pressure relative to the other, and subsequently remove the barrier, as in the following figure. These convection currents are illustrated in the following figure.

 

Figure 8.4: Flow of material through a pressure difference

When the barrier is removed, material in the high pressure (high density) area will flow to the low pressure (low density) area. If the low pressure region was originally created by heating of the material, one sees that movement of material in this way is an example of heat flow by convection.

An important example of convection currents that can be interpreted in this manner is the creation of breezes over land masses next to large bodies of water. Water has a larger heat capacity than land, and subsequently holds heat better. It therefore takes longer to change its temperature, either upward or downward. Thus, during the day the air above the water will be cooler than that over the land. This creates a low pressure area over the land, relative to the high pressure area over the water, and subsequently one finds breezes blowing from the water to the land. On the other hand, during the night water cools off more slowly than the land, and the air above the water is slightly warmer than over the land. This creates a low pressure area over the water relative to the high pressure area over the land, and breezes will blow from the land to the water. These are illustrated in the following figure.

 

Figure 8.5: Formation of breezes near large bodies of water