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Section 23.2 Convection

Convection occurs in fluids when higher energy molecules from one part of a fluid move to another part resulting in a bulk motion of the fluid. If the movement is caused by forcing the hot fluid with a mechanical stirrer or a blower, the convection is called forced convection, otherwise it is called the free or natural convection. For example, when you heat water on a stove, higher energy molecules from the bottom rise and mix with lower energy molecules above due to natural or free convection (Figure 23.2.1).

Figure 23.2.1. (a) Convection currents in water on a heater carry energy from the larger temperature bottom to the cooler top. (b) Granular convection cells at the surface of sun that carry energy from the interior of the sun to the surface. Credits:

Since the molecules in the hot region have more kinetic energy, they tend to move around more and hence make less free space avialble for other molecules. As a result, the volume per molecule in the hotter region is more and so the density in the hotter region is less than the colder region. This one of the main reasons why hot water rises in a container. Similarly, hot air from the surface of the Earth rises in the atmosphere. The flow of the fluid creates a convection current that leads to a heat transfer from the hot region to the cold region.

Convection plays a major role in the atmosphere as hot air with water vapor from near the surface of the Earth rises and cools at a lower temperature above the Earth where they condense and form clouds. Convection is also thought to play an important role in bringing heat from the center of the Sun to the surface of the Sun. The convection cells photographed at the solar surface shown in Figure 23.2.1(b) are evidence of the role of convection in the solar dynamics.