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4. Convection in a Pan - Background
This process through which heat energy is transferred through currents within a liquid or gas is called convection. The cyclical nature of the process in an enclosed system like the mantle of the Earth results in convection cells – local regions of liquid or gas that form a relatively stable cycle (heating, rising, moving aside, cooling, and sinking in roughly the same location over and over again).
Convection takes place in many other systems. A pot of water boiling on the stove is a good example of convection. Watch spaghetti boiling in a large pot and you will see the noodles rise near the middle of the pot above the flames, spread out over the surface, and fall again near the edges where it is cool. In the Earth’s atmosphere, convection results in regional weather patterns and thermals (rising columns of heated air). Eagles and hang gliders both take advantage of thermals to stay aloft. In the Earth’s oceans, the warm ocean water near the equator tends to follow currents towards the poles while cold polar ocean water follow currents back again to the equator.
A key concept is that hot fluid and gas rises and cold fluid and gas sinks. To demonstrate this principle, you can create a hot air balloon in the classroom using a dry cleaner bag and a hand-held hair dryer.