|MadSci Network: Physics|
OK, Steve, the basic idea for convection is pretty straightforward. Hot fluids (air or water, for example) expand and become less dense, which means the hot stuff is lighter than the same volume of cold stuff. If the fluid is not heated evenly, bubbles of the hot stuff will rise, just like a hot air ballon. The heavier cold fluid above it gets pushed aside and drops down. Convection happens on the insides of stars, in the Earth's atmosphere, and in a boiling pot of water. In each case, the hot area is on the bottom, so hot material is constantly bubbling upward from there. This carries the heat up, and also forces the cold material down closer to the heat source. Because convection moves and mixes the material, it is a much faster way for heat to travel than the other option, called "conduction." In conduction, hot particles are wiggling around really fast next to slower, colder particles. When the particles bump into one another, some of the energy from the hot particle is given to the cold one. Then that one bumps into the colder particle next to it, and so on. Conduction is how heat gets transported in solid things, like through the walls and windows of your house in the winter, or from a warm cup of cocoa to your hands. Convection is what drives the weather. Hot air moves up and cooler air moves in to replace it, making wind. If you live near the ocean, like I do, or a big lake (is Montepelier, Indiana very close to Lake Michigan?), the sun warms up the land faster than the water. The air over the land gets warm and moves up, and the cold air over the water blows in. At night, the land cools off faster, so the air over the water is then warmer, moves up, and the wind blows the other way. Convection is why fire travels much faster upward than to the side, and why you should stay low, below the hot smoke, if caught in a fire. Heat moves faster upward through convection than it can sideways by conduction. You might take a look at the Educator's Guide to Convection on the Web. It has some neat pictures and MPEG movies of computer simulations of convection. These were made by scientists trying to understand how heat gets transported in the insides of stars like the Sun.
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