MadSci Network: Physics

Re: Explain convection to me in terms I can understand

Area: Physics
Posted By: Jay H. Hartley, Grad Student,Lawrence Livermore National Lab
Date: Thu Mar 13 22:56:18 1997
Message ID: 858116832.Ph

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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