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Name: Dolores G.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 4/1/2004


Question:
Our class is studying about the movement of heat. We would like to find out about insulators. How they are made and how they work.


Replies:
Dolores,

Heat energy is stored in molecules as vibrations. More vibration means higher temperature. For some materials, it is easy for one molecule to make a neighbor start to vibrate. This new molecule then makes more molecules vibrate. Pretty soon, all the molecules are vibrating. The whole object has a higher temperature. This is a heat conductor. Metals are good at this. Many liquids are good at it as well.

The best insulator is a vacuum, completely empty space. If there are no molecules, there can be no vibrations. A very good insulator is air. Gasses like air do no transfer heat very well because the molecules are so far apart from each other. Wind can make air transfer heat because the air molecules carry the energy as they move. If the air molecules cannot move much, they cannot carry the heat energy very far. A bunch of air-filled plastic bubbles arranges in a honeycomb pattern is considered an excellent insulator. Dry wood has a great deal of empty space inside it. Dry wood is a good insulator, too.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College


Dolores,

Heat moves about by three mechanisms -- conduction, convection, and radiation. The first two work if there is an intervening medium to enable them.

In conduction, heat flows thorough the medium because the atoms and molecules in the medium are in contact with each other. Heat (thermal agitation) of the medium's atoms and molecules is passed along from the warmer parts toward the cooler areas by a relay process wherein agitation of one atom/molecule is passed along to its neighboring atom/molecules.

In convection, warmed atoms and/or molecules which are free to move about -- such as those in a gas or liquid -- circulate (move through) through the medium in a discernible and rather predictable route. You can nicely demonstrate convection by sprinkling a little sawdust atop water in a beaker. Heat the beaker gently at the bottom edge. As the water along the side of the beaker heats up, it will rise, move across the top, and then descend to cross the bottom and rise again. In the process, sawdust grains will be entrained in the moving warmer-to-cooler water. Thus, a convection current becomes visible. In radiation, heat energy is transmitted via electromagnetic waves. The radiative process requires no medium. In other words, radiation (for example, light and heat) can move through empty space. Insulators can be any material or means by which any or all of the three heat transfer mechanisms can be frustrated.

In the conductive case, an insulator might be something like glass -- a rather poor conductor of heat. Thus, separating the hot region from the cooler might be affected by putting a piece of glass (or any other heat-tolerant, poor heat conductor) in contact with both regions. Heat flow is frustrated because the adjacent areas are separated by a poor conductor.

In the case of convection, we want to use a poor heat conductor with many dead air spaces within it. Thus, things like fiberglass and foam board insulation work well because they interfere with the convective flow essential to that mechanism. The tiny air- or gas-filled spaces in the insulator frustrate heat flow because they greatly extend the time necessary for convective and conductive flow to occur. Foe heat to move through such a material, each cell -- and there are millions -- must relay the heat across it. The longer it takes, the slower the heat flow.

In the case of radiation, we want intervening surfaces that are highly reflective so that heat radiation bounces off rather than being absorbed. In fact, that is how a thermos bottle works. The shiny mirrored surface reflects heat back toward the source. Hot things in a thermos stay hot because heat is reflected back toward the hot contents. Cool things inside a thermos stay cool because the heat from the outside is reflected away from the contents.

In the case of fiberglass, the insulation is simply a mat of fine glass strands in a suitable containing wrap. Some fiberglass insulation comes wrapped in an aluminized (reflective) material to frustrate the radiative pathway.

Foams are simply frothy plastic materials containing a gas within non-connected, very tiny cells. Like fiberglass batting, some foam boards can be also obtained clad in a reflective, aluminized skin.

Regards,
ProfHoff 838



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