Ask A Scientist Physics Archive

   
name         Dolores G.
status       educator
age          50s

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.
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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
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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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