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INSULATION
"R" VALUES
This is only one
of three "HEAT TRANSFER" factors.
Heat naturally flows
from warm areas to cooler area, regardless of direction. In winter, heat
flows from the inside of a building to the outside and in the summer
high heat from roofs and walls travels from outside to insede. This flow
of heat can never be stopped completely, but the rate at which it flows
can be reduced by using materials which have a high resistance to heat
flow (R value = resistance).
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Heat Radiation
Radiation is heat
transfer by the emission of electromagnetic waves which carry energy
away from the emitting object. For ordinary temperatures
(less than red hot"),
the radiation is in the infrared
region of the electromagnetic
spectrum. The relationship governing radiation from hot objects is called
the Stefan-Boltzmann
law:
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Heat Conduction
Conduction is heat
transfer by means of molecular agitation within a material without any
motion of the material as a whole. If one end of a metal rod is at a higher
temperature,
then energy will be transferred down the rod toward the colder end because
the higher speed particles will collide with the slower ones with a net transfer
of energy to the slower ones. For heat transfer between two plane surfaces,
such as heat loss through the wall of a house, the rate of conduction heat
transfer is:
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| HyperPhysics***** Thermodynamics
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Heat Convection
Convection is heat
transfer by mass motion of a fluid such as air or water when the heated
fluid is caused to move away from the source of heat, carrying energy with
it. Convection above a hot surface occurs because hot air expands, becomes
less dense,
and rises (see Ideal
Gas Law). Hot water is likewise less dense than cold water and rises,
causing convection currents which transport energy.
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Convection can also lead to circulation in a liquid, as
in the heating of a pot of water over a flame. Heated water expands and becomes
more buoyant. Cooler, more dense water near the surface descends and patterns
of circulation can be formed, though they will not be as regular as suggested
in the drawing. |
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Convection cells are visible in the heated cooking oil in
the pot at left. Heating the oil produces changes in the index
of refraction of the oil, making the cell boundaries visible. Circulation
patterns form, and presumably the wall-like structures visible are the boundaries
between the circulation patterns. |
| Convection is thought to play a major role in transporting
energy from the center of the Sun to the surface, and in movements of the
hot magma beneath the surface of the earth. The visible surface of the Sun
(the photosphere) has a granular appearance with a typical dimension of a
granule being 1000 kilometers. The image at right is from the NASA Solar
Physics website and is credited to G. Scharmer and the Swedish Vacuum Solar
Telescope. The granules are described as convection
cells which transport heat from the interior of the Sun to the surface. |
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| In ordinary heat transfer on the Earth, it is difficult to quantify
the effects of convection since it inherently depends upon small nonuniformities
in an otherwise fairly homogeneous medium. In modeling things like the cooling
of the human body, we usually just lump it in with conduction. |
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