Rene-NASA-Mooned-America

Explaining Heat & Cold / Chap. 10 p. 83
Radiant Heat — Heat energy transferred by an electromagnetic wave.
The only way heat energy can be transferred through a vacuum is by radiation. The
Stefan-Bolzmann law is used to calculate the quantity of heat being radiated, or received, by
a substance. 1
The radiant heat transmitted from a unit area of surface is proportional to, and thereby
mostly dependent on the fourth power of the absolute (Kelvin) temperature of that surface.
The words "fourth power" sound complicated, but they simply mean multiplying a number
by itself four times. For example the fourth power of 2 is 2 X 2 X 2 X 2 which equals 16.
The fourth power of 3 is 81. The number 3 is only 1.5 times greater than 2. 2 However, if we
divide the fourth power of 3 by the fourth power of 2 we find it is 5 times as great. 3
Therefore, a body radiating heat at 3 degrees K radiates five times as much heat as a body at
2 degrees K. This ratio drops quickly as the numbers increase.
The heat emitted is also dependent upon the coefficient of emissivity. This is a number
which ranges from zero to one. A perfect emitter would be 1 and the perfect mirror would be
0 because it would reflect all the heat that hit it. It doesn't matter if the surface is emitting
or absorbing radiant heat the coefficient is the same.
A constant, called Stefan's constant, is also necessary to produce numerically correct
answers. The Stefan-Bolzmann formula produces numerical answers in watts. It can converted
to calories, a heat unit we're more familiar with, by multiplying the watts by 860.
The Sun's surface temperature is estimated at 6000° K. 4 The radiant energy at this
extremely high temperature is truly awesome. By using Stefan-Bolzmann's law we find that
73,487,090 watts per-square-meter is transmitted into space. After it has traveled 93 million
miles to the Earth, this figure has been reduced to an average of 1353 watts per square meter
above the atmosphere. 5
Boiling — The vaporizing of a liquid by the addition of heat.
When we boil any liquid we produce a vapor of that liquid. In addition to the sensible
heat (detected by a thermometer), each gram of vapor carries with it a much greater amount
of non-sensible heat which is called the Heat-of-Vaporization. If the vapor is physically
removed from the area the remaining liquid becomes cooler. The temperature at which a
liquid boils is also varied to a great degree by the pressure. On top of a mountain where the
atmospheric pressure is less, water boils at much lower temperatures. The freezing point of
a liquid is also affected by pressure in a similar manner, but to a much lesser degree.
A tumbler of water will start to boil away without added heat as you increase the vacuum.
In fact, if you had a thermometer in the tumbler, you would see the temperature of the
remaining liquid drop as the vapor was pumped out. At a low enough pressure or a high
enough vacuum you would also see some of the water turning into ice at the same time the
rest was boiling. In effect, the remaining water is being refrigerated by the heat energy it is
losing. Once boiling commences the pressure will drop much more slowly than in the beginning.
The lower the pressure the harder the pump must work. Since each volume of water
vapor is 1200-times greater than the water so vaporized, the pump must evacuate that much
more volume.
NASA MOONED AMERICA! / Rene