Joseph Herbert on wed 28 may 97
For any who are trying to follow the bouncing ball in the heat rises
discussion, a few definitions. First Temperature. In the atomic theory of
heat, accepted in most of the U.S. (outlawed in Alabama as is calculus), the
temperature of an object, body, or mass of something is a measure of the
average speed of the atoms or molecules that comprise the material. For
fluid masses, like water or air, this is the linear rate of movement of the
particles. For solid objects it is the velocity of vibration of the atoms
around their average position in the material mass. The speed of such a
molecule or atom at room temperature is pretty fast, like 3600 mph (maybe).
While it doesn t have physical meaning to us, a single atom can be thought
of as having a temperature if it is traveling at some rate of speed. This
leads to the Intuitively contradictory situation in near-earth space where
the temperature, in terms of molecular velocity is high, but objects exposed
there are very cold. There aren t many molecules out there so the velocity
temperature of hundreds or thousands of degrees doesn t "heat" the
spacecraft.
The other consequence of temperature as a measure of velocity is the concept
of absolute zero, or no temperature. This is the absence of molecular
movement either linearly or in vibration. The fractional temperatures stated
by scientists using liquid helium are an indication of very slow moving
atoms.
Heat, in the second place, is the energy contained in the kinetic energy, the
energy of movement, of a mass of material. No only is the temperature
important, so is the mass. The mass of all the moving particles in a body of
matter, moving linearly and vibrating, each with their own personal inertial
contribution is heat. An easily understood example of the importance of mass
in this concept of heat is derived from the mental comparison of the result
of holding one s hand in the 450 degree air of a pizza oven and of holding
one s hand in the 450 degree oil of the french fry machine. The temperatures
(particle velocities) are the same but the masses available to transfer heat
to your hand is very different. The oil is no hotter but it contains much
more energy (heat).
The "flow" of heat is something we all observe and pretend to understand. We
have been exposed to the idea that heat (energy) is transferred by three
means - radiation, conduction, and convection. People seem to have the least
trouble with conduction - the spoon is in the hot coffee and the heat runs up
the handle and the handle gets hot. Rich people (silver spoons) have an
easier time with this than do we patrons of Mcdonalds who don t know the
coffee is hot until it hit our lap. (lapse) In our atomic version of
things, the atoms in the silver spoon start to vibrate rapidly and bang
against each other. The banging is passed from atom to atom very easily, a
property of the material, and the entire spoon is soon vibrating at the same
speed as the coffee. Insulating materials don t pass the banging from atom
to atom as easily and the "heat" stays put for a while. The physical model
is pretty easy to imagine.
Radiation is usually accepted pretty easily because of the general
acknowledgment that the energy that heats the earth gets from the sun somehow
and it isn t the other two. Also the common experience of the HOT CAR
probably sells the idea of radiative heat transfer pretty easily. The
physical reality is a little more difficult to deal with. A massless packet
of energy arrives after traveling millions of miles at fantastic speed and
strikes an atom here in my car. That atom, as a result of the "impact" of a
massless thing starts to vibrate. Not only that, the original atom can give
off another massless packet of energy which slows the first atoms vibration
but will cause another to vibrate (not quite as fast) more. The complex
interactions of radiative heat transfer, especially at higher temperature,
are not intuitively accessible and are usually accepted an ignored outside
the engineering community. By the way, some of these communing engineers use
ordinary kitchen microwave ovens to melt metallurgical samples. Just need a
molybdenum cup and some magnesium oxide bricks. Not necessary to defrost
first. Also voids the warranty.
One of the things we don t easily accept is ourselves as radiators. People
sitting beside a window that has a landscape at 40 degrees below zero will
say of the chill, "There must be a draft."- assuming convection. Actually,
their bodies little atoms are radiating some packets of energy to the
landscape and getting nothing back. Frost on the car on a clear spring night
when the air temperature never approaches freezing? Outer space is cold!
Mars may want women but the universe will take all the heat it can get.
Convective heat transfer, which has been the subject of these Heat Rises
exchanges, is a fluid phenomenon. This transfer of energy can only in a mass
of fluid, that is a gas or liquid, that can move. Heating of one part of the
mass of fluid causes the atoms in that part to move faster and to each take
up more space. The same number of particles in more space means a less dense
mass. Things that are less dense, in a fluid situation, tend to rise,
regardless of their temperature.
We tend to think we have a good understanding of the processes because this
is the area where we have the most direct physical experience. We see smoke
rising from the chimney, we feel the heat from a candle flame (maybe not G.
Gordon), and can watch cigarette smoke circulate in the rising and falling
currents of warm and cool air in a sunny winter room. Actually, we mostly
understand the warm part of the equation and ignore the cold. For every
warmed molecule above a flame a cool one has been drawn in near the base and
a cool entrained current surrounds the flame. For every Gulf Stream, famous
to all school children and to navigators for hundreds of years, there is a
cold return current flowing to the Antarctic along the unseen canyons and
plains of the abyss. Many subtle interchanges escape our notice. So our
smuggness about knowing that heat rises is as much a prejudice as an
experience. Would the Montgolfier brothers have been famous if they had
dreamed of plummeting to the floor of the ocean, slowly dragged there by a
contained mass of 28 degree seawater?
I had intended to include something about the Pholgistine theory of heat, but
since I can t spell it (and neither can the millionaire Microsoft spell check
dweeb) I ll stop here.
Joseph Herbert
JJHerb@aol.com
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