Forces of Air Pressure
Published 4-30-2017
During my tenure as an earth science teacher weather was one of my favorite units of study. I hoped it would become a student favorite as well. Many of the experiments we assembled to teach weather concepts possessed an element of the “wow factor.” My reticence about over-stressing the pure entertainment value of science demonstrations took a back seat as we realized that learning real world scientific facts could be wholly instructional and entertaining at the same time. Air pressure demonstrations fell into the dual category of instruction plus entertainment.
Pedagogically, science instructors could argue that learning book facts should not precede a classroom demonstration. This depends on the nature of the facts being learned. If facts concerning the existence and action of air molecules are not readily apparent, the teacher may initially discuss the existence of billions of invisible air molecules randomly zig-zagging through the air striking neighboring molecules and surfaces in the environment. Their movement is due to the heat energy present in any sample of matter above the temperature of absolute zero. Molecules always possess kinetic energy—the energy of motion. Collectively, impacts of billions of whirling molecules against surfaces result in the constant force of air pressure. Ordinarily we do not perceive these considerable forces, but classroom demonstrations enable the teacher to affirm their existence.
As a prelude to lessons concerning air pressure it may be appropriate to promote wonder in students about the beauty of our air blanket. Air is low in density but its attraction by earth’s gravity is necessary for life on earth and makes air pressure phenomena possible. It is a protective layer against harmful radiation from outer space such as ultraviolet. The atmosphere helps warm the earth and protects planetary life from extreme heat or cold. The 21% oxygen level is life-giving for animals. The trace amount of CO2 is life-giving for plants. Oxygen permits combustion of fuels. Our world would be without sound except for the presence of our atmosphere. Circulation of air distributes life-sustaining precipitation. Mastery of facts about our atmosphere is a life long project for professional atmospheric scientists. The air pressure topic is of vital significance in atmospheric science.
Each cubic meter of atmosphere contains 7.5 septillion molecules of mostly nitrogen and oxygen. The pressure results in a substantial force of 14.7 lb. per square inch. This is illustrated by a 14.7 lb. weight pressing down on an area of one square inch. The average man’s body has about 2500 square inches of surface area. This calculates to roughly 18 tons of total pressure. Man does not feel this enormous pressure because his body’s internal pressure pushes back with equal force. The forces are usually balanced so he does not feel pressure from any direction. Only if internal pressure were reduced or increased or if external pressure were reduced or increased would our subject feel any effects. Ordinarily such pressure changes do not occur.
We begin with of one of the most spectacular demonstrations of the force of air pressure. This demonstration had a strong visual impact. We needed old-fashioned two gallon rectangular methanol or motor oil cans for our “wow factor” experiment, making sure no volatile vapors were left in the can.
The can with a small amount of water was heated with a propane burner until the water boiled long enough for steam to escape from the open can for at least a minute. After extinguishing the torch we tightly capped the can. I told the students to be quiet, watch, and listen. After a little while the can began to make noises as the can crinkled and collapsed before our eyes. The invisible force of air pressure was collapsing the can before our eyes—the same force that presses on our bodies every moment. (The can had been filled with water vapor after boiling the water. It began to condense back to a small amount of liquid water leaving a vacuum in the closed can. The force pushing out was no longer equal to the external force pushing in. Strength of the can was no match for strength of the outside air pressure.) The strongest boys in the classroom admitted they might be unable to match the destructive force of air pressure!
The behavior of molecules in air is tantamount to mayhem. They speed around chaotically, colliding with other molecules or with surrounding surfaces multiple times per second at a speed of about 1000 mph. On the scale of molecular sizes the distance between molecules is very great. Over 99.9% of air in our room is empty space. But trillions of tiny particles colliding with our bodies and with our demonstration can uncounted times per second at 1000 mph produced a startling visible and audible effect. The behavior of any one air molecule unpredictably moving about with kinetic energy is impossible to describe. In contrast, the collective behavior of gas molecules in our atmosphere may be accurately described by numerous gas laws.
Was the collapsed can experiment an example of magic? Or did it fuel our desire to explain phenomena with scientific inquiry? What is the basis of the “wow factor?” Laws of science, including the laws governing the behavior of air, were established by the Creator who initiated time, space, matter, and energy in the beginning. The Creator allows us to discover these laws and to describe their effects on humanity.
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