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Preston MacDougall: Chemical Eye on the Air Force
Published on Feb 23, 2008 - 7:37:53 AM
By: Preston MacDougall
Love was in the air last week, not just here in Tennessee, but all around the world people celebrated St. Valentine's Day on February 14. The week before that, however, flying toasters weren't just on computer screens. On February 5, from Memphis to Nashville and beyond, it seemed like everything was up in the air, including kitchen sinks.
Amazingly, the symbolism and the timing of the events could fit right into a glorious B. B. King song, since after the tornadoes had done their damage, "love came to town".
Several extremely lucky witnesses, including a college student at Union University, told a similar story: "I was being pulled out into the tornado!"
Even before the tornado ravaged its campus in Jackson, Tennessee, I had a soft spot for Union, whose Chemistry Club is among the most active in the country. And how could a chemist not love a university that goes by the motto "A Catalyst for Change", and uses an Erlenmeyer flask as their Web site's key icon? Nevertheless I must correct the student.
Although I hope I never have to confirm this, I am sure that is what it felt like. But that is not what was happening. The student was actually being pushed out.
As far as I know, the student in question wasn't a troublesome member of some fraternity. He wasn't being pushed out by anyone or anything that he could see, but by the same air molecules that he had been comfortably breathing minutes earlier.
Just like a fish does not know what it's like to get wet, most of us are unaware of the latent energy that surrounds us. That's because, on a windless day, it is equally distributed in all directions. Lots of it!
As you may have heard on the Weather Channel, wind is caused by so-called pressure gradients - pressure is low in one place, high in another, and the wind goes to and fro, respectively. But what is pressure?
Pressure-sensitive Midwest farmers talk about "inches of mercury". But unless you've seen a wall-mounted barometer, with its 30-inch-tall column of liquid mercury (depending on the weather), it's hard to imagine how that is related to pressure, let alone energy.
Tire warnings may give you a better feel for pressure. They may read "do not inflate over 35 psi". As you learned in science class, "pressure is force over area" and psi means pounds per square inch. If your science teacher also told you that "2.2 pounds is equivalent to one kilogram", that might be why science seemed confusing. Kilograms are the metric unit of mass, but pounds are units of force, as your tire just told you.
A Coke can after being "hugged" by the air force around it when there were few molecules left inside to "push back". Credit: P. J. MacDougall
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Weight, also known as the "force due to gravity", is not the same thing as mass. Just ask an astronaut. With the help of a ruler, some trigonometry, and a calculator, you can convince yourself that there is a lot of force being exerted on the inside surface of your tire. If you drive a Ford Explorer, and were lucky to survive a tire "blow-out", you also know that there is lots of energy in there.
Pressure can also be considered as an energy density. In the metric system, normal atmospheric pressure is about 100,000 pascals, which is equivalent to 100,000 Joules of energy in each cubic meter of the air surrounding us. (That's about 25 Calories pre cubic meter for those on a very light diet.)
Oblivious to all the wars fought over it, energy tries to distribute itself evenly. You can get a sense of the power of the atmospheric air force by doing the following simple experiment. (Children must be supervised by an adult, and all should wear eye protection.)
Place a pan of cold water next to a stovetop heating element on high. Put a teaspoon of water in an empty Coke can, and place it on the pre-heated element. Let the water come to a boil, and wait until the steam is escaping rapidly. Then, using salad or BBQ tongs, quickly turn the can over, putting the top of the can just under the surface of the cold water.
What you will witness is literally a crushing air hug at Mach speed. As surprising as it is, this 300-pound display is a tiny fraction of the awesome and ever-present air force.
Preston MacDougall is a chemistry professor at Middle Tennessee State University. His "Chemical Eye" commentaries are featured in the Arts and Public Affairs portion of the Murfreesboro/Nashville NPR station WMOT (wmot.org).
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Comments
Greg Moody
05 Mar 2008, 03:08
Ok. So the air outside the can crushed the can because the void inside the
can was not exerting equal (or greater) pressure against the inside of the
can...but on the tornado where was the pressure being exerted? This study
http://docs.lib.noaa.gov/rescue/mwr/089/mwr-089-12-0533.pdf
seems to indicate that there is a considerable pressure differential that
could account for houses "exploding" but most current research points to
the extremely strong winds as the culprit for building demolition. So are
you saying the student was pushed out of the dorm because there was less
air molecules outside of the dorm than was inside of the dorm?
Respectfully,
Greg Moody
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