MadSci Network: Chemistry

Re: how does air exert pressure in all directions?

Date: Sun May 6 19:15:01 2001
Posted By: Charles Riner, Secondary School Teacher, Science, Memorial Day School
Area of science: Chemistry
ID: 987983724.Ch

Claire, Thank you for your very good question. As you know, air is a mixture of gases, principally nitrogen and oxygen. Gases, along with liquids, are referred to as 'fluids'. In physics, the study of the pressure exerted by fluids which are not moving is called hydrostatics. As for pressure itself, the definition is fairly straightforward: Pressure is force per unit area, P = F/A. The pressure exerted by the atmosphere is considerable. Standard pressure is 14.7 lbs/in2 (psi). This is equivalent to 1 atmosphere = 760.0 mm Hg = 101, 325 pascals (Pa) = 29.92 in Hg. This pressure would support a column of water 10.3 meters or 33.8 feet in height! Even though I have used this demonstration for years, your question caused me to re-visit the fundamentals of hydrostatics and this particular demonstration. We teachers tend to go on auto-pilot sometimes and don't think! Only when we have a student who is willing to question are we forced back to the "books". In this case, back to the "lab" (my kitchen sink!). I tried several vessels (vial, drinking glass, and an orange juice storage jar) filled with water, somewhere between half-filled and filled, approximately half-filled, less than half-filled and "empty", and "empty". I also used an index card in one set of trials and a cardboard square in a second set (excluding the vial). Further, I turned the vessels mouth downward and slowly through all angles from mouth downward to mouth upward. From my "experiments", I conclude that the pressure in the vessels regardless of the position was always less than the pressure outside the vessels as the index card/cardboard square stayed put [the cardboard square did come off once in the mouthdown position, but it was a water leak problem]. So, it would seem to me that the outside air pressure is greater than the pressure exerted by a vessel [at least up to a liter in volume] filled with water or water and air in any proportion. After the "experiment", I then looked at some references: [Arthur Beiser, Ph. D., APPLIED PHYSICS, 3rd Ed., p. 194, Schaum's Outline Series/McGraw-Hill, Inc., New York, 1995.] Dr. Beiser gives the following summary about the pressure in a fluid: "(1) The forces that a fluid exerts on the walls of its container, and those that the walls exert on the fluid, always act perpendicular to the walls; (2) The force exerted by the pressure in a fluid is the same in all directions at a given depth; (3) An external pressure exerted on a fluid is transmitted uniformly throughout the fluid. This does not mean that pressures in a fluid are the same everywhere, because the weight of the fluid itself exerts pressures that increase with increasing depth. The pressure at a depth h in a fluid of density d due to the weight of fluid above is p = dgh. Hence the total pressure at that depth is p = p (external) + dgh. When a body of fluid is in an OPEN container, that atmosphere exerts an external pressure on it." [Alvin Halpern, Ph. D., BEGINNING PHYSICS I, p. 337, Schaum's Outline Series/McGraw-Hill, Inc., New York, 1995.] Dr. Halpern makes these statements which seem relevant to your question: "The pressure exerted by the air at any given location is directly due to the bombardment of molecules and not directly due to the push of the air from above. This can be seen by considering an empty paper cup sitting on a table. If a steel plate is put across the top of the cup, there is no contact between the air above the cup and the air in the cup. Yet, the pressure of the air in the cup remains the same (otherwise the sides of the cup would get crushed in by the pressure of the air outside)." In summary, I would say continue to question. Use caution in drawing a conclusion from one experiment, but do multiple trials and variations. I hope the answer helped. Keep up the good work!

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