PRESSURES AND GASES Pressures can be determined and are described in liquids and gases alike. With gases, there is pressure exerted on everyone by the atmosphere but it is something we have become accustomed so it is generally ignored. Water pressure, as is seen with swimming, is greater than air pressure and is something that can be felt when moving through water. As mentioned, pressure is the force divided by the area on a firm substance against it. When a balloon is blown up, the balloon expands because its pressure is greater than atmospheric pressure against the walls of the balloon. When the balloon pops, the air flows from an area of high pressure to an area of low pressure. In water pressure, the pressure increases as one goes deeper, and in air pressure, the pressure decreases at high altitudes. At 20,000 feet above sea level, air pressure is cut in half because half of the whole atmosphere of the earth is above and below this level. At sea levels, the atmospheric pressure is about 14.7 pounds per square inch. Car tires need a pressure of about 30 pounds per square inch. Bicycle tires have a pressure of about 60 pounds per square inch or psi. Standard atmospheric pressure at sea level is exactly 760 millimeters of mercury of mm Hg; 760 torr is one atmosphere, which is the air pressure arbitrarily set at sea level. One atmosphere is about 1 x 105 Pascal (which is the SI unit for pressure---the same as about 1 kilogram per meter per second squared). In reality, one pascal is equivalent to one newton (1 N) of force applied over an area of one meter squared (1 m2). The barometric pressure is measured in mm Hg (or millimeters of mercury). The barometer was invented by Evangelista Torricelli in the early 1600s, and uses mercury as the standard of weight. As you have seen by the U-shaped tube in figure q6, the mercury is put into a U-shaped tube and is left open on one end to measure the pressure of a certain gas against the atmospheric pressure. When measuring the atmospheric pressure force equals mass times acceleration. The acceleration is based on the Earth’s gravitational pull, which is about 9.8 meters per second squared.
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