force and return promptly to its original conditionwhen the force is removed. These are theproperties of air that is used in pneumatic tires,tennis balls and other deformable objects whoseshapes are maintained by compressed air.KINETIC THEORY OF GASESIn an attempt to explain the compressibilityof gases, Bernoulli proposed the hypothesis thatis accepted as the kinetic theory of gases.According to this theory, the pressure exerted bya gas on the walls of a closed container is causedby continual bombardment of the walls bymolecules of the gas.Consider the container shown in figure 11-2as containing a gas. At any given time, somemolecules are moving in one direction, some aretraveling in other directions; some are travelingfast, some slow, and some may even be in a stateof rest. The average effect of the moleculesbombarding each container wall corresponds tothe pressure of the gas.As more gas is pumped into the container,more molecules are available to bombard thewalls; thus the pressure in the container increases.The gas pressure in a container can also beincreased by increasing the speed with which themolecules hit the walls. If the temperature of thegas is raised, the molecules move faster causingan increase in pressure. This can be shown byconsidering the automobile tire. When you takea long drive on a hot day, the pressure in the tiresincreases and a tire which appeared to besomewhat “soft” in cool morning temperaturemay appear normal at a higher midday tempera-ture.BOYLE’S LAWWhen the automobile tire is initially inflated,air which normally occupies a specific volume iscompressed into a smaller volume inside the tire.This increases the pressure on the inside of the tire.Charles Boyle, an English scientist, was amongthe first to experiment with the pressure-volumerelationship of gas. During an experiment whenhe compressed a volume of air he found that thevolume decreased as the pressure increased, andby doubling the force exerted on the air he coulddecrease the volume of the air by half. See figure11-3. Recall from the example of the automobiletire that changes in temperature of a gas alsochange the pressure and volume. Therefore, theexperiment must be performed at a constanttemperature. The relationship between pressureand volume is known as Boyle’s law. It states:When the temperature of a gas is kept constant,the volume of an enclosed gas varies inversely withits pressure.In equation form, this relationship may beexpressed as eitherorEquation 11-6where V1 and P1 are the original volume andpressure, and V2 and P2 are the final volumeand pressure (P1 and P2 are absolute pressures).Figure 11-3.-Gas compressed to half its original volume byFigure 11-2.—Molecular bombardment creating pressure.11-4a doubled force.
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