The Relationship Between Volume and Pressure Boyles Law

The generalization of Boyle s observations is known as Boyle s law at constant temperature, the volume occupied by a fixed amount of gas is inversely proportional to the applied (external) pressure, or 

The constant is the same for the great majority of gases. Thus, tripling the external pressure reduces the volume to one-third its initial value halving the external pressure doubles the volume and so forth. 

The wording of Boyle s law focuses on external pressure. In his experiment, however, adding more mercury caused the mercury level to rise until the pressure of the trapped air stopped the rise at some new level. At that point, the pressure exerted on the gas equaled the pressure exerted by the gas. In other words, by measuring the applied pressure, Boyle was also measuring the gas pressure. Thus, when gas volume doubles, gas pressure is halved. In general, if Fg s increases, Fgas decreases, and vice versa. 

One question raised by Boyle s work was why the pressure-volume relationship holds only at constant temperature. It was not until the early 19 century, through the separate work of the French scientists J. A. C. Charles and J. L. Gay-Lussac, that the relationship between gas volume and temperature was clearly understood. 

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