The Origin of Fluid Pressure

Before we consider the PVT behavior of pure gases let us pose the following question what do we really mean when we say that the pressure of a gas, say i-butane, is equal to some specified value  

To this purpose, let us consider the following mental experiment. A transparent cylinder, with a cross-sectional area of 10 cm, is fitted with a piston and immersed into a constant temperature bath at 420 K. The piston has a plate, on which we can place weights, and a volume indicator calibrated to read cm. We assume, for simplicity, that the piston is weightless and frictionless and that the outside pressure is zero. 

We now place one mole (58.124 grs) of i-butane in the cylinder, and the necessary weights on the plate, so that the volume becomes equal to some specified value, say 171 cm. We wait until thermal equilibrium is reached with the bath, adjusting the weights so that the volume of 171 cm is maintained. The final weight is 5.75-10 N, which - when divided by the piston area - gives a pressure of 57.5 bar. 

If we now increase the bath temperature, and thus that of i-butane, the kinetic energy of the molecules and, consequently, the force exerted by them when they strike the piston surface will also increase. Thus, if the volume is to be maintained constant, more weight must be added on the plate the pressure of the gas has increased. 

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Bredehoeft, J.D., Wesley, J.B. and Pouch, T.D. 1994. Simulations of the origin of fluid pressure, fracture generation, and the movement of fluids in the Uinta Basin, Utah. Am. Assoc. Pet. Geol. Bull., 78 1729-1747. 

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