Kinetic Theory : Cause Of Gaseous Pressure

It was shown by Joule in 1845 that if a gas is allowed to expand from one copper vessel into a second exhausted copper vessel, in such a way that it does no external work, it does not become appreciably warmed or cooled. He concluded that no appreciable work is done by, or against, forces of repulsion or attraction between the molecules, and hence that the molecules of gases exert practically no forces on one another.

The pressure exerted by the gas uniformly over the walls of the containing vessel must, therefore, be wholly kinetic in origin-it must be caused by molecular bombardment. On all parts of the surface there is a ceaseless hail of elastic molecules, which impinge on the surface and fly off again into the gas. Without going into detail one can see that this molecular bombardment, distributed over the surface, must appear to our coarse senses as a uniform pressure.

The molecules strike the wall at all angles, from a full normal blow to a glancing impact, and it is only the component of the velocity perpendicular, or normal, to the surface which is effective in producing pressure.

In the gas itself the molecules, since they exert practically no forces one upon another, will move in straight lines until they encounter the walls, or one molecule collides with another. The molecular collisions will occupy but a small fraction of the whole time in which the molecule is moving, because the particles are sparsely distributed, except in highly compressed gases.

One c.c. of water gives 1240 c.c. of vapour at 100° and 760 mm. pressure, so that less than one-thousandth of the whole space of the vapour is occupied by the volume of the molecules. In air at 0.001 mm. pressure, the molecules occupy only about 1 part in 580 millions of the total space.

The molecules may have all possible speeds from zero to infinity, but Clerk Maxwell (1859) showed that the speeds of the majority of molecules in a gas at a given temperature differ only slightly from a mean speed, denoted by Ω. The ordinates of the curve in

Fig: Distribution of molecular speeds in a gas

represent the fractions of the molecules which have speeds represented by the abscissae. It will be seen that molecules very rapidly become scarcer which have speeds deviating appreciably from the mean speed. If we follow any molecule along its zigzag path, we shall therefore find that they all describe these with an almost constant speed, Ω. The component velocities, of course, fluctuate repeatedly as the molecules undergo collisions, but the speed along the path of motion is nearly uniform the whole time.

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