Ideal gas basics

Gases are a form of matter in which the individual molecules are free to move independently of one another except for occasional collisions. Most of the time the individual molecules are in free flight out of the range of influence of their neighbors. Gases differ from liquids and solids in that the force between neighbors (on the average over time) is very weak, since the intermolecular force is of short range compared to the typical intermolecular distances for the individual gas molecules.

If an imaginary plate is held vertically in a gas as shown in Figure 2.1, there will be a force exerted on the thin plate from each side. The forces on opposite sides of the inserted plane are equal; otherwise, if forces on the opposing sides were unbalanced, the plate would experience an acceleration. The force on the left side of the plate is caused by the reflection of molecules as they hit the left face of the plate and rebound. These impulsive forces are so frequent that the resulting macroscale force is effectively steady. The force is perpendicular to the face and has the same value no matter how the face is oriented. This can be seen by considering the collisions with the wall and the tendency for no momentum to be transferred parallel to the plane surface. The perpendicular component of the force per unit area on the plane is called the pressure. Tangential components of the force cancel out (when averaged over many collisions with the wall) and therefore vanish when averages are taken over a large number of collisions with the surface.