Partial vs. Total Pressure
In the Vacuum Technology module, we treated reaction chamber pressure as a total gas pressure because we considered only the venting gas N2.  We did a similar thing so far in the Gas Flow module, where we have worked only with a single reactive gas species SiH4.  What happens when more than a single gas is involved, for then the total pressure alone cannot tell us the composition of the gas, nor how much of it is chemically reactive?  Clearly a more precise notion is required, which is that of partial pressure.

Partial pressure is the component of total pressure associated with a specific gas species, while the total pressure is the sum of partial pressures for all gas species contributing in a particular location where the pressure is measured.  In both cases, pressure means a force per unit area, i.e., the force associated with collisions of gas species against a solid surface, where the partial or total pressure of the gas (and its temperature) determines the concentration of molecules and the momentum associated with each molecule, and consequently the impingement rate at which the molecules hit a surface to cause the force.

Example:
Suppose we construct a method to introduce SiH4 into a chamber at 5 torr total pressure, and the remove it.
Then we develop a method to introduce N2 into a chamber at 10 torr total pressure, then remove it.
If we use these methods to introduce both species at the same time, the mixture in the chamber will then produce 15 torr total pressure, comprised of 5 torr SiH4 partial pressure and 10 torr N2 partial pressure.