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Well if a molecule getting trapped behind a pump is random, How likely is a molecule to randomly fly into a pump? That depends on how big the tube is leading to the pump.
Tube Size and Conductance
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Ok, so you know how I said molecules bounce off walls? Not actually true! 😬 I lied to keep it simple, but I think you're ready for the truth: It turns out that any time a gas the only way wall-molecule interaction plays out. most of the time a gas time any gas molecule hits a surface it actually sticks to that surface, just if only temporarily, like how your breath fogs the a mirror; this called adsorption (not to be confused with absorption).
And also just like fog on a mirror vanishes over time, those stuck molecules will eventually jump back off of the surface (i.e. evaporate, outgas, desorb), flying of off in a random direction just as fast as it was when it hit the surface..
How long a molecule stays stuck before desorbing from a surface depends on the type of molecule, the temperature, and the what the surface is made of. Evaporation of a molecule can happen any time between almost instantly and basically never. And we'll never be a able to predict the moment exactly, but we usually have a general idea for how long, based on things like temperature, pressure, and material.
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The trouble for lowering pressure comes when a given type molecule jumps around enough to raise the pressure, but not enough to easily find it's way into the pump. Check out the Application of Vacuum Theory page to see how we deal with troublesome molecules.
Here is a presentation created by Dan Peterswright that introduces some vacuum concepts and walks through Gas Laws. It also offers a few examples that illustrate the effects of pressure differentials created by establishing vacuum in flexible couplings and brief examples that illustrate the related gas laws.
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