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Vacuum basics

  1. Nov 2, 2013 #1
    Hi all, i was pottering away at work yesterday and realised just how little i truly understand about vacuum!! So i have a question, i have a piston in a cylinder, the piston is fully in the cylinder so there is very little air space and the cylinder port is blocked off, the piston has an area of 1 square inch.
    Now my mind says that due to atmospheric pressure there is 14.7 pounds of force on the back of the piston so if i apply a force in the opposite direction to atmoshperic pressure (i.e trying to pull the piston out) of more than 14.7 pounds it should move out generating a higher and higher vacuum but obviously it doesn't?!? So why not?

    i can logically see that of the cylinder had a perfect vacuum inside it would be hard to define how that nothingness could expand to take up the extra space but... as you pull on the piston and it starts to form a vacuum in the cylinder it becomes near enough impossible to pull the piston any further yet it still only has 14.7lb of force pushing against it??

    Please help my confused brain!
     
  2. jcsd
  3. Nov 2, 2013 #2

    UltrafastPED

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    Pistons generate a poor vacuum because they leak ... plus the lubricants degrade the vacuum, depending on their vapor pressure.

    When taking flanges off of my ultra-high vacuum chamber (10^-10 Torr) I had to wait until the pressure was almost equalized inside/outside before I could pull off an 8" flange - about 50 square inches, for a total force of 740 pounds required earlier.

    In your example you would have to pull the piston with 14.7 pounds of force: like picking up a bowling ball, but unless there is a convenient handle you also have to provide a force to grab the rod!

    You can also think about the very early vacuums that were generated with mercury in a narrow glass tube:

    This is interesting: http://tinyurl.com/ltvuks5
    And this: http://www.m-p.co.uk/muk/acrobat/0796000115.pdf
     
  4. Nov 2, 2013 #3
    ok let me rephrase, ignoring seal losses :) there is no airspace between the cylinder and piston (we can evacuate it with another pump first to check). It would take more than 14.7 lb of force to move the piston and what would we have when it did move? i know weird things happen under high vacuum and things degas etc but as theres a limit of atmoshperic pressure trying to keep the piston in place surely if we pulled with say 15lb of force the piston would just keep moving and the vacuum would keep getting purer and purer?
    This is theoretical question so the cylinder cant collapse its very strong:P

    What im trying to understand is that as you say 14.7lb of force is like picking up say a bowling ball which isnt hard but during the pistons stroke the vacuum got to a point were i could no longer pull it why?
     
  5. Nov 2, 2013 #4

    UltrafastPED

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    Unless there is some other mechanical impediment (like a piston rod still connected to the crankshaft!) - the force opposing your effort is [ambient air pressure - piston pressure]*piston area.

    So there must be an additional impediment. And what is happening on the inside of the piston? The gas and vapor trapped inside is becoming more dilute, minus any out-gassing from the piston & cylinder. But the only force is due to the pressure differential, which can never exceed ambient: about 14.7 psi.


    This general problem is discussed in some detail here: http://rogercortesi.com/ideas/public/gasspring.html
     
  6. Nov 2, 2013 #5
    Thank you for a fantastic link:)
     
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