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Homework Help: Cylinder gas, pressure question

  1. Apr 20, 2009 #1
    http://img15.imageshack.us/img15/9746/21478397.png [Broken]

    I know how to setup the entire problem i just dont know how to calculate the new pressure after the piston is compressed.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 21, 2009 #2


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    PV = nRT

    n, R, T remain the same. They give you everything else.
  4. Apr 21, 2009 #3
    how do i calculate the number of moles?
  5. Apr 21, 2009 #4
    maybe p1v1/t1 = p2v2/t2 is more appropriate?
  6. Apr 21, 2009 #5
    i dont think so, this is actually from my physics class and we never covered that
  7. Apr 21, 2009 #6
    ok fair enough
    just out of interest is the answer 234375 Pa?
  8. Apr 21, 2009 #7


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    What Rory is saying is that PV = nRT is the general equation but nRT and are constant
    So p1v1 = p2v2, rearrange a little and you have the answer
  9. Apr 21, 2009 #8
    not sure, has yet to be handed in
  10. Apr 21, 2009 #9
    ahhh yes even more simple! forgot the temperature remained constant!
    ok fair enough, would you be as kind enough to tell us the answer as i am revising for my physics A-level at the minute so all questions are useful!!!
  11. Apr 21, 2009 #10


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    The volume goes from 0.750 -> 0.480 m^3, the pressure starts at 1.50 x10^5 Pa
    so the final pressure is (0.750/0.480) * 1.50 x10^5 Pa = 2.34 x10^5 pa

    You understand why your answer is wrong?
  12. Apr 21, 2009 #11
    lol thats the answer i got, except yours is in standard form! lol
  13. Apr 21, 2009 #12


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    This is a 'physics' A level not maths
    2.34 x10^5 is correct 234375.000000000000000000000 pa is wrong - whatever your calculator says!

    Hint, whats the precision of the initial pressure measurment
  14. Apr 21, 2009 #13
    oooohhh slightly harsh!!
    but never the less thats fair enough!
    no wonder my physics teacher is always annoyed with me for leaving whole answers!!!
  15. Apr 21, 2009 #14
    yea this is the correct way to do it, thanks
  16. Apr 21, 2009 #15
    http://wug.physics.uiuc.edu/cc/IAState/Phys221/spring/homework/written%20homework/index.html [Broken]

    have at it.
    Last edited by a moderator: May 4, 2017
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