1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Calculating pressure fromV1 to V2 with a polytropic exponent

  1. Nov 6, 2011 #1
    1. The problem statement, all variables and given/known data

    I have a bicycle pump where I need to calculate the pressure in a certain volume. No heat is lost during compression so this is a isentropic system

    initial volume is 0.3L
    final volume is 0.0195
    Gas is air

    2. Relevant equations
    I don't know, that's the problem. I recognise this as a fairly simple question but I just don't know

    3. The attempt at a solution

  2. jcsd
  3. Nov 6, 2011 #2

    I like Serena

    User Avatar
    Homework Helper

    Welcome to PF, metiz1! :smile:

    An adiabatic process (for an ideal gas) has [itex]P V^\gamma = constant[/itex], where [itex]\gamma = {7 \over 5}[/itex] for air (as a diatomic ideal gas).
    Combined with the initial pressure as standard pressure, you can calculate the final pressure.
  4. Nov 6, 2011 #3
    Thank you for your reply.

    I can't say I realy understand your reply though...Shouldn't I enter the initial temperature (lets say 20C, 293K) somewhere in the equation?
  5. Nov 6, 2011 #4

    I like Serena

    User Avatar
    Homework Helper

    No, you don't need the temperature.

    Let me rephrase:
    [tex]P_{initial} (V_{initial})^{7 \over 5} = P_{final} (V_{final})^{7 \over 5}[/tex]

    Solve for [itex]P_{final}[/itex].

    You can find the formula for instance here:
    (Shouldn't it be in your notes or something? :confused:)
  6. Nov 6, 2011 #5
    Thanks for your help! I had to use a hypotetical situation (n=1.4) for my calculations and see how the real word measurements stacked up....The n value I got was like 0.8....I dun goofed the measurement I think :P
  7. Nov 6, 2011 #6

    I like Serena

    User Avatar
    Homework Helper

    Hmm, I just realized... you're talking about a pump.
    I suppose that means the amount of air changes?
    Kind of relevant, since the formula only works when the amount of air remains constant...
  8. Nov 6, 2011 #7
    Yes you are right, however, in this situation I had to asume all the air was being compressed in a smaller volume withouth any air or heat escaping, so all is good.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook