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: Thermal Properties: Pressure, Volume, Temp question

  1. Apr 22, 2009 #1
    1. The problem statement, all variables and given/known data

    A Jaguar XK8 convertible has an eight-cylinder engine. At the beginning of its compression stroke, one of the cylinders contains 499 cm cubed of air at atmospheric pressure (1.01E5 Pa) and a temperature of 27.0 degrees C. At the end of the stroke, the air has been compressed to a volume of 46.2 cm cubed and the gauge pressure has increased to 2.72E6.

    What is the final temperature

    2. Relevant equations

    P1*V1/T1 = P2*V2/T2

    3. The attempt at a solution

    I followed the equation that I posted above. Here is what it looked like.

    (1.01E5 * 499 cm cubed)/ 27 = (2.72E6 * 46.2)/x

    Solving for x, I got x = (2.72E6*46.2)/1866629.63 = 66.8 degrees C

    Problem is, that answer is apparently wrong. Am I missing something here?

    Thanks for the help!
  2. jcsd
  3. Apr 22, 2009 #2
    Try changing your temperature to kelvin :)
  4. Apr 22, 2009 #3
    well it wants the answer in celcius. Does that equation have to be done with Kelvin units?
  5. Apr 22, 2009 #4
    Were you given that equation? As you wont find T2 like that. Even if you use kelvin as the pv/pv is a ratio, it wont make any difference.

    Hint: Expansion and compression is isentropic.
  6. Apr 23, 2009 #5
    No I wasn't given any equation. I thought that that one would work though because I was only missing one variable.

    Do I use pv=nrt?
  7. Apr 23, 2009 #6
    If you've got a book, look up isentropic compression.

    pv^gamma = constant.

    where gamma is the polytropic constant you'll have to find the relationship between temperature and anther variable.
  8. Apr 23, 2009 #7
    Thats the Adiabatic process.

    Calculate the mass of air in the cylinder from pv=mRT

    Constants for air Cp=1005J/kgK, Cv=718J/kgK R=287J/kgK, calculate gamma from these.

    Reaarange the equation for an adiabatic process to find T
  9. Apr 23, 2009 #8
    its both acutally, as isentropic is adiabatic and reversible. to find the temperatre after compression you dont need the mass in the cylinder as it comes out in the wash.

    the relationships for isentropic processes for ideal gases are listed in any thermo textbook. but as this is homework help im not going to just give them out.
    Last edited: Apr 23, 2009
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook