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!

Piston and spring

  1. Mar 20, 2010 #1
    1. The problem statement, all variables and given/known data
    Ok the problem I have been set has a frictionless piston containing steam at 200kpa at 200 degrees c and it is originally at 0.5m^3. It has a linear spring above it just touching and exerting no force. Heat is added and the gas expands pushing the cylinder up and causing the spring to exert a force that seeing as it is linear is proportional to the distance it is pushed up. Finally it stops at 0.6m^3, 500kpa and I have to work out work, temp and heat input.


    2. Relevant equations
    Ok so its neither isobaric, isochoric or isothermal and we have not been told that it's ideal so im assuming you could use that. (However just using ideal gas law to find temp yields wrong answer) Also I know W=P dv but that again gives the wrong answer as P is not constant.


    3. The attempt at a solution

    To find work i plotted it on a P-v diagram and found the area: [(200kpa+500Kpa)/2]*(0.6-0.5) = 35Kj which i know is the correct answer.

    I have tried using ideal gas law at both pressures to find T but it's wrong, actual answer is 1132 degrees c. Also I tried saying Qin = Wout + delta U and then reading U from the steam tables and rearranging to find Qin and therefore final temp maybe but it also didnt work. Please could someone help me with this, we are not given area or spring constant of the piston or spring just what I have stated.
     
  2. jcsd
  3. Mar 20, 2010 #2

    kuruman

    User Avatar
    Homework Helper
    Gold Member

    How do you know? How did you get the answer that you got for the temperature?
     
  4. Mar 20, 2010 #3
    The problem sheet we have been given has the answers on it, we need to show our working as it is that which is important.
     
  5. Mar 20, 2010 #4

    kuruman

    User Avatar
    Homework Helper
    Gold Member

    OK then, show your work. What value for the final temperature did you get and how exactly did you get it?
     
  6. Mar 20, 2010 #5
    My work is wrong, it gives me the wrong answer.

    P1V1=nRT1 / P2V2=nRT2

    P1V1/P2V2 =T1/T2 T2 = T1P2V2 / P1V1 and I get T2= 1146 degrees C which is Slightly too high.
     
  7. Mar 20, 2010 #6

    kuruman

    User Avatar
    Homework Helper
    Gold Member

    The formula that you are using is correct. Maybe you put in the numbers in the wrong place, maybe you did the calculation incorrectly. If you don't show exactly what you did and how you got the numbers, I cannot find where the problem is.
     
  8. Mar 20, 2010 #7
    I solved it, this is what I did:

    From the temperature we know it's superhated steam so looking at the steam tables we can find it's specific volume, 1/Specific volume = Density, density * volume = mass

    So now we have the mass of the steam in the system.

    Now mass / final volume = density ... 1/density = specific volume at the final state and now looking at the steam tables and interpolating you can find final temperature :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook