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Find the change of internal energy of the air.

  1. May 23, 2015 #1
    1. The problem statement, all variables and given/known data
    A car rubber has the volume 50 liters when it is bloated in a pressur 1.8 atm and in a temperature 293 K. After some hours of journey as an effect of friction the pressure will become 2atm. Find the change of internal energy of the air inside the rubber (U)

    2. Relevant equations
    U=3/2nRT

    3. The attempt at a solution
    I thought to sue the formula U=3/2nRT, but I don't know the n.
     
  2. jcsd
  3. May 23, 2015 #2

    TSny

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    Don't forget the ideal gas law. Also, in the formula for U, are you sure the fraction is 3/2? Nitrogen and oxygen are diatomic gases.
     
  4. May 24, 2015 #3
    Ok but how can I find the U with this compound of gases like air?
     
  5. May 24, 2015 #4
    I am writing my solution: pV=nRT=>n=(1.8*10^5*0.05)/(8.31*293)=3.7
    1.8/293=2/x=>x=325.5555
    U= 5/2* 3.7*8.31*(325.56-293)=2500, but in my book the answer is 2537J
     
  6. May 24, 2015 #5

    TSny

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    Looks good. Some of the data in the problem is given to only 2 significant figures. So, the textbook should have rounded the answer to 2 significant figures which agrees with yours.
     
  7. May 25, 2015 #6
    If you use 1.01325 x 105 for atm you find a better number. Your calculation is correct.
     
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