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Thermodynamics Work from pressure

  1. Feb 2, 2017 #1
    1. The problem statement, all variables and given/known data
    The tires on a bicycle require an air pressure of 80 psig. When isothermally pumped up the bicycle tires, the volume of the air (that was originally in the atmosphere) is reduced by a factor of 5.7. Please determine the work that must be done on each lbm of air that is pumped into the tires. Please report your answer in lbf-ft/lbm.
    .

    2. Relevant equations
    W=nRT*ln(V2/V1)
    Pv=nRT

    3. The attempt at a solution
    My attempt might be very wrong, but this is what i tried.
    I simplified Pv=nRT to T=PV/nR
    Plugged that into W=nRT*ln(V2/V1)
    W=PV*ln(V2/V1)
    I know the standard pressure (14.7psi) and the pressure inside the tire (80psig+14.7psi=94.7psi), but I don't know the volume of to plug in for the V.
     
  2. jcsd
  3. Feb 2, 2017 #2
    Use nRT, with T equal to room temperature.
     
  4. Feb 2, 2017 #3
    What do I use for n?
     
  5. Feb 2, 2017 #4
    For isothermal compression P1/P2 =V2/V2, instead of using volume, you can use pressure ratio
     
  6. Feb 2, 2017 #5
    You are trying to get the work per pound mass. So you don't need to know n. You are solving for W/n.
     
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