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Homework Help: Filling the wheel with air (ideal gas)

  1. Jan 23, 2012 #1
    1. The problem statement, all variables and given/known data
    A wheel has volume 1.2 x 10-3 m3 when being full. A pump has working volume of 9 x 10-5 m3. How many times do you need to push the pump to fill the air to the wheel (initially there is no air in the wheel) until the pressure is 3.0 x 105 Pa? The atmospheric pressure is 1.0 x 105 Pa and there is no change in temperature

    2. Relevant equations
    PV/T = constant

    3. The attempt at a solution
    I think my attempt is really really weird, but I'll post it anyway....

    P1.V1 = P2.V2
    1.0 x 105.V1 = 3.0 x 105 . 1.2 x 10-3
    V1 = 3.6 x 10-3

    ΔV = V1 - V2 = 2.4 x 10-3

    Number of pumping = 2.4 x 10-3 / 9 x 10-5 = 80 / 3 ≈ 27 times

    Actually, my idea is to find the initial volume of air in the wheel. The difference between the initial and final volume is the volume needed to be pumped to the wheel. But I think the final volume should be greater (my calculation shows the opposite). At the beginning, I take the pressure of the wheel the same as atmospheric pressure.

    Another thing that bothers me is the sentence "initially there is no air in the wheel". It means the initial volume is zero ????
  2. jcsd
  3. Jan 23, 2012 #2

    Determine how much air (density*volume) is in the tire at specified pressure.
    Last edited: Jan 23, 2012
  4. Jan 23, 2012 #3
    Hi. I have the same idea with you.
    The volume of the wheel is 1,2E-3
    The volume of the pump is 9E-5
    The pressure requested on the wheel is 3E5 and
    The output pressure of the air is 1E5
    since the internal pressure have to be equal to the external pressure,
    P1V1 = P2V2 and than
    3E5*1,2E-3=1E5*V ==> from there we got V=3,6E-3 this have to be the volume of the pump to fill the wheel with air at just one push. However, our pump's volume is 9E-5 so we need to devide it.
    (3,6E-3)/(9E-5)=40 That means we have to push our pump for 40 times to obtain the result.
    I think you do not need to make any substraction since it says the wheel is initially empty.
  5. Jan 23, 2012 #4
    At P = 1.0 x 105, the volume is 3.6 x 10-3. Taking density of air = 1 kg m-3, the mass is 3.6 x 10-3 kg

    AT P = 3.0 x 105, the volume is 1.2 x 10-3 so the mass is 1.2 x 10-3 kg

    Then what should I do?

    3.6 x 10-3 is the volume at 1.0 x 105 Pa? How can that be the volume needed to be pumped to the wheel while the question states to find the number of pumping when the pressure is 3.0 x 105?

    When the wheel is empty, is the pressure the same as atmospheric pressure?

  6. Jan 24, 2012 #5
    ohh..I think i misunderstand something. due to rush, it may be wrong. sorry for wasting your time.
  7. Jan 24, 2012 #6
    Another hint:

    Write an expression for the mass of air in the inflated tire. It will be a function of temperature and pressure. You do not know the temperature.

    Write another expression for the mass of air in the pump cylinder at atmospheric pressure. It will be a function of temperature and pressure. You do not know the temperature.

    The problem states the entire process is isothermal.
    Last edited: Jan 24, 2012
  8. Jan 25, 2012 #7
    Not sure how to do it but here's what I've done:
    *inflated tire
    P1V1 = n1RT1 ; n = m/M where M is molecular mass
    P1V1M = m1RT1
    m = P1V1M / RT1 --> here the mass is the function of pressure, volume and temperature

    *pump cylinder
    got the same as above; m2 = P2V2M / RT2

    Am I correct this far?
  9. Jan 25, 2012 #8
    Yes, so far so good. Now you have an expression for the mass in the tire when inflated. You also have an expression for the mass that is delivered by a single pump stroke. You know the process is isothermal........
  10. Jan 26, 2012 #9

    m1 = P1V1M / RT1
    1 / T1 = m1 R / P1 V1 M

    m2 = P2V2M / RT2
    1 / T2 = m2 R / P2 V2 M

    Isothermal : T1 = T2
    m1 R / P1 V1 M = m2 R / P2 V2 M

    m1 / P1 V1 = m2 / P2 V2

    Is P1 = 3 x 105 , V1 = 1.2 x 10-3 , P2 = 1.0 x 105 , V2 = 3.6 x 10-3 ?

    If yes, the number of pumping = m1 / m2 and I got 40 as the answer
  11. Jan 27, 2012 #10
    Your answer is correct but you typed

    "V2 = 3.6 x 10-3"

    when it should have been

    V2=9.0 x 10-5
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