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What is meant by no gas enters or leaves the cylinder ?

  1. Oct 8, 2012 #1
    What is meant by "no gas enters or leaves the cylinder"?

    I have the following question.

    A platform is supported on a piston by compressed air in a cylinder.
    The mass of the platform and equipment being lifted is 1100kg. The pressure needed to
    support this mass is 800kPa, and it rests initially at a height of 0.5m. The pressure supply
    is closed so that no gas enters or leaves the cylinder
    . One of the pieces of equipment with a mass of 50kg is moved o the platform. The entire system is at room temperature (300K)

    I am confused with the significance of the bolded line. I'm not sure whether it's implying constant volume, constant pressure, temperature, etc. Can someone explain this?
     
  2. jcsd
  3. Oct 8, 2012 #2
    Re: What is meant by "no gas enters or leaves the cylinder"?

    P*V=n*R*T

    from question; P dont change, and there is a finite amount of gas in chamber (i.e. n dont change)

    so P dont change, n dont change (and R is a const. so it dont change), then what else could change (hint. if T change, then V change)
     
  4. Oct 8, 2012 #3
    Re: What is meant by "no gas enters or leaves the cylinder"?

    Would the area change in this case? Because I need the area to find the volume, and according to my calculations it seems like it should, but it also seems a bit weird for the area of a platform to change
     
  5. Oct 9, 2012 #4

    ehild

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    Re: What is meant by "no gas enters or leaves the cylinder"?

    No, the area of the piston does not change. But the piston can move up or down. Some weight is removed from the platform: that means the external force decreases. To get into equilibrium at the same temperature (300 K) what parameter of the gas has to change?
    Initially the internal and external forces acting on the piston are in equilibrium: The weight of the 1100 kg load is balanced by he force the gas exerts when its pressure is 800 kPa. So what is the area of the piston?

    ehild
     
    Last edited: Oct 9, 2012
  6. Oct 9, 2012 #5
    Re: What is meant by "no gas enters or leaves the cylinder"?

    sorry the question is

    what is the new position of the platform?

    so i was referring to the area of the platform, not the piston.
     
  7. Oct 9, 2012 #6

    ehild

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    Re: What is meant by "no gas enters or leaves the cylinder"?

    The platform is supported on the piston. What does it mean?

    ehild
     
  8. Oct 9, 2012 #7
    Re: What is meant by "no gas enters or leaves the cylinder"?

    If the area is unchanging can we simplify the equation PV=nRT become

    h=T since everything else is a constant or unchanging?

    How would this relate to the mass though?

    edit:i meant h=t when comparting the two heights so for this case

    0.5m/300K = height2/T2?
     
  9. Oct 9, 2012 #8

    ehild

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    Re: What is meant by "no gas enters or leaves the cylinder"?

    The temperature does not change. The pressure does.

    ehild
     
  10. Oct 9, 2012 #9
    Re: What is meant by "no gas enters or leaves the cylinder"?

    Oh so according to the equation PV = nRT the only variables that change are P and V (since h changes)?

    I'm a bit confused because I thought the above poster told me that pressure remains constant.
     
  11. Oct 9, 2012 #10

    ehild

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    Re: What is meant by "no gas enters or leaves the cylinder"?

    Yes.
    He was mistaken.

    Think. What does it mean that some of the load is removed?

    ehild
     
  12. Oct 9, 2012 #11
    Re: What is meant by "no gas enters or leaves the cylinder"?

    If the load is removed then the height will increase.

    So iare there any errors in the following solution?

    find area using Pi=mig/A
    A*h=Vi

    find Pf using Pf = mfg/A

    PiVi = PfA*h to solve for the unknown height?

    thanks.
     
  13. Oct 9, 2012 #12

    ehild

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    Re: What is meant by "no gas enters or leaves the cylinder"?

    It is correct if the whole thing is in vacuum. Otherwise the 800 kPa is the gauge pressure, and the absolute pressure of the gas is 800 kPa +atmospheric pressure. As it was not mentioned, I think your solution is correct.


    ehild
     
  14. Oct 9, 2012 #13
    Re: What is meant by "no gas enters or leaves the cylinder"?

    ok thanks alot for the help
     
  15. Oct 9, 2012 #14

    ehild

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    Re: What is meant by "no gas enters or leaves the cylinder"?

    You are welcome.

    ehild
     
  16. Oct 9, 2012 #15
    Re: What is meant by "no gas enters or leaves the cylinder"?

    sorry.

    just one last question

    the question asks for the maximum velocity

    according to W=PdV
    W should remain constant

    and then using W=.5mv^2

    wouldn't that mean you use mf since m is inversely proportional to v and m is smalest at mf?
     
  17. Oct 9, 2012 #16

    ehild

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    Re: What is meant by "no gas enters or leaves the cylinder"?

    Write the original question, please.

    ehild
     
  18. Oct 9, 2012 #17
    Re: What is meant by "no gas enters or leaves the cylinder"?

    A platform is supported on a piston by compressed air in a cylinder.
    The mass of the platform and equipment being lifted is 1100kg. The pressure needed to
    support this mass is 800kPa, and it rests initially at a height of 0.5m. The pressure supply
    is closed so that no gas enters or leaves the cylinder. One of the pieces of equipment with a mass of 50kg is moved o the platform. The entire system is at room temperature (300K)

    A)what is the height of the platform after the weight is removed?
    B)what is the maximum velocity?
     
  19. Oct 9, 2012 #18

    ehild

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    Re: What is meant by "no gas enters or leaves the cylinder"?

    The gas exerts force and does work on the platform. That work increases both the potential and kinetic energy. The kinetic energy is maximum at the equilibrium height you calculated before, when the resultant force on the platform is zero, that is, the acceleration is zero. The platform will rise above the equilibrium height till the velocity becomes zero; after that, it moves backwards (ignoring friction).

    ehild
     
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