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Homework Help: Work Done By Friction on a Piston, and Change in Enthalpy Help Please!

  1. Jan 27, 2010 #1
    Hi, I have an assignment due tomorrow and have been working for hours and hours on it, and have done all of the questions pretty much, except for these 2 parts of questions, since it is due tomorrow morning if someone can help me out that would be great! Here they are:

    1.Consider a system consisting of a cylinder containing 0.2 kilomoles of an ideal gas and fitted with a massless piston of area 0.5m2. The force of friction between the piston and the cylinder is 10 N. The gas is initially at a pressure of 1atm and the system is to be maintained at 300 K. The volume of the system is slowly decreases by 10 percent by an external force.

    The question asks to find the configuration work done on the system, which I found, and it also asks to find the work done on the system by the external force and the dissipative work done on the system. I believe the work done on the system is the configuration work plus the dissipative work, but I can't figure out how to find the dissipative work. I know the force of friction, and I know W=FD, but we don't know the distance, but we know the new volume is 10% less, and I know the area...but this doesn't help with finding the distance since I don't know the height of the piston. Also I am not sure if I am suppose to integrate the FD, because I think the force changes with respect to the distance since as you push more more of the piston comes in contact with the cylinder...can someone please help! :)

    2. A steam turbine receives a steam flow of 5000 kg/h and its power output is 500 KW. Neglect any heat loss from the turbine. Find the change in specific enthalpy of the steam flowing through the turbine if a) if the entrance and exit are at the same elevation and entry and exit velocities are negligible (I found this answer already) and b) if the entrance velocity is 60m/s and the exit velocity is 360m/s, and the inlet pipe is 3m above the exhaust.

    Part b I can't figure out, I know Bernoulli's equation, but when I plug in the given velocities and heights it doesn't give me the right answer for the enthalpy. Can someone help me out please?!

    Thank you so much :D
  2. jcsd
  3. Jan 27, 2010 #2
    Any help please? Assignment is due tommorow :(
  4. Jan 27, 2010 #3
  5. Jan 27, 2010 #4
    you guys are in my class, could you wear bright colors tomorrow to solve the mystery of who you are?

    good luck figuring it out. We guessed that it was the change in volume divided by the area, to calculate the change in displacement.
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