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Thermodynamic Piston problem. 
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#1
Oct1006, 02:42 PM

P: 185

A cylinder contains 1kg of saturated H20 at 30C. This piston has a crosssectional area of .065m^2, a mass of 40kg, and rests on stops. With the piston in this position, the volume of the H20 is .1m^3. The external atmospheric pressure is 94Kpa and g is 9.75m/s^2. Heat is transferred to the system until the cylinder contains saturated H2O vapor.
1. Sketch the problem on Tv and pv diagrams. 2. What is the water temperature when the piston just moves off of the stops? I've been stumped on part 2 for hours. Here's how far I am: The first thing I did was calculate the pressure at which the piston would move up off of the stops. Since at that exact moment the piston won't actually be moving, I can say that Psys*Apiston  Patm*Apiston  m(piston)*g = 0. Solving, I get Psys = (Patm*Apiston + m(piston)*g)/Apiston. This calculation yields 100KPa. Help? 


#2
Oct1006, 02:59 PM

Sci Advisor
HW Helper
P: 3,033

I can't visualize the pistion you are describing. I assume it is completely full of water, but I don't get the geometry. What moves when the water evaporates? What doesn't move? Where are these "stops"



#3
Oct1006, 03:26 PM

P: 185

well here is about the best I can do away from home...
The H20 is in the bottom. The piston is frictionless thus the space between it and the cylinder. Hope that helps a bit. 


#4
Oct1610, 07:08 AM

P: 1

Thermodynamic Piston problem.
Hey if the H2O is at 100kpa and the piston is free to move with the expansion from saturated liquid to saturated vapor then the pressure is a constant and the temperature during the expansion will be the saturated temp. Due to the fact that the substance will be moving along the isobar in a pt diagram and along the isotherm in the pv diagram.
Hope that makes sense and helps 


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