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Pressure of a piston cylinder device after heated

  • #1

Homework Statement


I am confused on when will pressure of two states be the same for a piston cylinder device. Below are two problems where one's final pressure equals the initial pressure and one is not.
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Homework Equations




The Attempt at a Solution



Initially, I thought a piston-cylinder device will have the same initial pressure and final pressure as long as there is no rigid container or some physical limit that prevent the volume form expanding, if the volume is allowed to change, then when the piston is at rest, it will have the same final pressure as the initial pressure. But after doing some examples, I feel like P_f= P_i is kind rare. so how do you know if the pressure of state 2 is going to change? [/B]
 

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Answers and Replies

  • #2
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It should be clearly specified in the problem statement or the accompanying figure.
 
  • #3
It should be clearly specified in the problem statement or the accompanying figure.
is it clearly specified in the above two problems? the only difference I noticed is that the first problem is in a single phase ( super-heated), and the second problem is in two phase region. is that why we can assume the second problem's final pressure does not equal initial pressure?
 
  • #4
19,813
4,050
is it clearly specified in the above two problems? the only difference I noticed is that the first problem is in a single phase ( super-heated), and the second problem is in two phase region. is that why we can assume the second problem's final pressure does not equal initial pressure?
The second problem is very clearcut because of the specification of saturated vapor in the final state. The first problem is not properly specified (unless there is a figure that goes along with the problem, that hasn't been included).
 

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