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Homework Help: Thermodynamics- piston cylinder- 2nd law of thermodyamics

  1. May 7, 2012 #1
    ( part a only I don't understand how you know from this question that it is a constant pressure process.

    I thought it is a polytropic process. So it will have a pVγ curve. I was drawing a pV curve.

    The answer is a straight line parallel to x- axis.

    My question: How do I know from looking at the question what kind of graph it will be and what process it will be? constant pressure, constant volume or etc?


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  3. May 7, 2012 #2


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    Hi student. Consider a force balance on the piston. Note that they say the piston has negligible weight. How would the force up correlate to the force down as the piston moves within the cylinder and before it hits a step (ie: while it is sliding)?
  4. May 9, 2012 #3
    Following on Q_Goest: If you do a free body diagram on the cylinder just before it starts to move, the force on top of the cylinder is 800 N and the force on the bottom of the cylinder is also 800 N so the pressure are also equal. Now let the volume increase just a little. A free body diagram shows that the forces (and pressures) stay the same. Let the volume increase a little more, same result, a free body diagram shows the forces (and pressures) above and below the cylinder are still 800 N. Since the pressures don't change, the cylinder must be under going a constant pressure process.
  5. May 11, 2012 #4

    rude man

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    The p-V curve will not be pVγ. That would be the case for an adiabatic (isentropic) process. You're adding heat Q. And yes, p stays constant as RTW69 points out. It's always atmospheric pressure.
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