1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Thermodynamics cycles quick question

  1. Feb 6, 2015 #1
    I'm unsure of part b of the question 7 below;
    ImageUploadedByPhysics Forums1423230802.914015.jpg

    My attempt:
    ImageUploadedByPhysics Forums1423230835.870688.jpg

    I'm confused as to what direction the arrows are meant to go.
    If heat transfer occurs to the gas, and the piston rises, the volume must have decreased in the cylinder.

    So from state 1 - 2, the gas is cooling or heating up? This is a isobaric process right?
    And from states 2- 3 the piston moves up, known as isochoric?

    Finally therefore is my process path correct?

    Thanks, kind regards,
  2. jcsd
  3. Feb 6, 2015 #2
    Hi Jay. Welcome to Physics Forums.

    I have bad news for you. Sorry. Your analysis is completely incorrect.

    The weight of the piston is supported on the ridge inside the cylinder, until the pressure within the cylinder is high enough to start lifting the piston. So when heat is being added to the gas in the first stage of the process, the pressure of the gas is increasing at constant volume. How high does the temperature of the gas have to get before the pressure is high enough to start lifting the piston? In the second stage of the process, heat continues to be added to the gas, but now the piston is rising, so that its pressure stays constant. How high does the temperature of the gas have to get in this stage of the process to bring about the desired increase in volume?

    Now, re-evaluate the PV diagram and see what you come up with.

  4. Feb 8, 2015 #3
    Still slightly confused;

    So the gas is actually under the piston in the cylinder? Can you provide pictures as I can't visualise what you mean.
    What about the gas above the piston?

    Although if the gas is under the piston, I guess heating it up, doing work on it would increase pressure, which would produce force up on piston which makes sense, or am I completely wrong again?

  5. Feb 8, 2015 #4
    The cylinder is vertical, and the gas is within the cylinder. The piston sits on top of the gas, and there is air outside. The bore of the cylinder has two sections: (1) drilled from the top all the way down the length of the cylinder, at a diameter slightly less than that of the piston (2) drilled half way down the length of the cylinder from the top, with a diameter essentially equal to that of the piston (so that the piston slides freely). Initially, the piston sits on the ridge between the two sections, half way down the length.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted