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Homework Help: Isobaric, Isochoric, Isothermal, and Adiabatic Processes

  1. Nov 30, 2008 #1

    Which of the following statements are true about the first half of this process, just going from
    state A to state F?

    Both T and U increase.
    W provides energy input.
    Q provides energy input.
    Q is larger (in magnitude) than W.

    I know that Q provides energy input, but none of the other ones sound corrects to me. . .

    please help.
    Last edited: Nov 30, 2008
  2. jcsd
  3. Dec 1, 2008 #2
    OK. Here's how to approach this:

    1) Is there an increase or decrease in temperature, and hence is there an increase or decrease in U? (Hint: Think ideal gas eqn.)
    2) Is there any work done on or by the gas? How do you know?
    3) Relate the signs and magnitudes of U and W, to the sign and magnitude of Q by the first law of thermodynamics.
  4. Dec 1, 2008 #3

    Andrew Mason

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    Science Advisor
    Homework Helper

    I will assume this is an ideal gas. The PV graph shows P declining linearly as V increases. Plot the isotherm (ie a path in which T is constant: ie P = nRT/V). Now what can you say about paths that are ABOVE the isotherm and paths that are below it (ie what happens to the temperature jn moving along the path?). Is A-F above or below the isotherm?

    Now plot the adiabatic path:

    [tex]PV^\gamma = K[/tex]

    What can you say about Q on the adiabatic path? What can you say about a path that is above the adiabatic path? Below? Is A-F above or below the adiabatic path? What does that tell you about Q?'

    Now consider the work done from A-F. How do you measure that? How does that compare to the work done with the adiabatic path? (Q=0). What does that tell you about the work done compared to heat flow?

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