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Q for Isobaric Processes

  1. Nov 20, 2013 #1
    Just wanted to ask why is it that for isobaric processes, when we wanted to find Q, sometimes we use Q = nCpΔT and sometimes we use Q = nCvΔT + p(Vf - Vi)?

    How do we determine which should be used and when?
     
  2. jcsd
  3. Nov 20, 2013 #2
    They are not different things .Both of the above expressions are correct and can be used simultaneously , or separately depending on the situation.

    For an isobaric process ΔQ = nCPΔT.

    From First law of Thermodynamics ΔQ = ΔU+ΔW .

    Now ΔU=nCVΔT applies to all kinds of processes involving an ideal gas.

    So ,putting values of ΔQ and ΔU ,we have nCPΔT = nCVΔT + pΔV


    To elaborate it further-

    Case 1) What is the heat required to raise the temperature of 'n' moles of an ideal monoatomic gas by 'ΔT' .

    You can simply use Q = nCPΔT

    Case 2) How much heat is supplied to 'n' moles of an ideal monoatomic gas in a chamber fitted with a light piston , when the temperature changes by 'ΔT' and volume changes by ΔV ? Consider the process to be isobaric

    Here ,use ΔQ = nCVΔT + pΔV

    Case 3) What is the change in the volume of 'n' moles of an ideal monoatomic gas in a chamber fitted with a light piston , when the temperature changes by 'ΔT' ? Consider the process to be isobaric .

    Now use nCPΔT = nCVΔT + pΔV

    Edit:Removed the word - "insulated" and removed a typo
     
    Last edited: Nov 20, 2013
  4. Nov 20, 2013 #3
    Hi Tanya. You need to "lose" the word insulated from cases 2 and 3. If Q is not equal to zero, the chamber is not insulated. In all three cases, you need to lose the word monoatomic, and in case 1 you need to add the words at "at constant volume". Also, in item 2 there is a typo: replace ΔU with ΔV.

    Chet
     
  5. Nov 20, 2013 #4
    Hi Chet...

    Yes you are right :redface: .I have edited my post.

    Why? These are just examples i have taken using monoatomic ideal gas.

    Why ? The heat supplied at constant pressure is nCPΔT
     
    Last edited: Nov 20, 2013
  6. Nov 20, 2013 #5
    It might give the impression that the relationship applies only to a monoatomic ideal gas.

    Oh, sorry. I misread the Cp as Cv. I think it would be worthwhile adding the words "at constant pressure" to Case 1.

    Chet
     
  7. Nov 20, 2013 #6
    Well..these are just random questions/cases I formed .But...you may be correct.

    Right.

    Thanks for your valuable suggestions :smile: .I very much like reading your posts .They are full of insight.
     
    Last edited: Nov 20, 2013
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