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(Thermodynamics) Why Do I Use U For Constant Volume and H For Constant Pressure

  1. Oct 30, 2011 #1
    Why Do I Use U For Constant Volume and H For Constant Pressure?

    Also, which can be calculated H, Q, U, W?
     
  2. jcsd
  3. Oct 30, 2011 #2

    Andrew Mason

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    For an ideal gas:

    [itex]\Delta U = nC_v\Delta T[/itex]

    [itex]\Delta H = \Delta U + \Delta (PV) = nC_v\Delta T + nR\Delta T = nC_p\Delta T[/itex]

    I am not sure what you mean by your last question. They can all be calculated if you have sufficient information.

    AM
     
  4. Oct 30, 2011 #3
    I can't tell you why you do that but if there is no non-volumetric work the change of U or H is equal to the exchanged heat if you do so.
     
  5. Oct 30, 2011 #4

    I like Serena

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    In general we have the equations for small changes dP, dV, dU, dH:
    dU=dQ - PdV
    dH=dQ + VdP


    For constant volume dV=0, so we can write the first as:
    ΔU=ΔQ

    For constant pressure dP=0, so we can write the second as:
    ΔH=ΔQ

    These are the quantities that we can actually measure.


    Depending on circumstances we can calculate each of H, Q, U, and W.
     
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