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1st law of thermo

  1. Nov 8, 2009 #1
    What is the difference between [tex]\Delta[/tex]H and [tex]\Delta[/tex]E in a constant volume process ?????
     
  2. jcsd
  3. Nov 8, 2009 #2
    By definition it is always true that
    [tex]
    H=E+pV
    [/tex]
    At constant volume you have
    [tex]
    \Delta_V H=\Delta E+V\Delta p
    [/tex]
    Here you see the difference in the term.
    What exactly did you want to know?
     
  4. Nov 14, 2009 #3
    but acc to first law Heat supplied = internal energy + work done
    i.e.
    i.e. [tex]\Delta[/tex]h = [tex]\Delta[/tex]U + [tex]\Delta[/tex]W
    and [tex]\Delta[/tex]W = [tex]\Delta[/tex](pv) and
    [tex]\Delta[/tex](pv) = P[tex]\Delta[/tex]V + V[tex]\Delta[/tex]P and
    for work done V[tex]\Delta[/tex]p is taken as zero so how come can we say here V[tex]\Delta[/tex]P is not taken as zero here?
     
  5. Nov 14, 2009 #4

    Mapes

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    Hi jeedoubts, welcome to PF. Can you give a reference for these two equations? I doubt very much that they're correct. For example, work is defined as [itex]P\Delta V[/itex], not [itex]\Delta(PV)[/itex].

    I agree with Gerenuk's answer.
     
  6. Nov 14, 2009 #5
    what does the quantity v[tex]\Delta[/tex]p refers to then????
     
  7. Nov 14, 2009 #6

    Mapes

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    The difference between enthalpy change and energy change in a constant-volume process.
     
  8. Nov 14, 2009 #7
    physically what does it represent??
     
  9. Nov 14, 2009 #8

    Mapes

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    I don't know of a simpler description (other than the literal "volume multiplied by pressure change"). What are you looking for?
     
  10. Nov 14, 2009 #9
    Neither equation is correct in general. The correct equations are
    [tex]
    \mathrm{d}W=p\mathrm{d}V
    [/tex]
    (or with the other sign if you consider the work done on the system) and if you wish
    [tex]
    \mathrm{d}(pV)=p\mathrm{d}V+V\mathrm{d}p
    [/tex]
    It follows that only for constant volume or constant pressure processes the work can be described by
    [tex]
    W=p\Delta V\qquad\text{(const. p or const. V)}
    [/tex]
    Is it very important to know what is the general equation and what the special case. These special cases only apply if the conditions are met.

    Mapes is correct.
     
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