1st law of thermo

  • Thread starter phymatter
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  • #1
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Main Question or Discussion Point

What is the difference between [tex]\Delta[/tex]H and [tex]\Delta[/tex]E in a constant volume process ?????
 

Answers and Replies

  • #2
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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?
 
  • #3
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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?
 
  • #4
Mapes
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i.e. [tex]\Delta[/tex]h = [tex]\Delta[/tex]U + [tex]\Delta[/tex]W
and [tex]\Delta[/tex]W = [tex]\Delta[/tex](pv)
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.
 
  • #5
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what does the quantity v[tex]\Delta[/tex]p refers to then????
 
  • #6
Mapes
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The difference between enthalpy change and energy change in a constant-volume process.
 
  • #7
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physically what does it represent??
 
  • #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?
 
  • #9
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[tex]\Delta[/tex]W = [tex]\Delta[/tex](pv) and
[tex]\Delta[/tex](pv) = P[tex]\Delta[/tex]V + V[tex]\Delta[/tex]P
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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