Thermodynamics: Proofs of work done on/by gas during adiabatic process

  • Thread starter Matty R
  • Start date
  • #1
83
0
Hello

I'm really confused with this and would appreciate any help.

Homework Statement



a) Show that the work done on a gas during a quasistatic adiabatic compression is given by:

[tex]W = \frac{P_f V_f - P_i V_i}{\gamma - 1}[/tex]

b) Show that the work done by a gas during a quasistatic adiabatic compression is given by:

[tex]w = \frac{P_i V_i - P_f V_f}{\gamma - 1}[/tex]


Homework Equations



[tex]w = -W[/tex]
[tex]PV^{\gamma} = K[/tex]


The Attempt at a Solution



a) [tex]dW = -PdV[/tex]
[tex]P = KV^{- \gamma}[/tex]
[tex]W = -K \left[\frac{V^{1- \gamma}}{1 - \gamma} \right]^{V_f} _{V_i}[/tex]
[tex]W = - \frac{P_f V_f - P_i V_i}{1 - \gamma}[/tex]
[tex]W = \frac{P_i V_i - P_f V_f}{\gamma - 1}[/tex]

I don't understand where I've gone wrong.

b) [tex]w = -W[/tex]
[tex]w = - \frac{P_i V_i - P_f V_f}{\gamma - 1}[/tex]
[tex]w = \frac{P_f V_f - P_i V_i}{1 - \gamma}[/tex]

I also get this answer if I derive w from:

[tex]dw = PdV[/tex]

I've probably got a minus sign mixed up, or something like that, but I've been through the derivation so many times it's invading my dreams. Okay, maybe it isn't that bad, but I'm really struggling.
 

Answers and Replies

  • #2
ehild
Homework Helper
15,543
1,913
Hello

I'm really confused with this and would appreciate any help.

Homework Statement



a) Show that the work done on a gas during a quasistatic adiabatic compression is given by:

[tex]W = \frac{P_f V_f - P_i V_i}{\gamma - 1}[/tex]


[tex]w = -W[/tex]
[tex]PV^{\gamma} = K[/tex]


The Attempt at a Solution



a) [tex]dW = -PdV[/tex]
[tex]P = KV^{- \gamma}[/tex]
[tex]W = -K \left[\frac{V^{1- \gamma}}{1 - \gamma} \right]^{V_f} _{V_i}[/tex]
[tex]W = - \frac{P_f V_f - P_i V_i}{1 - \gamma}[/tex]
[tex]W = \frac{P_i V_i - P_f V_f}{\gamma - 1}[/tex]

I don't understand where I've gone wrong.


You have changed sign too many times:tongue2:.
[tex]W = - \frac{P_f V_f - P_i V_i}{1 - \gamma}=\frac{P_f V_f - P_i V_i}{\gamma-1}[/tex]
 
  • #3
83
0
Thanks for the reply.

So, erm, is this true:

[tex]W = - \frac{P_f V_f - P_i V_i}{1 - \gamma}[/tex]
[tex]= \frac{P_f V_f - P_i V_i}{\gamma-1}[/tex]
[tex]= \frac{P_i V_i - P_f V_f}{1 - \gamma}[/tex]
 
  • #4
ehild
Homework Helper
15,543
1,913
So, erm, is this true:

[tex]W = - \frac{P_f V_f - P_i V_i}{1 - \gamma}[/tex]
[tex]= \frac{P_f V_f - P_i V_i}{\gamma-1}[/tex]
[tex]= \frac{P_i V_i - P_f V_f}{1 - \gamma}[/tex]

Yes.

ehild
 
  • #5
83
0
Thank you very much for your help.

I'm feeling quite embarrassed now, though most of the threads I've created here have been about really basic mistakes, so I should be used to it. :rolleyes:
 

Related Threads on Thermodynamics: Proofs of work done on/by gas during adiabatic process

Replies
2
Views
3K
Replies
3
Views
2K
Replies
5
Views
3K
Replies
2
Views
3K
  • Last Post
Replies
8
Views
24K
  • Last Post
Replies
7
Views
6K
Replies
2
Views
4K
  • Last Post
Replies
4
Views
6K
Replies
2
Views
319
Top