• Support PF! Buy your school textbooks, materials and every day products Here!

Determine the amount of energy transfer by work in btu

  • #1
249
0

Homework Statement


A closed system consisting of 14.5 lb of air undergoes a polytropic process from p1 = 80 lbf/in2, v1 = 4 ft3/lb to a final state where p2= 20 lbf/in2, v2 = 11 ft3/lb. Determine the amount of energy transfer by work, in Btu, for the process


Homework Equations


W = ∫v1v2p dv = ∫v1v2 c/vn dv

The Attempt at a Solution



Using the relationship p1v1n = p2vn2

I solved for n with:

n = ln(p2/p1)/ln(v2/v1) which gives me

n = -1.3704 but other people I've spoken to have gotten +1.3704 which, when used in:

W = p2v2 - p1v1/(1-n)

gives me 563714.9 lbf-ft, which then gives me 725 Btu when converted to Btu.

Why do I get -1.3704 instead of a positive?
 

Answers and Replies

  • #2
rude man
Homework Helper
Insights Author
Gold Member
7,766
760
You did not solve for n correctly. It's
n = ln(p1/p2)/ln(V1/V2) > 0.
 

Related Threads on Determine the amount of energy transfer by work in btu

Replies
4
Views
4K
Replies
1
Views
6K
Replies
7
Views
3K
  • Last Post
Replies
5
Views
2K
Replies
5
Views
819
Replies
6
Views
1K
Replies
13
Views
3K
Replies
1
Views
651
  • Last Post
Replies
1
Views
3K
Top