Reviewing for the FE exam, came across questionable thermo problem.


by jasc15
Tags: exam, questionable, reviewing, thermo
jasc15
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#1
Apr4-08, 12:18 PM
P: 156
Im studying for the Fundamentals of Engineering exam (coming up April 12!!) and came across this Thermodynamics problem in my review book.

A gas goes through the following processes:

A to B: isothermal compression
B to C: isochoric compression
C to A: isobaric expansion

PC = PA = 1.4bar
VC = VB = 0.028m3
The net work during the C-to-A process is WCA = 10.5kJ

What work is performed in the A-to-B process?
So the work for an isothermal process is RTA*ln(VB/VA), where VA was previously determined to be 0.103m3.

In the solution to the problem, the RTA term is replaced with PA*VA, where it should be (PA*VA)/n from the ideal gas law.

Is this solution wrong? As I see it, this cant be solved without knowing the number of moles of gas (or mass of the gas depending on which R is used). I was unable to get the correct answer, and the solution seems wrong to me.
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Mapes
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#2
Apr4-08, 01:04 PM
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The solution should have replaced nRTA with PAVA. You don't need to know the amount of gas to solve this problem.

EDIT: I get -18.8 kJ for the work done in the A-to-B process.


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