Tuesday, April 11, 2006
HW #4 - P #1 - Boundary Work as a Gas Expands
During an expansion process, the pressure of a gas changes from 15 to 100 psia according to the relation P = aV + b, where a = 5 psia/ft3 and b is a constant. If the initial volume of the gas is 7 ft3, calculate the boundary work done during the process.
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6 comments:
I can't find the pressure for the work integral for problem #1. Should I use average P here? I tried it but the answer was way off.
P = a V + b
Don't use an average P, you need to integrate with respect to V from the initial volume to the final volume to determine Wb.
Okay, so the integral is easy 'nuf. How should we deal with the pressure? Do we need to covert it back from pisa to psi (and at what stage?) or can we just leave it alone and continue with the conversion to Btu's?
Graham, actually I think you're okay. I assume 977.5 is in psi-cu.foot (or psia, I'm still not sure which). All you need to do is convert to foot-pounds and then to Btu's.
graham 9:15 PM
Steve is correct. You have a units problem. What are the units on 977.5 ? You need to convert from psia-ft^3 to Btu.
Steve 9:16 & 9:25
You can either convert the pressure to lbf/ft^2 before the integration or after. The energy unit you get is ft-lbf. This is a lot like N-m or J and it is an energy unit. Do the unit conversion to get from ft-lbf to Btu and you are done.
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