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Refrigirator OR Heat Engine

by dav2008
Tags: engine, heat, refrigirator
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dav2008
#1
Apr15-03, 07:57 PM
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ok..Im doing this review packet for Physics B...I did all of a Thermo problem except one part...

Basically an ideal monoatomic gas first expands in an isobaric process, then the pressure is lowered down isometrically, keeping the volume constant, and finally it is returned to state A isothermally where temperature stays constant...

Part D of the question asks whether this device is a refrigerator or a heat engine... This is what got me confused...I dont even know where to begin...Does it have to do with work being negative or positive?

Part C asked for the net work sign, and i found it was negative(by the new convention at least) because more work was done in the first process (negative, by the gas) than in the third process(positive, on the gas)

..Thanks for any replies..basically i just need to know how to determine if a PV diagram cycle is a refrigirator or a heat engine..tx
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dav2008
#2
Apr15-03, 08:31 PM
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OK..i realized it has to do with entropy..but how would i encorporate that with the given information?
xyz
#3
Apr16-03, 01:44 PM
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Hi,dav2008...
there are something I know after I look through ur questions...the gas frist expands in an isobaric process,it means that the temperature grows up...because PV=nRT,the pressure fixes,but the volumn gets bigger.Q_in=W_out. W_out=nR ln(V_final/V_inital ).

Then the pressure lowered down,and the volumn keep constant, so the temperature also lowered down.W_out=o, Q_in=nC_v T. T---stands for the amount of changed temperature.and C_v=3/2 R, R=8.31J/(mol*K).

Finally, the temperature stays constant,the final temperature lower than the inital.
So I guess the device is refrigerator. I hope those will be useful for u !!! please reply to me if u know the answer...

dav2008
#4
Apr16-03, 04:01 PM
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Refrigirator OR Heat Engine

Yea lol..its actually a heat engine.i figured it out with teh help of some people....

Since work is negative and done by the gas that means that work is coming out of the system..and in a head engine work is being taken out..Unlike a fridge where work is added


And you are right that the final temperature IS lower than the initial...BUT in a fridge the final temperature is HIGHER than the initial..because u are moving heat from low temperature with the help of work to a higher temperature.
xyz
#5
Apr17-03, 12:11 AM
P: n/a
hummm...oic...thanks for ur reply...
I always confuse refrigirator and heat engine...


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