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dUDEonAfORUM
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An ideal gas at initial state has temperature 300 K has been compressed under constant pressure of 30 Pa from volume 3 cubic meters to 1.8 cubic meters. In the process 75 J of heat was lost.
A. Find the change in internal energy
B. Find the final temperature
I tried two methods in A and they give out different answers
Method I
formula: ΔU = Q- W = Q - PΔV
answer -39 J
Method II
formula: ΔU = 3/2 nRΔT = 3/2 PΔV
since this is an isobaric process PΔV=nRΔT
But the answer from this is -54 Jwhich method is wrong? If Method II is wrong, but then when can I use ΔU=3/2 PΔV
I think Method II is wrong as P in PΔV=nRΔT refers to the pressure of the ideal gas, but the pressure given is the external pressure acting on the gas...is this true?
As for B, I don't know whether to use PΔV=nRΔT or ΔU = 3/2 nRΔT
A. Find the change in internal energy
B. Find the final temperature
I tried two methods in A and they give out different answers
Method I
formula: ΔU = Q- W = Q - PΔV
answer -39 J
Method II
formula: ΔU = 3/2 nRΔT = 3/2 PΔV
since this is an isobaric process PΔV=nRΔT
But the answer from this is -54 Jwhich method is wrong? If Method II is wrong, but then when can I use ΔU=3/2 PΔV
I think Method II is wrong as P in PΔV=nRΔT refers to the pressure of the ideal gas, but the pressure given is the external pressure acting on the gas...is this true?
As for B, I don't know whether to use PΔV=nRΔT or ΔU = 3/2 nRΔT