(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A cylinder contains 0.2mol of Helium at 30 degrees C and is heated different ways.

How much heat is needed to raise the temperature to 70C while keeping thevolume constant?

2. Relevant equations

dQ=dU+dT

nCpdT=nCvdT+nRdT

3. The attempt at a solution

What I am confused about is whether to use the nCpdT (heat added) or nCvdT (change in internal energy). If it is a constant volume process, wouldnt that mean dQ=dU therefore nCpdT=nCvdT?

Obviously that is not the case as the moles (0.2) are constant as is the change in temperature (70-30=40C or 40K). As as result Cp=Cv which should be untrue for a monatomic gas.

What is the cause for this discrepancy and how can I determine which equation to use?

Thanks

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# Homework Help: Heat capacities of ideal gases

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