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Old Sep 10th 2009, 12:30 PM   #1
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Join Date: Aug 2008
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Help with Ideal Gases and Engines

This is my problem:

A three-step cycle is undergone by 2.9 mol of an ideal diatomic gas: (1) the temperature of the gas is increased from 160 K to 450 K at constant volume; (2) the gas is then isothermally expanded to its original pressure; (3) the gas is then contracted at constant pressure back to its original volume. Throughout the cycle, the molecules rotate but do not oscillate. What is the efficiency of the cycle?

Neither my book or the lectures i have attended have been able to help me completely understand all the relationsships between work internal energy and heat, so i'm really fuzzy on how to even approach this problem.

I know for the first process that since it is constant volume, W=0. And since it is a diatomic molecule, i can use Q=nCv(dT) to find the heat for this process. Or atleast i think that is what i can do.

But then i get to the second step and I'm not really sure how to tackle that one. The only thing i know is the Q=W and W=nRTln(Vf/Vi) but i don't know the volumes. Any help on fixing my current process and or help on getting me past this mental block?
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