I am confused..can anyone help me please..

[sab421]

TWo containers each containing 2 m oles of th same ideal gas at the same pressure and tempreature are kept isolated from one another. Two procedures are performed:300 J of heat enery is added to the gas in container 1 and container two is allowed to expand, the gas inside doing 300 j of work work on the container walls. What is the difference between internal energies of the two samples of gas after these procedures?


This question is from Kaplan Q bank and the answer is 600J of work. I dont understand kaplan's explanation for this question.
Regards

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[RPedigo]

So we have two containers, A and B. Let's say they are both at 0J initially.

If you add heat to container A, you are raising the energy of the system, so it is now +300J.

If you perform work in container B, that means we're losing energy to perform that work. Doing work takes energy. If it does 300J of work, that means it is now -300J.

The difference is 600J between the two.



If you add heat to something, it's + energy
If you remove heat from something, it's - energy
If the system does work, it's - energy
If something does work on the system, it's + energy



Hope this helped!

 

[bluemonkey]

If you add heat to something, it's + energy
If you remove heat from something, it's - energy
If the system does work, it's - energy
If something does work on the system, it's + energy

Hope this helped!

It depends on the equation that you use. If you use the equation U=Q+W these are the signs. If you use the other (really irritating), U=Q-W, then the work signs get flipped.

 

[Kaustikos]

It depends on the equation that you use. If you use the equation U=Q+W these are the signs. If you use the other (really irritating), U=Q-W, then the work signs get flipped.

That's the screwed up part, the MCAT can use both, which sucks. They can say work done by the system is positive or work done on the system is positive.

THe point, however, is that in the situation above, you're having one system doing work and the other having work done on it - 600 difference.

 

[sab421]

but how to u know that work done in container 1 is 0. I used equation : E= q+W , q=+300 but how do we know that work is zero and not the E, which is internal energy for container 1.

 

[Vihsadas]

That's the screwed up part, the MCAT can use both, which sucks. They can say work done by the system is positive or work done on the system is positive.

THe point, however, is that in the situation above, you're having one system doing work and the other having work done on it - 600 difference.

Did you see this discrepancy on different practice exams? I don't remember seeing it used two different ways. As far as I know, the commonly used convention is U = Q + W and this is the only definition you will see on the MCAT.

 

[sab421]

Thanks all of you...I really appreciate ur help