Christopher Moore is an Assistant Professor of Physics at Longwood University in Farmville, VA.Carlwin has the following question about energy:
I dont know if our solution is right, but here’s the problem: Compute for the energy given off by a gas doing 200 J of work and losing 1350 J of its internal energy to the environment
Normally, I don’t respond to these types of questions for two reasons: (1) I don’t do other people’s homework, and (2) no mention was given to what their solution was, so it makes it hard to comment on where their reasoning is correct or incorrect. The forums are sometimes useful for these types of questions, but usually without having shown some initial effort, most questions like this die from lack of response.
So why am I going to respond? Because the question is either a trick question or awkwardly worded, which means it is the type of question that I hate and the type that typically leads to very little learning on the part of the student. This isn’t Carlwin’s fault. This is the fault of the person that assigned this question.
Why is this question a poorly formed question? Because, I could pose this question to a room full of PhDed physicists and get 2 or 3 different answers, all of which could be justified.
The question specifically asks how much energy the system “gives off”. The answer is 1350 J.
Anytime you see the buzzwords “internal energy” and “work”, think of the 1st Law of Thermodynamics: the change in the internal energy equals the sum of the change in the heat and the change in the work, or
.
A system has a certain amount of internal energy. That is all the energy it has available to “give off”. If it looses 1350 J of energy, then it lost 1350 J of energy. Period. It doesn’t matter whether that energy was work or heat.
However, it seems like the questioner meant to ask how much heat does the gas give off. In that case, the answer would be 1150 J.
So, either the question is a “trick” question or the question is a poorly formed question. Either way, I’m not sure how you would properly assess a student’s understanding via this question.