Conceptual question about internal energy

[Telemachus]

Hi there. Here is the deal, today morning in my thermodynamic class we were making some exercises of stability, and then surged a question about internal energy. The thing is I thought that the internal energy should be always positive in sign. This doesn't mean that the variation must be positive, what I thought is that internal energy as a "thing" must be positive, as volume must be positive. But my teacher and some of my classmates refuted me. But it still not clear to me how is that internal energy can be negative, at that moment I said that it depends on a reference frame condescending with the others opinions, as the potential energy does depends on a reference frame (now this is not clear to me because potential energy is transformed in kinetic energy). But anyway I want to clarify the concepts, so I wanted to start a debate an read some opinions of yours about this, maybe you can give me an example that make this clear. I still in doubt about this, because if I think on the second principle energy can't be created nor destroyed, so how can it be negative? I mean, I can't take energy out of a system, but that doesn't mean there is such a thing as "negative energy". The same discussion was stated about entropy, my classmates and the teacher said that it could be negative. We weren't talking about variations, we were talking about the internal energy and the entropy as entities.

Bye, and thanks for posting.

Sorry, I shouldn't post this here. If it can be moved to the proper section I'll be thanked.

 

[ideasrule]

Here's an example: suppose I consider internal energy to be composed of kinetic energy, and gravitational potential energy. Now I chill some hydrogen atoms to nearly absolute zero, so they have nearly no kinetic energy. Since the zero point for potential energy is arbitrary, I arbitrarily define the atoms to have -10^9 J of potential energy. My hydrogen atoms now have an energy of -10^9 J.

 

[Telemachus]

Thanks.