what is energy?!

cesiumfrog

What irks me about the definition "capacity to do work" is the thermodynamic impossibility of powering a boat with the heat energy from the ocean.

Tom Mattson

I like the first response the best. Energy is the generator of time translations.

ZapperZ

Unless, of course, one has a spontaneous cold reservoir at, let's say, liquid helium temperature!

:)

Still, you are highlighting an important point that many beginners forget, and that is the 2nd Law of thermo, that not every "energy" can be made to do the conventional "work" that we know of.

I believe the problem with this type of question (and note that the OP hasn't come back since) is knowing the level of complexity that the questioner requires, or can understand. I think it is safe to say that if I'm teaching a high school physics class, one can answer this in a relatively simple (naive?) manner without added complications (Chi Meson, how would you answer such a question in your class?). One can even make a case that such a question should also have a rather straightforward answer in college intro Physics, with the appropriate caveats.

However, if we are talking about the term to be as accurate as possible over all realm of known physics, then not only is the answer not as simple, but the question itself then requires a more appropriate clarification and maybe even redefined. It does mean that within a certain area of physics, the concept of energy is well-known and well-defined, especially mathematically. However, it doesn't mean that the same concept applies to every other areas of physics. So it is not that "energy" has no clear definition. It is just that it means different things in different context, and trying to make an overall definition that works all the time is rather questionable. I think this is what most people outside of physics are not aware of.

Zz.