# Why are there so many different formulations for energy? When do I use each one? [closed]

I understand that energy is a fairly arbitrary concept, and that it completely depends on your definition of energy. So far in my class, we have discussed:

• Entropy, $$S$$ (not really energy, but it seems to be closely related);
• Total internal energy, $$U$$, for which it is impossible to find an absolute value for;
• Enthalpy, $$H=U+PV$$;
• Gibbs energy, $$G=H-TS$$.

My questions are (and sorry if this is kind of loaded):

• Where do these formulations arise? Why is there not just one version that trumps the rest? (Total internal energy sounds like a candidate)
• In which scenarios would I be using one version over another?
• If you are not going to elaborate the question by your effort to answer the question yourself, you may rather want to post it to Quora site. As it is, it does not fit the site policy and will be probably voted to be closed. – Poutnik Mar 3 at 3:09
• Why do we use kinetic energy, potential energy, thermal energy and not just total energy ? – Poutnik Mar 3 at 3:13
• @Poutnik Has it exactly right: the answer to all three of your questions is simply to pay close attention to what the teacher/professor says. – Ed V Mar 3 at 3:16