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 3 replaced http://physics.stackexchange.com/ with https://physics.stackexchange.com/ edited Apr 13 '17 at 12:39 "Free energy" in the sense of "Gibbs Free Energy" or "Helmholtz Free Energy" is a rigorously defined concept. By contrast, the word "Energy" is ill defined and means different things in different contexts. I think you will have to state your question more precisely for us to help. From Lagerbaer in a comment: One way to think about energy vs free energy is that the former is a measure of all the work you'd have to do to assemble the system, whereas the free energy, which you get from the energy itself by subtracting $$T\cdot S$$ where $$T$$ is temperature and $$S$$ is entropy, tells you how much work you could get out of a system as "useful" work (in contrast to the "random" energy in heat) I did a quick search for "what is energy" and http://physics.stackexchange.com/questions/3014What Is Energy? Where did it come from? looks an informative thread. have a look at this discussion and see if it casts any light on your question. "Free energy" in the sense of "Gibbs Free Energy" or "Helmholtz Free Energy" is a rigorously defined concept. By contrast, the word "Energy" is ill defined and means different things in different contexts. I think you will have to state your question more precisely for us to help. From Lagerbaer in a comment: One way to think about energy vs free energy is that the former is a measure of all the work you'd have to do to assemble the system, whereas the free energy, which you get from the energy itself by subtracting $$T\cdot S$$ where $$T$$ is temperature and $$S$$ is entropy, tells you how much work you could get out of a system as "useful" work (in contrast to the "random" energy in heat) I did a quick search for "what is energy" and http://physics.stackexchange.com/questions/3014 looks an informative thread. have a look at this discussion and see if it casts any light on your question. "Free energy" in the sense of "Gibbs Free Energy" or "Helmholtz Free Energy" is a rigorously defined concept. By contrast, the word "Energy" is ill defined and means different things in different contexts. I think you will have to state your question more precisely for us to help. From Lagerbaer in a comment: One way to think about energy vs free energy is that the former is a measure of all the work you'd have to do to assemble the system, whereas the free energy, which you get from the energy itself by subtracting $$T\cdot S$$ where $$T$$ is temperature and $$S$$ is entropy, tells you how much work you could get out of a system as "useful" work (in contrast to the "random" energy in heat) I did a quick search for "what is energy" and What Is Energy? Where did it come from? looks an informative thread. have a look at this discussion and see if it casts any light on your question. 2 added quote from Lagerbaer's comment edit approved Mar 6 '12 at 20:48 AJP 16911 gold badge11 silver badge77 bronze badges "Free energy" in the sense of "Gibbs Free Energy" or "Helmholtz Free Energy" is a rigorously defined concept. By contrast, the word "Energy" is ill defined and means different things in different contexts. I think you will have to state your question more precisely for us to help. From Lagerbaer in a comment: One way to think about energy vs free energy is that the former is a measure of all the work you'd have to do to assemble the system, whereas the free energy, which you get from the energy itself by subtracting $$T\cdot S$$ where $$T$$ is temperature and $$S$$ is entropy, tells you how much work you could get out of a system as "useful" work (in contrast to the "random" energy in heat) I did a quick search for "what is energy" and http://physics.stackexchange.com/questions/3014 looks an informative thread. have a look at this discussion and see if it casts any light on your question. "Free energy" in the sense of "Gibbs Free Energy" or "Helmholtz Free Energy" is a rigorously defined concept. By contrast, the word "Energy" is ill defined and means different things in different contexts. I think you will have to state your question more precisely for us to help. I did a quick search for "what is energy" and http://physics.stackexchange.com/questions/3014 looks an informative thread. have a look at this discussion and see if it casts any light on your question. "Free energy" in the sense of "Gibbs Free Energy" or "Helmholtz Free Energy" is a rigorously defined concept. By contrast, the word "Energy" is ill defined and means different things in different contexts. I think you will have to state your question more precisely for us to help. From Lagerbaer in a comment: One way to think about energy vs free energy is that the former is a measure of all the work you'd have to do to assemble the system, whereas the free energy, which you get from the energy itself by subtracting $$T\cdot S$$ where $$T$$ is temperature and $$S$$ is entropy, tells you how much work you could get out of a system as "useful" work (in contrast to the "random" energy in heat) I did a quick search for "what is energy" and http://physics.stackexchange.com/questions/3014 looks an informative thread. have a look at this discussion and see if it casts any light on your question. 1 answered Mar 4 '12 at 10:47 John Rennie 286k4545 gold badges583583 silver badges833833 bronze badges "Free energy" in the sense of "Gibbs Free Energy" or "Helmholtz Free Energy" is a rigorously defined concept. By contrast, the word "Energy" is ill defined and means different things in different contexts. I think you will have to state your question more precisely for us to help. I did a quick search for "what is energy" and http://physics.stackexchange.com/questions/3014 looks an informative thread. have a look at this discussion and see if it casts any light on your question.