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<p>Thermodynamics is one of those subjects whose underpinnings are so abstruse as to be the subject of ridicule. As Arnold Sommerfeld once famously joked:</p>
<p>Thermodynamics is a funny subject. The first time you go through it, you don’t understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don’t understand it, but by that time you are so used to it, it doesn’t bother you any more.</p>
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<p>Yet manipulating symbols is inevitable. Apart from the concept of entropy - which is readily calculated but which likely only a tiny fraction of nonacademic engineers understand what it actually is - there are seemingly mundane quantities such as “free energy” that are readily calculated but which practically nobody in industry actually understands what it really is. </p>
<p>To those who would disagree, pop quiz: what exactly is “free” about '“free energy”? It’s not the thermodynamically available energy, it’s certainly not free in the financial sense, as you almost always have to pay to obtain more free energy, so what exactly is it? (Note, this has nothing to do with the mathematical definition of free energy, which we all know is found by simply subtracting the quantity TdS from some form of energy, either internal energy or enthalpy. But I can take any quantity and arbitrarily add or subtract a bunch of random terms to it. To specifically take energy/enthalpy and then specifically subtract TdS should produce some quantity that has some useful form that is readily understandable by the engineering students who are forced to learn how to calculate it, so what exactly is it? I defy anybody to pick a random selection of practicing engineers, or even engineering students, and ask them what free energy actually is, and I suspect that not one of them will know.}</p>