Hi, I'm new here, I was just wondering if anyone could help clarify a subject I'm having difficulties teaching myself.... In thermo we perform a "Legendre transform" on the internal energy with respect to entropy. The stated purpose of this is so that we don't have to work in the entropy variable, since it is difficult to measure entropy. But the result of the transform (F = U - TS) still has entropy (S) in it. So how is this helpful? Thanks!