1. The problem statement, all variables and given/known data Hi! I have this question "how is the curve of U(internal energy) with volume if the temperature is negative"? 2. Relevant equations 3. The attempt at a solution i cnsidered T=(partial U/partial S) at constant V and N (number of particules, but now i cant say the change of U with V if T<0...can somebody help me? i also thought that T = slope of the tangent line to the function U(S) but i cant say how is the curve U(V) when T<0 as required (note: i know the temperature cant be negative but we want to know what theoricaly happens) thanks!