My professor says the internal energy of a system can be calculated as $U=n T c_v$. Now I always thought that we could only calculate a change in internal energy $\Delta U = n c_v \Delta T$.
Following this idea, he says that a recipient that contains gas and loses a certain number of moles at constant temperautre undergoes a change in internal energy of $\Delta U = T c_v \Delta n$.
I've never seen any of this in any book. I believe he is assuming that the internal energy at 0 Kelvin is zero, which I believe is not true. However, in the case of the recipient losing gas,I wouldn't know how to calculate $\Delta U$.
Any thoughts? thanks!