# Why is internal energy a function of temperature and volume?

Why is internal energy a function of temperature and volume? $u=f(T,V)$. In my reasoning, I can only understand that internal energy is a function of Temperature, because it changes directly with Temperature. If temperature raises, $u$ raises, if $T$ decreases, $u$ decreases.

What about enthalpy? Why is it a function of temperature and pressure? $h=f(T,p)$

• That may be true for an ideal gas but it's not true in general... – lemon Apr 22 '17 at 16:32
• From the phase rule, it takes two independent degrees of freedom to specify the thermodynamic equilibrium state of a single phase, single component substance. – Chet Miller Apr 22 '17 at 19:12
• It may be worth looking up 'state variables' and 'extensivity'. – MPath Apr 23 '17 at 8:43