# Comparison between entropy and internal energy

Why is entropy change a better way of determining a spontaneous process compared to the change in internal energy?

• There is a law of Thermodynamics that tells us something about the direction in which entropy evolves. Does that give you a hint? Nov 5, 2014 at 2:40

The change in internal energy is not a relevant quantity for spontaneous evolution of a system. Consider an isolated system made of two blocks of the same material at two different temperatures such that $T_1>T_2$. Heat will flow from $1$ to $2$ but the total change in internal energy is $\Delta U_{1+2}=0$. This information is therefore not useful.
On the other hand, the change in total entropy is $\Delta S_{1+2}=Q/T_2-Q/T_1>0$ and can tell us about the direction of the process.