Laws of Thermodynamics

Empirical laws that explain the behavior of thermodynamic systems.

0th Law

Defines that temperature is equal for 2 systems in thermal equilibrium.

If 2 systems have the same temperature, then they are in thermal equilibrium.

If systems $A$ and $B$ are in thermal equilibrium then any system $C$ in thermal equilibrium with $A$ is also in thermal equilibrium with $B$.

Note

1st and 2nd laws are explained in detail in the next pages.

3rd Law

As the absolute temperature approaches zero, the entropy of a system approaches a constant minimum value.

Establishes that it is impossible to reach absolute zero in a finite number of steps or processes. As a system approaches absolute zero, it becomes increasingly difficult to remove additional heat.

$$\lim_{T \to 0} S = S_0$$

Here:

• $S$ is the entropy of the system
• $S_0$ is the minimum entropy value
• $T$ is the absolute temperature

Note

$S_0=0$ for perfect crystals (an ideal solid where every atom is arranged in a completely ordered, flawless pattern with no defects).

More of an interest in statistical mechanics; and not much in engineering.