Laws of Thermodynamics

Zeroth law of thermodynamics

If both thermodynamic system A and thermodynamic system B are in thermal equilibrium (no net heat flow) with a third thermodynamic system C, then A and B are in thermal equilibrium with each other.

That defines the physical property "temperature". Any two objects with the same temperature will not show heat flow when they contact with one another.

$$T_A=T_C, T_B=T_C\Rightarrow T_A=T_B$$

First law of thermodynamics

The first law of thermodynamics is an application of conservation of energy to thermodynamic processes. When mass does not enter or leave the system, it is called a closed system. In a closed system, change in internal energy of the system $\Delta U$ is equal to the sum of the heat transferred to the system $Q$ and the work done to the system $W$:

$$\Delta U=Q+W$$

Second law of thermodynamics

Entropy is a measure of how spread out mass-energy is in a system. In many cases, this spreading out is interpreted as chaos and disorder.

The second law of thermodynamics states that the total entropy of an isolated system can only remain constant or increase.

Assume the whole universe is an isolated system, then its entropy never decreases:

$$\Delta S_{universe}\geq 0$$

Another perspective is that no spontaneous process (without the need of work done) in nature decreases entropy.

Third law of thermodynamics

Absolute zero temperature is expressed as $0 K=-273.15\ ^\circ C=-459.67\ ^\circ F$. At that temperature, no energy can be removed from matter as heat, and the entropy of the system is zero.

The third law of thermodynamics states while the entropy of a perfect system is zero when the temperature of the system is absolute zero, no process with finite steps can lead a system to absolute zero. Experiments show absolute zero can be approached very closely, but not reached.