Zeroth Law
If two systems are both in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.
- Is essentially transitive relation between different systems.
First Law
In a closed system (i.e. there is no transfer of matter into or out of the system), the first law states that the change in internal energy of the system (ΔU) is equal to the difference between the heat supplied to the system (Q) and the work (W) done by the system on its surroundings.
- Is essentially a law of conservation of energy.
Second Law
When two initially isolated systems in separate but nearby regions of space, each in thermodynamic equilibrium with itself but not necessarily with each other, are then allowed to interact, they will eventually reach a mutual thermodynamic equilibrium. The sum of the entropies of the initially isolated systems is less than or equal to the total entropy of the final combination. Equality occurs just when the two original systems have all their respective intensive variables (temperature, pressure) equal; then the final system also has the same values.
- The second law implies the existence of a physical quantity- entropy.
Third Law
A system’s entropy approaches a constant value as its temperature approaches absolute zero.