The four fundamental laws of thermodynamics express empirical facts and define physical quantities, such as temperature, heat, thermodynamic work, and entropy, that characterize thermodynamic processes and thermodynamic systems in thermodynamic equilibrium. They describe the relationships between these quantities, and form a basis for precluding the possibility of certain phenomena, such as perpetual motion. In addition to their use in thermodynamics, the laws have interdisciplinary applications in physics and chemistry.
Traditionally, thermodynamics has stated three fundamental laws: the first law, the second law, and the third law.[1][2][3] A more fundamental statement was later labelled the 'zeroth law'. The law of conservation of mass is also an equally fundamental concept in the theory of thermodynamics, but it is not generally included as a law of thermodynamics.
The zeroth law of thermodynamics defines thermal equilibrium and forms a basis for the definition of temperature. It says that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.
