any of three principles governing the relationships between different forms of energy. The first law of thermodynamics (law of conservation of energy) states that the change in the internal energy of a system is equal to the sum of the heat added to the system and the work done on it. The second law of thermodynamics states that heat cannot be transferred from a colder to a hotter body within a system without net changes occurring in other bodies within that system; in any irreversible process, entropy always increases. The third law of thermodynamics (Nernst heat theorem) states that it is impossible to reduce the temperature of a system to absolute zero in a finite number of steps

the principle that if two bodies are each in thermal equilibrium with a third body then the first two bodies are in thermal equilibrium with eachother