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The zeroth law of thermodynamics states that if twothermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other.
It is called the zeroth (0th) law of thermodynamics because it was discovered after the others, but people thought it was more important so they wanted to give it a number before the other laws.
The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes:
If Q is the energy supplied to the system U the change in internal energy, and W the work done on the environment, First Law of Thermodynamics states that