Engineering Thermodynamics/Zeroth Law
The Zeroth law of Thermodynamics can be stated as:
If two thermodynamic systems A and B are in thermal equilibrium, and B is also in thermal equilibrium with another system C, then A and C are in thermal equilibrium. OR
If a body isolated from the other environment is in thermal equilibrium with one body & is separately in thermal equilibrium with the another body then three bodies are said to be in the thermal equilibrium with each other.—Max Planck, The Theory of Heat Radiation.
This may seem obvious as we are quite familiar with this experiment. When we place in a cup of water (System A) a thermometer (System B) we wait a period of time until they reach equilibrium then read the measurement on the thermometer.
It is called the Zeroth Law as it is not derivable from the other laws and is often useful to understand the concept before presenting the other laws of thermodynamics.
It also states that at absolute zero temperature (i.e. zero Kelvin) all molecular motion inside a crystal ceases.
The application of zeroth law is mainly seen in the thermodynamic properties.