ZEROTH LAW OF THERMODYNAMICS

Statement 

"The zeroth law of thermodynamics state that if each of the two given systems are in thermal equilibrium with third system then the two given systems are in thermal equilibrium with each other."

• Zeroth law of thermodynamics gives us the concept of temperature and thermometer.
• A thermometer is a portable device whose thermal state is related linearly to some simple property, for example its density or pressure. 
• Once a suitable temperature scale is defined for the device, one can use it to measure the temperature of a variety of disparate systems in thermal equilibrium. Temperature thus characterizes thermal equilibrium.
• Thermal equilibrium means, the object will approach the same temperature and in the absence of loss to the other objects, they will maintain a single constant temperature. 
• Zeroth law of thermodynamics was discovered way after the first second and third law of thermodynamics were discovered. But zeroth law is the most basic law. That's why this law is named as zeroth law of thermodynamics.
• For example: 

1. If A and C are in thermal equilibrium with B, then A is in thermal equilibrium with B. If an object with a higher temperature comes in contact with a lower temperature object, it will transfer heat to the lower temperature object. That means all three bodies are at the same temperature.
2. Consider two separate cups of boiling water. If we place a thermometer into the first cup, it gets warmed up by the water until it reads 100°C. We now say that the thermometer is in thermal equilibrium with the first cup of water. Next, we move the thermometer into the second cup of boiling water, and it continues to read 100°C. The thermometer is therefore also in thermal equilibrium with the second cup of water. Using the logic of the zeroth law, we can conclude that the two separate cups of boiling water are in thermal equilibrium with each other. The zeroth law therefore enables us to use thermometers to compare the temperatures of any objects we like.