If the temperature of 'A' is equal to the temperature of 'B' and 'C’, then the temperature of 'B' will be equal to the temperature of 'C'. This is known as:  

Concept:

Zeroth law of thermodynamics:

• Zeroth law of thermodynamics states that "if two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other" as shown below.

• This figure shows that when body A is in thermal equilibrium with body B, and also separately with body C, then B and C will be in thermal equilibrium with each other.
• This is the basis of temperature measurement and is used to explain the concept of temperature.

First law of thermodynamics:

• The first law of thermodynamics enables to calculate the change in internal energy of the system
•  As, δQ = ΔU + δW
• or, ΔU = δQ – δW
• The first law of thermodynamics only gives information about the energy balance when a system undergoes a change of state.
• But it does not help to predict whether the change of state is feasible or not.
• The second law of thermodynamics provides the criterion for the probability of various processes.
• The first law of thermodynamics enables one to determine the property, the internal energy.

Second law of thermodynamics:

• Second law of thermodynamics leads to expressions that involve inequalities. The important inequality that has a major consequence is Clausius inequality.
• It is expressed as,  
• For a reversible process,  
• Since the cyclic integral of a property is zero, therefore the quantity  must represent a property, this property is named as entropy as designated as S.
• Therefore entropy is related to second law of thermodynamics.

Important Points 

1. Zeroth law – concept of temperature
2. First law – concept of internal energy
3. Second law – concept of entropy