Thermodynamics is that branch of physics which is concerned with transformation of heat into mechanical work.
It deals with the concepts of heat, temperature and interconversion of heat into other forms of energy i.e., electrical, mechanical, chemical magnetic etc.
Thermodynamics does not take any account of atomic or molecular constitution of matter and it deals with the bulk systems.
State of any thermodynamic system can be described in terms of certain know macroscopic variables known as thermodynamic variables.
Thermodynamic variables determine the thermodynamic behaviour of a system . Quantities like pressure(P), volume(V), and temperature(T) are thermodynamic variables. Some other thermodynamic variables are entropy, internal energy etc. described in terms of P, V and T
A thermodynamic system is said to be in thermal equilibrium if all parts of it are at same temperature.
Thus two systems are said to be in thermal equilibrium if they are at same temperature.
2. Concept of Heat
Heat may be defined as energy in transit.
Word heat is used only if there is a transfer of energy from one thermodynamic system to the another.
When two systems at different temperatures are kept in contact with each other then after some time temperatures of both the systems become equal and this phenomenon can be described by saying that energy has flown from one system to another.
This flow of energy from one system to another on account of temperature difference is called heat transfer.
Flow of heat is a non-mechanical mode of energy transfer.
Heat flow depends not only on initial and find states but also on path it's.