Enthalpy and Entropy and Internal energy

Internal energy in thermodynamics 

Internal energy is defined as the sum of microscopic energies of all the molecules of a system which may appear in several complex forms. These forms may include 

  • Inter-molecular potential energy, associated with   forces between the molecules. 
  • Molecular K.E associated with transitional and rotational velocity of each molecules.
  • Intra molecular energy within the molecular/atomic structures and related forces. 
Internal energy is primarily and function of temperature. The term internal energy was first used    by Rudolph  Clausius And William Rankin in 1852. 


Definition of Enthalpy

Enthalpy is mathematical combination of internal energy and pressure energy of the fluid. It is expressed as H= U + pV in extensive or h= u+pv in intensive forms. Common expression in S.I system has a unit of kJ/kg (intensive). 
Enthalpy was introduced by German scientist R.Mollier, who is also famous for the Mollier charts expressing properties of stem. Initially he was Enthalpy was called Heat content. 


Definition of entropy : 

Entropy (S.s) represents the disorder of uncertainty of microscopic level, even yet it is used as a property at macroscopic level. Entropy is specified for every equilibrium state of a pure substance. Entropy is also mainly a function of temperature. In 1865 Rudolph Clausius first introduced the term entropy.

I hope understand the difference between enthalpy and entropy. Don't get confused. Try to understand Enthalpy vs. Entropy.

These three properties mentioned are more like mathematical entities having important physical significance. We are more interested in changes of these properties  i.e. ds. du etc. rather than individual values which may depend on reference of calculation.



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