Second Law of Thermodynamics:

  1. First law of thermodynamics states the equivalence of heat and energy.
  2. It does not state anything about the limitation in the conversion of heat into work or about the condition necessary for such conversion.
  3. The second law of thermodynamics is a generalization of certain experience and observation and is concerned with time direction in which energy flow takes place.
  4. This law can be stated in a number of ways. Although differently said, they are essentially equivalent.
    1. Kelvin Plank Statement:
      “It is impossible to construct a device which, operating in a cycle, has a sole effect of extracting heat from a reservoir and performing an equivalent amount of work”.
    2. Clausius Statement:
      “It is impossible for a self acting machine, unaided by external agency, to transfer heat from a colder body to a hotter body”.
  5. It can be proved that these two statements of second law are completely equivalent and violation of Kelvin Plank statement leads to violation of Clausius statement and vice versa.

Reversibility and irreversibility

  1. The reversible process is the one which can be retracted in opposite order by changing external conditions slightly.

Necessary conditions for a reversible process:

  1. The process must be quasi- static. For this, the process must be carried out infinitesimally and mechanical equilibrium with the surroundings throughout.
  2. The dissipative forces such as viscosity, friction, inelasticity, etc. should be absent.
  3. Those processes which cannot be retracted in opposite order by reversing the controlling factors are known as the irreversible process.


  1. Diffusion of gases.
  2. Dissolution of salt in water.
  3. Rusting of iron.
  4. Sudden expansion or contraction of a gas.
  5. It is a consequence of the second law that all the natural processes are the irreversible process.
  6. Conditions for reversibility of a process are:
  7. The process is performed quasi-statically.
  8. It is not accompanied by any dissipative effects.
  9. It is impossible to satisfy these two conditions perfectly, thus the reusable process is purely an ideal abstraction.


Please enter your comment!
Please enter your name here