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Thermodynamics



11. Principle of a Refrigerator


  • Refrigerators works in reverse direction of heat engines.
  • In refrigerators working substance extracts heat Q2 from sink at lower temperature T2
  • Some external work is performed by the compressor of refrigerator and then heat Q1 is rejected to the
    source, to the radiator of the refrigerator.

    Principle of a Refrigerator
    Coefficent of performance :
         β= Amount of heat absorbed from the cold reservoir / work done in running the mechinery
    Q2 - heat absorbed from cold reservoir.
    Q1 - heat rejected to hot reservoir during one complete cycle
    W = (Q1-Q2 ) is the work done in running the machinery
    thus,
         β= Q2/W =Q2/(Q1-Q2)          (18)
  • Like heat engines refrigerators can not work without some external work done on the system. Hence coefficent of performance can not be infinite.

12. Second law of thermodynamics


  • First law of thermodynamics states the equivalance of heat and energy.
  • It does not state anything about the limitation in the conversion of heat into work or about the condition necessary for such conversion.
  • Second law of thermodynamics is generalization of certain experience and observation and is concerned with tine direction in which energy flow takes place.
  • This law can be stated in number of ways. Although differently said, they are essentially equvalent.
    (i)Kelvin Plank Statment :
    "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".
    (ii)Clasius Statement :
    "It is impossible for a self acting machine, unaided by enternal agency, to transfer heat from a colder body to a hotter body".
  • It can ne proved that these two statements of second law are completely equivalent and voilation of Kelvin Plank statement leads to voilation of Clasius statement and vice-versa.

13. Reversibility and irreversibility


  • Reversible process is the one which can be retraced in opposite order by changing external conditions slightly.
  • Those processes which can not be retraced in opposite order by reversing the controling factors are known as irreversible process.
  • It is a consequence of second law that all the natural processes are irreversible process.
  • Conditions for reversibility of a process are
         (i) Process is performed quasi-statically
         (ii) it is not accompained by any dissipative effects.
  • It is impossible to satisfy these two conditions perfectly, thus requessible process is purely an ideal abstraction.



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