# 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.

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.