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
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.