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
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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