(a) What are the limitations of first law of thermodynamics?
(b) Two identical samples of a gas are expanded so that volume is increased to twice. One is expanded isothermally and second adiabatically. In which sample, pressure is greater? Explain.
Limitations of the First Law of Thermodynamics are:
(a) First law of thermodynamics does not tell us about the direction in which a given process can take place and up to what extent it can take place.
(b) Let P and V be the pressure and volume of gas.
Let Pi be the pressure of gas when expanded isothermally.
Pa be the pressure of gas when expanded adiabatically. 
∴ 
∴ 
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Here, we can see, 
Therefore, 
.
So, pressure is greater in the isothermal process.
