Second law of thermodynamics can be stated in many ways:
i) “It is impossible to construct an engine which operated in a complete cycle will absorb heat from a single body and convert it completely to work without leaving some changes in the working system”.
This is called as the Kelvin – Planck statement of II law of thermodynamics.
ii) “It is impossible to transfer heat from a cold body to a hot body by a machine without doing some work”.
This is called as the clausius statement of II law of thermodynamics.
iii) ‘A process accompanied by increase in entropy tends to be spontaneous”.
This statement is called as the entropy statement of II law of thermodynamics. Entropy is a measure of randomness or disorder of the molecules of a system and it is a thermodynamic state function. A system always spontaneously changes from ordered to a disordered state. Therefore entropy of a spontaneous process is constantly increasing.
iv) “Efficiency of a machine can never be cent percent”.
v) The heat Efficiency of any machine is given by the value of ratio of output to input energies. Output can be in the form of any measurable energy or temperature change while input can be in the form of heat energy or fuel amount which can be converted to heat energy.