Kelvin’s statement (second law of thermodynamics)
According to Kelvin, the second law of thermodynamics can be stated as follows:
“It is impossible to get a continuous supply of work from a body by cooling it to a temperature lower than that of its surroundings.”
This statement is applicable to heat engines. In a heat engine, the working substance absorbs some heat from the source (at higher temperature), converts a part of heat into mechanical work and rejects the remaining part to the sink (at lower temperature). Therefore, presence of sink is essential for continuous conversion of heat into work. We cannot obtain continuous supply of work from a single body i.e. the source of heat alone.
Clausius statement (second law of thermodynamics)
According to Clausius, the second law of thermodynamics can be stated as follows:
“It is impossible for a self acting machine by any agency to covey heat from one body to another at a higher temperature i.e. we cannot make heat flow from a colder body to a warmer body without doing external work.”
This statement is based upon the performance of a refrigerator, which is a heat engine working in the backward direction. In a refrigerator the working substance takes in heat from a cold body, a certain amount of work is done on it by some outside agency and rejects a larger amount of heat to a hotter body. Therefore, a refrigerator transfers heat from a cold body to a hot body with the aid of an outside agency.