The second law of thermodynamics denies a system the possibility of utilizing energy in a particular or arbitrary way. There are two basic definitions of the second law which were stated at different times by different people, historically speaking.
It is impossible to construct an engine which is operating in a cycle that will produce no effect other than the extraction of energy from a reservoir and the performance of an equivalent amount of work. Thus an efficiency of 100% is not a possibility.
It is impossible to construct a device which while operating in a cycle will produce no effect other than the transfer of energy from a colder to a hotter body. Thus a refrigeration unit requires energy input as work; otherwise it cannot function. These two statements are equivalent and the assumption of the validity of one leads to the situation given by the other. Consider that the Kelvin-Planck statement is valid. If one now constructs a refrigerator running with the work of the engine and extracting energy [-Qc] from the cold reservoir, then the engine-refrigerator system is one which absorbs energy from a cold reservoir and transfers it to the hot reservoir without any external work, a situation which the Clau-sius statement refutes. If this were possible, the energy in the oceans at a low and cold temperature could be utilized to run all kinds of equipment without any external work; an impossible situation known to all.
If now the Clausius statement is considered to be valid, then the refrigerator functions without any external work. At the same time, an engine placed between the same two reservoirs produces work. The refrigerator-engine system leads to a situation where an amount of energy is absorbed from a hot reservoir and an equivalent amount of work is performed, a situation that the Kelvin-Planck statement refutes. If this were valid, all the fuel consumed in the engine of a car would be used in the form of work without any losses whatsoever another impossible situation that is well known to all. All physical systems must function in such a way as not to violate either one of these statements. Otherwise a perpetual motion machine of the second kind will result.
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