The origin of the second law of thermodynamics was in Carnot's principle. It refers to  Carnot cycle of heat engine, fictively operated in the limiting mode of extreme slowness known as quasi-static, so that the heat and work transfers are between subsystems that are always in their own internal states of thermodynamic equilibrium. The carnot cycle is an ideal process.

The second law of thermodynamics may be expressed in many specific ways, the most well known statements being the statement by Rudolf Clausius (1854) and the statement by Lord Kelvin (1851). These statements cast the law in general physical terms citing the impossibility of certain processes. The Clausius and the Kelvin statements have been shown to be equal.


                     

The 2 statements of second law of thermodynamics are :
  1. Kelvin-Plank statement
  2. Clausius statement.

Kelvin-Plank statement

It is impossible to create a heat engine which converts all the supplied heat to it into equivalent work.

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 statement is regarding heat engines. The main function of an engine is to convert heat energy (combustion) into mechanical work. However, all amount of heat or input given to the engine doesn’t translate into equivalent amount of work. This is due to energy losses caused in the process. The major losses include friction and heat lost to the surroundings. As show in figure below.The Fig shows the kelvin-plank statement working through an example of a car engine.

Clausius statement

It is impossible to create a heat pump or refrigerator which removes heat from low temperature body and delivers it to a high temperature body except that energy in the form of work is given to the heat pump or refrigerator.

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A refrigerator’s work is to cool, and it does so by removing heat from the body. However, heat naturally flows from high level to low level thus, to do the opposite ie. remove heat from low level and deliver it to high level the refrigerator requires energy to do so. This energy is supplied to it in the form of electricity. As the refrigerator removes heat from low level, the low level naturally goes on becomig cooler and cooler as shown in fig. below. The Fig shows the clausius statement working through an example of a refrigerator compressor.

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