Second law of thermodynamics- statements and examples | The Mechanical post

Welcome back to The  Mechanical post! Today well see What is the Second law of thermodynamics? both the statements as well as examples of it.         

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Second law of thermodynamics- statements and examples | The Mechanical post


Introduction to Second law of thermodynamics

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 Kelvin statements are equal.                     

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

Kelvin-Plank statement

Given below is the Kelvin-Plank statement:
It is impossible to create a heat engine which converts all the supplied heat to it into equivalent work.

Kelvin-plank statement of Second law of thermodynamics

Explanation:

The Kelvin statement deals with heat engines. First, let us get acquainted heat with a heat engine. The main function of an engine is to convert heat energy (combustion) into mechanical work.

However, all amount of heat or input which is given to the engine doesn’t translate into an equivalent amount of work. This is due to the energy losses caused by the process. The major losses include frictional losses and heat lost to the surroundings. The above fig shows the kelvin-plank statement working through an example of a car engine. 

The input to the engine is the heat energy of the fuel. Due to the friction between the moving parts of the engine as well as heat lost to the surroundings result in less energy or power available at the end of the process i.e. the movement of the car.

Thus, all the supplied input energy cannot be converted into equivalent work.

Clausius statement

Given below is the 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.

Clausius statement of Second law of thermodynamics

Explanation:

This statement deals with refrigeration. A refrigerator’s work is to cool, and it does so by removing heat from the body. However, heat flows naturally from high level to lower level thus, to do the opposite ie. remove heat from low level and deliver it to a high level the refrigerator requires energy in the form of work to do so. 

This energy is supplied to it in the form of electricity. As the refrigerator removes heat from a low level, the lower level naturally goes on becoming cooler and cooler as shown in the figure above. Fig shows the Clausius statement working through an example of a refrigerator compressor. 

We have written a post on the Vapour compression refrigeration cycle which explains how the refrigeration or cooling effect is obtained by the refrigerator. 

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