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Chapter: Mechanical - Engineering Thermodynamics - The Second Law of Thermodynamics

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Limitations of First Law of Thermodynamics

If a well insulated tank of fluid is stirred by a rotating paddle wheel, the energy of the fluid increases. If the stirrer is stopped, however the energy of the fluid will not decrease and cause the stirrer to rotate in the opposite direction.


Limitations of First Law of Thermodynamics


If a well insulated tank of fluid is stirred by a rotating paddle wheel, the energy of the fluid increases. If the stirrer is stopped, however the energy of the fluid will not decrease and cause the stirrer to rotate in the opposite direction. The possibility of this process proceeding in the opposite direction is not excluded by the first law of Thermodynamics. Hence first law of thermodynamics does not allow us to predict whether a proposed conceived energy conversion is possible or not.

 

In all the internal combustion engines fuel and air mixture is supplied at room temperature. This mixture undergoes combustion inside the engine and gives out work. Exhaust gases coming out of the engine are always at higher temperature, indicating that some heat is taken away into atmosphere. Hence, in all the IC engines only a part of the heat is converted into work. From our experience we know that if any attempt is made to convert all the heat into work, our effort will go in vain. This limitation in the extent of energy conversion has also not been addressed in first law of thermodynamics.

 

The first law of thermodynamics tells us that energy can be changed from  one form to another but can be neither created nor destroyed in any process. The mathematical expression which is used to represent the first law of thermodynamics is DE = q-w, where DE refers to the internal energy change of the system when 'w' amount of work is done by the system when it absorbs 'q' amount of heat and carries out 'w' amount of work. However, this law possesses many limitations such as given below.

1. The first law of thermodynamics merely indicates that in any process there is a transformation between the various forms of energy involved in the process but provides no information regarding the feasibility of such transformation.

2. First law does not provide any information regarding the direction a processes will take whether it is a spontaneous or a non spontaneous process.


The Second law of Thermodynamics

 

Kelvin Planck’s: It is impossible statement to constructa device that, operating continuously, will produce no effect other than transfer of heat from a single thermal reservoir and performance of an equal amount of work.

 

The term thermal reservoir refers to a very large system in stable equilibrium, to which or from which, any amount of heat can be transferred at constant temperature.

 

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