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THE SECOND LAW OF THERMODYNAMICS - LECTURE - 8


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


Kelvin Planck’s statement :

It is impossible to construct a 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.

Clausius statement : 

Unaided by an external agency heat can not be transferred from a body at lower temperature to a body at higher temperature.

A thermal reservoir supplying heat continuously at constant temperature is known as source. (Example : Sun)

A thermal reservoir receiving heat continuously at constant temperature is known as sink. (Examples : River, Sea)

Reversible Process

A process is said to be reversible if it can be reversed without leaving any trace on the surroundings.

For example, let a system be taken from state 1 to state 2 with a work transfer of +5 kJ and heat transfer of −10 kJ. If the process is reversible, while taking the system from state 2 to state 1, the work transfer must be −5 kJ and heat transfer must be +10 kJ. So that, both the system and surroundings are returned to their initial states at the end of the process 2 to 1.


Irreversibility and Causes of Irreversibility

The factors that make a process irreversible are known as irreversibilities. Various forms of irreversibilities are listed below.

a) Friction

: Friction occurs at the interface of two bodies moving relative to each other. It is the main cause of irreversibility in many processes. Energy spent in overcoming friction is dissipated in the form of heat which can never be restored.

b) Heat transfer:

Once heat is transferred from a body at higher temperature to a body at lower temperature, it can never be reversed without the aid of an external agency.

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