Second law of thermodynamics

Second law of thermodynamics

1. Thermodynamics Second Law of Thermodynamics Prepared By, Jay Joshi (17MSE005) SSE, PDPU

2. The Necessity of The S econd l aw The first law of thermodynamics does not give any information on whether that change of state or the process is at all feasible or not. It is the second law of thermodynamics which provides the criterion as to the probability of various processes.

3. Second Law of Thermodynamics A process can not happen unless it satisfies both the first and second law of thermodynamics. The first law defines about the “quantity” of the energy, while the second law defines the direction which the process can take place and its “quality”. There are two statements defining the second law of thermodynamics. Kelvin-Planck statement of second law and Clausius statement of second law

4. Heat Engines A heat engine is a device which converts heat or thermal energy to mechanical energy while operating in a cycle. Heat engine Q H Q L T H T L Net work done W net = Q H -Q L Thermal efficiency, h th is defined as h th = W net /Q H =(Q H -Q L )/Q H = 1-(Q L /Q H )

5. Kelvin-Plank Statement It state that “It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce net work.” In other words, it is impossible to build a heat engine with a 100% efficiency. Heat engine Q H T H W net A heat engine has to reject some energy into a lower temperature sink in order to complete the cycle. The higher the reservoir temperature, the higher the quality of the energy and more work is produced. This statement doesn’t have any proof, yet it hasn’t been violated yet.

6. Heat Pumps and Refrigerators A “heat pump” is defined as a device that transfers heat from a low-temperature source to a high-temperature one. E.g. a heat pump is used to extract energy from outside cold outdoor air into the warm indoors. A refrigerator performs the same function; the difference between the two is in the type of heat transfer that needs to be optimized. Heat pump/ Refrigerator Q H Q L T H T L W net

7. Heat Pumps and Refrigerators For a Heat Pump: COP HP =Q H / W net =Q H /(Q H -Q L ) = 1/(1-Q L /Q H ) For a Refrigerator: COP R =Q L / W net =Q L /(Q H -Q L ) = 1/(Q H /Q L -1) Note : COP HP = COP R + 1 COP HP >1 The efficiencies of heat pumps and refrigerators are denoted by the Coefficient of Performance (COP) where

8. Clausius Statement The Clausius statement is another expression of the second law of thermodynamics. It states that, It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body. In other words, heat can not be transferred from low temperature to higher temperature unless external work is supplied. Heat pump Q H Q L T H T L Therefore, it is impossible to build a heat pump or a refrigerator which can work without a work input.

9. Thank You Sources : Basics and Applied Thermodynamics by P. K. Nag Thermodynamics : An Engineering Approach by Y. A. Cengel