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ME2320_CHAPTERVI

# ME2320_CHAPTERVI - CH VI ME2320 Thermo I The Second Law of...

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CH. VI ME2320 Thermo I The Second Law of Thermodynamics 68 THE SECOND LAW OF THERMODYNAMICS 1. INTRODUCTION TO THE SECOND LAW The Second Law of Thermodynamics states that processes occur in a certain direction and not in any direction. A typical example can be observed every morning in a cup of coffee. If a cup of hot coffee is left during a period of time in the kitchen, its temperature will decrease until it reaches the same temperature as that in the kitchen. Thus, the heat released by the cup of coffee is absorbed by the air in the kitchen. This concurs with the first law of thermodynamics. However, the inverse process, heat from the kitchen going into the cup of coffee, that it would not violate the first law of thermodynamics, will never occur spontaneously. From this and some other examples is possible to establish that processes proceed in certain direction and not in the reverse direction. The first law of thermodynamic places no restriction on the direction of a process, therefore, satisfying the first law of thermodynamics does not ensure that a certain process can actually occur. Thus, the second law of thermodynamics is a general principle that determines whether a process can occur. And finally, a process cannot occur unless satisfies the first and second laws of thermodynamics. In addition to establishing whether a process can occur or not, the second law of thermodynamics can be used to determine the quality of the energy. 2. THERMAL ENERGY RESERVOIRS In order to establish the second law of thermodynamics is necessary to define a hypothetical body with large thermal energy capacity (mass × specific heat) that can supply or absorb finite amounts of heat without undergoing any change in temperature. This body is called thermal energy reservoir . In practice, large bodies of water, such as oceans, lakes or rivers can be modeled accurately as thermal energy reservoirs because of their large energy storage capabilities. A reservoir that supplies energy in the form of heat is called Source and one that absorbs energy in the form of heat is called sink . The thermal energy reservoirs are called heat reservoirs . 3. HEAT ENGINES Work can be easily converted into other forms of energy; however, converting other forms of energy into work is not simple. That is, work can be easily converted into heat, but transforming heat into work requires the use of special devices called heat engines . Heat engines differ from each other depending of the task that they perform, but in general they are characterized by:

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CH. VI ME2320 Thermo I The Second Law of Thermodynamics 69 They receive heat from a high temperature source They transform part of this heat into work They reject the remaining heat waste to a low temperature sink They operate on a cycle Heat engines and other cyclic devices usually involve a fluid to and from which heat is transferred while undergoing a cycle. This fluid is called working fluid . The classic and typical example of a heat engine is a steam power plant. A steam power plant is an
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ME2320_CHAPTERVI - CH VI ME2320 Thermo I The Second Law of...

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