SEM314 Topic 5 - 2nd Law of Thermodynamics

SEM314 Topic 5 - 2nd Law of Thermodynamics - Topic 5 The...

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Deakin University 2003 Topic 5 The Second Law of Thermodynamics Throughout this topic you will need to refer to chapter 4 in your textbook. Introduction The following section will help you to deal with the Second Law of Thermodynamics, entropy, and irreversibility. Before we go into details about the Second Law of Thermodynamics, let us first see what is meant by a heat engine and a heat pump. The heat engine A heat engine is a device which operates a cycle and develops work when some heat is supplied to it. The key words here are ‘cycle’, ‘heat’ and ‘work’ (namely a heat engine converts heat into work through a cycle ). A system diagram for a ‘heat engine’ is shown in figure 5.1. It should be noted that the term ‘heat engine’ does not necessarily require mechanical components such as gears, shafts, or piston cylinders, but it is any device that converts heat into work. It can be as big as a whole power station or as small as a car engine. In the operation of a heat engine, it is essential that heat is transferred from some high temperature source (which may physically be within the system boundary or may be more than one source) to the heat engine. The heat engine then converts the transferred energy (heat) to mechanical work and dumps the excess into a low temperature sink. This is the general operation of all heat engines.
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5-2 Principles of Thermodynamics Deakin University 2003 Figure 5.1 System diagram for a heat engine According to the First Law of Thermodynamics, Net heat supplied = Net work done Σ dQ = Σ dW Hence Q 1 – Q 2 = W The heat pump/refrigerator The heat pump/refrigerator (figure 5.2) is actually a reversed heat engine, which can transfer heat from cold reservoir to hot reservoir. However, in order to do this, some external work input is necessary. For the reversed heat engine cycle, if our interest is at the hot end, that is, supply of heat to hot reservoir, then the cycle is called a heat pump cycle: pumping heat from low (temperature) to high (temperature). If, for the same cycle, our interest is at the cold end, that is, extraction of heat from cold reservoir, then the cycle is a refrigerator cycle. Figure 5.2 System diagram for a heat pump/refrigerator
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Topic Five The Second Law of Thermodynamics 5-3 Deakin University 2003 For the heat pump/refrigerator cycle the first law gives Q 1 = Q 2 + W And the second law tells us that W > 0 Note: In the above first law equations, the state of the heat engine (reversed heat engine) does not change after a cycle, so that the internal energy item Δ u in the (original) energy equation has been eliminated. The statements of the Second Law of Thermodynamics We know that the first law is based on the conservation of energy principle. The Second Law of Thermodynamics says that although the net heat supplied in a cycle is equal to the net work done, the gross heat supplied must be greater than the net work done.
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SEM314 Topic 5 - 2nd Law of Thermodynamics - Topic 5 The...

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