ME235Lecture_9

# ME235Lecture_9 - Thermodynamics Vm235 Lecture 9 Chapter 8...

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1 Thermodynamics Vm235 Lecture 9 Chapter 8 – Entropy What we have learned so far about the 2 nd Law: ¾ The 2 nd Law tell us the direction of spontaneous change. ¾ The 2 nd Law gives us the maximum possible efficiency of a heat engine and a the maximum possible coefficient of performance (COP) of a refrigerator. ¾ To get the maximum possible efficiency we must have a reversible engine. [Frictionless, adiabatic, quasi-steady Æ always in equilibrium] ¾ A reversible process is one in which both the system and the surroundings can be returned to their original state. ¾ One realization of such an engine is a Carnot Engine. ¾ All reversible engines have the same efficiency. ¾ This maximum efficiency depends only on the temperature of the reservoirs – this yielded the Absolute Temp. Scale (K)

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2 The Second Law says that spontaneous change will occur only in the direction indicated. Always from the state of lower disorder to higher disorder Hot Body Cold Body Thermal Equilibrium (same T) YES NO High Pressure Vacuum Uniform pressure throughout YES NO Oxygen Nitrogen Uniform mixture throughout YES NO To use the 2nd Law in a quantitative manner we need to find a property that will measure the level of disorder . We did the same for the 1st Law where we defined a measure of energy – internal energy. This property is called ENTROPY Kelvin-Plank Statement of the 2 nd Law: It is impossible to construct a device Q H T H The recognition that there are 2 class of processes – spontaneous & non-spontaneous is summarized in the 2 nd Law: that operates in a cycle and produce no effect other than raising of a weight and exchange heat with a single reservoir. W Impossible Spontaneous changes are always accompanied by a dispersal of energy into a more disordered form. Entropy is a measure of disorder – it is a property that increases during spontaneous change.
3 The Clausius Inequality 0 T Q δ : engine reversible a For Consider an engine : 0 > = L H that showed we scale e temperatur absolute the defining While Q Q Q since : = < L H rev irr W Q Q W W engine le irreversib an For 0 : = = = = L L H H L L H H L H L H T Q T Q T Q thus T Q T Q or

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ME235Lecture_9 - Thermodynamics Vm235 Lecture 9 Chapter 8...

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