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Second Law of Thermodynamics Physics 202 Professor Lee Carkner Lecture 18

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PAL #17 Internal Energy II 3 moles of He at 300 K, raised to 400 K Fixed piston Constant pressure H 2 gas, constant volume H 2 gas, constant pressure Rank by heat: d > c = b > a

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Irreversible Free Expansion

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Irreversible and Reversible Processes

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Second Law of Thermodynamics No real process is truly reversible (due to friction, turbulence etc.), so we can say: This is the second law of thermodynamics Entropy always increases

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Hero’s Door Opener (1 AD)

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Steam Engines (18th century)

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Internal Combustion Engine (late 19th century)

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Engines An engine is a device for converting temperature differences into work by continuously repeating a set of processes

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Engine Elements

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p-V and T-S Engine Diagrams

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The Stirling Engine As an example, we will examine the Stirling Engine In between is an insulated chamber which can temporarily store energy

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Stirling Engine Diagram QHQH QCQC THTH TCTC Hot Piston Cold Piston

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The First 2 Strokes 1) Isothermal Expansion 2) Isochoric process

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The Last 2 Strokes 3) Isothermal Compression 4) Isochoric process

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Sterling Engine Diagram

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Heat and Work Over the course of one cycle positive work is done and heat is transferred Since the total heat is Q H -Q C from the first law of thermodynamics E int =(Q H -Q C )-W =0

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Efficiency We get work out of an engine, what do we put into it? Q H is what you put in, W is what you get out so the efficiency is: = W/Q H

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Efficiency and Heat Since W=Q H -Q C we can rewrite efficiency as: The efficiency depends on how much of Q H is transformed into W and how much is lost in Q C :

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Efficiency and Entropy If we consider and engine as a closed system we must include the high and low temperature reservoir If all the processes are reversible, the change in entropy between the two reservoirs must be zero so: We can use this to rewrite the efficiency equation as:

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Ideal and Perfect Engines The above equations hold only for ideal engines It is also impossible to produce an engine where Q H is completely converted into work Called a perfect engine (no energy lost to heat)

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Perfect Engine

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Entropy and Real Engines On a practical level, you always have heat losses in an engine In other words the second law of thermodynamics can be stated:

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