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Refrigerators Physics 313 Professor Lee Carkner Lecture 13
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Exercise #12 Engines V 1 = 6.25X10 -4 m 3, P 1 = 12X10 6 Pa, n = 3 moles P 1 V 1 = P 2 V 2 P 2 = P 1 V 1 /V 2 = 385 MPa = 1 – T 1 /T 2 = 0.75 W = Q H, Q H = nc V T 23 P 3 = 500X10 6 Pa, V 3 = 7.8X10 -5 m 3 Q H = (3)(3/2)(8.31)(1564-1205) = 13424 J
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Limits on Engines Engines convert heat into work and waste heat Second Law of Thermodynamics An engine cannot have 100% efficiency
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1st and 2nd Laws Converting heat completely into work does not violate the 1st law The second law is an independent statement
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Refrigerators A refrigerator is a device that uses work to move heat from low to high temperature A heat pump does this to heat a room (want large Q H )
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How a Refrigerator Works Fluid flows through the cold chamber and evaporates, adding heat Q L to the fluid from the chamber The fluid is pumped into the hot chamber and compressed, adding work W The fluid condenses releasing heat Q H
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Refrigerator Cycle Liquid Gas Compressor (work =W) Expansion Valve Heat removed from fridge by evaporation Heat added to room by condensation High Pressure Low Pressure QLQL QHQH
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Refrigerator Performance The equivalent of efficiency for a refrigerator is the coefficient of performance K K = Q L /(Q H -Q L ) Unlike efficiency, K can be greater than 1
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Air Conditioner Air conditioners also have condensers that dry out the air
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Heat Pump The heat removed from the inside of a refrigerator is ejected into the kitchen A refrigerator that has the cold chamber as the outdoors and the hot chamber as the house is called a heat pump Many heat pumps can be reversed in summer to function as air conditioners
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Refrigerators and the Second Law You cannot move heat from low to high temperature without the addition of work
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Statements of the Second Law Kelvin-Planck Statement: Clausius Statement:
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Equivalence One implies the other For example: A 100% efficient engine connected to a high T reservoir powering a refrigerator cooling a low T reservoir to the same high T reservoir The refrigerator by itself is “legal” but the net effect to is move Q L from low to high T with no other effect
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Engines and Refrigerators Efficiency: = W/Q H = (Q H -Q L )/Q H = 1 - (Q L /Q H ) Can rewrite using: · · Coefficient of performance: K = Q L /W = Q L /(Q H -Q L ) (refrigerator)
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