Refrigerators Physics 313 Professor Lee Carkner Lecture 13.

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Presentation transcript:

Refrigerators Physics 313 Professor Lee Carkner Lecture 13

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)( ) = J 

Limits on Engines  Engines convert heat into work and waste heat   Second Law of Thermodynamics   An engine cannot have 100% efficiency

1st and 2nd Laws  Converting heat completely into work does not violate the 1st law   The second law is an independent statement 

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 )

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

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

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  

Air Conditioner   Air conditioners also have condensers that dry out the air 

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

Refrigerators and the Second Law  You cannot move heat from low to high temperature without the addition of work  

Statements of the Second Law  Kelvin-Planck Statement:   Clausius Statement: 

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

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) 