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MECH 213 - 10.2.07

MECH 213 - 10.2.07 - o Impossible to have a device that...

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MECH 213 – October 2, 2007 Chapter 6 Begins (Second Law of Thermodynamics) Second Law of Thermodynamics o Governs direction of heat transfer o Heat moves from hot to cold in order to decrease entropy Thermal Reservoir o Region with uniform and constant temperature that only undergoes heat transfer interactions o Oceans, atmosphere Heat engine o Takes heat in from a high temperature reservoir o Converts some of the heat into work o Rejects the remainder of the heat into a lower temperature reservoir o Operates on a thermodynamic cycle (i.e. beginning and end are the same) First law for a cyclic process Q net-in = W net-out Q in – W out = 0 o Q H – Q L = W net-out η th = (desired output)/(required input) = W net-out /Q H = (Q H – Q L )/Q H = 1 – Q L /Q H = W net /Q H o where η th is thermal efficiency Kelvin – Planck – First form of the Second Law of thermodynamics
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Unformatted text preview: o Impossible to have a device that interacts with a single reservoir and produces net work out Always must reject heat Efficiency can never be 1.0 must be < 1.0 Reverse Heat Engine o Refrigerator /Air Conditioner o Heat Pump First Law • W in = Q H- Q L Coefficient of Performance • COP Ref = Q L /W in = Q L /(Q H – Q L ) = 1/[(Q H /Q L ) – 1] • COP HP = Q H /W in = Q H /(Q H – Q H ) = 1/[1 – (Q L /Q H )] • COP HP = COP Ref + 1 Clausius – Second Form of the Second Law of Thermodynamics o Impossible to take heat from a lower temperature reservoir and move it to a high temperature reservoir without work input T H Q H Q L T L W net-out Q L } Ref T L Ref/HP W net-in T H Q H } HP...
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