1Chapter 5: The 2ndLaw of ThermodynamicsAn IntroductionThis 1000 hp engine photo is courtesy of Bugatti automobiles.Motivating The 2nd Law of Thermodynamics• The 1stlaw of thermodynamics alone does not predict the direction of a process, e.g.– Can a hot object naturally cool down to a temperature below its surrounding?– Can a hot mass return to its initial position by losing its internal energy?• The first law does not distinguish between reversible and irreversible processes• The 2ndlaw can be used in conjunction with the 1stlaw to determine the capability (e.g., max efficiency) of a process.2Spontaneous ProcessesObjects spontaneously tend to cool Fluids move from higher to lower pressure environments spontaneouslyObjects spontaneously fall from elevated positionsSpontaneous processes allows occur in a predictable direction, and have the potential to produce workComments• A spontaneous process takes place on its own but its inverse would not take place spontaneously• There is an opportunity to develop work from an spontaneous process that otherwise would be lost (e.g., turbine, pulley)• If work is developed from s spontaneous process– What is the max theoretical limit– What factors would preclude its realization3The Many Uses of the 2ndLaw• Predict process direction• Establish equilibrium conditions• Determine theoretical best performance• Evaluate factors limiting best performance• Define a temperature scale independent of properties• Develop means for property evaluation for derived properties, such as h and uStatements of the 2ndLaw• Kelvin-Planck Statement• It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy from a single thermal reservoir• Clausius Statement• It is impossible for any system to operate in such a way that the sole result would be an energy transfer by heat from a cooler to a hotter body4Proof Illustration•K-P⇒Clausius • Clausius⇒K-PReversible vs. Irreversible• A process is called irreversible if the system and all parts of its surroundings cannot be exactly restored to their initial values• A Process is reversible if both the system and surroundings can be returned to its initial states.• An irreversible process may be returned to initial state but not if combined with surroundings• All real-world processes are irreversible5Irreversibilities• Heat transfer through a finite temperature difference• Unrestrained expansion of a gas or liquid• Spontaneous chemical reaction• Spontaneous mixing• Friction (sliding and flow)• Electric current flow through a resistance• Magnetization or polarization with hysteresis• Inelastic deformation• And many more …ClipArt courtesy of PowerPoint 2002ClipArt courtesy of PowerPoint 2002ClipArt courtesy of PowerPoint 2002Internal vs. External Irreversibilities• For engineering analyses the internalirreversibilities may be considered as opposed to total.• A process is internally reversible if there are no internal irreversibilities• An internally reversible process can return to its initial state• It consists of a series equilibrium states, i.e., quasiequilibrium prcocess6How to Prove Irreversibility• The proof is typically by contradiction• First suppose the process is reversible• Put together a series of additional reversible (ideal) processes to form a thermodynamic cycle• Show that existence of such a cycle would violate the Kevin-Planck Statement• Example: Heating due to FrictionExample: Irreversibility of Friction7Examples of Reversible Processes• Frictionless mass-spring or pendulum• Adiabatic expansion or compression in friction-less piston cylinder• Statble equilibrium statesPower Cycles (Heat Engines)1cycleLHHWQQQη==−8Refrigeration and Heat Pump CyclesRefrigeration Heat PumpsCCcycle H CQQWQQβ==−HHcycle H CQQWQQγ==−Carnot Corollaries• The thermal efficiency of an irreversible power cycle is always less than that of a reversible one when each operates between the same two reservoirs.• All reversible power cycles between the same two thermal reservoirs have the same thermal efficiency.• There are similar corollaries for refrigeration and heat pump cycles.9Proof Of Carnot CorollaryKelvin Scale• Carnot corollaries imply that for a reversible power cycle Qc/QH= ψ(Tc,TH)–Qc: Heat from system to cold reservoir–QH: Heat from hot reservoir to system–Tc,TH: cold and hot reservoir temperature– ψ: unspecified function• Kelvin scale: ψ(Tc,TH)=Tc/TH10The Kelvin Temperature ScaleCCHHrevQTQT⎛⎞=⎜⎟⎝⎠273.16revHcycleQTQ⎛⎞=⎜⎟⎝⎠273.16 K is the Triple Point temperature of waterClipArt courtesy of PowerPoint 2002Maximum Performancemax1CHTTη=−Heat Engines Refrigerators & Heat PumpsmaxCHCTTTβ=−maxHHCTTTγ=−11Carnot CycleReversible power cycle: Two adiabatic processes alternated with two isothermal processesCarnot power cycles operated in reverse may be regarded as a reversible refrigeration or heat pump cycle, with maximum coefficient of performanceExample: Problem 5.3812Problem 5.40Problem