15-1 The First Law of ThermodynamicsThe change in internal energy of a closedsystem will be equal to the energy added to thesystem minus the work done by the system onits surroundings. (15-1)This is the law of conservation of energy,written in a form useful to systems involvingheat transfer.
15-2 Thermodynamic Processes and the First Law An isothermal process is one where the temperature does not change.
15-2 Thermodynamic Processes and the First LawIn order for an isothermal process to takeplace, we assume the system is in contactwith a heat reservoir.In general, we assume that the systemremains in equilibrium throughout allprocesses.
15-2 Thermodynamic Processes and the First LawAn adiabatic process is one where there is noheat flow into or out of the system.
15-2 Thermodynamic Processes and the First LawAn isobaric process (a) occurs at constantpressure; an isovolumetric one (b) at constantvolume.
15-2 Thermodynamic Processes and the First LawIf the pressure is constant, the work done is thepressure multiplied by the change in volume: (15-3)In an isometric process, the volume does notchange, so the work done is zero. 10-6
15-2 Thermodynamic Processes and the First LawFor processes where the pressure varies, thework done is the area under the P-V curve.
15-4 The Second Law of Thermodynamics – IntroductionThe absence of the process illustrated aboveindicates that conservation of energy is not thewhole story. If it were, movies run backwardswould look perfectly normal to us!
15-4 The Second Law of Thermodynamics – IntroductionThe second law of thermodynamics is astatement about which processes occur andwhich do not. There are many ways to state thesecond law; here is one:Heat can flow spontaneously from a hot objectto a cold object; it will not flow spontaneously from a cold object to a hot object.
15-5 Heat Engines It is easy to produce thermal energy using work, but how does one produce work using thermal energy?This is a heat engine;mechanical energy canbe obtained fromthermal energy onlywhen heat can flow froma higher temperature toa lower temperature. 10-7
15-5 Heat EnginesWe will discuss only engines that run in arepeating cycle; the change in internal energyover a cycle is zero, as the system returns to itsinitial state.The high temperature reservoir transfers anamount of heat QH to the engine, where part ofit is transformed into work W and the rest, QL, isexhausted to the lower temperature reservoir.Note that all three of these quantities arepositive.
15-5 Heat EnginesA steam engine is one type of heat engine.
15-5 Heat EnginesThe internal combustion engine is a type of heatengine as well.
15-5 Heat EnginesWhy does a heat engine need a temperaturedifference?Otherwise the work done on the system in onepart of the cycle will be equal to the work doneby the system in another part, and the net workwill be zero.
15-5 Heat EnginesThe efficiency of the heat engine is the ratio ofthe work done to the heat input:Using conservation of energy to eliminate W,we find: (15-4a) (15-4b)
15-5 Heat EnginesThe Carnot engine was created to examine theefficiency of a heat engine. It is idealized, as ithas no friction. Each leg of its cycle is reversible.The Carnot cycle consists of:• Isothermal expansion• Adiabatic expansion• Isothermal compression 10-9• Adiabatic compressionAn example is on the next slide.
15-5 Heat EnginesFor an ideal reversible engine, the efficiency canbe written in terms of the temperature: (15-5)From this we see that 100% efficiency can beachieved only if the cold reservoir is at absolutezero, which is impossible.Real engines have some frictional losses; thebest achieve 60-80% of the Carnot value ofefficiency.