Simple Nature - Light and Matter

5.4.6 Summary of the laws of thermodynamicsHere is a summary of the laws of thermodynamics:The zeroth law of thermodynamics (page 303) If object A isat the same temperature as object B, and B is at the sametemperature as C, then A is at the same temperature as C.The first law of thermodynamics (page 298) Energyconserved.isThe second law of thermodynamics (page 314) The entropyof a closed system always increases, or at best stays the same:∆S ≥ 0.The third law of thermodynamics (page 327) The entropy ofa system approaches zero as its temperature approaches absolutezero.From a modern point of view, only the first law deserves to be calleda fundamental law of physics. Once Boltmann discovered the microscopicnature of entropy, the zeroth and second laws could beunderstood as statements about probability: a system containing alarge number of particles is overwhelmingly likely to do a certainthing, simply because the number of possible ways to do it is extremelylarge compared to the other possibilities. The third lawis also now understood to be a consequence of more basic physicalprinciples, but to explain the third law, it’s not sufficient simply toknow that matter is made of atoms: we also need to understand thequantum-mechanical nature of those atoms, discussed in chapter 13.Historically, however, the laws of thermodynamics were discoveredin the eighteenth century, when the atomic theory of matter wasgenerally considered to be a hypothesis that couldn’t be tested experimentally.Ideally, with the publication of Boltzmann’s work onentropy in 1877, the zeroth and second laws would have been immediatelydemoted from the status of physical laws, and likewise thedevelopment of quantum mechanics in the 1920’s would have donethe same for the third law.5.5 More About Heat EnginesSo far, the only heat engine we’ve discussed in any detail has beena fictitious Carnot engine, with a monoatomic ideal gas as its workinggas. As a more realistic example, figure l shows one full cycleof a cylinder in a standard gas-burning automobile engine. Thisfour-stroke cycle is called the Otto cycle, after its inventor, Germanengineer Nikolaus Otto. The Otto cycle is more complicated than aCarnot cycle, in a number of ways:328 Chapter 5 Thermodynamics