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Unformatted text preview: Physics 53 Thermal Physics 3 The future lies ahead. Mort Sahl Ordered and disordered energy The 1 st law tells us that internal energy can be changed either by heat fow or by work done. It makes no diFFerence to the internal energy which oF these methods we use. But there is an important practical diFFerence: Heat fow is a transFer oF energy oF random motion From one system to another. Work done by the system on an external load changes random motion oF molecules oF the system into ordered motion oF the load. Work done on the system by an external object does the reverse, converting ordered motion into random motion. Conversion between ordered and disordered energy is not the same in both directions. One can easily convert energy oF ordered motion completely into that oF random motion, but not the reverse. In Fact, processes that convert energy From ordered motion into random motion are very common. An example is the slowing and eventual stopping oF a block sliding on a rough surFace. riction converts the kinetic energy oF its original motion ordered motion, with its particles moving (on average) together into thermal energy disordered thermal motion oF the particles oF the block and surFace. But the reverse does not happen spontaneously. One never sees a block at rest absorb thermal energy From itselF and the surFace, and start to move spontaneously. A process that can happen in one order in time but not in the reverse order is called irreversible . The slowing oF the block by Friction is an irreversible process. One way to visualize the diFFerence between reversible and irreversible is to imagine the process being shown in a motion picture. IF the projector is run backwards (time is reversed) a reversible process will appear to be a diFFerent but possible process. An irreversible process will appear absurd, because we know it does not happen. A perFectly elastic collision is an example oF the Former; an egg being cooked is an example oF the latter. Although complete conversion oF disordered motion into ordered motion cannot occur, it is possible to make a device that achieves partial conversion oF random (thermal) energy into ordered energy. Such a device is called a heat engine . Heat engines oF PHY 53 1 Thermal Physics 3 which the steam engine was the frst to make an important impact have greatly changed society since the 18th century. The science oF thermodynamics grew out oF the study oF heat engines in an attempt to make them more eFfcient. We will begin by discussing heat engines, and their reverse counterpart, reFrigerators, in general. This leads to an account oF the seminal work oF Sadi Carnot in the early 1800s. Then we will discuss these issues in a more general theoretical Framework, using the concept oF entropy....
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