![]() No matter how the attempt is made or what machine or technology is em- ployed, it is not possible to recapture all of the thermal energy and do work with it. ![]() Some of this thermal energy can be used to do work, but not all. Friction and air resistance generates heat, and although the energy of motion that produces this heat is not destroyed, the conversion is not fully reversible. What the second law of thermodynam- ics says is that some of this loss is not reversible.Ĭonsider pushing a cart across a level street, an example discussed in the earlier sidebar, “The First Law of Thermodynam- ics.” Pushing a cart involves work, but unlike lifting a weight, there is little or no energy “stored” in the position of the cart-it cannot return across the street on its own or do work in the process. A lot of heat is generated by friction, for example this is wasteful, for more effort is required in the presence of friction than without it. This is one of the most important constraints of physics. The second law of thermodynamics says that it is impossible to make a machine that functions by completely converting heat, drawn from some body or object at a given temperature, into work (in the sense of physics, where a force moves something over a distance).
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