Category:Second Law and Its Applications

From ThermoWiki

…from ThermoWiki, an online resource for thermodynamics located at www.thermowiki.org; Site Revision #1263; 6 January 2009.

The industrial revolution began when people started to develop machines that extract useful work from heat rather than from changes in potential or kinetic energies. To design such processes, we must select a working fluid (such as water), provide a way to transfer heat to the fluid to increase its enthalpy, and then allow the fluid to expand so that useful work is done. Typically, the work would rotate a shaft that would drive a pump, or rotate a coil in an electric generator, or turn a screw to propel a ship. The earliest such devices—steam engines—were notoriously inefficient; usually, no more than 2% of the heat added to the fluid would be extracted as useful work. This motivates obvious questions about such devices: How can the efficiency be improved? Is there any limit to the efficiency; that is, is there a limit to the fraction of heat that can be converted into useful work? In particular, can all the heat added to the working fluid be extracted as useful work?

It is not immediately obvious what the answer to this last question might be. After all, it is easy to do the inverse; that is, we can readily convert all the work from a process into heat. For example, you can start your car's engine and let it sit idling in the driveway. All the work being done by the pistons to rotate the crankshaft would be converted into heat that is expelled to the surroundings through the radiator and engine casing. So is easy. Further, the first law (ΔU = Q + W) implies symmetry between heat and work: many changes in U can be produced by only transferring heat, or by only doing work, or by many different combinations of the two. Moreover, Joule could have gathered experimental evidence for conservation of energy by starting with workfree experiments rather than adiabatic ones. Given this apparent symmetry and the fact that is easy, it seems likely that a 100% conversion of is at least possible, if not easy. These are the kinds of questions that motivated the original development of the second law.