Chapter 10: Section 3

 Describe the Second Law of Thermodynamics  Explain how to calculate an engine’s efficiency  Relate entropy to an engine’s efficiency

 A heat engine is a device that uses heat to do mechanical work  This is accomplished by transferring energy from a high temperature substance to a substance with lower temperature

 Because heat engines operate in the real world, some energy always escapes into the outside world  This leads to both inefficiency and the application of the Second Law  The Second Law of Thermodynamics says that no process can convert heat energy entirely into work  Some energy must always be transferred as heat to the surroundings

 No process can completely convert heat into work, nor can heat flow from a colder body to a hotter body without work being done.  Thus, you cannot keep a continual flow of heat to work without adding energy to the system  In machine terms, you have to add energy to get more work, and the ratio of heat to work will never equal 100%

 A measure of how well an engine operates is given by the engine’s efficiency (eff)  Efficiency is a measure of the work done by an engine relative to the total energy put into the system as heat

 Efficiency is a unitless quantity  To have 100% efficiency Q c would have to equal zero  Qc = energy removed as heat  Qh = energy added as heat

 No engine has 100 percent efficiency  The smaller the fraction of usable energy an engine can provide, the lower its efficiency

 Find the efficiency of a gasoline engine that, during one cycle, receives 204 J of energy from combustion and loses 153 J of heat to the exhaust  0.250

 In any system, the particles tends to go from a very ordered set of energies to one with less order  The measure of a systems disorder is called entropy

 Since the entropy of a system naturally tends to increase, heat engines are limited in the amount of work they can do  Not all gas particles move in an orderly fashion toward the piston, giving up their energy along the way  Instead, they move in all available directions, transferring energy through collisions  Although energy is conserved, not all of it is available to do useful work