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Download Ch 14.3 PPT - Using Heat
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Heat and Temperature Section 3 Using Heat Chapter 14.3 Heat and Temperature Section 3 Laws of Thermodynamics 〉What happens to heat energy when it is transferred? 〉The first law of thermodynamics - total energy used in any process is conserved, whether that energy is transferred as a result of work, heat, or both. 〉The second law of thermodynamics - energy transferred as heat always moves from an object at a higher temperature to an object at a lower temperature. Heat and Temperature Section 3 Laws of Thermodynamics • Work can increase average kinetic energy by – mechanical processes: processes in which energy is transferred by work • The disorder of a system tends to increase. – Over time, in any given system left to itself, the entropy of that system will tend to increase. • entropy: a measure of the randomness or disorder of a system • Usable energy decreases in all energy transfers. – As entropy increases – usable energy decreases • Conduction and radiation may cause energy to be transferred to the surrounding. Heat and Temperature Section 3 Heat Engines 〉Heat Engine: a machine that transforms heat into mechanical energy, or work 〉 2 main types: 〉 Internal-combustion engines 〉 External-combustion engines 〉What do heat engines do? 〉 In a heat engine, potential chemical energy and internal kinetic energy are converted to mechanical energy by the process of combustion. Heat and Temperature Section 3 Heat Engines, continued • Internal-combustion engines burn fuel inside the engine. – An automobile engine is a four-stroke engine, because four strokes take place for each cycle of the piston. – The four strokes are: • intake • compression • power • exhaust • Internal-combustion engines always generate heat. – Friction and other forces cause much of the energy to be lost to the atmosphere as heat. Heat and Temperature Section 3 Heat Engines, continued • Automobile engines (Internal-combustion) use carburetors or fuel injectors. – A carburetor is the part of the engine in which liquid gasoline becomes vaporized. – Some engines have fuel injectors instead of carburetors. Heat and Temperature Section 3 Internal Combustion Engine Heat and Temperature Section 3 External-Combustion Engines • External-Combustion Engine – an (internal) working fluid is heated through the engine wall or a heat exchanger by combustion in an external source. – The fluid (by expanding and acting on the mechanism of the engine) produces motion and usable work. – The fluid is then cooled, compressed and reused (closed cycle), or (less commonly) dumped, and cool fluid pulled in (open cycle air engine). • Example: Steam Engine Heat and Temperature Section 3 External-Combustion Engine